2 files changed, 105 insertions, 0 deletions

diff --git a/src/lib/shared/routine-metrics.test.ts b/src/lib/shared/routine-metrics.test.ts
index 4a84eda..a63069c 100644
--- a/src/lib/shared/routine-metrics.test.ts
+++ b/src/lib/shared/routine-metrics.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect } from 'vitest';
 import {
 	computeRoutineState,
 	computeHealthScore,
+	computeHealthSeries,
 	parseIntervalToDays,
 } from './routine-metrics';
 
@@ -64,6 +65,51 @@ describe('computeRoutineState', () => {
 	});
 });
 
+describe('computeHealthSeries', () => {
+	it('decays from 100 at creation to 0 two periods late when never done', () => {
+		const now = Date.parse(CREATED) + 15 * DAY_MS;
+		const series = computeHealthSeries(routine, [], now);
+		expect(series[0].health).toBe(100);
+		const at8d = series.find((p) => p.at === Date.parse(CREATED) + 8 * DAY_MS);
+		expect(at8d?.health).toBe(86); // 1/7 of a period overdue
+		const last = series[series.length - 1];
+		expect(last.at).toBe(now);
+		expect(last.health).toBe(0); // more than a full period overdue
+	});
+
+	it('resets to 100 right after a completion, then decays again (sawtooth)', () => {
+		const doneAt = Date.parse(CREATED) + 5 * DAY_MS;
+		const now = doneAt + 13 * DAY_MS; // 13 days after the completion
+		const series = computeHealthSeries(
+			routine,
+			[{ routineId: 1, doneAt: new Date(doneAt).toISOString() }],
+			now,
+		);
+		const rightAfter = series.find((p) => p.at >= doneAt);
+		expect(rightAfter?.health).toBe(100);
+		// 8 days after the completion: 1 day into the 7-day period overdue.
+		const at8d = series.find((p) => p.at === doneAt + 8 * DAY_MS);
+		expect(at8d?.health).toBe(86);
+	});
+
+	it('a second completion resets the decay again', () => {
+		const first = Date.parse(CREATED) + 3 * DAY_MS;
+		const second = Date.parse(CREATED) + 10 * DAY_MS;
+		const now = Date.parse(CREATED) + 11 * DAY_MS;
+		const series = computeHealthSeries(
+			routine,
+			[
+				{ routineId: 1, doneAt: new Date(first).toISOString() },
+				{ routineId: 1, doneAt: new Date(second).toISOString() },
+			],
+			now,
+		);
+		// Just after the second completion the routine is back at full health.
+		const nearEnd = series[series.length - 1];
+		expect(nearEnd.health).toBe(100);
+	});
+});
+
 describe('parseIntervalToDays', () => {
 	it('converts units to days', () => {
 		expect(parseIntervalToDays('1d')).toBe(1);
diff --git a/src/lib/shared/routine-metrics.ts b/src/lib/shared/routine-metrics.ts
index 2ac245e..99e5c56 100644
--- a/src/lib/shared/routine-metrics.ts
+++ b/src/lib/shared/routine-metrics.ts
@@ -84,6 +84,65 @@ export function computeHealthScore(states: { health: number }[]): number {
 	return Math.round(total / states.length);
 }
 
+export interface HealthPoint {
+	/** Epoch ms of the sample. */
+	at: number;
+	/** Health (0-100) at that instant. */
+	health: number;
+}
+
+/**
+ * Daily health samples from the routine's creation to now: 100 right after a
+ * completion, then a linear decay to 0 once a full period is overdue (sawtooth).
+ * Each sample anchors on the latest completion at or before it (or creation).
+ * Uses the routine's current frequency across the whole history.
+ */
+export function computeHealthSeries(
+	routine: RoutineInput,
+	completions: CompletionInput[],
+	now: number,
+): HealthPoint[] {
+	const createdMs = Date.parse(routine.createdAt);
+	const startMs = Number.isFinite(createdMs) ? createdMs : now;
+	const periodMs = routine.frequencyDays * DAY_MS;
+
+	const completionMs = completions
+		.filter((c) => c.routineId === routine.id)
+		.map((c) => Date.parse(c.doneAt))
+		.sort((a, b) => a - b);
+
+	const points: HealthPoint[] = [];
+	if (now <= startMs) {
+		return [{ at: now, health: 100 }];
+	}
+
+	// Reference anchor: latest completion at-or-before the sampled instant.
+	let refMs = startMs;
+	let nextCompletion = 0;
+	const sampleAt = (t: number): number => {
+		while (
+			nextCompletion < completionMs.length &&
+			completionMs[nextCompletion] <= t
+		) {
+			refMs = completionMs[nextCompletion];
+			nextCompletion++;
+		}
+		const dueMs = refMs + periodMs;
+		const rawRatio = (t - dueMs) / periodMs;
+		return Math.max(0, Math.round(100 - Math.max(0, rawRatio) * 100));
+	};
+
+	for (let t = startMs; t <= now; t += DAY_MS) {
+		points.push({ at: t, health: sampleAt(t) });
+	}
+
+	// Final unaligned sample so the curve ends at the present instant.
+	if (points[points.length - 1].at < now) {
+		points.push({ at: now, health: sampleAt(now) });
+	}
+	return points;
+}
+
 export function formatFrequency(days: number): string {
 	if (days === 1) return 'chaque jour';
 	if (days === 7) return 'chaque semaine';