{ lib, pkgs }:
let
  inherit (import ./semver.nix { inherit lib ireplace; }) satisfiesSemver;
  inherit (builtins) genList length;

  # Replace a list entry at defined index with set value
  ireplace = idx: value: list: (
    genList (i: if i == idx then value else (builtins.elemAt list i)) (length list)
  );

  # Do some canonicalisation of module names
  moduleName = name: lib.toLower (lib.replaceStrings [ "_" "." ] [ "-" "-" ] name);

  # Get a full semver pythonVersion from a python derivation
  getPythonVersion = python:
    let
      pyVer = lib.splitVersion python.pythonVersion ++ [ "0" ];
      ver = lib.splitVersion python.version;
      major = l: lib.elemAt l 0;
      minor = l: lib.elemAt l 1;
      joinVersion = v: lib.concatStringsSep "." v;
    in
    joinVersion (if major pyVer == major ver && minor pyVer == minor ver then ver else pyVer);

  # Compare a semver expression with a version
  isCompatible = version:
    let
      operators = {
        "||" = cond1: cond2: cond1 || cond2;
        "," = cond1: cond2: cond1 && cond2; # , means &&
        "&&" = cond1: cond2: cond1 && cond2;
      };
      splitRe = "(" + (builtins.concatStringsSep "|" (builtins.map (x: lib.replaceStrings [ "|" ] [ "\\|" ] x) (lib.attrNames operators))) + ")";
    in
    expr:
    let
      tokens = builtins.filter (x: x != "") (builtins.split splitRe expr);
      combine = acc: v:
        let
          isOperator = builtins.typeOf v == "list";
          operator = if isOperator then (builtins.elemAt v 0) else acc.operator;
        in
        if isOperator then (acc // { inherit operator; }) else {
          inherit operator;
          state = operators."${operator}" acc.state (satisfiesSemver version v);
        };
      initial = { operator = "&&"; state = true; };
    in
    if expr == "" then true else (builtins.foldl' combine initial tokens).state;
  fromTOML = builtins.fromTOML or
    (
      toml: builtins.fromJSON (
        builtins.readFile (
          pkgs.runCommand "from-toml"
            {
              inherit toml;
              allowSubstitutes = false;
              preferLocalBuild = true;
            }
            ''
              ${pkgs.remarshal}/bin/remarshal \
                -if toml \
                -i <(echo "$toml") \
                -of json \
                -o $out
            ''
        )
      )
    );
  readTOML = path: fromTOML (builtins.readFile path);

  #
  # Returns the appropriate manylinux dependencies and string representation for the file specified
  #
  getManyLinuxDeps = f:
    let
      ml = pkgs.pythonManylinuxPackages;
    in
    if lib.strings.hasInfix "manylinux1" f then { pkg = [ ml.manylinux1 ]; str = "1"; }
    else if lib.strings.hasInfix "manylinux2010" f then { pkg = [ ml.manylinux2010 ]; str = "2010"; }
    else if lib.strings.hasInfix "manylinux2014" f then { pkg = [ ml.manylinux2014 ]; str = "2014"; }
    else { pkg = [ ]; str = null; };

  # Predict URL from the PyPI index.
  # Args:
  #   pname: package name
  #   file: filename including extension
  #   hash: SRI hash
  #   kind: Language implementation and version tag
  predictURLFromPypi = lib.makeOverridable (
    { pname, file, hash, kind }:
    "https://files.pythonhosted.org/packages/${kind}/${lib.toLower (builtins.substring 0 1 file)}/${pname}/${file}"
  );


  # Fetch the wheels from the PyPI index.
  # We need to first get the proper URL to the wheel.
  # Args:
  #   pname: package name
  #   file: filename including extension
  #   hash: SRI hash
  #   kind: Language implementation and version tag
  fetchWheelFromPypi = lib.makeOverridable (
    { pname, file, hash, kind, curlOpts ? "" }:
    let
      version = builtins.elemAt (builtins.split "-" file) 2;
    in
    (pkgs.stdenvNoCC.mkDerivation {
      name = file;
      nativeBuildInputs = [
        pkgs.curl
        pkgs.jq
      ];
      isWheel = true;
      system = "builtin";

      preferLocalBuild = true;
      impureEnvVars = lib.fetchers.proxyImpureEnvVars ++ [
        "NIX_CURL_FLAGS"
      ];

      predictedURL = predictURLFromPypi { inherit pname file hash kind; };
      inherit pname file version curlOpts;

      builder = ./fetch-wheel.sh;

      outputHashMode = "flat";
      outputHashAlgo = "sha256";
      outputHash = hash;
    })
  );

  # Fetch the artifacts from the PyPI index. Since we get all
  # info we need from the lock file we don't use nixpkgs' fetchPyPi
  # as it modifies casing while not providing anything we don't already
  # have.
  #
  # Args:
  #   pname: package name
  #   file: filename including extension
  #   hash: SRI hash
  #   kind: Language implementation and version tag https://www.python.org/dev/peps/pep-0427/#file-name-convention
  fetchFromPypi = lib.makeOverridable (
    { pname, file, hash, kind }:
    if lib.strings.hasSuffix "whl" file then fetchWheelFromPypi { inherit pname file hash kind; }
    else
      pkgs.fetchurl {
        url = predictURLFromPypi { inherit pname file hash kind; };
        sha256 = builtins.elemAt (builtins.match "sha256:(.*)" hash) 0; # nix 2.0 backwards compatibility.
      }
  );
  getBuildSystemPkgs =
    { pythonPackages
    , pyProject
    }:
    let
      buildSystem = lib.attrByPath [ "build-system" "build-backend" ] "" pyProject;
      drvAttr = moduleName (builtins.elemAt (builtins.split "\\.|:" buildSystem) 0);
    in
    if buildSystem == "" then [ ] else (
      [ pythonPackages.${drvAttr} or (throw "unsupported build system ${buildSystem}") ]
    );

  # Find gitignore files recursively in parent directory stopping with .git
  findGitIgnores = path:
    let
      parent = path + "/..";
      gitIgnore = path + "/.gitignore";
      isGitRoot = builtins.pathExists (path + "/.git");
      hasGitIgnore = builtins.pathExists gitIgnore;
      gitIgnores = if hasGitIgnore then [ gitIgnore ] else [ ];
    in
    lib.optionals (builtins.toString path != "/" && ! isGitRoot) (findGitIgnores parent) ++ gitIgnores;

  /*
  Provides a source filtering mechanism that:

  - Filters gitignore's
  - Filters pycache/pyc files
  - Uses cleanSourceFilter to filter out .git/.hg, .o/.so, editor backup files & nix result symlinks
  */
  cleanPythonSources = { src }:
    let
      gitIgnores = findGitIgnores src;
      pycacheFilter = name: type:
        (type == "directory" && ! lib.strings.hasInfix "__pycache__" name)
        || (type == "regular" && ! lib.strings.hasSuffix ".pyc" name)
      ;
    in
    lib.cleanSourceWith {
      filter = lib.cleanSourceFilter;
      src = lib.cleanSourceWith {
        filter = pkgs.nix-gitignore.gitignoreFilterPure pycacheFilter gitIgnores src;
        inherit src;
      };
    };
in
{
  inherit
    fetchFromPypi
    fetchWheelFromPypi
    getManyLinuxDeps
    isCompatible
    readTOML
    getBuildSystemPkgs
    satisfiesSemver
    cleanPythonSources
    moduleName
    getPythonVersion
    ;
}