port flake-compat to make flake inputs accessible (#386)

this makes a bunch of changes with regard to readability and reusability:
- use more descriptive names in the datetime conversion
- extract interesting parts of the business logic into a library

the formatting made by Alejandra is terrible...
most of these assignments can be one-liners, and it breaks doc comments.

default.nix

@@ -0,0 +1,14 @@
+{
+  sources ? (import ./flake-compat.nix {root = ./.;}).inputs,
+  system ? builtins.currentSystem,
+  pkgs ?
+    import sources.nixpkgs {
+      config = {};
+      overlays = [];
+      inherit system;
+    },
+}: {
+  shell = pkgs.mkShellNoCC {
+    packages = [];
+  };
+}

flake-compat.nix

@@ -0,0 +1,4 @@
+let
+  lib = import ./lib.nix {lib = {};};
+in
+  lib.flake-inputs

lib.nix

@@ -1,4 +1,4 @@
-{lib}: {
+{lib}: rec {
   # Take an attrset of arbitrary nesting and make it flat
   # by concatenating the nested names with the given separator.
   flattenAttrs = separator: let
@@ -9,4 +9,277 @@
       else {${path + name} = value;};
   in
     f "";
+
+  /*
+    *
+  Convert number of seconds in the Unix epoch to a Gregorian calendar date and time
+
+  This does not take into account leap seconds, which would require a table lookup.
+  */
+  datetime-from-timestamp = timestamp: let
+    remainder = x: y: x - x / y * y;
+    seconds-per-day = 86400;
+    day-of-epoch = timestamp / seconds-per-day;
+    seconds-of-day = remainder timestamp seconds-per-day;
+    hours = seconds-of-day / 3600;
+    minutes = (remainder seconds-of-day 3600) / 60;
+    seconds = remainder timestamp 60;
+
+    # Courtesy of http://howardhinnant.github.io/date_algorithms.html via https://stackoverflow.com/a/32158604
+    day' = day-of-epoch + 719468; # internal representation of days, based on number of days between 0000-03-01 and 1970-01-01
+    days-per-era = 146097;
+    era =
+      # 400-year interval of the Gregorian calendar
+      (
+        if day' >= 0
+        then day'
+        else day' - (days-per-era - 1)
+      )
+      / days-per-era;
+    day-of-era = day' - era * days-per-era;
+    year-of-era = (day-of-era - day-of-era / 1460 + day-of-era / 36524 - day-of-era / (days-per-era - 1)) / 365;
+    year' = year-of-era + era * 400; # internal representation of years
+    day-of-year = day-of-era - (365 * year-of-era + year-of-era / 4 - year-of-era / 100);
+    month' = (5 * day-of-year + 2) / 153; # internal representation of months
+    day = day-of-year - (153 * month' + 2) / 5 + 1;
+    month =
+      month'
+      + (
+        if month' < 10
+        then 3
+        else -9
+      );
+    year =
+      year'
+      + (
+        if month <= 2
+        then 1
+        else 0
+      );
+  in {
+    inherit year month day hours minutes seconds;
+  };
+
+  # Format number of seconds in the Unix epoch as %Y%m%d%H%M%S.
+  format-timestamp = timestamp: let
+    pad = n: s: let
+      str = toString s;
+    in
+      with builtins; concatStringsSep "" (genList (_: "0") (n - stringLength str)) + str;
+  in
+    with builtins.mapAttrs
+    (name: s:
+      if name == "year"
+      then pad 4 s
+      else pad 2 s)
+    (datetime-from-timestamp timestamp); "${year}${month}${day}${hours}${minutes}${seconds}";
+
+  /*
+    *
+  Polyfill for the experimental `builtins.fetchTree`
+
+  https://nix.dev/manual/nix/latest/language/builtins#builtins-fetchTree
+  */
+  fetchTree = info:
+    if info.type == "github"
+    then {
+      outPath = fetchTarball (
+        {url = "https://api.${info.host or "github.com"}/repos/${info.owner}/${info.repo}/tarball/${info.rev}";}
+        // (
+          if info ? narHash
+          then {sha256 = info.narHash;}
+          else {}
+        )
+      );
+      rev = info.rev;
+      shortRev = builtins.substring 0 7 info.rev;
+      lastModified = info.lastModified;
+      lastModifiedDate = format-timestamp info.lastModified;
+      narHash = info.narHash;
+    }
+    else if info.type == "git"
+    then
+      {
+        outPath = builtins.fetchGit (
+          {
+            url = info.url;
+            shallow = true;
+            allRefs = true;
+          }
+          // (
+            if info ? rev
+            then {inherit (info) rev;}
+            else {}
+          )
+          // (
+            if info ? submodules
+            then {inherit (info) submodules;}
+            else {}
+          )
+        );
+        lastModified = info.lastModified;
+        lastModifiedDate = format-timestamp info.lastModified;
+        narHash = info.narHash;
+      }
+      // (
+        if info ? rev
+        then {
+          rev = info.rev;
+          shortRev = builtins.substring 0 7 info.rev;
+        }
+        else {}
+      )
+    else if info.type == "path"
+    then {
+      outPath = builtins.path {
+        path =
+          if builtins.substring 0 1 info.path != "/"
+          # XXX: relative paths require an additional `root` attribute!
+          #      this is supplied by our own flake-inputs, but may not work elsewhere
+          then "${info.root}/${info.path}"
+          else info.path;
+      };
+      narHash = info.narHash;
+    }
+    else if info.type == "tarball"
+    then {
+      outPath = fetchTarball (
+        {inherit (info) url;}
+        // (
+          if info ? narHash
+          then {sha256 = info.narHash;}
+          else {}
+        )
+      );
+    }
+    else if info.type == "gitlab"
+    then {
+      inherit (info) rev narHash lastModified;
+      outPath = fetchTarball (
+        {url = "https://${info.host or "gitlab.com"}/api/v4/projects/${info.owner}%2F${info.repo}/repository/archive.tar.gz?sha=${info.rev}";}
+        // (
+          if info ? narHash
+          then {sha256 = info.narHash;}
+          else {}
+        )
+      );
+      shortRev = builtins.substring 0 7 info.rev;
+    }
+    # TODO: Mercurial, tarball inputs, ...
+    else throw "flake input has unsupported input type '${info.type}'";
+
+  /*
+  *
+  Compatibility layer to allow `flake.lock` files to be used with stable Nix.
+
+  Modified from https://github.com/nix-community/dream2nix/blob/main/dev-flake/flake-compat.nix
+  https://github.com/nix-community/flake-compat is not actively maintained
+  */
+  flake-inputs = {
+    root,
+    overrides ? {},
+  }: let
+    lockFilePath = root + "/flake.lock";
+    lockFile = builtins.fromJSON (builtins.readFile lockFilePath);
+
+    tree = 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 (root + "/.git");
+      isShallow = builtins.pathExists (root + "/.git/shallow");
+    in
+      {
+        lastModified = 0;
+        lastModifiedDate = format-timestamp 0;
+      }
+      // (
+        if root ? outPath
+        then root
+        else tryFetchGit root
+      );
+
+    # we can't import those from the Nixpkgs `lib`,
+    # since flake inputs are used to fetch Nixpkgs to begin with
+    nameValuePair = name: value: {inherit name value;};
+    mapAttrs' = f: set:
+      builtins.listToAttrs (map (attr: f attr set.${attr}) (builtins.attrNames set));
+
+    rootOverrides =
+      mapAttrs'
+      (input: lockKey': let
+        lockKey =
+          if builtins.isList lockKey'
+          then builtins.concatStringsSep "/" lockKey'
+          else lockKey';
+      in
+        nameValuePair lockKey (overrides.${input} or null))
+      lockFile.nodes.${lockFile.root}.inputs;
+
+    allNodes =
+      builtins.mapAttrs
+      (
+        key: node: let
+          sourceInfo =
+            if key == lockFile.root
+            then tree
+            else if rootOverrides.${key} or null != null
+            then {
+              type = "path";
+              outPath = rootOverrides.${key};
+              narHash = throw "flake-inputs: overriding narHash not implemented";
+            }
+            else fetchTree (node.info or {} // removeAttrs node.locked ["dir"] // {inherit root;});
+
+          inputs =
+            builtins.mapAttrs
+            (_inputName: inputSpec: allNodes.${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);
+        in
+          sourceInfo
+          // {
+            inherit inputs;
+            inherit sourceInfo;
+            _type = "flake";
+          }
+      )
+      lockFile.nodes;
+  in
+    if lockFile.version >= 5 && lockFile.version <= 7
+    then
+      allNodes.${lockFile.root}
+      // {
+        overrideInputs = ov:
+          flake-inputs {
+            inherit root;
+            overrides = ov;
+          };
+      }
+    else throw "flake-inputs: lock file '${lockFilePath}' has unsupported version ${toString lockFile.version}";
 }