fars: FARS Data Functions

#' Read FARS Data 
#'
#' Reads a Fatality Analysis Reporting System (FARS) file in csv format and creates a dplyr tbl_df from it.
#'  
#' @param filename A character string giving the name of the file to read. 
#' 
#' @return This function returns a \code{dplyr::tbl_df} of the file.
#' 
#' @import dplyr
#' 
#' @note If the file or permission to it does not exist, an error is produced.
#' 
#' @examples
#' test_data <- system.file("extdata", "accident_2014.csv.bz2", package = "fars")
#' fars_read(filename=test_data)
#' 
#' @references \url{https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars}
#' 
#' @export
fars_read <- function(filename) {
        if(!file.exists(filename))
                stop("file '", filename, "' does not exist")
        data <- suppressMessages({
                readr::read_csv(filename, progress = FALSE)
        })
        dplyr::tbl_df(data)
}

#' Generate FARS Filename
#'
#' Generates a standard Fatality Analysis Reporting System (FARS) filename for the year provided.
#'  
#' @param year A 4 digit year, coerced to integer.
#' 
#' @return This function returns a FARS filename in character string format.
#' 
#' @note If the year cannot be coerced to integer, NA is used.
#' 
#' @examples
#' make_filename('2013')
#' 
#' @references \url{https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars}
#' 
#' @export
make_filename <- function(year) {
        year <- as.integer(year)
        sprintf("accident_%d.csv.bz2", year)
}

#' Read FARS Data for One or More Years
#'
#' Reads a series of Fatality Analysis Reporting System (FARS) files and returns a dplyr tbl_df of the month and year columns for each file.
#'  
#' @param years A vector of 4 digit years, coerced to integers. 
#' 
#' @return This function returns a \code{dplyr::tbl_df} with month and year columns.
#' 
#' @import dplyr
#' 
#' @note If the file for a specified year, or permission to it, does not exist, an error is produced.
#' 
#' @examples
#' \dontrun{fars_read_years(2013:2015)}
#' 
#' @references \url{https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars}
#' 
#' @export
fars_read_years <- function(years) {
        lapply(years, function(year) {
                file <- make_filename(year)
                tryCatch({
                        MONTH=NULL
                        dat <- fars_read(file)
                        dplyr::mutate(dat, year = year) %>% 
                                dplyr::select(MONTH, year)
                }, error = function(e) {
                        warning("invalid year: ", year)
                        return(NULL)
                })
        })
}

#' Summarize FARS Data for One or More Years
#'
#' Reads a series of Fatality Analysis Reporting System (FARS) files and returns a frequency table of fatal crashes by month and year.
#'  
#' @inheritParams fars_read_years
#' 
#' @return This function returns a data frame of the fatality crash frequency by month (row) and year (column).
#' 
#' @import dplyr 
#' @importFrom tidyr spread
#' 
#' @note If the file for a specified year, or permission to it, does not exist, an error is produced.
#' 
#' @examples
#' \dontrun{fars_summarize_years(2013:2015)}
#' 
#' @references \url{https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars}
#' 
#' @export
fars_summarize_years <- function(years) {
        dat_list <- fars_read_years(years)
        MONTH=NULL
        year=NULL
        dplyr::bind_rows(dat_list) %>% 
                dplyr::group_by(year, MONTH) %>% 
                dplyr::summarize(n = n()) %>%
                tidyr::spread(year, n)
}

#' Map FARS Crash Fatalities
#'
#' Reads a Fatality Analysis Reporting System (FARS) file and plots a map of fatal crashes.
#'  
#' @param state.num Two digit FARS state reference number.   
#' @param year Two digit year, coerced to integer. 
#'   
#' @return This function produces a plot of the fatal crashes on a map of the state.
#' 
#' @import dplyr
#' @import maps 
#' @importFrom graphics points
#' 
#' @note If the \code{state.num} provided is not in the FARS file, an error is produced. 
#' @note If there are no fatal crashes for the state and year, a message is provided.
#' @note LONGITUD values > 900 are set to NA, LATITUDE values > 90 are set to NA
#' 
#' @examples
#' \dontrun{fars_map_state('01','2014')}
#' 
#' @references 
#' FARS data: \url{https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars};
#' State reference numbers: \url{https://crashstats.nhtsa.dot.gov} see publication 812296
#' 
#' @export
fars_map_state <- function(state.num, year) {
        filename <- make_filename(year)
        data <- fars_read(filename)
        state.num <- as.integer(state.num)
        STATE=NULL # to avoid R CMD Check note
        if(!(state.num %in% unique(data$STATE)))
                stop("invalid STATE number: ", state.num)
        data.sub <- dplyr::filter(data, STATE == state.num)
        if(nrow(data.sub) == 0L) {
                message("no accidents to plot")
                return(invisible(NULL))
        }
        is.na(data.sub$LONGITUD) <- data.sub$LONGITUD > 900
        is.na(data.sub$LATITUDE) <- data.sub$LATITUDE > 90
        with(data.sub, {
                maps::map("state", ylim = range(LATITUDE, na.rm = TRUE),
                          xlim = range(LONGITUD, na.rm = TRUE))
                graphics::points(LONGITUD, LATITUDE, pch = 46)
        })
}