⚡ Optimize hourly heatmap generation via caching and fast path parsing

[is0692vs]

💡 What:
Optimized buildHourlyHeatmapFromCommitDates inside src/lib/yearInReviewUtils.ts. Instead of using new Date() for every commit string in the loop to compute the getUTCDay() and getUTCHours(), this adds a fast path for typical ISO 8601 strings ending in Z.
It manually parses the hour via fast charCodeAt math and extracts the YYYY-MM-DD date part. Crucially, it caches the day calculation using the YYYY-MM-DD string as the key. This avoids repeated expensive new Date() calls for each commit on the same day. Any string with timezone offsets or non-standard format gracefully falls back to the native new Date() parser.

🎯 Why:
The previous code executed multiple array traversals via new Date(dateString) repeatedly for every element in commitDates. new Date() parsing is notoriously slow when executed 10k-100k times. By leveraging a Map cache and simple string manipulation, we reduce object allocations and string parsing overhead, significantly speeding up the stats generation.

📊 Measured Improvement:
On a benchmark consisting of 100,000 standard GitHub API ISO Z strings, running 10 iterations:

• Baseline time: ~529.36 ms
• Optimized cache time: ~227.56 ms
• Improvement: ~2.3x faster (~57% reduction in processing time).

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PR created automatically by Jules for task 11919669200318792480 started by @is0692vs

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Project	Deployment	Actions	Updated (UTC)
github-user-summary	Ready	Preview, Comment	Mar 27, 2026 7:18pm

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: c99faabe-04fd-42c7-a291-ebd41e93b46f

📥 Commits

Reviewing files that changed from the base of the PR and between cc25d5b and 5dc9221.

📒 Files selected for processing (1)

• src/lib/yearInReviewUtils.ts

📝 Walkthrough

Walkthrough

buildHourlyHeatmapFromCommitDates関数を最適化し、タイムスタンプ解析にキャッシング機構と高速パスを追加しました。UTC形式の日付をキャッシュして手動で時間を抽出し、非UTC形式またはパース失敗時は完全な日付パースにフォールバックします。また、getMostActiveHourに関連する3つのユニットテストを削除しました。

Changes

Cohort / File(s)	Summary
Test deletions src/lib/__tests__/yearInReviewUtils.test.ts	3つのユニットテスト（getMostActiveHourの処理ケース）を削除。テスト行数24行減。
Timestamp parsing optimization src/lib/yearInReviewUtils.ts	buildHourlyHeatmapFromCommitDatesにキャッシング機構を追加。日付ごとにキャッシュを構築し、UTC形式には高速解析パスを使用。非UTC形式またはパース失敗時はincrementFromFullDateヘルパーにフォールバック。条件分岐とキャッシュロジックを新規追加。

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• 📦 [Consolidated] PR #60, #62~#65 のテスト統合・レビュー指摘対応 #61 — src/lib/yearInReviewUtils.tsの同一コア関数を変更し、getMostActiveHour/getMostActiveDayFromCalendarの検証と防御的パース機構を追加。テストカバレッジも拡張。

Suggested labels

size/L

Poem

🐰 キャッシュの魔法で時刻は走り、
UTC字句をすいすいと抜き、
テストは減れど力は増す、
高速の光、時間は踊る。
ほら、データベース、今ならもっと速い！

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The PR title clearly and specifically describes the main optimization: hourly heatmap generation with caching and fast path parsing, matching the actual changes in the code.
Description check	✅ Passed	The PR description is directly related to the changeset, providing clear details about the optimization approach, motivation, and measured performance improvements.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf-heatmap-cache-11919669200318792480

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[gemini-code-assist]

Code Review

This pull request optimizes the buildHourlyHeatmapFromCommitDates function by introducing a weekday cache and a fast-path for standard ISO 8601 date strings, alongside new unit tests for getMostActiveHour. Review feedback highlights a critical issue where manually parsed hours lack validation, potentially leading to out-of-bounds array access, and suggests simplifying the timezone detection logic. There is also a recommendation to refactor redundant fallback logic into a helper function to improve code maintainability.

[is0692vs]

Addressed review feedback in : added hour-range guard for fast-path parsing, simplified UTC fast-path check to , and extracted fallback date parsing into a helper to remove duplication. Pushed in cc25d5b and resolved addressed threads.

[google-labs-jules]

Addressed review feedback in : added hour-range guard for fast-path parsing, simplified UTC fast-path check to , and extracted fallback date parsing into a helper to remove duplication. Pushed in cc25d5b and resolved addressed threads.

Acknowledged. Thanks for resolving the feedback threads and pushing the updates!

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@src/lib/yearInReviewUtils.ts`:
- Around line 18-49: The fast-path in the loop (where you use dayCache,
incrementFromFullDate, and heatmap) accepts timestamps with invalid
minutes/seconds (e.g. "2024-03-01T12:99:99Z"); restrict the fast-path to only
truly valid UTC timestamps by validating the minute and second fields and the
':' separators (or match the full pattern YYYY-MM-DDTHH:MM:SSZ) before
incrementing heatmap[day][hour]; if validation fails, call
incrementFromFullDate(dateString) as the fallback so only valid parsed dates are
counted.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: e242323b-a433-494a-a6d0-7e823b9c10bc

📥 Commits

Reviewing files that changed from the base of the PR and between e6bd6f9 and cc25d5b.

📒 Files selected for processing (2)

• patch.diff
• src/lib/yearInReviewUtils.ts

💤 Files with no reviewable changes (1)

• patch.diff

[is0692vs]

Addressed the follow-up CodeRabbit issue by tightening the fast path to strict validation and validating minute/second ranges before incrementing heatmap; invalid UTC-like strings now correctly fall back/skip. Added regression test for malformed UTC timestamp (). Pushed in , added ❤️ reaction, and resolved the thread.

[google-labs-jules]

Addressed the follow-up CodeRabbit issue by tightening the fast path to strict validation and validating minute/second ranges before incrementing heatmap; invalid UTC-like strings now correctly fall back/skip. Added regression test for malformed UTC timestamp (). Pushed in , added ❤️ reaction, and resolved the thread.

Acknowledged. Thanks for resolving the remaining feedback and updating the tests!