# https://github.com/siers/nix-gitignore/

{ lib, runCommand }:

# An interesting bit from the gitignore(5):
# - A slash followed by two consecutive asterisks then a slash matches
# - zero or more directories. For example, "a/**/b" matches "a/b",
# - "a/x/b", "a/x/y/b" and so on.

let
  inherit (builtins) filterSource;

  inherit (lib)
    concatStringsSep elemAt filter head isList length optionals optionalString
    pathExists readFile removePrefix replaceStrings stringLength sub substring
    toList trace;

  inherit (lib.strings) match split typeOf;

  debug = a: trace a a;
  last = l: elemAt l ((length l) - 1);
in rec {
  # [["good/relative/source/file" true] ["bad.tmpfile" false]] -> root -> path
  filterPattern = patterns: root:
    (name: _type:
      let
        relPath = removePrefix ((toString root) + "/") name;
        matches = pair: (match (head pair) relPath) != null;
        matched = map (pair: [ (matches pair) (last pair) ]) patterns;
      in last (last ([[ true true ]] ++ (filter head matched))));

  # string -> [[regex bool]]
  gitignoreToPatterns = gitignore:
    let
      # ignore -> bool
      isComment = i: (match "^(#.*|$)" i) != null;

      # ignore -> [ignore bool]
      computeNegation = l:
        let split = match "^(!?)(.*)" l;
        in [ (elemAt split 1) (head split == "!") ];

      # regex -> regex
      handleHashesBangs = replaceStrings [ "\\#" "\\!" ] [ "#" "!" ];

      # ignore -> regex
      substWildcards = let
        special = "^$.+{}()";
        escs = "\\*?";
        splitString = let
          recurse = str:
            [ (substring 0 1 str) ] ++ (optionals (str != "")
              (recurse (substring 1 (stringLength (str)) str)));
        in str: recurse str;
        chars = s: filter (c: c != "" && !isList c) (splitString s);
        escape = s: map (c: "\\" + c) (chars s);
      in replaceStrings
      ((chars special) ++ (escape escs) ++ [ "**/" "**" "*" "?" ])
      ((escape special) ++ (escape escs) ++ [ "(.*/)?" ".*" "[^/]*" "[^/]" ]);

      # (regex -> regex) -> regex -> regex
      mapAroundCharclass = f: r: # rl = regex or list
        let slightFix = replaceStrings [ "\\]" ] [ "]" ];
        in concatStringsSep ""
        (map (rl: if isList rl then slightFix (elemAt rl 0) else f rl)
          (split "(\\[([^\\\\]|\\\\.)+])" r));

      # regex -> regex
      handleSlashPrefix = l:
        let
          split = (match "^(/?)(.*)" l);
          findSlash = l: optionalString ((match ".+/.+" l) == null) l;
          hasSlash = mapAroundCharclass findSlash l != l;
        in (if (elemAt split 0) == "/" || hasSlash then "^" else "(^|.*/)")
        + (elemAt split 1);

      # regex -> regex
      handleSlashSuffix = l:
        let split = (match "^(.*)/$" l);
        in if split != null then (elemAt split 0) + "($|/.*)" else l;

      # (regex -> regex) -> [regex, bool] -> [regex, bool]
      mapPat = f: l: [ (f (head l)) (last l) ];
    in map (l: # `l' for "line"
      mapPat (l:
        handleSlashSuffix (handleSlashPrefix
          (handleHashesBangs (mapAroundCharclass substWildcards l))))
      (computeNegation l))
    (filter (l: !isList l && !isComment l) (split "\n" gitignore));

  gitignoreFilter = ign: root: filterPattern (gitignoreToPatterns ign) root;

  # string|[string|file] (→ [string|file] → [string]) -> string
  gitignoreCompileIgnore = file_str_patterns: root:
    let
      onPath = f: a: if typeOf a == "path" then f a else a;
      str_patterns = map (onPath readFile) (toList file_str_patterns);
    in concatStringsSep "\n" str_patterns;

  gitignoreFilterPure = predicate: patterns: root: name: type:
    gitignoreFilter (gitignoreCompileIgnore patterns root) root name type
    && predicate name type;

  # This is a very hacky way of programming this!
  # A better way would be to reuse existing filtering by making multiple gitignore functions per each root.
  # Then for each file find the set of roots with gitignores (and functions).
  # This would make gitignoreFilterSource very different from gitignoreFilterPure.
  # rootPath → gitignoresConcatenated
  compileRecursiveGitignore = root:
    let
      dirOrIgnore = file: type:
        baseNameOf file == ".gitignore" || type == "directory";
      ignores = builtins.filterSource dirOrIgnore root;
    in readFile (runCommand "${baseNameOf root}-recursive-gitignore" { } ''
      cd ${ignores}

      find -type f -exec sh -c '
        rel="$(realpath --relative-to=. "$(dirname "$1")")/"
        if [ "$rel" = "./" ]; then rel=""; fi

        awk -v prefix="$rel" -v root="$1" -v top="$(test -z "$rel" && echo 1)" "
          BEGIN { print \"# \"root }

          /^!?[^\\/]+\/?$/ {
            match(\$0, /^!?/, negation)
            sub(/^!?/, \"\")

            if (top) { middle = \"\" } else { middle = \"**/\" }

            print negation[0] prefix middle \$0
          }

          /^!?(\\/|.*\\/.+$)/ {
            match(\$0, /^!?/, negation)
            sub(/^!?/, \"\")

            if (!top) sub(/^\//, \"\")

            print negation[0] prefix \$0
          }

          END { print \"\" }
        " "$1"
      ' sh {} \; > $out
    '');

  withGitignoreFile = patterns: root:
    toList patterns ++ [ ".git" ] ++ [ (root + "/.gitignore") ];

  withRecursiveGitignoreFile = patterns: root:
    toList patterns ++ [ ".git" ] ++ [ (compileRecursiveGitignore root) ];

  # filterSource derivatives

  gitignoreFilterSourcePure = predicate: patterns: root:
    filterSource (gitignoreFilterPure predicate patterns root) root;

  gitignoreFilterSource = predicate: patterns: root:
    gitignoreFilterSourcePure predicate (withGitignoreFile patterns root) root;

  gitignoreFilterRecursiveSource = predicate: patterns: root:
    gitignoreFilterSourcePure predicate
    (withRecursiveGitignoreFile patterns root) root;

  # "Predicate"-less alternatives

  gitignoreSourcePure = gitignoreFilterSourcePure (_: _: true);
  gitignoreSource = patterns:
    let type = typeOf patterns;
    in if (type == "string" && pathExists patterns) || type == "path" then
      throw "type error in gitignoreSource(patterns -> source -> path), "
      ''use [] or "" if there are no additional patterns''
    else
      gitignoreFilterSource (_: _: true) patterns;

  gitignoreRecursiveSource = gitignoreFilterSourcePure (_: _: true);
}