(** Transform dune-generated opam files for opam-repo overlay. *)

(** Remove the "generated by dune" comment from the first line *)
let strip_dune_comment content =
  let lines = String.split_on_char '\n' content in
  match lines with
  | first :: rest
    when String.starts_with ~prefix:"# This file is generated by dune"
           (String.trim first) ->
      String.concat "\n" rest
  | _ -> content

(** Remove existing dev-repo line if present *)
let remove_dev_repo_line content =
  let lines = String.split_on_char '\n' content in
  let lines =
    List.filter
      (fun line ->
        let trimmed = String.trim line in
        not (String.starts_with ~prefix:"dev-repo:" trimmed))
      lines
  in
  String.concat "\n" lines

(** Remove existing url { ... } section if present *)
let remove_url_section content =
  let lines = String.split_on_char '\n' content in
  let rec process lines in_url_block acc =
    match lines with
    | [] -> List.rev acc
    | line :: rest ->
        let trimmed = String.trim line in
        if in_url_block then
          (* Inside url { ... }, skip until we see } *)
          if String.starts_with ~prefix:"}" trimmed then
            process rest false acc
          else process rest true acc
        else if trimmed = "url {" || String.starts_with ~prefix:"url {" trimmed
        then
          (* Start of url block *)
          if String.ends_with ~suffix:"}" trimmed then
            (* Single-line url block, skip it *)
            process rest false acc
          else process rest true acc
        else process rest false (line :: acc)
  in
  String.concat "\n" (process lines false [])

(** Trim trailing blank lines and ensure single trailing newline *)
let normalize_ending content =
  let lines = String.split_on_char '\n' content in
  let rec trim_trailing = function
    | [] -> []
    | [ "" ] -> []
    | "" :: rest -> (
        match trim_trailing rest with [] -> [] | trimmed -> "" :: trimmed)
    | x :: rest -> x :: trim_trailing rest
  in
  let lines = List.rev (trim_trailing (List.rev lines)) in
  String.concat "\n" lines

let transform ~content ~dev_repo ~url_src =
  (* Step 1: Strip the dune comment *)
  let content = strip_dune_comment content in

  (* Step 2: Remove any existing dev-repo and url sections *)
  let content = remove_dev_repo_line content in
  let content = remove_url_section content in

  (* Step 3: Normalize ending *)
  let content = normalize_ending content in

  (* Step 4: Append dev-repo and url section *)
  let dev_repo_line = Printf.sprintf {|dev-repo: "%s"|} dev_repo in
  let url_section =
    Printf.sprintf "url {\n  src: \"%s\"\n}" url_src
  in
  content ^ "\n" ^ dev_repo_line ^ "\n" ^ url_section ^ "\n"