1 files changed, 122 insertions, 0 deletions
diff --git a/.github/scripts/compare_benchmarks.py b/.github/scripts/compare_benchmarks.py
new file mode 100644
index 00000000..5d4176f6
--- /dev/null
+++ b/.github/scripts/compare_benchmarks.py
@@ -0,0 +1,122 @@
+#!/usr/bin/env python3
+"""
+Compare JMH benchmark results (with heapMetric) against a stored baseline and
+print a markdown table suitable for a GitHub PR comment.
+
+Usage:
+    python compare_benchmarks.py <current_results.json> [<baseline_latest.json>]
+
+If the baseline file is absent or empty the table shows current values only.
+"""
+
+import json
+import sys
+from pathlib import Path
+
+
+def short_name(full: str) -> str:
+    """Strip package prefix, keep ClassName.methodName."""
+    parts = full.rsplit(".", 2)
+    return ".".join(parts[-2:]) if len(parts) >= 2 else full
+
+
+def fmt_delta(current: float, baseline: float) -> str:
+    if baseline == 0:
+        return "-"
+    pct = (current - baseline) / baseline * 100
+    return f"{pct:+.1f}%"
+
+
+def fmt_float(value: float | None, decimals: int = 1) -> str:
+    if value is None:
+        return "N/A"
+    return f"{value:,.{decimals}f}"
+
+
+def load_results(path: Path) -> dict[str, dict]:
+    """Return dict keyed by benchmark name.
+
+    Accepts either a raw results array or a benchmark-history array, in which
+    case the last entry's results are used as the baseline.
+    """
+    if not path.exists():
+        return {}
+    data = json.loads(path.read_text())
+    if not isinstance(data, list) or not data:
+        return {}
+    # History format: list of {commit, timestamp, results: [...]}
+    if "results" in data[-1]:
+        data = data[-1]["results"]
+    return {entry["benchmark"]: entry for entry in data if "benchmark" in entry}
+
+
+def main() -> None:
+    if len(sys.argv) < 2:
+        print("Usage: compare_benchmarks.py <current.json> [baseline.json]", file=sys.stderr)
+        sys.exit(1)
+
+    current_path = Path(sys.argv[1])
+    baseline_path = Path(sys.argv[2]) if len(sys.argv) > 2 else None
+
+    current = load_results(current_path)
+    baseline = load_results(baseline_path) if baseline_path else {}
+
+    if not current:
+        print("## Benchmark Results\n\n_No benchmark results found._")
+        return
+
+    has_baseline = bool(baseline)
+
+    lines = ["## Benchmark Results\n"]
+    if has_baseline:
+        # Extract the baseline commit SHA if stored in the file (update_history stores it)
+        first = next(iter(baseline.values()), {})
+        sha = first.get("_meta_commit", "")
+        lines.append(f"> Compared against master{f' @ `{sha[:7]}`' if sha else ''}\n")
+    else:
+        lines.append("> No baseline found — showing current values only.\n")
+
+    header = (
+        "| Benchmark | Time (ms) | Δ Time"
+        " | Avg Heap (MB) | Δ Avg Heap"
+        " | Max Heap (MB) | Δ Max Heap |"
+    )
+    separator = (
+        "|-----------|----------:|-------:"
+        "|--------------:|-----------:"
+        "|--------------:|-----------:|"
+    )
+    lines += [header, separator]
+
+    for name, entry in sorted(current.items(), key=lambda x: x[0]):
+        score = entry.get("primaryMetric", {}).get("score")
+        heap = entry.get("heapMetric", {})
+        avg_mb = heap.get("avgMb")
+        max_mb = heap.get("maxMb")
+
+        base = baseline.get(name, {})
+        base_score = base.get("primaryMetric", {}).get("score") if base else None
+        base_heap = base.get("heapMetric", {}) if base else {}
+        base_avg = base_heap.get("avgMb")
+        base_max = base_heap.get("maxMb")
+
+        delta_time = fmt_delta(score, base_score) if (has_baseline and score is not None and base_score is not None) else "-"
+        delta_avg = fmt_delta(avg_mb, base_avg) if (has_baseline and avg_mb is not None and base_avg is not None) else "-"
+        delta_max = fmt_delta(max_mb, base_max) if (has_baseline and max_mb is not None and base_max is not None) else "-"
+
+        lines.append(
+            f"| {short_name(name)}"
+            f" | {fmt_float(score)}"
+            f" | {delta_time}"
+            f" | {fmt_float(avg_mb)}"
+            f" | {delta_avg}"
+            f" | {fmt_float(max_mb)}"
+            f" | {delta_max} |"
+        )
+
+    lines.append("\n_Positive Δ = slower / more memory. No threshold gate — informational only._")
+    print("\n".join(lines))
+
+
+if __name__ == "__main__":
+    main()
+\ No newline at end of file