Customize the "edit" link in bookdown

I use bookdown with the gitbook ouput to build a book. The .Rmd files used to build this book are in turn auto-generated from other source files in the following workflow: An R-Script takes folder1/file.txt and generates the file folder2/file.Rmd, which is in turn used to build the book.

So far, so good. However, when I use the edit option in _bookdown.yaml the link (naturally) refers to folder2/file.Rmd. It would be a simple thing to replace folder2 with folder1 and .Rmd with .txt, but I don't know where I should start.

In R/html.R (in the bookdown package), the link is generated within the function build_chapter using rmd_cur as input (which is "The Rmd filename of the current chapter") and the function source_link.

source_link = function(target, type) {
  if (length(target) == 0) return()
  setting = source_link_setting(type = type)
  if (is.null(setting)) return()
  button_link(sprintf(setting$link, target), setting$text)
}

How can I use this information to generate my custom link that points to folder1/file.txt instead of folder2/file.Rmd?

If you want the history and view to also link to the text file

source_link = function(target, type) {
  if (length(target) == 0) return()
  setting = source_link_setting(type = type)
  if (is.null(setting)) return()
  button_link(sprintf(setting$link, sub("folder2/(.+?)\\.[Rr]md$", "folder1/\\1.txt", target)), setting$text)
}

otherwise overload the build_chapter function:

build_chapter = function(
  head, toc, chapter, link_prev, link_next, rmd_cur, html_cur, foot
) {
  # add a has-sub class to the <li> items that has sub lists
  toc = gsub('^(<li>)(.+<ul>)$', '<li class="has-sub">\\2', toc)
  paste(c(
    head,
    '<div class="row">',
    '<div class="col-sm-12">',
    toc,
    '</div>',
    '</div>',
    '<div class="row">',
    '<div class="col-sm-12">',
    chapter,
    '<p style="text-align: center;">',
    button_link(link_prev, 'Previous'),
    source_link(sub("folder2/(.+?)\\.[Rr]md$", "folder1/\\1.txt", rmd_cur), type = 'edit'),
    source_link(rmd_cur, type = 'history'),
    source_link(rmd_cur, type = 'view'),
    button_link(link_next, 'Next'),
    '</p>',
    '</div>',
    '</div>',
    foot
  ), collapse = '\n')
}