#' @export
run_regex <- function(
  text,
  pattern,
  ignore.case = FALSE,
  perl = FALSE,
  fixed = FALSE,
  useBytes = FALSE,
  invert = FALSE
) {
  m <- regexec(pattern, text, ignore.case, perl, fixed, useBytes)
  x <- purrr::map(purrr::set_names(m, text), function(mi) list('idx' = purrr::map2(mi, attr(mi, "match.length"), ~ if(.x[1] != -1) c(.x, .x + .y - 1L))))
  y <- purrr::map(purrr::set_names(text), ~ list(text = .))
  z <- purrr::map(purrr::set_names(regmatches(text, m), text), ~ list(m = .))
  x <- utils::modifyList(y, x, TRUE)
  utils::modifyList(z, x, TRUE)
}

wrap_span <- function(x, escape = FALSE) {
  if (is.null(x$idx[[1]])) return(x$text)
  text <- x$text
  idx <- x$idx
  len_idx <- length(idx)
  inserts <- data.frame(
    i = 1:len_idx - 1,
    start = purrr::map_int(idx, ~ .[1]),
    end = purrr::map_int(idx, ~ .[2]) + 1
  ) %>%
    mutate(
      class = sprintf("group g%02d", i),
      pad = 0
    )
  for (j in seq_len(nrow(inserts))) {
    if (inserts$i[j] == 0) next
    overlap <- filter(
      inserts,
      i != 0,
      start <= !!inserts$start[j] & end > !!inserts$end[j])
    inserts[j, 'pad'] <- inserts$pad[j] + nrow(overlap)
  }
  inserts <- inserts %>%
    tidyr::gather(type, loc, start:end) %>%
    mutate(
      class = ifelse(pad > 0, sprintf("%s pad%02d", class, pad), class),
      insert = ifelse(type == 'start', sprintf('<span class="%s">', class), "</span>")
    ) %>%
    group_by(loc, type) %>%
    summarize(insert = paste(insert, collapse = ''))
  idx_split <- c(0 - (inserts$loc[1] == 0), inserts$loc)
  if (!(nchar(text) + 1) %in% idx_split) idx_split <- c(idx_split, nchar(text) + 1)
  text_split <- c()
  for (k in seq_along(idx_split[-1])) {
    text_split <- c(text_split, substr(text, idx_split[k], idx_split[k+1] - 1))
  }
  out <- c()
  for (j in seq_along(text_split)) {
    out <- c(
      out,
      ifelse(escape, escape_html(text_split[j]), text_split[j]),
      if (!is.na(inserts$insert[j])) inserts$insert[j]
    )
  }
  paste(out, collapse = '')
}

#' View grouped regex results
#'
#' @param text Text to search
#' @param pattern Regex pattern to look for
#' @param render Render results to an HTML doc and open in RStudio viewer?
#' @param escape Escape HTML-related characters in `text`?
#' @param knitr Print into knitr doc? If `TRUE`, marks text as `asis_output` and
#'   sets `render = FALSE` and `escape = TRUE`.
#' @param ... Passed to [run_regex]
#' @export
view_regex <- function(text, pattern, ..., render = TRUE, escape = render, knitr = FALSE) {
  if (knitr) {
    render <- FALSE
    escape <- TRUE
  }
  res <- run_regex(text, pattern, ...)
  res <- purrr::map_chr(res, wrap_span, escape = escape)
  res <- paste("<p class='results'>", res, "</p>")
  if (knitr) return(knitr::asis_output(res))
  if (!render) return(res)
  head <- c(
    "---", "pagetitle: View Regex", "---",
    "<h5>Regex</h5>",
    "<p><pre>", escape_html(pattern), "</pre></p>",
    "<h5>Results</h5>"
  )
  res <- c(head, res)
  tmp <- tempfile(fileext = ".Rmd")
  cat(res, file = tmp, sep = "\n")
  tmp_html <- suppressWarnings(
    rmarkdown::render(
      tmp,
      output_format = rmarkdown::html_document(css = system.file('style.css', package='regexhelp')),
      quiet = TRUE
  ))
  rstudioapi::viewer(tmp_html)
}