R/MsBackend.R

In rformassspectrometry/Spectra: Spectra Infrastructure for Mass Spectrometry Data

#' @include hidden_aliases.R

#' @title Mass spectrometry data backends
#'
#' @aliases class:MsBackend MsBackend-class MsBackendDataFrame-class
#' @aliases MsBackendMzR-class [,MsBackend-method
#' @aliases uniqueMsLevels,MsBackend-method
#' @aliases MsBackendMemory-class
#' @aliases supportsSetBackend
#' @aliases backendBpparam
#' @aliases backendInitialize
#' @aliases backendParallelFactor,MsBackendMzR-method
#' @aliases backendParallelFactor,MsBackendHdf5Peaks-method
#' @aliases dataStorageBasePath
#' @aliases dataStorageBasePath,MsBackendMzR-method
#' @aliases dataStorageBasePath<-
#' @aliases dataStorageBasePath<-,MsBackendMzR-method
#' @aliases extractByIndex
#' @aliases msLeveL<-,MsBackend-method
#' @aliases backendRequiredSpectraVariables
#' @aliases backendRequiredSpectraVariables,MsBackend-method
#'
#' @description
#'
#' Note that the classes described here are not meant to be used
#' directly by the end-users and the material in this man page is
#' aimed at package developers.
#'
#' `MsBackend` is a virtual class that defines what each different
#' backend needs to provide. `MsBackend` objects provide access to
#' mass spectrometry data. Such backends can be classified into
#' *in-memory* or *on-disk* backends, depending on where the data, i.e
#' spectra (m/z and intensities) and spectra annotation (MS level,
#' charge, polarity, ...) are stored.
#'
#' Typically, in-memory backends keep all data in memory ensuring fast
#' data access, while on-disk backends store (parts of) their data on
#' disk and retrieve it on demand.
#'
#' The *Backend functions and implementation notes for new backend
#' classes* section documents the API that a backend must implement.
#'
#' Currently available backends are:
#'
#' - `MsBackendMemory` and `MsBackendDataFrame`: store all data in memory. The
#'   `MsBackendMemory` is optimized for accessing and processing the peak data
#'   (i.e. the numerical matrices with the m/z and intensity values) while the
#'   `MsBackendDataFrame` keeps all data in a `DataFrame`.
#'
#' - `MsBackendMzR`: stores the m/z and intensities on-disk in raw
#'   data files (typically `mzML` or `mzXML`) and the spectra
#'   annotation information (header) in memory in a `DataFrame`. This
#'   backend requires the `mzR` package.
#'
#' - `MsBackendHdf5Peaks`: stores the m/z and intensities on-disk in custom hdf5
#'   data files and the remaining spectra variables in memory (in a
#'   `DataFrame`). This backend requires the `rhdf5` package.
#'
#' See below for more details about individual backends.
#'
#' @param acquisitionNum for `filterPrecursorScan()`: `integer` with the
#'     acquisition number of the spectra to which the object should be
#'     subsetted.
#'
#' @param BPPARAM for `backendBpparam()`: parameter object from the
#'     `BiocParallel` package defining the parallel processing setup.
#'     Defaults to `BPPARAM = bpparam()`. See [bpparam()] for more
#'     information.
#'
#' @param columns For `spectraData()` accessor: optional `character` with
#'     column names (spectra variables) that should be included in the
#'     returned `DataFrame`. By default, all columns are returned.
#'     For `peaksData()` accessor: optional `character` with requested columns
#'     in the individual `matrix` of the returned `list`. Defaults to
#'     `peaksVariables(object)` and depends on what *peaks variables* the
#'     backend provides.
#'
#' @param match For `filterRanges()` and `filterValues()`: `character(1) `
#'     defining whether the condition has to match for all provided
#'     `ranges`/`values` (`match = "all"`; the default), or for any of them
#'     (`match = "any"`) for spectra to be retained.
#'
#' @param data For `backendInitialize()`: `DataFrame` with spectrum
#'     metadata/data. This parameter can be empty for `MsBackendMzR` backends
#'     but needs to be provided for `MsBackendDataFrame` backends.
#'
#' @param dataOrigin For `filterDataOrigin()`: `character` to define which
#'     spectra to keep.
#'     For `filterAcquisitionNum()`: optionally specify if filtering should
#'     occur only for spectra of selected `dataOrigin`.
#'
#' @param dataStorage For `filterDataStorage()`: `character` to define which
#'     spectra to keep.
#'     For `filterAcquisitionNum()`: optionally specify if filtering should
#'     occur only for spectra of selected `dataStorage`.
#'
#' @param drop For `[`: not considered.
#'
#' @param f `factor` defining the grouping to split `x`. See [split()]. For
#'     `filterPrecursorScan()`: factor defining from which original data files
#'     the spectra derive to avoid selecting spectra from different
#'     samples/files. Defaults to `f = dataOrigin(object)`.
#'
#' @param file For `filterFile()`: index or name of the file(s) to which the
#'     data should be subsetted. For `export()`: `character` of length 1 or
#'     equal to the number of spectra.
#'
#' @param initial For `tic()`: `logical(1)` whether the initially
#'     reported total ion current should be reported, or whether the
#'     total ion current should be (re)calculated on the actual data
#'     (`initial = FALSE`).
#'
#' @param i For `[`: `integer`, `logical` or `character` to subset the object.
#'
#' @param j For `[`: not supported.
#'
#' @param msLevel `integer` defining the MS level of the spectra to which the
#'     function should be applied. For `filterMsLevel()`: the MS level to which
#'     `object` should be subsetted.
#'
#' @param msLevel. same as `msLevel` above.
#'
#' @param mz For `filterIsolationWindow()`: `numeric(1)` with the m/z value to
#'     filter the object. For `filterPrecursorMzRange()`: `numeric(2)` with the
#'     lower and upper m/z boundary. For `filterPrecursorMzValues()`: `numeric`
#'     with the m/z value(s) to filter the object.
#'
#' @param peaksVariables For `backendInitialize()` for `MsBackendMemory`:
#'     `character` specifying which of the columns of the provided `data`
#'     contain *peaks variables* (i.e. information for individual mass
#'     peaks). Defaults to `peaksVariables = c("mz", "intensity")`. `"mz"`
#'     and `"intensity"` should **always** be specified.
#'
#' @param ppm For `filterPrecursorMzValues()`: `numeric(1)` with the
#'     m/z-relative maximal acceptable difference for a m/z to be considered
#'     matching. See [closest()] for details.
#'     For `filterValues()`: `numeric` of any length allowing to define
#'     a maximal accepted difference between user input `values` and the
#'     `spectraVariables` values.  If it is not equal to the length of the
#'     value provided with parameter `spectraVariables`, `ppm[1]` will be
#'     recycled.
#'
#' @param z For `filterPrecursorCharge()`: `integer()` with the precursor
#'     charges to be used as filter.
#'
#' @param n for `filterAcquisitionNum()`: `integer` with the acquisition
#'     numbers to filter for.
#'
#' @param name For `$` and `$<-`: the name of the spectra variable to return
#'     or set.
#'
#' @param object Object extending `MsBackend`.
#'
#' @param polarity For `filterPolarity()`: `integer` specifying the polarity to
#'     to subset `object`.
#'
#' @param ranges for `filterRanges()`: A `numeric` vector of paired values
#'     (upper and lower boundary) that define the ranges to filter the `object`.
#'     These paired values need to be in the same order as the
#'     `spectraVariables` parameter (see below).
#'
#' @param rt for `filterRt()`: `numeric(2)` defining the retention time range
#'     to be used to subset/filter `object`.
#'
#' @param spectraVariables For `selectSpectraVariables()`: `character` with the
#'     names of the spectra variables to which the backend should be subsetted.
#'     For `filterRanges()` and `filterValues()`: `character` vector specifying
#'     the column(s) from `spectraData(object)` on which to filter the data and
#'     that correspond to the the names of the spectra variables that should be
#'     used for the filtering.
#'
#' @param tolerance For `filterPrecursorMzValues()`: `numeric(1)` with the
#'     maximal absolute acceptable difference for a m/z value to be considered
#'     matching. See [closest()] for details. For `filterValues()`: `numeric`
#'     accepted tolerance between the `values` and the spectra variables.
#'     Defaults to `tolerance = 0`. If it is not equal to the length of the
#'     value provided with parameter `spectraVariables`, `tolerance[1]` will
#'     be recycled.
#'
#' @param use.names For `lengths()`: whether spectrum names should be used.
#'
#' @param value replacement value for `<-` methods. See individual
#'     method description or expected data type.
#'
#' @param values for `filterValues()`: A `numeric` vector that define the
#'     values to filter the `object`. `values` needs to be of same length than
#'     parameter `spectraVariables` and in the same order.
#'
#' @param x Object extending `MsBackend`.
#'
#' @param ... Additional arguments.
#'
#'
#' @section Implementation notes:
#'
#' Backends extending `MsBackend` **must** implement all of its methods (listed
#' above). Developers of new `MsBackend`s should follow the
#' `MsBackendMemory` implementation. To ensure a new implementation being
#' conform with the `MsBackend` definition, developers should included test
#' suites provided by this package in their unit test setup. For that a variable
#' `be` should be created in the package's `"testthat.R"` file that represents
#' a (initialized) instance of the developed backend. Then the path to the
#' test suites should be defined with
#' `test_suite <- system.file("test_backends", "test_MsBackend",
#' package = "Spectra")` followed by `test_dir(test_suite)` to run all test
#' files in that directory. Individual unit test files could be run with
#' `test_file(file.path(test_suite, "test_spectra_variables.R"),
#' stop_on_failure = TRUE)` (note that without `stop_on_failure = TRUE` tests
#' would fail silently) . Adding this code to the packages `"testthat.R"` file
#' ensures that all tests checking the validity of an `MsBackend` instance
#' defined in the `Spectra` package are also run on the newly develped backend
#' class.
#'
#' The `MsBackend` defines the following slots:
#'
#' - `@readonly`: `logical(1)` whether the backend supports writing/replacing
#'   of m/z or intensity values.
#'
#' @section Backend functions:
#'
#' New backend classes **must** extend the base `MsBackend` class will have to
#' implement some of the following methods (see the `MsBackend` vignette for
#' detailed description and examples):
#'
#' - `[`: subset the backend. Only subsetting by element (*row*/`i`) is
#'   allowed. Parameter `i` should support `integer` indices and `logical`
#'   and should throw an error if `i` is out of bounds. The
#'   `MsCoreUtils::i2index` could be used to check the input `i`.
#'   For `i = integer()` an empty backend should be returned. Implementation
#'   of this method is optional, as the default calls the `extractByIndex()`
#'   method (which has to be implemented as the main subsetting method).
#'
#' - `$`, `$<-`: access or set/add a single spectrum variable (column) in the
#'   backend. Using a `value` of `NULL` should allow deleting the specified
#'   spectra variable. An error should be thrown if the spectra variable is not
#'   available.
#'
#' - `[[`, `[[<-`: access or set/add a single spectrum variable (column) in the
#'   backend. The default implementation uses `$`, thus these methods don't have
#'   to be implemented for new classes extending `MsBackend`.
#'
#' - `acquisitionNum()`: returns the acquisition number of each
#'   spectrum. Returns an `integer` of length equal to the number of
#'   spectra (with `NA_integer_` if not available).
#'
#' - `backendBpparam()`: return the parallel processing setup supported by
#'   the backend class. This function can be used by any higher
#'   level function to evaluate whether the provided parallel processing
#'   setup (or the default one returned by `bpparam()`) is supported
#'   by the backend. Backends not supporting parallel processing (e.g.
#'   because they contain a connection to a database that can not be
#'   shared across processes) should extend this method to return only
#'   `SerialParam()` and hence disable parallel processing for (most)
#'   methods and functions. See also `backendParallelFactor()` for a
#'   function to provide a preferred splitting of the backend for parallel
#'   processing.
#'
#' - `backendInitialize()`: initialises the backend. This method is
#'   supposed to be called rights after creating an instance of the
#'   backend class and should prepare the backend (e.g. set the data
#'   for the memory backend or read the spectra header data for the
#'   `MsBackendMzR` backend). Parameters can be defined freely for each
#'   backend, depending on what is needed to initialize the backend. It
#'   is however suggested to also support a parameter `data` that can be
#'   used to submit the full spectra data as a `DataFrame` to the
#'   backend. This would allow the backend to be also usable for the
#'   [setBackend()] function from `Spectra`. Note that eventually (for
#'   *read-only* backends) also the `supportsSetBackend` method would need
#'   to be implemented to return `TRUE`.
#'   The `backendInitialize()` method has also to ensure to correctly set
#'   spectra variable `dataStorage`.
#'
#' - `backendMerge()`: merges (combines) `MsBackend` objects into a single
#'   instance. All objects to be merged have to be of the same type (e.g.
#'   [MsBackendDataFrame()]).
#'
#' - `backendParallelFactor()`: returns a `factor` defining an optimal
#'   (preferred) way how the backend can be split for parallel processing
#'   used for all peak data accessor or data manipulation functions.
#'   The default implementation returns a factor of length 0 (`factor()`)
#'   providing thus no default splitting. `backendParallelFactor()` for
#'   `MsBackendMzR` on the other hand returns `factor(dataStorage(object))`
#'   hence suggesting to split the object by data file.
#'
#' - `backendRequiredSpectraVariables()`: returns a `character` with spectra
#'   variable names that are mandatory for a specific backend. The default
#'   returns an empty `character()`. The implementation for `MsBackendMzR`
#'   returns `c("dataStorage", "scanIndex")` as these two spectra variables
#'   are required to load the MS data on-the-fly. This method needs only to
#'   be implemented if a backend requires specific variables to be defined.
#'
#' - `dataOrigin()`: gets a `character` of length equal to the number of
#'   spectra in `object` with the *data origin* of each spectrum. This could
#'   e.g. be the mzML file from which the data was read.
#'
#' - `dataStorage()`: gets a `character` of length equal to the number of
#'   spectra in `object` with the data storage of each spectrum. Note that
#'   missing values (`NA_character_`) are not supported for `dataStorage`.
#'
#' - `dataStorageBasePath()`, `dataStorageBasePath<-: gets or sets the common
#'   *base* path of the directory containing all data files. If supported,
#'   the function is expected to return (or accept) a `character` of length 1.
#'   Most backends (such as for example the `MsBackendMemory` will not support
#'   this function and `dataStorageBasePath()` will return `NA_character_`.
#'   For `MsBackendMzR`, this function allows to get or change the path to the
#'   directory containing the original data files, which is required if e.g.
#'   a serialized `MsBackendMzR` instance gets copied to another computer or
#'   file system.
#'
#' - `dropNaSpectraVariables()`: removes spectra variables (i.e. columns in the
#'   object's `spectraData` that contain only missing values (`NA`). Note that
#'   while columns with only `NA`s are removed, a `spectraData()` call after
#'   `dropNaSpectraVariables()` might still show columns containing `NA` values
#'   for *core* spectra variables.
#'
#' - `centroided()`, `centroided<-`: gets or sets the centroiding
#'   information of the spectra. `centroided()` returns a `logical`
#'   vector of length equal to the number of spectra with `TRUE` if a
#'   spectrum is centroided, `FALSE` if it is in profile mode and `NA`
#'   if it is undefined. See also `isCentroided()` for estimating from
#'   the spectrum data whether the spectrum is centroided.  `value`
#'   for `centroided<-` is either a single `logical` or a `logical` of
#'   length equal to the number of spectra in `object`.
#'
#' - `collisionEnergy()`, `collisionEnergy<-`: gets or sets the
#'   collision energy for all spectra in `object`. `collisionEnergy()`
#'   returns a `numeric` with length equal to the number of spectra
#'   (`NA_real_` if not present/defined), `collisionEnergy<-` takes a
#'   `numeric` of length equal to the number of spectra in `object`.
#'
#' - `export()`: exports data from a `Spectra` class to a file. This method is
#'   called by the `export,Spectra` method that passes itself as a second
#'   argument to the function. The `export,MsBackend` implementation is thus
#'   expected to take a `Spectra` class as second argument from which all data
#'   is exported. Taking data from a `Spectra` class ensures that also all
#'   eventual data manipulations (cached in the `Spectra`'s lazy evaluation
#'   queue) are applied prior to export - this would not be possible with only a
#'   [MsBackend] class. An example implementation is the `export()` method
#'   for the `MsBackendMzR` backend that supports export of the data in
#'   *mzML* or *mzXML* format. See the documentation for the `MsBackendMzR`
#'   class below for more information.
#'
#' - `extractByIndex()`: function to subset a backend to selected elements
#'   defined by the provided index. Similar to `[`, this method should allow
#'   extracting (or to subset) the data in any order. In contrast to `[`,
#'   however, `i` is expected to be an `integer` (while `[` should also
#'   support `logical` and eventually `character`). While being apparently
#'   redundant to `[`, this methods avoids package namespace errors/problems
#'   that can result in implementations of `[` being not found by R (which
#'   can happen sometimes in parallel processing using the [SnowParam()]). This
#'   method is used internally by `Spectra` to extract/subset its backend.
#'   Implementation of this method is mandatory.
#'
#' - `filterAcquisitionNum()`: filters the object keeping only spectra matching
#'   the provided acquisition numbers (argument `n`). If `dataOrigin` or
#'   `dataStorage` is also provided, `object` is subsetted to the spectra with
#'   an acquisition number equal to `n` **in spectra with matching dataOrigin
#'   or dataStorage values** retaining all other spectra.
#'
#' - `filterDataOrigin()`: filters the object retaining spectra matching the
#'   provided `dataOrigin`. Parameter `dataOrigin` has to be of type
#'   `character` and needs to match exactly the data origin value of the
#'   spectra to subset.
#'   `filterDataOrigin()` should return the data ordered by the provided
#'   `dataOrigin` parameter, i.e. if `dataOrigin = c("2", "1")` was provided,
#'   the spectra in the resulting object should be ordered accordingly (first
#'   spectra from data origin `"2"` and then from `"1"`).
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterDataStorage()`: filters the object retaining spectra matching the
#'   provided `dataStorage`. Parameter `dataStorage` has to be of type
#'   `character` and needs to match exactly the data storage value of the
#'   spectra to subset.
#'   `filterDataStorage()` should return the data ordered by the provided
#'   `dataStorage` parameter, i.e. if `dataStorage = c("2", "1")` was provided,
#'   the spectra in the resulting object should be ordered accordingly (first
#'   spectra from data storage `"2"` and then from `"1"`).
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterEmptySpectra()`: removes empty spectra (i.e. spectra without peaks).
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterFile()`: retains data of files matching the file index or file name
#'    provided with parameter `file`.
#'
#' - `filterIsolationWindow()`: retains spectra that contain `mz` in their
#'   isolation window m/z range (i.e. with an `isolationWindowLowerMz` `<=` `mz`
#'   and `isolationWindowUpperMz` `>=` `mz`.
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterMsLevel()`: retains spectra of MS level `msLevel`.
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterPolarity()`: retains spectra of polarity `polarity`.
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterPrecursorMzRange()` (previously `filterPrecursorMz`): retains
#'   spectra with a precursor m/z within the provided m/z range.
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterPrecursorMzValues()`: retains spectra with a precursor m/z matching
#'   any of the provided m/z values (given `ppm` and `tolerance`).
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterPrecursorCharge()`: retains spectra with the defined precursor
#'   charge(s).
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterPrecursorScan()`: retains parent (e.g. MS1) and children scans (e.g.
#'    MS2) of acquisition number `acquisitionNum`. Parameter `f` is supposed to
#'   define the origin of the spectra (i.e. the original data file) to ensure
#'   related spectra from the same file/sample are selected and retained.
#'   Implementation of this method is optional since a default implementation
#'   for `MsBackend` is available.
#'
#' - `filterRanges()`: allows filtering of the `Spectra` object based on user
#'    defined *numeric* ranges (parameter `ranges`) for one or more available
#'    spectra variables in object (spectra variable names can be specified with
#'    parameter `spectraVariables`). Spectra for which the value of a spectra
#'    variable is within it's defined range are retained. If multiple
#'    ranges/spectra variables are defined, the `match` parameter can be used
#'    to specify whether all conditions (`match = "all"`; the default) or if
#'    any of the conditions must match (`match = "any"`; all spectra for which
#'    values are within any of the provided ranges are retained).
#'    Implementation of this method is optional since a default implementation
#'    for `MsBackend` is available.
#'
#' - `filterRt()`: retains spectra of MS level `msLevel` with retention times
#'    within (`>=`) `rt[1]` and (`<=`) `rt[2]`.
#'    Implementation of this method is optional since a default implementation
#'    for `MsBackend` is available.
#'
#' - `filterValues()`: allows filtering of the `Spectra` object based on
#'    similarities of *numeric* values of one or more `spectraVariables(object)`
#'    (parameter `spectraVariables`) to provided values (parameter `values`)
#'    given acceptable differences (parameters tolerance and ppm). If multiple
#'    values/spectra variables are defined, the `match` parameter can be used
#'    to specify whether all conditions (`match = "all"`; the default) or if
#'    any of the conditions must match (`match = "any"`; all spectra for which
#'    values are within any of the provided ranges are retained).
#'    Implementation of this method is optional since a default implementation
#'    for `MsBackend` is available.
#'
#' - `intensity()`: gets the intensity values from the spectra. Returns
#'   a [NumericList()] of `numeric` vectors (intensity values for each
#'   spectrum). The length of the `list` is equal to the number of
#'   `spectra` in `object`.
#'
#' - `intensity<-`: replaces the intensity values. `value` has to be a `list`
#'   (or [NumericList()]) of length equal to the number of spectra and the
#'   number of values within each list element identical to the number of
#'   peaks in each spectrum (i.e. the `lengths(x)`). Note that just
#'   writeable backends support this method.
#'
#' - `ionCount()`: returns a `numeric` with the sum of intensities for
#'   each spectrum. If the spectrum is empty (see `isEmpty()`),
#'   `NA_real_` is returned.
#'
#' - `isCentroided()`: a heuristic approach assessing if the spectra in
#'   `object` are in profile or centroided mode. The function takes
#'   the `qtl` th quantile top peaks, then calculates the difference
#'   between adjacent m/z value and returns `TRUE` if the first
#'   quartile is greater than `k`. (See `Spectra:::.peaks_is_centroided` for
#'   the code.)
#'
#' - `isEmpty()`: checks whether a spectrum in `object` is empty
#'   (i.e. does not contain any peaks). Returns a `logical` vector of
#'   length equal number of spectra.
#'
#' - `isolationWindowLowerMz()`, `isolationWindowLowerMz<-`: gets or sets the
#'   lower m/z boundary of the isolation window.
#'
#' - `isolationWindowTargetMz()`, `isolationWindowTargetMz<-`: gets or sets the
#'   target m/z of the isolation window.
#'
#' - `isolationWindowUpperMz()`, `isolationWindowUpperMz<-`: gets or sets the
#'   upper m/z boundary of the isolation window.
#'
#' - `isReadOnly()`: returns a `logical(1)` whether the backend is *read
#'   only* or does allow also to write/update data.
#'
#' - `length()`: returns the number of spectra in the object.
#'
#' - `lengths()`: gets the number of peaks (m/z-intensity values) per
#'   spectrum.  Returns an `integer` vector (length equal to the
#'   number of spectra). For empty spectra, `0` is returned.
#'
#' - `msLevel()`: gets the spectra's MS level. Returns an `integer`
#'   vector (of length equal to the number of spectra) with the MS
#'   level for each spectrum (or `NA_integer_` if not available).
#'
#' - `msLevel<-`: replaces the spectra's MS level.
#'
#' - `mz()`: gets the mass-to-charge ratios (m/z) from the
#'   spectra. Returns a [NumericList()] or length equal to the number of
#'   spectra, each element a `numeric` vector with the m/z values of
#'   one spectrum.
#'
#' - `mz<-`: replaces the m/z values. `value` has to be a `list` of length equal
#'   to the number of spectra and the number of values within each list element
#'   identical to the number of peaks in each spectrum (i.e. the
#'   `lengths(x)`). Note that just writeable backends support this method.
#'
#' - `polarity()`, `polarity<-`: gets or sets the polarity for each
#'   spectrum.  `polarity()` returns an `integer` vector (length equal
#'   to the number of spectra), with `0` and `1` representing negative
#'   and positive polarities, respectively. `polarity<-` expects an
#'   integer vector of length 1 or equal to the number of spectra.
#'
#' - `precursorCharge()`, `precursorIntensity()`, `precursorMz()`,
#'   `precScanNum()`, `precAcquisitionNum()`: get the charge (`integer`),
#'   intensity (`numeric`), m/z (`numeric`), scan index (`integer`)
#'   and acquisition number (`interger`) of the precursor for MS level
#'   2 and above spectra from the object. Returns a vector of length equal to
#'   the number of spectra in `object`. `NA` are reported for MS1
#'   spectra of if no precursor information is available.
#'
#' - `peaksData()` returns a `list` with the spectras' peak data, i.e. m/z and
#'   intensity values or other *peak variables*. The length of the list is
#'   equal to the number of spectra in `object`. Each element of the list has
#'   to be a two-dimensional array (`matrix` or `data.frame`)
#'   with columns depending on the provided `columns` parameter (by default
#'   `"mz"` and `"intensity"`, but depends on the backend's available
#'   `peaksVariables`). For an empty spectrum, a `matrix` (`data.frame`) with
#'   0 rows and columns according to `columns` is returned. The optional
#'   parameter `columns`, if supported by the backend, allows to define which
#'   peak variables should be returned in the `numeric` peak `matrix`. As a
#'   default `c("mz", "intensity")` should be used.
#'
#' - `peaksData<-` replaces the peak data (m/z and intensity values) of the
#'   backend. This method expects a `list` of two dimensional arrays (`matrix`
#'   or `data.frame`) with columns representing the peak variables. All
#'   existing peaks data is expected to be replaced with these new values. The
#'   length of the `list` has to match the number of spectra of `object`.
#'   Note that only writeable backends need to support this method.
#'
#' - `peaksVariables()`: lists the available variables for mass peaks. Default
#'   peak variables are `"mz"` and `"intensity"` (which all backends need to
#'   support and provide), but some backends might provide additional variables.
#'   All these variables are expected to be returned (if requested) by the
#'   `peaksData()` function.
#'
#' - `reset()` a backend (if supported). This method will be called on the
#'   backend by the `reset,Spectra` method that is supposed to restore the data
#'   to its original state (see `reset,Spectra` for more details). The function
#'   returns the *reset* backend. The default implementation for `MsBackend`
#'   returns the backend as-is.
#'
#' - `rtime()`, `rtime<-`: gets or sets the retention times for each
#'   spectrum (in seconds). `rtime()` returns a `numeric` vector (length equal
#'   to the number of spectra) with the retention time for each spectrum.
#'   `rtime<-` expects a numeric vector with length equal to the
#'   number of spectra.
#'
#' - `scanIndex()`: returns an `integer` vector with the *scan index*
#'   for each spectrum. This represents the relative index of the
#'   spectrum within each file. Note that this can be different to the
#'   `acquisitionNum()` of the spectrum which is the index of the
#'   spectrum as reported in the mzML file.
#'
#' - `selectSpectraVariables()`: reduces the information within the backend to
#'   the selected spectra variables. It is suggested to **not** remove values
#'   for the `"dataStorage"` variable, since this might be required for some
#'   backends to work properly (such as the `MsBackendMzR`).
#'
#' - `smoothed()`,`smoothed<-`: gets or sets whether a spectrum is
#'   *smoothed*. `smoothed()` returns a `logical` vector of length equal
#'   to the number of spectra. `smoothed<-` takes a `logical` vector
#'   of length 1 or equal to the number of spectra in `object`.
#'
#' - `spectraData()`, `spectraData<-`: gets or sets general spectrum
#'   metadata (annotation, also called header).  `spectraData()` returns
#'   a `DataFrame`, `spectraData<-` expects a `DataFrame` with the same number
#'   of rows as there are spectra in `object`. Note that `spectraData()` has to
#'   return the full data, i.e. also the m/z and intensity values (as a `list`
#'   or `SimpleList` in columns `"mz"` and `"intensity"`.
#'
#' - `spectraNames()`: returns a `character` vector with the names of
#'   the spectra in `object` or `NULL` if not set. `spectraNames<-` allows to
#'   set spectra names (if the object is not read-only).
#'
#' - `spectraVariables()`: returns a `character` vector with the
#'   available spectra variables (columns, fields or attributes)
#'   available in `object`. This should return **all** spectra variables which
#'   are present in `object`, also `"mz"` and `"intensity"` (which are by
#'   default not returned by the `spectraVariables,Spectra` method).
#'
#' - `split()`: splits the backend into a `list` of backends (depending on
#'   parameter `f`). The default method for `MsBackend` uses [split.default()],
#'   thus backends extending `MsBackend` don't necessarily need to implement
#'   this method.
#'
#' - `supportsSetBackend()`: whether a `MsBackend` supports the `Spectra`
#'   `setBackend()` function. For a `MsBackend` to support `setBackend()` it
#'   needs to have a parameter called `data` in its `backendInitialize()` method
#'   that support receiving all spectra data as a `DataFrame` from another
#'   backend and to initialize the backend with this data. In general
#'   *read-only* backends do not support `setBackend()` hence, the default
#'   implementation of `supportsSetBackend()` returns `!isReadOnly(object)`. If
#'   a read-only backend would support the `setBackend()` and being initialized
#'   with a `DataFrame` an implementation of this method for that backend could
#'   be defined that returns `TRUE` (see also the `MsBackend` vignette for
#'   details and examples).
#'
#' - `tic()`: gets the total ion current/count (sum of signal of a
#'   spectrum) for all spectra in `object`. By default, the value
#'   reported in the original raw data file is returned. For an empty
#'   spectrum, `NA_real_` is returned.
#'
#' - `uniqueMsLevels()`: gets the unique MS levels of all spectra in `object`.
#'   The default implementation calls `unique(msLevel(object))` but more
#'   efficient implementations could be defined for specific backends.
#'
#' @section Subsetting and merging backend classes:
#'
#' Backend classes must support (implement) the `[` method to subset the object.
#' This method should only support subsetting by spectra (rows, `i`) and has
#' to return a `MsBackend` class.
#'
#' Backends extending `MsBackend` should also implement the `backendMerge()`
#' method to support combining backend instances (only backend classes of the
#' same type should be merged). Merging should follow the following rules:
#'
#' - The whole spectrum data of the various objects should be merged. The
#'   resulting merged object should contain the union of the individual objects'
#'   spectra variables (columns/fields), with eventually missing variables in
#'   one object being filled with `NA`.
#'
#' @section In-memory data backends: `MsBackendMemory` and `MsBackendDataFrame`:
#'
#' The `MsBackendMemory` and `MsBackendDataFrame` objects keep all MS data in
#' memory are thus ideal for fast data processing. Due to their large memory
#' footprint they are however not suited for large scale experiments. The two
#' backends store the data different. The `MsBackendDataFrame` stores
#' all data in a `DataFrame` and thus supports also S4-classes as
#' spectra variables. Also, sepratate access to m/z or intensity values (i.e.
#' using the `mz()` and `intensity()` methods) is faster for the
#' `MsBackendDataFrame`. The `MsBackendMemory` on the other hand, due to the
#' way the data is organized internally, provides much faster access to the
#' full peak data (i.e. the numerical matrices of m/z and intensity values).
#' Also subsetting and access to any spectra variable (except `"mz"` and
#' `"intensity"`) is fastest for the `MsBackendMemory`.
#'
#' Thus, for most use cases, the `MsBackendMemory` provides a higher
#' performance and flexibility than the `MsBackendDataFrame` and should thus be
#' preferred. See also issue
#' [246](https://github.com/rformassspectrometry/Spectra/issues/246) for a
#' performance comparison.
#'
#' New objects can be created with the `MsBackendMemory()` and
#' `MsBackendDataFrame()` function, respectively. Both backends can be
#' subsequently initialized with the `backendInitialize()` method, taking a
#' `DataFrame` (or `data.frame`) with the (full) MS data as first parameter
#' `data`. The second parameter `peaksVariables` allows to define which columns
#' in `data` contain *peak variables* such as the m/z and intensity values of
#' individual peaks per spectrum. The default for this parameter is
#' `peaksVariables = c("mz", "intensity")`. Note that it is not supported to
#' provide either `"mz"` or `"intensity"`, if provided, both need to be
#' present in the data frame. Alternatively, the function also supports a data
#' frame without m/z and intensity values, in which case a `Spectra` without
#' mass peaks is created.
#'
#' Suggested columns of this `DataFrame` are:
#'
#' - `"msLevel"`: `integer` with MS levels of the spectra.
#' - `"rt"`: `numeric` with retention times of the spectra.
#' - `"acquisitionNum"`: `integer` with the acquisition number of the spectrum.
#' - `"scanIndex"`: `integer` with the index of the scan/spectrum within the
#'   *mzML*/*mzXML*/*CDF* file.
#' - `"dataOrigin"`: `character` defining the *data origin*.
#' - `"dataStorage"`: `character` indicating grouping of spectra in different
#'   e.g. input files. Note that missing values are not supported.
#' - `"centroided"`: `logical` whether the spectrum is centroided.
#' - `"smoothed"`: `logical` whether the spectrum was smoothed.
#' - `"polarity"`: `integer` with the polarity information of the spectra.
#' - `"precScanNum"`: `integer` specifying the index of the (MS1) spectrum
#'   containing the precursor of a (MS2) spectrum.
#' - `"precursorMz"`: `numeric` with the m/z value of the precursor.
#' - `"precursorIntensity"`: `numeric` with the intensity value of the
#'   precursor.
#' - `"precursorCharge"`: `integer` with the charge of the precursor.
#' - `"collisionEnergy"`: `numeric` with the collision energy.
#' - `"mz"`: [NumericList()] of `numeric` vectors representing the m/z values
#'   for each spectrum.
#' - `"intensity"`: [NumericList()] of `numeric` vectors representing the
#'   intensity values for each spectrum.
#'
#' Additional columns are allowed too.
#'
#' The `peaksData()` function for `MsBackendMemory` and `MsBackendDataFrame`
#' returns a `list` of `numeric` `matrix` by default (with parameter
#' `columns = c("mz", "intensity")`). If other peak variables are requested,
#' a `list` of `data.frame` is returned (ensuring m/z and intensity values
#' are always `numeric`).
#'
#'
#' @section `MsBackendMzR`, on-disk MS data backend:
#'
#' The `MsBackendMzR` keeps only a limited amount of data in memory,
#' while the spectra data (m/z and intensity values) are fetched from
#' the raw files on-demand. This backend uses the `mzR` package for
#' data import and retrieval and hence requires that package to be
#' installed. Also, it can only be used to import and represent data
#' stored in *mzML*, *mzXML* and *CDF* files.
#'
#' The `MsBackendMzR` backend extends the `MsBackendDataFrame` backend using
#' its `DataFrame` to keep spectra variables (except m/z and intensity) in
#' memory.
#'
#' New objects can be created with the `MsBackendMzR()` function which
#' can be subsequently filled with data by calling `backendInitialize()`
#' passing the file names of the input data files with argument `files`.
#'
#' This backend provides an `export()` method to export data from a `Spectra` in
#' *mzML* or *mzXML* format. The definition of the function is:
#'
#' `export(object, x, file = tempfile(), format = c("mzML", "mzXML"),
#'         copy = FALSE)`
#'
#' The parameters are:
#' - `object`: an instance of the `MsBackendMzR` class.
#' - `x`: the [Spectra] object to be exported.
#' - `file`: `character` with the (full) output file name(s). Should be
#'   of length 1 or equal `length(x)`. If a single file is specified, all
#'   spectra are exported to that file. Alternatively it is possible to specify
#'   for each spectrum in `x` the name of the file to which it should be
#'   exported (and hence `file` has to be of length equal `length(x)`).
#' - `format`: `character(1)`, either `"mzML"` or `"mzXML"` defining the output
#'   file format.
#' - `copy`: `logical(1)` whether general file information should be copied from
#'   the original MS data files. This only works if `x` uses a `MsBackendMzR`
#'   backend and if `dataOrigin(x)` contains the original MS data file names.
#' - `BPPARAM`: parallel processing settings.
#'
#' See examples in [Spectra] or the vignette for more details and
#' examples.
#'
#' The `MsBackendMzR` ignores parameter `columns` of the `peaksData()`
#' function and returns **always** m/z and intensity values.
#'
#'
#' @section `MsBackendHdf5Peaks`, on-disk MS data backend:
#'
#' The `MsBackendHdf5Peaks` keeps, similar to the `MsBackendMzR`, peak data
#' (i.e. m/z and intensity values) in custom data files (in HDF5 format) on
#' disk while the remaining spectra variables are kept in memory. This backend
#' supports updating and writing of manipulated peak data to the data files.
#'
#' New objects can be created with the `MsBackendHdf5Peaks()` function which
#' can be subsequently filled with data by calling the object's
#' `backendInitialize()` method passing the desired file names of the HDF5 data
#' files along with the spectra variables in form of a `DataFrame` (see
#' `MsBackendDataFrame` for the expected format). An optional parameter
#' `hdf5path` allows to specify the folder where the HDF5 data files should be
#' stored to. If provided, this is added as the path to the submitted file
#' names (parameter `files`).
#'
#' By default `backendInitialize()` will store all peak data into a single HDF5
#' file which name has to be provided with the parameter `files`. To store peak
#' data across several HDF5 files `data` has to contain a column
#' `"dataStorage"` that defines the grouping of spectra/peaks into files: peaks
#' for spectra with the same value in `"dataStorage"` are saved into the same
#' HDF5 file. If parameter `files` is omitted, the value in `dataStorage` is
#' used as file name (replacing any file ending with `".h5"`. To specify the
#' file names, `files`' length has to match the number of unique elements in
#' `"dataStorage"`.
#'
#' For details see examples on the [Spectra()] help page.
#'
#' The `MsBackendHdf5Peaks` ignores parameter `columns` of the `peaksData()`
#' function and returns **always** m/z and intensity values.
#'
#'
#' @section Implementation notes:
#'
#' Backends extending `MsBackend` **must** implement all of its methods (listed
#' above). Developers of new `MsBackend`s should follow the
#' `MsBackendDataFrame` implementation.
#'
#' The [MsBackendCached()] backend provides a caching mechanism to allow
#' *read only* backends to add or change spectra variables. This
#' backend shouldn't be used on its own, but is meant to be extended. See
#' [MsBackendCached()] for details.
#'
#' The `MsBackend` defines the following slots:
#'
#' - `@readonly`: `logical(1)` whether the backend supports writing/replacing
#'   of m/z or intensity values.
#'
#' @name MsBackend
#'
#' @return See documentation of respective function.
#'
#' @author Johannes Rainer, Sebastian Gibb, Laurent Gatto, Philippine Louail
#'
#' @md
#'
#' @exportClass MsBackend MsBackendDataFrame MsBackendMzR
#'
#' @examples
#'
#' ## The MsBackend class is a virtual class and can not be instantiated
#' ## directly. Below we define a new backend class extending this virtual
#' ## class
#' MsBackendDummy <- setClass("MsBackendDummy", contains = "MsBackend")
#' MsBackendDummy()
#'
#' ## This class inherits now all methods from `MsBackend`, all of which
#' ## however throw an error. These methods would have to be implemented
#' ## for the new backend class.
#' try(mz(MsBackendDummy()))
#'
#' ## See `MsBackendDataFrame` as a reference implementation for a backend
#' ## class (in the *R/MsBackendDataFrame.R* file).
#'
#' ## MsBackendDataFrame
#' ##
#' ## The `MsBackendDataFrame` uses a `S4Vectors::DataFrame` to store all MS
#' ## data. Below we create such a backend by passing a `DataFrame` with all
#' ## data to it.
#' data <- DataFrame(msLevel = c(1L, 2L, 1L), scanIndex = 1:3)
#' data$mz <- list(c(1.1, 1.2, 1.3), c(1.4, 54.2, 56.4, 122.1), c(15.3, 23.2))
#' data$intensity <- list(c(3, 2, 3), c(45, 100, 12.2, 1), c(123, 12324.2))
#'
#' ## Backends are supposed to be created with their specific constructor
#' ## function
#' be <- MsBackendDataFrame()
#'
#' be
#'
#' ## The `backendInitialize()` method initializes the backend filling it with
#' ## data. This method can take any parameters needed for the backend to
#' ## get loaded with the data (e.g. a file name from which to load the data,
#' ## a database connection or, in this case, a data frame containing the data).
#' be <- backendInitialize(be, data)
#'
#' be
#'
#' ## Data can be accessed with the accessor methods
#' msLevel(be)
#'
#' mz(be)
#'
#' ## Even if no data was provided for all spectra variables, its accessor
#' ## methods are supposed to return a value.
#' precursorMz(be)
#'
#' ## The `peaksData()` method is supposed to return the peaks of the spectra as
#' ## a `list`.
#' peaksData(be)
#'
#' ## List available peaks variables
#' peaksVariables(be)
#'
#' ## Use columns to extract specific peaks variables. Below we extract m/z and
#' ## intensity values, but in reversed order to the default.
#' peaksData(be, columns = c("intensity", "mz"))
#'
#' ## List available spectra variables (i.e. spectrum metadata)
#' spectraVariables(be)
#'
#' ## Extract precursor m/z, rtime, MS level spectra variables
#' spectraData(be, c("precursorMz", "rtime", "msLevel"))
#'
#' ## MsBackendMemory
#' ##
#' ## The `MsBackendMemory` uses a more efficient internal data organization
#' ## and allows also adding arbitrary additional peaks variables (annotations)
#' ## Below we thus add a column "peak_ann" with arbitrary names/ids for each
#' ## peak and add the name of this column to the `peaksVariables` parameter
#' ## of the `backendInitialize()` method (in addition to `"mz"` and
#' ## `"intensity"` that should **always** be specified.
#' data$peak_ann <- list(c("a", "", "d"), c("", "d", "e", "f"), c("h", "i"))
#' be <- backendInitialize(MsBackendMemory(), data,
#'     peaksVariables = c("mz", "intensity", "peak_ann"))
#' be
#'
#' spectraVariables(be)
#'
#' ## peak_ann is also listed as a peaks variable
#' peaksVariables(be)
#'
#' ## The additional peaks variable can be accessed using the `peaksData()`
#' ## function
#' peaksData(be, "peak_ann")
#'
#' ## The $<- method can be used to replace values of an existing peaks
#' ## variable. It is important that the number of elements matches the
#' ## number of peaks per spectrum.
#' be$peak_ann <- list(1:3, 1:4, 1:2)
#'
#' ## A peaks variable can again be removed by setting it to NULL
#' be$peak_ann <- NULL
#'
#' peaksVariables(be)
NULL

setClass(
    "MsBackend",
    contains = "VIRTUAL",
    slots = c(
        readonly = "logical",
        version = "character"),
    prototype = prototype(readonly = FALSE, version = "0.1"))

#' @importFrom methods .valueClassTest is new validObject
#'
#' @noRd
setValidity("MsBackend", function(object) {
    msg <- .valid_ms_backend_data_storage(dataStorage(object))
    if (length(dataStorage(object)) != length(object))
        msg <- c(msg, "length of object and 'dataStorage' have to match")
    if (is.null(msg)) TRUE
    else msg
})

#' @importMethodsFrom ProtGenerics backendBpparam
#'
#' @exportMethod backendBpparam
#'
#' @rdname MsBackend
#'
#' @export
setMethod("backendBpparam", signature = "MsBackend",
          function(object, BPPARAM = bpparam()) {
              BPPARAM
          })

#' @exportMethod backendInitialize
#'
#' @importMethodsFrom ProtGenerics backendInitialize
#'
#' @rdname MsBackend
#'
#' @export
setMethod("backendInitialize", signature = "MsBackend", function(object, ...) {
    validObject(object)
    object
})

#' @importMethodsFrom ProtGenerics backendMerge
#'
#' @rdname MsBackend
setMethod("backendMerge", "list", function(object, ...) {
    backendMerge(object[[1]], object[-1])
})

#' @exportMethod backendMerge
#'
#' @rdname MsBackend
#'
#' @export
setMethod("backendMerge", "MsBackend", function(object, ...) {
    stop("Not implemented for ", class(object), ".")
})

#' @importMethodsFrom ProtGenerics backendParallelFactor
#'
#' @exportMethod backendParallelFactor
#'
#' @rdname MsBackend
#'
#' @export
setMethod("backendParallelFactor", "MsBackend", function(object, ...) {
    factor()
})

#' @export
setMethod("backendRequiredSpectraVariables", "MsBackend",
          function(object, ...) {
              character()
          })

#' @rdname MsBackend
#'
#' @export
setMethod("export", "MsBackend", function(object, ...) {
    stop(class(object), " does not support export of data; please provide a ",
         "backend that supports data export with parameter 'backend'.")
})

#' @exportMethod acquisitionNum
#'
#' @importMethodsFrom ProtGenerics acquisitionNum
#'
#' @rdname MsBackend
#'
#' @export
setMethod("acquisitionNum", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod peaksData
#'
#' @importMethodsFrom ProtGenerics peaksData
#'
#' @rdname MsBackend
#'
#' @export
setMethod("peaksData", "MsBackend", function(object,
                                             columns = c("mz", "intensity")) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod peaksVariables
#'
#' @importMethodsFrom ProtGenerics peaksVariables
#'
#' @rdname MsBackend
#'
#' @export
setMethod("peaksVariables", "MsBackend", function(object) {
    c("mz", "intensity")
})

#' @exportMethod centroided
#'
#' @aliases centroided<-,MsBackend-method
#'
#' @importMethodsFrom ProtGenerics centroided
#'
#' @rdname MsBackend
#'
#' @export
setMethod("centroided", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod centroided<-
#'
#' @importMethodsFrom ProtGenerics centroided<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("centroided", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod collisionEnergy
#'
#' @importMethodsFrom ProtGenerics collisionEnergy
#'
#' @rdname MsBackend
#'
#' @export
setMethod("collisionEnergy", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod collisionEnergy<-
#'
#' @importMethodsFrom ProtGenerics collisionEnergy<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("collisionEnergy", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod dataOrigin
#'
#' @importMethodsFrom ProtGenerics dataOrigin
#'
#' @rdname MsBackend
#'
#' @export
setMethod("dataOrigin", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod dataOrigin<-
#'
#' @importMethodsFrom ProtGenerics dataOrigin<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("dataOrigin", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod dataStorage
#'
#' @importMethodsFrom ProtGenerics dataStorage
#'
#' @rdname MsBackend
#'
#' @export
setMethod("dataStorage", "MsBackend", function(object) {
    stop("Method 'dataStorage' is not implemented for ", class(object), ".")
})

#' @exportMethod dataStorage<-
#'
#' @importMethodsFrom ProtGenerics dataStorage<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("dataStorage", "MsBackend", function(object, value) {
    stop("Method 'dataStorage' is not implemented for ", class(object), ".")
})

#' @exportMethod dropNaSpectraVariables
#'
#' @rdname MsBackend
#'
#' @export
setMethod("dropNaSpectraVariables", "MsBackend", function(object) {
    svs <- spectraVariables(object)
    req_cols <- c(backendRequiredSpectraVariables(object), c("mz", "intensity"))
    svs <- svs[!(svs %in% req_cols)]
    spd <- spectraData(object, columns = svs)
    keep <- !vapply1l(spd, function(z) {
        allna <- all(is.na(z))
        if (length(allna) > 1)
            FALSE
        else allna
    })
    selectSpectraVariables(object, c(svs[keep], req_cols))
})

#' @rdname MsBackend
#'
#' @importFrom methods existsMethod
#'
#' @export
setMethod("extractByIndex", c("MsBackend", "ANY"), function(object, i) {
    if (existsMethod("[", class(object)[1L]))
        object[i = i]
    else stop("'extractByIndex' not implemented for ", class(object)[1L], ".")
})

#' @rdname MsBackend
#'
#' @export
setMethod("extractByIndex", c("MsBackend", "missing"), function(object, i) {
    object
})

#' @exportMethod filterAcquisitionNum
#'
#' @importMethodsFrom ProtGenerics filterAcquisitionNum
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterAcquisitionNum", "MsBackend", function(object, n, file, ...) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod filterDataOrigin
#'
#' @importMethodsFrom ProtGenerics filterDataOrigin
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterDataOrigin", "MsBackend",
          function(object, dataOrigin = character()) {
              if (length(dataOrigin)) {
                  object <- object[dataOrigin(object) %in% dataOrigin]
                  if (is.unsorted(dataOrigin))
                      object[order(match(dataOrigin(object), dataOrigin))]
                  else object
              } else object
          })

#' @exportMethod filterDataStorage
#'
#' @importMethodsFrom ProtGenerics filterDataStorage
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterDataStorage", "MsBackend",
          function(object, dataStorage = character()) {
              if (length(dataStorage)) {
                  object <- object[dataStorage(object) %in% dataStorage]
                  if (is.unsorted(dataStorage))
                      object[order(match(dataStorage(object), dataStorage))]
                  else object
              } else object
          })

#' @exportMethod filterEmptySpectra
#'
#' @importMethodsFrom ProtGenerics filterEmptySpectra
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterEmptySpectra", "MsBackend", function(object, ...) {
    if (!length(object)) return(object)
    object[as.logical(lengths(object))]
})

#' @exportMethod filterIsolationWindow
#'
#' @importMethodsFrom ProtGenerics filterIsolationWindow
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterIsolationWindow", "MsBackend",
          function(object, mz = numeric(), ...) {
              if (length(mz)) {
                  if (length(mz) > 1)
                      stop("'mz' is expected to be a single m/z value")
                  keep <- which(isolationWindowLowerMz(object) <= mz &
                                isolationWindowUpperMz(object) >= mz)
                  object[keep]
              } else object
          })

#' @exportMethod filterMsLevel
#'
#' @importMethodsFrom ProtGenerics filterMsLevel
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterMsLevel", "MsBackend",
          function(object, msLevel = integer()) {
              if (length(msLevel)) {
                  object[msLevel(object) %in% msLevel]
              } else object
          })

#' @exportMethod filterPolarity
#'
#' @importMethodsFrom ProtGenerics filterPolarity
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterPolarity", "MsBackend",
          function(object, polarity = integer()) {
              if (length(polarity))
                  object[polarity(object) %in% polarity]
              else object
          })

#' @exportMethod filterPrecursorMzRange
#'
#' @importMethodsFrom ProtGenerics filterPrecursorMzRange
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterPrecursorMzRange", "MsBackend",
          function(object, mz = numeric()) {
              if (length(mz)) {
                  mz <- range(mz)
                  keep <- which(between(precursorMz(object), mz))
                  object[keep]
              } else object
          })

#' @importMethodsFrom ProtGenerics filterPrecursorMz
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterPrecursorMz", "MsBackend",
          function(object, mz = numeric()) {
              filterPrecursorMzRange(object, mz)
          })

#' @exportMethod filterPrecursorMzValues
#'
#' @importMethodsFrom ProtGenerics filterPrecursorMzValues
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterPrecursorMzValues", "MsBackend",
          function(object, mz = numeric(), ppm = 20, tolerance = 0) {
              if (length(mz)) {
                  object[.values_match_mz(precursorMz(object), mz = mz,
                                          ppm = ppm, tolerance = tolerance)]
              } else object
          })

#' @exportMethod filterPrecursorCharge
#'
#' @importMethodsFrom ProtGenerics filterPrecursorCharge
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterPrecursorCharge", "MsBackend",
          function(object, z = integer()) {
              if (length(z)) {
                  keep <- which(precursorCharge(object) %in% z)
                  object[keep]
              } else object
          })

#' @exportMethod filterPrecursorScan
#'
#' @importMethodsFrom ProtGenerics filterPrecursorScan
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterPrecursorScan", "MsBackend",
          function(object, acquisitionNum = integer(), f = dataOrigin(object)) {
              if (length(acquisitionNum) && length(f)) {
                  if (!is.factor(f))
                      f <- factor(f, exclude = character())
                  keep <- unsplit(lapply(split(object, f = f), function(z, an) {
                      .filterSpectraHierarchy(acquisitionNum(z),
                                              precScanNum(z),
                                              an)
                  }, an = acquisitionNum), f = f)
                  object[keep]
              } else object
          })

#' @exportMethod filterRanges
#'
#' @importMethodsFrom ProtGenerics filterRanges
#' @importFrom MsCoreUtils between
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterRanges", "MsBackend",
          function(object, spectraVariables = character(), ranges = numeric(),
                   match = c("all", "any")){
              if (!length(spectraVariables) || !length(ranges))
                  return(object)
              if (!is.numeric(ranges))
                  stop("filterRanges only support filtering for numerical ",
                  "'spectraVariables'")
              match <- match.arg(match)
              if (is.character(spectraVariables)){
                  if(!all(spectraVariables %in% spectraVariables(object)))
                      stop("One or more values passed with parameter ",
                           "'spectraVariables' are not available as spectra ",
                           "variables in object. Use the 'spectraVariables()' ",
                           "function to list possible values.")
              } else
                  stop("The 'spectraVariables' parameter needs to be of type ",
                  "'character'.")
              if (length(spectraVariables) != length(ranges) / 2)
                  stop("Length of 'ranges' needs to be twice the length of ",
                       "the parameter 'spectraVariables' and define the lower ",
                       "and upper bound for values of each spectra variable ",
                       "defined with parameter 'spectraVariables'.")
              query <- spectraData(object, columns = spectraVariables)
              nc <- ncol(query)
              within_ranges <- vapply(seq_len(nc), function(i) {
                  pairs <-  c(ranges[2*i - 1], ranges[2*i])
                  between(query[[i]], pairs)
              }, logical(nrow(query)))
              if (match == "all")
                  keep <- which(rowSums(within_ranges, na.rm = FALSE) == nc)
              else
                  keep <- which(rowSums(within_ranges, na.rm = FALSE) > 0)
              object <- object[keep]
          })

#' @exportMethod filterRt
#'
#' @importMethodsFrom ProtGenerics filterRt
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterRt", "MsBackend",
          function(object, rt = numeric(), msLevel. = uniqueMsLevels(object)) {
              if (length(rt)) {
                  rt <- range(rt)
                  sel_ms <- msLevel(object) %in% msLevel.
                  sel_rt <- between(rtime(object), rt) & sel_ms
                  object[sel_rt | !sel_ms]
              } else object
          })

#' @exportMethod filterValues
#'
#' @importMethodsFrom ProtGenerics filterValues
#'
#' @importFrom MsCoreUtils ppm
#'
#' @rdname MsBackend
#'
#' @export
setMethod("filterValues", "MsBackend",
          function(object, spectraVariables = character(), values = numeric(),
                   ppm = 0, tolerance = 0, match = c("all", "any")){
              if (!is.numeric(values))
                  stop("filterValues only support filtering for numerical ",
                       "'spectraVariables'")
              nsv <- length(spectraVariables)
              if (is.character(spectraVariables)){
                  if(!all(spectraVariables %in% spectraVariables(object)))
                      stop("One or more values passed with parameter ",
                           "'spectraVariables' are not available as spectra ",
                           "variables in object. Use the 'spectraVariables()' ",
                           "function to list possible values.")
              } else
                  stop("'spectraVariables' needs to be of type character.")
              if (nsv != length(values))
                  stop("Length of 'values' needs to be same length as the ",
                       "parameter 'spectraVariables'.")
              if (length(ppm) != nsv){
                  ppm <- rep(ppm[1], nsv)
                  warning("Length of 'ppm' does not match the amount of ",
                          "'spectraVariables', the first value of the vector ",
                          "will be recycled")
              }
              if (length(tolerance) != nsv){
                  tolerance <- rep(tolerance[1], nsv)
                  warning("Length of 'tolerance' does not match the amount of ",
                          "'spectraVariables', the first value of the vector ",
                          "will be recycled.")
              }

              ## create ranges
              lower_bounds <- values - (tolerance + ppm(values, ppm))
              upper_bounds <- values + (tolerance + ppm(values, ppm))
              ranges <- c(rbind(lower_bounds, upper_bounds))

              object <- filterRanges(object, spectraVariables, ranges,
                                     match)
          })

#' @exportMethod intensity
#'
#' @importMethodsFrom ProtGenerics intensity
#'
#' @rdname MsBackend
#'
#' @export
setMethod("intensity", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod intensity<-
#'
#' @importMethodsFrom ProtGenerics intensity<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("intensity", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod ionCount
#'
#' @importMethodsFrom ProtGenerics ionCount
#'
#' @importFrom MsCoreUtils vapply1d
#'
#' @rdname MsBackend
#'
#' @export
setMethod("ionCount", "MsBackend", function(object) {
    vapply1d(intensity(object), sum, na.rm = TRUE)
})

#' @exportMethod isCentroided
#'
#' @importMethodsFrom ProtGenerics isCentroided
#' @importFrom MsCoreUtils vapply1l
#'
#' @rdname MsBackend
#'
#' @export
setMethod("isCentroided", "MsBackend", function(object, ...) {
    vapply1l(peaksData(object), .peaks_is_centroided)
})

#' @exportMethod isEmpty
#'
#' @rdname MsBackend
#'
#' @importMethodsFrom S4Vectors isEmpty
#'
#' @export
setMethod("isEmpty", "MsBackend", function(x) {
    stop("Not implemented for ", class(x), ".")
})

#' @exportMethod isolationWindowLowerMz
#'
#' @importMethodsFrom ProtGenerics isolationWindowLowerMz
#'
#' @rdname MsBackend
#'
#' @export
setMethod("isolationWindowLowerMz", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod isolationWindowLowerMz<-
#'
#' @importMethodsFrom ProtGenerics isolationWindowLowerMz<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("isolationWindowLowerMz", "MsBackend", function(object,
                                                                 value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod isolationWindowTargetMz
#'
#' @importMethodsFrom ProtGenerics isolationWindowTargetMz
#'
#' @rdname MsBackend
#'
#' @export
setMethod("isolationWindowTargetMz", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod isolationWindowTargetMz<-
#'
#' @importMethodsFrom ProtGenerics isolationWindowTargetMz<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("isolationWindowTargetMz", "MsBackend", function(object,
                                                                  value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod isolationWindowUpperMz
#'
#' @importMethodsFrom ProtGenerics isolationWindowUpperMz
#'
#' @rdname MsBackend
#'
#' @export
setMethod("isolationWindowUpperMz", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod isolationWindowUpperMz<-
#'
#' @importMethodsFrom ProtGenerics isolationWindowUpperMz<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("isolationWindowUpperMz", "MsBackend", function(object,
                                                                 value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod isReadOnly
#'
#' @importMethodsFrom ProtGenerics isReadOnly
#'
#' @rdname MsBackend
#'
#' @export
setMethod("isReadOnly", "MsBackend", function(object) {
    object@readonly
})

#' @exportMethod length
#'
#' @rdname MsBackend
#'
#' @export
setMethod("length", "MsBackend", function(x) {
    stop("Not implemented for ", class(x), ".")
})

#' @exportMethod msLevel
#'
#' @importMethodsFrom ProtGenerics msLevel
#'
#' @rdname MsBackend
#'
#' @export
setMethod("msLevel", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @importMethodsFrom ProtGenerics msLevel<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("msLevel", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod mz
#'
#' @importMethodsFrom ProtGenerics mz
#'
#' @rdname MsBackend
#'
#' @export
setMethod("mz", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod mz<-
#'
#' @importMethodsFrom ProtGenerics mz<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("mz", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @rdname MsBackend
setMethod("lengths", "MsBackend", function(x, use.names = FALSE) {
    stop("Not implemented for ", class(x), ".")
})

#' @exportMethod polarity
#'
#' @importMethodsFrom ProtGenerics polarity
#'
#' @rdname MsBackend
#'
#' @export
setMethod("polarity", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod polarity<-
#'
#' @importMethodsFrom ProtGenerics polarity<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("polarity", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod precScanNum
#'
#' @importMethodsFrom ProtGenerics precScanNum
#'
#' @rdname MsBackend
#'
#' @export
setMethod("precScanNum", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod precursorCharge
#'
#' @importMethodsFrom ProtGenerics precursorCharge
#'
#' @rdname MsBackend
#'
#' @export
setMethod("precursorCharge", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod precursorIntensity
#'
#' @importMethodsFrom ProtGenerics precursorIntensity
#'
#' @rdname MsBackend
#'
#' @export
setMethod("precursorIntensity", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod precursorMz
#'
#' @importMethodsFrom ProtGenerics precursorMz
#'
#' @rdname MsBackend
#'
#' @export
setMethod("precursorMz", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod precursorMz<-
#'
#' @importMethodsFrom ProtGenerics precursorMz<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("precursorMz", "MsBackend", function(object, ..., value) {
    object$precursorMz <- value
    object
})

#' @exportMethod peaksData<-
#'
#' @importMethodsFrom ProtGenerics peaksData<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("peaksData", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod reset
#'
#' @rdname MsBackend
#'
#' @export
setMethod("reset", "MsBackend", function(object) {
    object
})

#' @exportMethod rtime
#'
#' @importMethodsFrom ProtGenerics rtime
#'
#' @rdname MsBackend
#'
#' @export
setMethod("rtime", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod rtime<-
#'
#' @importMethodsFrom ProtGenerics rtime<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("rtime", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod scanIndex
#'
#' @importMethodsFrom ProtGenerics scanIndex
#'
#' @rdname MsBackend
#'
#' @export
setMethod("scanIndex", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod selectSpectraVariables
#'
#' @rdname MsBackend
#'
#' @export
setMethod(
    "selectSpectraVariables", "MsBackend",
    function(object, spectraVariables = spectraVariables(object)) {
        stop("Not implemented for ", class(object), ".")
    })

#' @exportMethod smoothed
#'
#' @importMethodsFrom ProtGenerics smoothed
#'
#' @rdname MsBackend
#'
#' @export
setMethod("smoothed", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod smoothed<-
#'
#' @aliases smoothed<-,MsBackend-method
#'
#' @importMethodsFrom ProtGenerics smoothed<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("smoothed", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod spectraData
#'
#' @rdname MsBackend
#'
#' @export
setMethod(
    "spectraData", "MsBackend",
    function(object, columns = spectraVariables(object)) {
        stop("Not implemented for ", class(object), ".")
    })

#' @exportMethod spectraData<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("spectraData", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod spectraNames
#'
#' @importMethodsFrom ProtGenerics spectraNames
#'
#' @rdname MsBackend
#'
#' @export
setMethod("spectraNames", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod spectraNames<-
#'
#' @importMethodsFrom ProtGenerics spectraNames<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("spectraNames", "MsBackend", function(object, value) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod spectraVariables
#'
#' @importMethodsFrom ProtGenerics spectraVariables
#'
#' @rdname MsBackend
#'
#' @export
setMethod("spectraVariables", "MsBackend", function(object) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod split
#'
#' @importMethodsFrom S4Vectors split
#'
#' @rdname MsBackend
#'
#' @export
setMethod("split", "MsBackend", function(x, f, drop = FALSE, ...) {
    split.default(x, f, drop = drop, ...)
})

#' @importMethodsFrom ProtGenerics supportsSetBackend
#'
#' @exportMethod supportsSetBackend
#'
#' @rdname MsBackend
#'
#' @export
setMethod("supportsSetBackend", "MsBackend", function(object, ...) {
    !isReadOnly(object)
})

#' @exportMethod tic
#'
#' @importMethodsFrom ProtGenerics tic
#'
#' @rdname MsBackend
#'
#' @export
setMethod("tic", "MsBackend", function(object, initial = TRUE) {
    stop("Not implemented for ", class(object), ".")
})

#' @exportMethod [
#'
#' @rdname MsBackend
#'
#' @export
setMethod("[", "MsBackend", function(x, i, j, ..., drop = FALSE) {
    extractByIndex(x, i2index(i, length = length(x)))
})

#' @exportMethod $
#'
#' @rdname MsBackend
#'
#' @export
setMethod("$", "MsBackend", function(x, name) {
    stop("Not implemented for ", class(x), ".")
})

#' @exportMethod $<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("$", "MsBackend", function(x, name, value) {
    stop("Not implemented for ", class(x), ".")
})

#' @exportMethod [[
#'
#' @rdname MsBackend
#'
#' @export
setMethod("[[", "MsBackend", function(x, i, j, ...) {
    if (!is.character(i))
        stop("'i' is supposed to be a character defining the spectra ",
             "variable to access.")
    if (!missing(j))
        stop("'j' is not supported.")
    do.call("$", list(x, i))
})

#' @exportMethod [[<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod("[[", "MsBackend", function(x, i, j, ..., value) {
    if (!is.character(i))
        stop("'i' is supposed to be a character defining the spectra ",
             "variable to replace or create.")
    if (!missing(j))
        stop("'j' is not supported.")
    do.call("$<-", list(x, i, value))
})

#' @exportMethod uniqueMsLevels
#'
#' @importMethodsFrom ProtGenerics uniqueMsLevels
#'
#' @rdname MsBackend
#'
#' @export
setMethod("uniqueMsLevels", "MsBackend", function(object, ...) {
    unique(msLevel(object))
})

#' @exportMethod dataStorageBasePath
#'
#' @rdname MsBackend
#'
#' @export
setMethod("dataStorageBasePath", "MsBackend", function(object) {
    NA_character_
})

#' @exportMethod dataStorageBasePath<-
#'
#' @rdname MsBackend
#'
#' @export
setReplaceMethod(
    "dataStorageBasePath", "MsBackend", function(object, value) {
        warning(class(object)[1L], " does not support changing",
                " 'dataStorageBasePath'.")
        object
    })