# Builds a skill source directory into a derivation, `SKILL.md` at the `$out` root.
{
  pkgs,
  src,
  name ? builtins.baseNameOf (toString src),
  commands ? [ ],
}:
let
  lib = pkgs.lib;

  patchEnvDefault = command: envName: value:
    let
      marker = "# mkSkill:${envName}";
      replacement = '': "''${${envName}:=${value}}" ${marker}'';
    in
    ''
      if ! awk -v marker=${lib.escapeShellArg marker} -v replacement=${lib.escapeShellArg replacement} '
        index($0, marker) { print replacement; found = 1; next }
        { print }
        END { if (!found) exit 42 }
      ' "$_mkskill_command" > "$_mkskill_command.tmp"; then
        rm -f "$_mkskill_command.tmp"
        echo "mkSkill: command '${command.path}' has no ${marker} marker" >&2
        exit 1
      fi
      mv "$_mkskill_command.tmp" "$_mkskill_command"
      chmod +x "$_mkskill_command"
    '';

  patchCommand = command:
    let
      env = command.env or { };
    in
    ''
      _mkskill_command="$out/${command.path}"
      test -f "$_mkskill_command" \
        || { echo "mkSkill: command '${command.path}' does not exist" >&2; exit 1; }
      test -x "$_mkskill_command" \
        || { echo "mkSkill: command '${command.path}' is not executable" >&2; exit 1; }
      ${lib.concatStringsSep "\n" (lib.mapAttrsToList (patchEnvDefault command) env)}
    '';

  patchCommands = lib.concatStringsSep "\n" (map patchCommand commands);
in
pkgs.stdenvNoCC.mkDerivation {
  inherit name src;

  # A skill is just files to copy, with nothing to unpack, configure, or build.
  dontUnpack = true;
  dontConfigure = true;
  dontBuild = true;

  installPhase = ''
    runHook preInstall
    mkdir -p "$out"
    cp -R "$src"/. "$out/"
    test -f "$out/SKILL.md" \
      || { echo "mkSkill: skill '${name}' has no SKILL.md at its root" >&2; exit 1; }
${patchCommands}
    runHook postInstall
  '';

  # The name as an eval-time attribute, readable without building `$out`
  # (no import-from-derivation).
  passthru.skillName = name;
}