nvf/lib/dag.nix

# From home-manager: https://github.com/nix-community/home-manager/blob/master/modules/lib/dag.nix
# A generalization of Nixpkgs's `strings-with-deps.nix`.
#
# The main differences from the Nixpkgs version are
#
#  - not specific to strings, i.e., any payload is OK,
#
#  - the addition of the function `entryBefore` indicating a "wanted
#    by" relationship.
{lib}: let
  inherit (lib) all filterAttrs nvim mapAttrs toposort;
in {
  empty = {};

  isEntry = e: e ? data && e ? after && e ? before;
  isDag = dag:
    builtins.isAttrs dag && all nvim.dag.isEntry (builtins.attrValues dag);

  /*
  Takes an attribute set containing entries built by entryAnywhere,
  entryAfter, and entryBefore to a topologically sorted list of
  entries.

  Internally this function uses the `toposort` function in
  `<nixpkgs/lib/lists.nix>` and its value is accordingly.

  Specifically, the result on success is

     { result = [ { name = ?; data = ?; } … ] }

  For example

     nix-repl> topoSort {
                 a = entryAnywhere "1";
                 b = entryAfter [ "a" "c" ] "2";
                 c = entryBefore [ "d" ] "3";
                 d = entryBefore [ "e" ] "4";
                 e = entryAnywhere "5";
               } == {
                 result = [
                   { data = "1"; name = "a"; }
                   { data = "3"; name = "c"; }
                   { data = "2"; name = "b"; }
                   { data = "4"; name = "d"; }
                   { data = "5"; name = "e"; }
                 ];
               }
     true

  And the result on error is

     {
       cycle = [ { after = ?; name = ?; data = ? } … ];
       loops = [ { after = ?; name = ?; data = ? } … ];
     }

  For example

     nix-repl> topoSort {
                 a = entryAnywhere "1";
                 b = entryAfter [ "a" "c" ] "2";
                 c = entryAfter [ "d" ] "3";
                 d = entryAfter [ "b" ] "4";
                 e = entryAnywhere "5";
               } == {
                 cycle = [
                   { after = [ "a" "c" ]; data = "2"; name = "b"; }
                   { after = [ "d" ]; data = "3"; name = "c"; }
                   { after = [ "b" ]; data = "4"; name = "d"; }
                 ];
                 loops = [
                   { after = [ "a" "c" ]; data = "2"; name = "b"; }
                 ];
               }
     true
  */
  topoSort = dag: let
    dagBefore = dag: name:
      builtins.attrNames
      (filterAttrs (n: v: builtins.elem name v.before) dag);
    normalizedDag =
      mapAttrs (n: v: {
        name = n;
        data = v.data;
        after = v.after ++ dagBefore dag n;
      })
      dag;
    before = a: b: builtins.elem a.name b.after;
    sorted = toposort before (builtins.attrValues normalizedDag);
  in
    if sorted ? result
    then {
      result = map (v: {inherit (v) name data;}) sorted.result;
    }
    else sorted;

  # Applies a function to each element of the given DAG.
  map = f: mapAttrs (n: v: v // {data = f n v.data;});

  entryBetween = before: after: data: {inherit data before after;};

  # Create a DAG entry with no particular dependency information.
  entryAnywhere = nvim.dag.entryBetween [] [];

  entryAfter = nvim.dag.entryBetween [];
  entryBefore = before: nvim.dag.entryBetween before [];
}