perf(metadata-minifier): defer copying versions out of expand

MetadataMinifier::expand now returns ExpandedVersions, which holds the
minified input plus, for each expanded version, a table of references to
where its fields live. A version is copied only when materialize() asks
for it. ComposerRepository::load_async_packages runs its constraint and
stability filters straight off that view through the new VersionFields
trait, so the versions it rejects are never copied at all.

Benchmarks are new under crates/shirabe/benches. load_packages against
real packagist p2 metadata, before -> after:

  symfony/console (768 versions)
      0 accepted     9.01 ms ->   4.40 ms   -51%
     50 accepted    10.70 ms ->   6.30 ms   -41%
    147 accepted    13.17 ms ->   9.62 ms   -27%
    329 accepted    18.58 ms ->  16.89 ms    -9%
    663 accepted    27.27 ms ->  27.89 ms    +2%

  laravel/framework (1277 versions)
      0 accepted    39.44 ms ->  11.22 ms   -72%
     81 accepted    44.76 ms ->  18.25 ms   -59%
    840 accepted   125.44 ms -> 120.12 ms    -4%
   1266 accepted   163.18 ms -> 182.01 ms   +12%

The crossover sits near 80% acceptance. Past it the view loses, because
materialize rebuilds a map where the old code cloned one, and the
minified input stays alive alongside the copies; the 1266-of-1277 case
measured between +5% and +12% across runs. Loads with a real constraint
sit far below the crossover.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 98 insertions, 0 deletions

diff --git a/crates/shirabe/benches/metadata_minifier.rs b/crates/shirabe/benches/metadata_minifier.rs
new file mode 100644
index 00000000..dff37740
--- /dev/null
+++ b/crates/shirabe/benches/metadata_minifier.rs
@@ -0,0 +1,98 @@
+//! Benchmarks for expanding minified package metadata.
+//!
+//! The workload mirrors `ComposerRepository::load_async_packages`: every expanded version is
+//! inspected, but only the ones accepted by the constraint and stability filters are kept. The
+//! `keep` parameter is that acceptance rate, which is what decides how much of the expansion has
+//! to be copied out.
+
+use criterion::BatchSize;
+use criterion::BenchmarkId;
+use criterion::Criterion;
+use criterion::criterion_group;
+use criterion::criterion_main;
+use indexmap::IndexMap;
+use shirabe::json::json_file::JsonFile;
+use shirabe_metadata_minifier::MetadataMinifier;
+use shirabe_php_shim::PhpMixed;
+use std::hint::black_box;
+
+#[path = "packagist_fixture.rs"]
+mod packagist_fixture;
+
+/// Real packagist metadata, fetched on first run. `symfony/console` and `laravel/framework` are
+/// long-lived packages whose p2 files carry hundreds of versions each.
+const PACKAGES: &[&str] = &["monolog/monolog", "symfony/console", "laravel/framework"];
+
+/// Percentages of the expanded versions the caller keeps.
+const KEEP_PERCENTS: &[u32] = &[0, 10, 50, 100];
+
+/// The minified version list for `package`, in the shape `load_async_packages` hands to the
+/// minifier.
+fn minified_versions(package: &str) -> Vec<IndexMap<String, PhpMixed>> {
+    let response = JsonFile::parse_json(Some(&packagist_fixture::fetch(package)), None)
+        .expect("invalid fixture JSON");
+    let response = response.as_array().expect("the fixture is not an object");
+    assert_eq!(
+        response.get("minified").and_then(|v| v.as_string()),
+        Some("composer/2.0"),
+        "{package} is not served in the minified format",
+    );
+
+    response
+        .get("packages")
+        .and_then(|v| v.as_array())
+        .and_then(|packages| packages.get(package))
+        .and_then(|v| v.as_list())
+        .expect("the fixture has no version list for the package")
+        .iter()
+        .map(|version| {
+            version
+                .as_array()
+                .expect("a version is not an object")
+                .clone()
+        })
+        .collect()
+}
+
+/// Reads one field of every expanded version and keeps the newest `keep` of them, the way the
+/// repository keeps only the versions its filters accept.
+fn consume(
+    versions: Vec<IndexMap<String, PhpMixed>>,
+    keep: usize,
+) -> Vec<IndexMap<String, PhpMixed>> {
+    let expanded = MetadataMinifier::expand(versions);
+    let first_kept = expanded.len() - keep;
+    let mut kept: Vec<IndexMap<String, PhpMixed>> = Vec::with_capacity(keep);
+    for index in 0..expanded.len() {
+        black_box(expanded.version(index).get("version_normalized"));
+        if index >= first_kept {
+            kept.push(expanded.materialize(index));
+        }
+    }
+    kept
+}
+
+fn bench_expand(c: &mut Criterion) {
+    for package in PACKAGES {
+        let versions = minified_versions(package);
+        let mut group = c.benchmark_group(format!("expand/{package}"));
+
+        for keep_percent in KEEP_PERCENTS {
+            let keep = versions.len() * (*keep_percent as usize) / 100;
+            let parameter = format!("keep{keep_percent}%/{}of{}", keep, versions.len());
+
+            group.bench_function(BenchmarkId::from_parameter(parameter), |b| {
+                b.iter_batched(
+                    || versions.clone(),
+                    |versions| consume(versions, keep),
+                    BatchSize::PerIteration,
+                );
+            });
+        }
+
+        group.finish();
+    }
+}
+
+criterion_group!(benches, bench_expand);
+criterion_main!(benches);