{ lib, stdenv, buildPackages, fetchurl, bison, m4, autoreconfHook, help2man }:

# Avoid 'fetchpatch' to allow 'flex' to be used as a possible 'gcc'
# dependency during bootstrap. Useful when gcc is built from snapshot
# or from a git tree (flex lexers are not pre-generated there).

stdenv.mkDerivation rec {
  pname = "flex";
  version = "2.6.4";

  src = fetchurl {
    url =
      "https://github.com/westes/flex/releases/download/v${version}/flex-${version}.tar.gz";
    sha256 = "15g9bv236nzi665p9ggqjlfn4dwck5835vf0bbw2cz7h5c1swyp8";
  };

  # Also upstream, will be part of 2.6.5
  # https://github.com/westes/flex/commit/24fd0551333e
  patches = [
    (fetchurl {
      name = "glibc-2.26.patch";
      url =
        "https://raw.githubusercontent.com/lede-project/source/0fb14a2b1ab2f82ce63f4437b062229d73d90516/tools/flex/patches/200-build-AC_USE_SYSTEM_EXTENSIONS-in-configure.ac.patch";
      sha256 = "0mpp41zdg17gx30kcpj83jl8hssks3adbks0qzbhcz882b9c083r";
    })
  ];

  postPatch = ''
    patchShebangs tests
  '' + lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
    substituteInPlace Makefile.in --replace "tests" " "

    substituteInPlace doc/Makefile.am --replace 'flex.1: $(top_srcdir)/configure.ac' 'flex.1: '
  '';

  depsBuildBuild = [ buildPackages.stdenv.cc ];
  nativeBuildInputs = [ autoreconfHook help2man ];
  buildInputs = [ bison ];
  propagatedBuildInputs = [ m4 ];

  preConfigure =
    lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''
      export ac_cv_func_malloc_0_nonnull=yes
      export ac_cv_func_realloc_0_nonnull=yes
    '';

  postConfigure = lib.optionalString (stdenv.isDarwin || stdenv.isCygwin) ''
    sed -i Makefile -e 's/-no-undefined//;'
  '';

  dontDisableStatic = stdenv.buildPlatform != stdenv.hostPlatform;

  postInstall = ''
    ln -s $out/bin/flex $out/bin/lex
  '';

  meta = with lib; {
    homepage = "https://github.com/westes/flex";
    description = "A fast lexical analyser generator";
    license = licenses.bsd2;
    platforms = platforms.unix;
  };
}