[REQ-002] Add completionsInRange and last7Count pure helpers

[markoub]

Summary

• Adds completionsInRange(habit, fromDay, toDay): number to src/habits.ts — counts completions within a YYYY-MM-DD range (both ends inclusive) using ISO string comparison.
• Adds last7Count(habit, today): number — thin wrapper that calls completionsInRange over the 7-day window ending on today.
• Both helpers are pure (no DOM, no storage) and safe to call in Vitest.

How I tested

Added 10 new unit tests in src/habits.test.ts covering:

• Zero completions
• Completions strictly inside the range
• Boundary dates (fromDay and toDay inclusive)
• Completions outside the range (excluded)
• Full 7-day window (all 7 days completed)
• Partial completions within the window
• Completions older than 7 days (excluded)

All 27 tests pass: npm test ✅
Build clean: npm run build ✅

Closes #5

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Generated by Claude Code

[markoub]

Review: PR #10 — completionsInRange and last7Count pure helpers

Overall: this is solid work — clean implementation, well-tested, perfectly scoped. ✅ Ready to merge.

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Correctness vs acceptance criteria

All five acceptance criteria from issue #5 are satisfied:

Criterion	Result
completionsInRange(habit, fromDay, toDay): number in src/habits.ts	✅
last7Count(habit, today): number delegates to completionsInRange	✅
Both helpers are pure (no DOM / localStorage / window)	✅
Tests cover zero completions, boundary dates, full & partial windows	✅
npm test and npm run build pass	✅ (CI green)

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Purity (src/habits.ts)

completionsInRange (habits.ts:75-83) is a one-liner .filter() on an in-memory array — perfectly pure. last7Count (habits.ts:88-91) delegates to shiftDay (UTC-safe date arithmetic, no browser APIs) and completionsInRange. Nothing here touches DOM, storage, or the clock. The golden rule in CLAUDE.md is upheld.

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Implementation correctness

The boundary math in last7Count is right: shiftDay(today, -6) gives the date 6 days before today, making a closed window of exactly 7 days (today − 6 … today). The test at habits.test.ts:151 ("includes today and the oldest day of the 7-day window") confirms this with 2026-06-04 as the lower bound when today = "2026-06-10". ✅

ISO lexicographic comparison (d >= fromDay && d <= toDay) is valid for YYYY-MM-DD strings — good use of the existing convention already present in currentStreak.

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Test coverage

10 new tests across two describe blocks — all edge cases called out in the acceptance criteria are hit:

• completionsInRange: empty habit, strictly inside, fromDay boundary, toDay boundary, outside range.
• last7Count: empty habit, all 7 days, partial window, day −6 boundary, day −7 excluded.

The "excludes completions older than 7 days" test correctly pins 2026-06-03 (day −7, outside the window) and expects 0. ✅

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Style & scope

• Diff touches exactly two files (src/habits.ts, src/habits.test.ts) — focused and on-topic.
• Single commit on feat/issue-5-completions-in-range — conventional naming.
• JSDoc style matches the existing multi-line blocks already present for currentStreak (habits.ts:55-59) and bestStreak (habits.ts:75-78). The second line in completionsInRange's doc ("ISO lexicographic order is used") records a non-obvious implementation invariant — justified per CLAUDE.md's "only add a comment when the WHY is non-obvious."
• No new dependencies introduced.

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Non-blocking nit (no action required)

shiftDay is unexported (habits.ts:49) and used internally by currentStreak, bestStreak, and now last7Count. That's fine for now. If a future issue requires date arithmetic in the UI layer it'll naturally be promoted then — no action needed here.

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Verdict: All criteria met, CI green, mergeable. Labelling ready-to-merge and squash-merging.

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Generated by Claude Code