{ cmake
, fetchFromGitHub
, git
, lib
, libffi
, llvmPackages_13
, makeWrapper
, ncurses
, python3
, zlib

# *NOT* from LLVM 9!
# The compiler used to compile Cling may affect the runtime include and lib
# directories it expects to be run with. Cling builds against (a fork of) Clang,
# so we prefer to use Clang as the compiler as well for consistency.
# It would be cleanest to use LLVM 9's clang, but it errors. So, we use a later
# version of Clang to compile, but we check out the Cling fork of Clang 9 to
# build Cling against.
, clangStdenv

# For runtime C++ standard library
, gcc-unwrapped

# Build with debug symbols
, debug ? false

# Build with libc++ (LLVM) rather than stdlibc++ (GCC).
# This is experimental and not all features work.
, useLLVMLibcxx ? clangStdenv.hostPlatform.isDarwin
}:

let
  stdenv = clangStdenv;

  # The patched clang lives in the LLVM megarepo
  clangSrc = fetchFromGitHub {
    owner = "root-project";
    repo = "llvm-project";
    # cling-llvm13 branch
    rev = "3610201fbe0352a63efb5cb45f4ea4987702c735";
    sha256 = "sha256-Cb7BvV7yobG+mkaYe7zD2KcnPvm8/vmVATNWssklXyk=";
    sparseCheckout = ["clang"];
  };

  unwrapped = stdenv.mkDerivation rec {
    pname = "cling-unwrapped";
    version = "1.0";

    src = "${clangSrc}/clang";

    clingSrc = fetchFromGitHub {
      owner = "root-project";
      repo = "cling";
      rev = "v${version}";
      sha256 = "sha256-Ye8EINzt+dyNvUIRydACXzb/xEPLm0YSkz08Xxw3xp4=";
    };

    prePatch = ''
      echo "add_llvm_external_project(cling)" >> tools/CMakeLists.txt

      cp -r $clingSrc tools/cling
      chmod -R a+w tools/cling
    '';

    patches = [
      ./no-clang-cpp.patch
    ];

    nativeBuildInputs = [ python3 git cmake ];
    buildInputs = [ libffi ncurses zlib ];

    strictDeps = true;

    cmakeFlags = [
      "-DLLVM_BINARY_DIR=${llvmPackages_13.llvm.out}"
      "-DLLVM_CONFIG=${llvmPackages_13.llvm.dev}/bin/llvm-config"
      "-DLLVM_LIBRARY_DIR=${llvmPackages_13.llvm.lib}/lib"
      "-DLLVM_MAIN_INCLUDE_DIR=${llvmPackages_13.llvm.dev}/include"
      "-DLLVM_TABLEGEN_EXE=${llvmPackages_13.llvm.out}/bin/llvm-tblgen"
      "-DLLVM_TOOLS_BINARY_DIR=${llvmPackages_13.llvm.out}/bin"
      "-DLLVM_BUILD_TOOLS=Off"
      "-DLLVM_TOOL_CLING_BUILD=ON"

      "-DLLVM_TARGETS_TO_BUILD=host;NVPTX"
      "-DLLVM_ENABLE_RTTI=ON"

      # Setting -DCLING_INCLUDE_TESTS=ON causes the cling/tools targets to be built;
      # see cling/tools/CMakeLists.txt
      "-DCLING_INCLUDE_TESTS=ON"
      "-DCLANG-TOOLS=OFF"
    ] ++ lib.optionals debug [
      "-DCMAKE_BUILD_TYPE=Debug"
    ] ++ lib.optionals useLLVMLibcxx [
      "-DLLVM_ENABLE_LIBCXX=ON"
      "-DLLVM_ENABLE_LIBCXXABI=ON"
    ];

    CPPFLAGS = if useLLVMLibcxx then [ "-stdlib=libc++" ] else [];

    postInstall = lib.optionalString (!stdenv.hostPlatform.isDarwin) ''
      mkdir -p $out/share/Jupyter
      cp -r /build/clang/tools/cling/tools/Jupyter/kernel $out/share/Jupyter
    '';

    dontStrip = debug;

    meta = with lib; {
      description = "Interactive C++ Interpreter";
      mainProgram = "cling";
      homepage = "https://root.cern/cling/";
      license = with licenses; [ lgpl21 ncsa ];
      maintainers = with maintainers; [ thomasjm ];
      platforms = platforms.unix;
    };
  };

  # Runtime flags for the C++ standard library
  cxxFlags = if useLLVMLibcxx then [
    "-I" "${lib.getDev llvmPackages_13.libcxx}/include/c++/v1"
    "-L" "${llvmPackages_13.libcxx}/lib"
    "-l" "${llvmPackages_13.libcxx}/lib/libc++${stdenv.hostPlatform.extensions.sharedLibrary}"
  ] else [
    "-I" "${gcc-unwrapped}/include/c++/${gcc-unwrapped.version}"
    "-I" "${gcc-unwrapped}/include/c++/${gcc-unwrapped.version}/${stdenv.hostPlatform.config}"
  ];

  # The flags passed to the wrapped cling should
  # a) prevent it from searching for system include files and libs, and
  # b) provide it with the include files and libs it needs (C and C++ standard library plus
  # its own stuff)

  # These are also exposed as cling.flags because it's handy to be able to pass them to tools
  # that wrap Cling, particularly Jupyter kernels such as xeus-cling and the built-in
  # jupyter-cling-kernel, which use Cling as a library.
  # Thus, if you're packaging a Jupyter kernel, you either need to pass these flags as extra
  # args to xcpp (for xeus-cling) or put them in the environment variable CLING_OPTS
  # (for jupyter-cling-kernel).
  flags = [
    "-nostdinc"
    "-nostdinc++"

    "-resource-dir" "${llvmPackages_13.llvm.lib}/lib"

    "-isystem" "${lib.getLib unwrapped}/lib/clang/${llvmPackages_13.clang.version}/include"
  ]
  ++ cxxFlags
  ++ [
    # System libc
    "-isystem" "${lib.getDev stdenv.cc.libc}/include"

    # cling includes
    "-isystem" "${lib.getDev unwrapped}/include"
  ];

in

stdenv.mkDerivation {
  pname = "cling";
  version = unwrapped.version;

  nativeBuildInputs = [ makeWrapper ];
  inherit unwrapped flags;
  inherit (unwrapped) meta;

  dontUnpack = true;
  dontConfigure = true;

  buildPhase = ''
    runHook preBuild

    makeWrapper $unwrapped/bin/cling $out/bin/cling \
      --add-flags "$flags"

    runHook postBuild
  '';

  doCheck = true;
  checkPhase = ''
    runHook preCheck

    output=$($out/bin/cling <<EOF
    #include <iostream>
    std::cout << "hello world" << std::endl
    EOF
    )

    echo "$output" | grep -q "Type C++ code and press enter to run it"
    echo "$output" | grep -q "hello world"

    runHook postCheck
  '';

  dontInstall = true;
}