[REQ-001] Add currentStreak and bestStreak pure functions with unit tests

[markoub]

Summary

• Adds currentStreak(habit, today) to src/habits.ts: counts consecutive completed days ending on or just before today; returns 0 if neither today nor yesterday is completed.
• Adds bestStreak(habit) to src/habits.ts: returns the length of the longest ever consecutive run of completions; returns 0 for an empty habit.
• Both functions are pure (no DOM, no localStorage, no Date reads — today is always passed as a parameter).
• Adds a private shiftDay helper for date arithmetic using UTC to avoid DST issues.

How I tested

Ran npm ci && npm test && npm run build — all 17 tests pass, build succeeds with no TypeScript errors.

New test cases added to src/habits.test.ts:

• Empty habit → 0 (both functions)
• Single completion today → 1
• Single completion yesterday, today not done → currentStreak = 1, bestStreak = 1
• Streak broken by a gap → only the contiguous tail counts for currentStreak; best segment for bestStreak
• Active streak of 6 days ending today
• Past best-run longer than current run

Closes #1

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Generated by Claude Code

[markoub]

Review — PR #3: Add currentStreak and bestStreak

This is a clean, well-structured implementation. All acceptance criteria from issue #1 are met, CI is green, and the code is genuinely pleasant to read. Here's the full walkthrough.

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Correctness ✅

currentStreak (src/habits.ts:49–67) — logic is correct in all required cases:

• Starts from today; if not in the set, falls back one day; if still not found, returns 0 immediately. No infinite loop risk.
• The backwards while walk correctly counts the contiguous tail. Traced manually through all 6 test scenarios — all produce the right result.

bestStreak (src/habits.ts:74–88) — also correct:

• The early-return guard for empty completions is clean.
• The [...habit.completions].sort() defensive copy is good hygiene even though toggleCompletion already sorts; costs nothing.
• The in-place best / run scan is O(n) and readable. Confirmed it handles the single-element case (loop body never executes, returns best = 1) and the "past best longer than current run" case correctly.

shiftDay (src/habits.ts:48–53) — the UTC anchoring ("T00:00:00Z" + setUTCDate) is exactly right to avoid DST surprises. Good call.

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Purity ✅

src/habits.ts remains completely free of DOM, localStorage, window, and clock reads. today is always injected as a YYYY-MM-DD string parameter. shiftDay uses Date only for arithmetic, never to read wall-clock time. The purity invariant holds.

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Test coverage ✅

All six cases required by the acceptance criteria are present for both functions:

Case	currentStreak	bestStreak
Empty habit → 0	✅	✅
Single day today → 1	✅	✅
Single day yesterday, today not done	✅	✅
Streak broken in the middle	✅	✅
Active streak of several days ending today	✅ (3-day + 6-day)	✅ (6-day)
Best streak is a past run	n/a	✅
Gap two days ago → 0 for current	✅	n/a

12 tests total, all meaningful and non-redundant.

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Style & scope ✅

• Single focused commit feat: add currentStreak and bestStreak pure functions — conventional commit format, good body, Closes #1 wired up correctly.
• Diff touches only src/habits.ts and src/habits.test.ts. No stray changes.
• Immutable patterns consistent with the rest of the codebase.

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Non-blocking nit

The JSDoc blocks on currentStreak and bestStreak are multi-line (3 lines and 2 lines respectively), which is a minor departure from CLAUDE.md's "one short line max" comment guideline. The content is genuinely useful for exported API functions, so this is not a blocker — just worth keeping in mind for future contributions.

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Verdict: ready to merge. CI passed, all acceptance criteria satisfied, purity maintained, edge cases covered. Tagging ready-to-merge and squash-merging now.

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Generated by Claude Code