feat: add Nix versioning scheme

Signed-off-by: Samiser <github@me.samiser.xyz>

Author: samiser

README.rst

@@ -113,6 +113,11 @@ include:
 - Alpine linux: handled using the base Gentoo version support and extras
   specific to Alpine.
 
+- nix: handled by the embedded `nix module
+  <https://github.com/aboutcode-org/univers/blob/main/src/univers/nix.py.ABOUT>`_,
+  a Python port of Nix's ``builtins.compareVersions`` validated against the
+  reference implementation.
+
 
 The level of support for each ecosystem may not be even for now and new schemes
 and support for more package types are implemented on a continuous basis.

etc/scripts/gen_nix_version_cmp.py

@@ -0,0 +1,132 @@
+#
+# Copyright (c) nexB Inc. and others.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Visit https://aboutcode.org and https://github.com/aboutcode-org/univers for support and download.
+
+"""
+Regenerate tests/data/schema/nix_version_cmp.json from the Nix reference
+implementation. Requires the ``nix-instantiate`` command: every expected
+output is the answer of the real ``builtins.compareVersions``, evaluated over
+all pairs from a corpus of version strings chosen to exercise every branch of
+the comparison.
+
+Run from the repository root:
+    python etc/scripts/gen_nix_version_cmp.py
+"""
+
+import itertools
+import json
+import subprocess
+from pathlib import Path
+
+CORPUS = [
+    ".",
+    "-",
+    "1",
+    "007",
+    "1.0",
+    "1.0.0",
+    "1-0",
+    "1.0.1",
+    "1.9",
+    "1.10",
+    "1.1.1k",
+    "2.3",
+    "2.3.1",
+    "2.3a",
+    "2.3b",
+    "2.3pre",
+    "2.3pre1",
+    "2.3.pre1",
+    "2.3-pre.1",
+    "2.3preX",
+    "pre",
+    "pre1",
+    "a",
+    "abc",
+    "openssl",
+    "2147483647",
+    "2147483648",
+    "9223372036854775808",
+    "1.2147483648",
+    "1.2147483648.0",
+    "0.0.0",
+    "10.0.0",
+    "1..2",
+    "1.-2",
+    "1.0rc1",
+    "1.0.rc1",
+    "unstable-2024-01-01",
+    "0-unstable-2024-01-01",
+    "v1.0",
+    "V2",
+    "ABC",
+    "152.0.7977.60",
+    "152.0.7977.82",
+    "9.8p1",
+    "3.0.2-r10",
+]
+
+EXPR = (
+    "{pairs}: map"
+    " (p: builtins.compareVersions (builtins.elemAt p 0) (builtins.elemAt p 1))"
+    " (builtins.fromJSON pairs)"
+)
+
+OUTPUT = Path(__file__).parent.parent.parent / "tests" / "data" / "schema" / "nix_version_cmp.json"
+
+
+def main():
+    pairs = list(itertools.product(CORPUS, repeat=2))
+    out = subprocess.run(
+        [
+            "nix-instantiate",
+            "--eval",
+            "--strict",
+            "--json",
+            "--expr",
+            EXPR,
+            "--argstr",
+            "pairs",
+            json.dumps(pairs),
+        ],
+        capture_output=True,
+        text=True,
+        check=True,
+    ).stdout
+    results = json.loads(out)
+
+    entries = []
+    for (version1, version2), result in zip(pairs, results):
+        if result == 0:
+            entries.append(
+                {
+                    "description": "Equality test for Nix version, "
+                    "generated from builtins.compareVersions.",
+                    "test_group": "basic",
+                    "test_type": "equality",
+                    "input": {"input_scheme": "nix", "versions": [version1, version2]},
+                    "expected_output": True,
+                }
+            )
+        else:
+            ordered = [version1, version2] if result < 0 else [version2, version1]
+            entries.append(
+                {
+                    "description": "Comparison test for Nix version, "
+                    "generated from builtins.compareVersions.",
+                    "test_group": "basic",
+                    "test_type": "comparison",
+                    "input": {"input_scheme": "nix", "versions": [version1, version2]},
+                    "expected_output": ordered,
+                }
+            )
+
+    with open(OUTPUT, "w") as f:
+        json.dump(entries, f, indent=1)
+    print(f"wrote {len(entries)} entries to {OUTPUT}")
+
+
+if __name__ == "__main__":
+    main()