#' Converts an R reStructuredText document (Rrst) to R Markdown (Rmd)
#'
#' @param input character string with the path of the file to convert
#' @param output optional character string with the path for the output file
#' @param out_dir optional character string with the output directory. Not used
#'   if \code{output} is supplied.
#' @param output_file logical whether to return the output to a file or not.
#' If \code{FALSE}, then \code{output} and \code{out_dir} are ignored.
#'
#' @importFrom readr read_file write_file
#' @importFrom tools file_path_sans_ext

rrst_to_rmd <- function(input, output, out_dir, output_file = TRUE) {
    library(readr)
    library(tools)

    if (!file.exists(input))
        stop(sprintf('Unable to find %s', input))

    # Find basic file name and file path
    input_name <- file_path_sans_ext(basename(input))

    # Read file into R
    txt <- read_file(input)

    #### Conversions -----------------------------------------------------------
    # h1
    txt_edit <- gsub('\\n~~~~*\\n', '\n======\n', txt)
    # h2
    txt_edit <- gsub('\\n\\+\\+\\+\\+*\\n', '\n------\n', txt_edit)
    # h3
    txt_edit <- gsub('\\n\\!\\!\\!\\!*\\n', '\\!\\!\\!\\!\\!\\!\\!\\!\n',
                     txt_edit)
    txt_edit <- gsub('(\\n\\n){1}(.+?)(\\!\\!\\!\\!\\!\\!\\!\\!)',
                     '\n\n### \\2', txt_edit, perl = TRUE)

    # inline math
    txt_edit <- gsub('(?<!`)(?<=(?!``).)`(?!`)', '$', txt_edit, perl = TRUE)
    txt_edit <- gsub(":math:", '', txt_edit)

    # inline code chunks
    txt_edit <- gsub('``', '`', txt_edit)

    # display code chunks
    txt_edit <- gsub('\\n\\.\\. \\{', '\n```{', txt_edit)
    txt_edit <- gsub('\\n\\.\\. \\.\\.', '\n```', txt_edit)

    # display math
    txt_edit <- gsub('(math::)(\\n\\n)', '\\1', txt_edit, perl = TRUE)
    txt_edit <- gsub('(math::)(\\s+)', '\\1', txt_edit)
    txt_edit <- gsub('math::.+?\\K(\\n{1} +)', 'PLACEHOLDER', txt_edit,
                     perl = TRUE)
    txt_edit <- gsub('math::.*\\K\\n\\n', '\n$$\n\n', txt_edit, perl = TRUE)
    txt_edit <- gsub('.. math::', '\n$$\n    ', txt_edit, perl = TRUE)
    txt_edit <- gsub('PLACEHOLDER', '\n        ', txt_edit, perl = TRUE)

    # Cleanup
    txt_edit <- gsub('\\n\\n\\n', '\n\n', txt_edit)

    # Return output ------------------------------------------------------------
    if (isTRUE(output_file)) {
        if (missing(output)) {
            if (missing(out_dir))
                input_path <- dirname(input)
            else input_path <- out_dir
            output <- paste0(input_path, '/', input_name, '.Rmd')
        }
        write_file(txt_edit, path = output)
    }
    else
        return(txt_edit)
}