flake: add diff-plugins command

Prints the plugins added or removed when compared to a particular nixvim
revision.

flake/dev/devshell.nix

@@ -115,6 +115,11 @@
               command = ''${pkgs.python3.interpreter} ${./new-plugin.py} "$@"'';
               help = "Create a new plugin";
             }
+            {
+              name = "diff-plugins";
+              command = ''${pkgs.python3.interpreter} ${./diff-plugins.py} "$@"'';
+              help = "Compare available plugins with another nixvim commit";
+            }
           ];
       };
     };

flake/dev/diff-plugins.py

@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+
+import json
+import re
+import subprocess
+from argparse import ArgumentParser, ArgumentTypeError
+
+
+def main():
+    """
+    Main function to compare nixvim plugins with another revision.
+    """
+
+    parser = ArgumentParser(description="Compare nixvim plugins with another revision")
+    parser.add_argument(
+        "flakeref",
+        metavar="old",
+        help="the commit or flakeref to compare against",
+        type=flakeref,
+    )
+    parser.add_argument(
+        "--compact",
+        "-c",
+        help="produce compact json instead of prettifying",
+        action="store_true",
+    )
+    args = parser.parse_args()
+
+    after_plugins = list_plugins(".")
+    before_plugins = list_plugins(args.flakeref)
+    print(
+        json.dumps(
+            diff(before_plugins, after_plugins),
+            separators=((",", ":") if args.compact else None),
+            indent=(None if args.compact else 4),
+            sort_keys=(not args.compact),
+            default=list,
+        )
+    )
+
+
+def flakeref(arg):
+    """
+    An argparse type that represents a flakeref, or a partial flakeref that we can
+    normalise using sane defaults.
+    """
+    default_protocol = "github:"
+    default_repo = "nix-community/nixvim"
+    sha_rxp = re.compile(r"^[A-Fa-f0-9]{6,40}$")
+    repo_rxp = re.compile(
+        r"^(?P<protocol>[^:/]+:)?(?P<repo>(:?[^/]+)/(:?[^/]+))(?P<sha>/[A-Fa-f0-9]{6,40})?$"
+    )
+    if sha_rxp.match(arg):
+        return f"{default_protocol}{default_repo}/{arg}"
+    elif m := repo_rxp.match(arg):
+        protocol = m.group("protocol") or default_protocol
+        repo = m.group("repo")
+        sha = m.group("sha") or ""
+        return protocol + repo + sha
+    else:
+        raise ArgumentTypeError(f"Unsupported commit or flakeref format: {arg}")
+
+
+def diff(before: list[str], after: list[str]):
+    """
+    Compare the before and after plugin sets.
+    """
+    # TODO: also guess at "renamed" plugins heuristically
+    return {
+        "added": {n: after[n] - before[n] for n in ["plugins", "colorschemes"]},
+        "removed": {n: before[n] - after[n] for n in ["plugins", "colorschemes"]},
+    }
+
+
+def list_plugins(flake: str) -> list[str]:
+    """
+    Gets a list of plugins that exist in the flake.
+    Grouped as "plugins" and "colorschemes"
+    """
+    expr = """
+        options:
+        builtins.listToAttrs (
+          map
+            (name: {
+              inherit name;
+              value = builtins.attrNames options.${name};
+            })
+            [
+              "plugins"
+              "colorschemes"
+            ]
+        )
+    """
+    cmd = [
+        "nix",
+        "eval",
+        f"{flake}#nixvimConfiguration.options",
+        "--apply",
+        expr,
+        "--json",
+    ]
+    out = subprocess.check_output(cmd)
+    # Parse as json, converting the lists to sets
+    return {k: set(v) for k, v in json.loads(out).items()}
+
+
+if __name__ == "__main__":
+    main()