R/fansi-package.R

In fansi: ANSI Control Sequence Aware String Functions

## Copyright (C) Brodie Gaslam
##
## This file is part of "fansi - ANSI Control Sequence Aware String Functions"
##
## This program is free software: you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation, either version 2 or 3 of the License.
##
## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.
##
## Go to <https://www.r-project.org/Licenses> for copies of the licenses.

#' Details About Manipulation of Strings Containing Control Sequences
#'
#' Counterparts to R string manipulation functions that account for
#' the effects of some ANSI X3.64 (a.k.a. ECMA-48, ISO-6429) control sequences.
#'
#' @section Control Characters and Sequences:
#'
#' Control characters and sequences are non-printing inline characters or
#' sequences initiated by them that can be used to modify terminal display and
#' behavior, for example by changing text color or cursor position.
#'
#' We will refer to X3.64/ECMA-48/ISO-6429 control characters and sequences as
#' "_Control Sequences_" hereafter.
#'
#' There are four types of _Control Sequences_ that `fansi` can treat
#' specially:
#'
#' * "C0" control characters, such as tabs and carriage returns (we include
#'   delete in this set, even though technically it is not part of it).
#' * Sequences starting in "ESC&#91;", also known as Control Sequence
#'   Introducer (CSI) sequences, of which the Select Graphic Rendition (SGR)
#'   sequences used to format terminal output are a subset.
#' * Sequences starting in "ESC&#93;", also known as Operating System
#'   Commands (OSC), of which the subset beginning with "8" is used to encode
#'   URI based hyperlinks.
#' * Sequences starting in "ESC" and followed by something other than "&#91;" or
#'   "&rbrack;".
#'
#' _Control Sequences_ starting with ESC are assumed to be two characters
#' long (including the ESC) unless they are of the CSI or OSC variety, in which
#' case their length is computed as per the [ECMA-48
#' specification](https://www.ecma-international.org/publications-and-standards/standards/ecma-48/),
#' with the exception that [OSC hyperlinks](#osc-hyperlinks) may be terminated
#' with BEL ("\\a") in addition to ST ("ESC\\").  `fansi` handles most common
#' _Control Sequences_ in its parsing algorithms, but it is not a conforming
#' implementation of ECMA-48.  For example, there are non-CSI/OSC escape
#' sequences that may be longer than two characters, but `fansi` will
#' (incorrectly) treat them as if they were two characters long.  There are many
#' more unimplemented ECMA-48 specifications.
#'
#' In theory it is possible to encode CSI sequences with a single byte
#' introducing character in the 0x40-0x5F range instead of the traditional
#' "ESC&#91;".  Since this is rare and it conflicts with UTF-8 encoding, `fansi`
#' does not support it.
#'
#' Within _Control Sequences_, `fansi` further distinguishes CSI SGR and OSC
#' hyperlinks by recording format specification and URIs into string state, and
#' applying the same to any output strings according to the semantics of the
#' functions in use.  CSI SGR and OSC hyperlinks are known together as _Special
#' Sequences_.  See the following sections for details.
#'
#' Additionally, all _Control Sequences_, whether special or not,
#' do not count as characters, graphemes, or display width.  You can cause
#' `fansi` to treat particular _Control Sequences_ as regular characters with
#' the `ctl` parameter.
#'
#' @section CSI SGR Control Sequences:
#'
#' **NOTE**: not all displays support CSI SGR sequences; run
#' [`term_cap_test`] to see whether your display supports them.
#'
#' CSI SGR Control Sequences are the subset of CSI sequences that can be
#' used to change text appearance (e.g. color).  These sequences begin with
#' "ESC&#91;" and end in "m".  `fansi` interprets these sequences and writes new
#' ones to the output strings in such a way that the original formatting is
#' preserved.  In most cases this should be transparent to the user.
#'
#' Occasionally there may be mismatches between how `fansi` and a display
#' interpret the CSI SGR sequences, which may produce display artifacts.  The
#' most likely source of artifacts are _Control Sequences_ that move
#' the cursor or change the display, or that `fansi` otherwise fails to
#' interpret, such as:
#'
#' * Unknown SGR substrings.
#' * "C0" control characters like tabs and carriage returns.
#' * Other escape sequences.
#'
#' Another possible source of problems is that different displays parse
#' and interpret control sequences differently.  The common CSI SGR sequences
#' that you are likely to encounter in formatted text tend to be treated
#' consistently, but less common ones are not.  `fansi` tries to hew by the
#' ECMA-48 specification **for CSI SGR control sequences**, but not all
#' terminals do.
#'
#' The most likely source of problems will be 24-bit CSI SGR sequences.
#' For example, a 24-bit color sequence such as "ESC&#91;38;2;31;42;4" is a
#' single foreground color to a terminal that supports it, or separate
#' foreground, background, faint, and underline specifications for one that does
#' not.  `fansi` will always interpret the sequences according to ECMA-48, but
#' it will warn you if encountered sequences exceed those specified by
#' the `term.cap` parameter or the "fansi.term.cap" global option.
#'
#' `fansi` will will also warn if it encounters _Control Sequences_ that it
#' cannot interpret.  You can turn off warnings via the `warn` parameter, which
#' can be set globally via the "fansi.warn" option.  You can work around "C0"
#' tabs characters by turning them into spaces first with [`tabs_as_spaces`] or
#' with the `tabs.as.spaces` parameter available in some of the `fansi`
#' functions
#'
#' `fansi` interprets CSI SGR sequences in cumulative "Graphic Rendition
#' Combination Mode".  This means new SGR sequences add to rather than replace
#' previous ones, although in some cases the effect is the same as replacement
#' (e.g. if you have a color active and pick another one).
#'
#' @section OSC Hyperlinks:
#'
#' Operating System Commands are interpreted by terminal emulators typically to
#' engage actions external to the display of text proper, such as setting a
#' window title or changing the active color palette.
#'
#' [Some terminals](https://iterm2.com/documentation-escape-codes.html) have
#' added support for associating URIs to text with OSCs in a similar way to
#' anchors in HTML, so `fansi` interprets them and outputs or terminates them as
#' needed.  For example:
#'
#' ```
#' "\033]8;;xy.z\033\\LINK\033]8;;\033\\"
#' ```
#'
#' Might be interpreted as link to the URI "x.z".  To make the encoding pattern
#' clearer, we replace "\033]" with "&lt;OSC&gt;" and "\033\\\\" with
#' "&lt;ST&gt;" below:
#'
#' ```
#' <OSC>8;;URI<ST>LINK TEXT<OSC>8;;<ST>
#' ```
#'
#' @section State Interactions:
#'
#' The cumulative nature of state as specified by SGR or OSC hyperlinks means
#' that unterminated strings that are spliced will interact with each other.
#' By extension, a substring does not inherently contain all the information
#' required to recreate its state as it appeared in the source document. The
#' default `fansi` configuration terminates extracted substrings and prepends
#' original state to them so they present on a stand-alone basis as they did as
#' part of the original string.
#'
#' To allow state in substrings to affect subsequent strings set `terminate =
#' FALSE`, but you will need to manually terminate them or deal with the
#' consequences of not doing so (see "Terminal Quirks").
#'
#' By default, `fansi` assumes that each element in an input character vector is
#' independent, but this is incorrect if the input is a single document with
#' each element a line in it.  In that situation state from each line should
#' bleed into subsequent ones.  Setting `carry = TRUE` enables the "single
#' document" interpretation.
#'
#' To most closely approximate what `writeLines(x)` produces on your terminal,
#' where `x` is a stateful string, use `writeLines(fansi_fun(x, carry=TRUE,
#' terminate=FALSE))`.  `fansi_fun` is a stand-in for any of the `fansi` string
#' manipulation functions.  Note that even with a seeming "null-op" such as
#' `substr_ctl(x, 1, nchar_ctl(x), carry=TRUE, terminate=FALSE)` the output
#' control sequences may not match the input ones, but the output _should_ look
#' the same if displayed to the terminal.
#'
#' `fansi` strings will be affected by any active state in strings they are
#' appended to.  There are no parameters to control what happens in this case,
#' but `fansi` provides functions that can help the user get the desired
#' behavior.  `state_at_end` computes the active state the end of a string,
#' which can then be prepended onto the _input_ of `fansi` functions so that
#' they are aware of the active style at the beginning of the string.
#' Alternatively, one could use `close_state(state_at_end(...))` and pre-pend
#' that to the _output_ of `fansi` functions so they are unaffected by preceding
#' SGR.  One could also just prepend "ESC&#91;0m", but in some cases as
#' described in [`?normalize_state`][normalize_state] that is sub-optimal.
#'
#' If you intend to combine stateful `fansi` manipulated strings with your own,
#' it may be best to set `normalize = TRUE` for improved compatibility (see
#' [`?normalize_state`][normalize_state].)
#'
#' @section Terminal Quirks:
#'
#' Some terminals (e.g. OS X terminal, ITerm2) will pre-paint the entirety of a
#' new line with the currently active background before writing the contents of
#' the line.  If there is a non-default active background color, any unwritten
#' columns in the new line will keep the prior background color even if the new
#' line changes the background color.  To avoid this be sure to use `terminate =
#' TRUE` or to manually terminate each line with e.g.  "ESC&#91;0m".  The
#' problem manifests as:
#'
#' ```
#' " " = default background
#' "#" = new background
#' ">" = start new background
#' "!" = restore default background
#'
#' +-----------+
#' | abc\n     |
#' |>###\n     |
#' |!abc\n#####| <- trailing "#" after newline are from pre-paint
#' | abc       |
#' +-----------+
#' ```
#'
#' The simplest way to avoid this problem is to split input strings by any
#' newlines they contain, and use `terminate = TRUE` (the default).  A more
#' complex solution is to pad with spaces to the terminal window width before
#' emitting the newline to ensure the pre-paint is overpainted with the current
#' line's prevailing background color.
#'
#' @section Encodings / UTF-8:
#'
#' `fansi` will convert any non-ASCII strings to UTF-8 before processing them,
#' and `fansi` functions that return strings will return them encoded in UTF-8.
#' In some cases this will be different to what base R does.  For example,
#' `substr` re-encodes substrings to their original encoding.
#'
#' Interpretation of UTF-8 strings is intended to be consistent with base R.
#' There are three ways things may not work out exactly as desired:
#'
#' 1. `fansi`, despite its best intentions, handles a UTF-8 sequence differently
#'    to the way R does.
#' 2. R incorrectly handles a UTF-8 sequence.
#' 3. Your display incorrectly handles a UTF-8 sequence.
#'
#' These issues are most likely to occur with invalid UTF-8 sequences,
#' combining character sequences, and emoji.  For example, whether special
#' characters such as emoji are considered one or two wide evolves as software
#' implements newer versions the Unicode databases.
#'
#' Internally, `fansi` computes the width of most UTF-8 character sequences
#' outside of the ASCII range using the native `R_nchar` function.  This will
#' cause such characters to be processed slower than ASCII characters.  Unlike R
#' (at least as of version 4.1), `fansi` can account for graphemes.
#'
#' Because `fansi` implements its own internal UTF-8 parsing it is possible
#' that you will see results different from those that R produces even on
#' strings without _Control Sequences_.
#'
#' @section Overflow:
#'
#' The maximum length of input character vector elements allowed by `fansi` is
#' the 32 bit INT_MAX, excluding the terminating NULL.  As of R4.1 this is the
#' limit for R character vector elements generally, but is enforced at the C
#' level by `fansi` nonetheless.
#'
#' It is possible that during processing strings that are shorter than INT_MAX
#' would become longer than that. `fansi` checks for that overflow and will
#' stop with an error if that happens.  A work-around for this situation is to
#' break up large strings into smaller ones.  The limit is on each element of a
#' character vector, not on the vector as a whole.  `fansi` will also error on
#' your system if `R_len_t`, the R type used to measure string lengths, is less
#' than the processed length of the string.
#'
#' @section R < 3.2.2 support:
#'
#' Nominally you can build and run this package in R versions between 3.1.0 and
#' 3.2.1.  Things should mostly work, but please be aware we do not run the test
#' suite under versions of R less than 3.2.2.  One key degraded capability is
#' width computation of wide-display characters.  Under R < 3.2.2 `fansi` will
#' assume every character is 1 display width.  Additionally, `fansi` may not
#' always report malformed UTF-8 sequences as it usually does.  One
#' exception to this is [`nchar_ctl`] as that is just a thin wrapper around
#' [`base::nchar`].
#'
#' @useDynLib fansi, .registration=TRUE, .fixes="FANSI_"
#' @docType package
#' @aliases fansi-package
#' @name fansi

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