Adding anchor links to equations in Quarto

post_add_equation_links.R

## expects to be passed HTML that has already been read with readLines()
insert_anchor_JS <- function(lines) {
  
  ## we'll look for this JavaScript entry in the html
  pattern <- "const anchorJS = new window.AnchorJS();"

  ## this is the new JavaScript to insert
  insert <- "  const anchorJS_eq = new window.AnchorJS();
  anchorJS_eq.options = {
    placement: 'left',
    class: 'eq-link',
    icon: icon
  };
  anchorJS_eq.add('.anchored-eq');"
  
  ## Look for the existing JS line, and if we find it
  ## insert the new JS before the original declaration
  line <- grep(pattern = pattern, x = lines, fixed = TRUE)
  if(length(line) == 1) {
    lines[line] <- paste(insert, lines[line], sep = "\n")
  }
  
  return(lines)
}

## input is the path to an HTML file
add_equation_links <- function(input) {
  
  ## read HTML file and add the second AnchorJS declaration
  lines <- readLines(input, warn = FALSE) |>
    insert_anchor_JS()
  
  ## find all spans that have an equation identifier
  equation_lines <- grep(x = lines, "span.*eq-")
  
  ## iterate over equation lines and add the new class
  for(j in seq_along(equation_lines)) {
    
    ## extract only the equation id from the line
    equation_id <- sub(lines[equation_lines[j]], 
                       pattern = ".*(eq-[[:alnum:]]*).*", 
                       replacement = "\\1")
    
    ## create a new span with additional class and data-anchor-id attributes
    new_line <- sub(x = lines[equation_lines[j]], 
                    pattern = "span", 
                    replacement = sprintf('span class="anchored-eq" data-anchor-id="%s"', equation_id))
    
    ## replace the old equation line 
    lines[equation_lines[j]] <- new_line
  }

  ## overwrite the input HTML file
  writeLines(text = lines, con = input)
}

html_files <- list.files(path = Sys.getenv("QUARTO_PROJECT_OUTPUT_DIR"), 
                         pattern = ".html",
                         full.names = TRUE)

invisible(lapply(html_files, FUN = add_equation_links))