# Compatibility function to allow flakes to be used by
# non-flake-enabled Nix versions. Given a source tree containing a
# 'flake.nix' and 'flake.lock' file, it fetches the flake inputs and
# calls the flake's 'outputs' function. It then returns an attrset
# containing 'defaultNix' (to be used in 'default.nix'), 'shellNix'
# (to be used in 'shell.nix').
{
  src,
  system ? builtins.currentSystem or "unknown-system",
}: let
  lockFilePath = src + "/flake.lock";

  lockFile = builtins.fromJSON (builtins.readFile lockFilePath);

  fetchTree = info:
    if info.type == "github"
    then {
      outPath =
        fetchTarball
        (
          {url = "https://api.${info.host or "github.com"}/repos/${info.owner}/${info.repo}/tarball/${info.rev}";}
          // (
            if info ? narHash
            then {sha256 = info.narHash;}
            else {}
          )
        );
      rev = info.rev;
      shortRev = builtins.substring 0 7 info.rev;
      lastModified = info.lastModified;
      lastModifiedDate = formatSecondsSinceEpoch info.lastModified;
      narHash = info.narHash;
    }
    else if info.type == "git"
    then
      {
        outPath =
          builtins.fetchGit
          (
            {url = info.url;}
            // (
              if info ? rev
              then {inherit (info) rev;}
              else {}
            )
            // (
              if info ? ref
              then {inherit (info) ref;}
              else {}
            )
            // (
              if info ? submodules
              then {inherit (info) submodules;}
              else {}
            )
          );
        lastModified = info.lastModified;
        lastModifiedDate = formatSecondsSinceEpoch info.lastModified;
        narHash = info.narHash;
      }
      // (
        if info ? rev
        then {
          rev = info.rev;
          shortRev = builtins.substring 0 7 info.rev;
        }
        else {
        }
      )
    else if info.type == "path"
    then {
      outPath = builtins.path {path = info.path;};
      narHash = info.narHash;
    }
    else if info.type == "tarball"
    then {
      outPath =
        fetchTarball
        (
          {inherit (info) url;}
          // (
            if info ? narHash
            then {sha256 = info.narHash;}
            else {}
          )
        );
    }
    else if info.type == "gitlab"
    then {
      inherit (info) rev narHash lastModified;
      outPath =
        fetchTarball
        (
          {url = "https://${info.host or "gitlab.com"}/api/v4/projects/${info.owner}%2F${info.repo}/repository/archive.tar.gz?sha=${info.rev}";}
          // (
            if info ? narHash
            then {sha256 = info.narHash;}
            else {}
          )
        );
      shortRev = builtins.substring 0 7 info.rev;
    }
    else
      # FIXME: add Mercurial, tarball inputs.
      throw "flake input has unsupported input type '${info.type}'";

  callFlake4 = flakeSrc: locks: let
    flake = import (flakeSrc + "/flake.nix");

    inputs = builtins.mapAttrs (n: v:
      if v.flake or true
      then callFlake4 (fetchTree (v.locked // v.info)) v.inputs
      else fetchTree (v.locked // v.info))
    locks;

    outputs = flakeSrc // (flake.outputs (inputs // {self = outputs;}));
  in
    assert flake.edition == 201909; outputs;

  callLocklessFlake = flakeSrc: let
    flake = import (flakeSrc + "/flake.nix");
    outputs = flakeSrc // (flake.outputs {self = outputs;});
  in
    outputs;

  rootSrc = let
    # Try to clean the source tree by using fetchGit, if this source
    # tree is a valid git repository.
    tryFetchGit = src:
      if isGit && !isShallow
      then let
        res = builtins.fetchGit src;
      in
        if res.rev == "0000000000000000000000000000000000000000"
        then removeAttrs res ["rev" "shortRev"]
        else res
      else {outPath = src;};
    # NB git worktrees have a file for .git, so we don't check the type of .git
    isGit = builtins.pathExists (src + "/.git");
    isShallow = builtins.pathExists (src + "/.git/shallow");
  in
    {
      lastModified = 0;
      lastModifiedDate = formatSecondsSinceEpoch 0;
    }
    // (
      if src ? outPath
      then src
      else tryFetchGit src
    );

  # Format number of seconds in the Unix epoch as %Y%m%d%H%M%S.
  formatSecondsSinceEpoch = t: let
    rem = x: y: x - x / y * y;
    days = t / 86400;
    secondsInDay = rem t 86400;
    hours = secondsInDay / 3600;
    minutes = (rem secondsInDay 3600) / 60;
    seconds = rem t 60;

    # Courtesy of https://stackoverflow.com/a/32158604.
    z = days + 719468;
    era =
      (
        if z >= 0
        then z
        else z - 146096
      )
      / 146097;
    doe = z - era * 146097;
    yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
    y = yoe + era * 400;
    doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
    mp = (5 * doy + 2) / 153;
    d = doy - (153 * mp + 2) / 5 + 1;
    m =
      mp
      + (
        if mp < 10
        then 3
        else -9
      );
    y' =
      y
      + (
        if m <= 2
        then 1
        else 0
      );

    pad = s:
      if builtins.stringLength s < 2
      then "0" + s
      else s;
  in "${toString y'}${pad (toString m)}${pad (toString d)}${pad (toString hours)}${pad (toString minutes)}${pad (toString seconds)}";

  allNodes =
    builtins.mapAttrs
    (
      key: node: let
        sourceInfo =
          if key == lockFile.root
          then rootSrc
          else fetchTree (node.info or {} // removeAttrs node.locked ["dir"]);

        subdir =
          if key == lockFile.root
          then ""
          else node.locked.dir or "";

        flake = import (sourceInfo
          + (
            if subdir != ""
            then "/"
            else ""
          )
          + subdir
          + "/flake.nix");

        inputs =
          builtins.mapAttrs
          (inputName: inputSpec: allNodes.${resolveInput inputSpec})
          (node.inputs or {});

        # Resolve a input spec into a node name. An input spec is
        # either a node name, or a 'follows' path from the root
        # node.
        resolveInput = inputSpec:
          if builtins.isList inputSpec
          then getInputByPath lockFile.root inputSpec
          else inputSpec;

        # Follow an input path (e.g. ["dwarffs" "nixpkgs"]) from the
        # root node, returning the final node.
        getInputByPath = nodeName: path:
          if path == []
          then nodeName
          else
            getInputByPath
            # Since this could be a 'follows' input, call resolveInput.
            (resolveInput lockFile.nodes.${nodeName}.inputs.${builtins.head path})
            (builtins.tail path);

        outputs = flake.outputs (inputs // {self = result;});

        result =
          outputs
          // sourceInfo
          // {
            inherit inputs;
            inherit outputs;
            inherit sourceInfo;
          };
      in
        if node.flake or true
        then assert builtins.isFunction flake.outputs; result
        else sourceInfo
    )
    lockFile.nodes;

  result =
    if !(builtins.pathExists lockFilePath)
    then callLocklessFlake rootSrc
    else if lockFile.version == 4
    then callFlake4 rootSrc (lockFile.inputs)
    else if lockFile.version >= 5 && lockFile.version <= 7
    then allNodes.${lockFile.root}
    else throw "lock file '${lockFilePath}' has unsupported version ${toString lockFile.version}";
in rec {
  # expose flake inputs
  inherit (result) inputs;

  defaultNix =
    result
    // (
      if result ? defaultPackage.${system}
      then {default = result.defaultPackage.${system};}
      else {}
    )
    // (
      if result ? packages.${system}.default
      then {default = result.packages.${system}.default;}
      else {}
    );

  shellNix =
    defaultNix
    // (
      if result ? devShell.${system}
      then {default = result.devShell.${system};}
      else {}
    )
    // (
      if result ? devShells.${system}.default
      then {default = result.devShells.${system}.default;}
      else {}
    );
}