docs: add problem yearly AC rate feature design spec

docs/superpowers/specs/2026-05-11-problem-yearly-ac-rate-design.md

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+# Problem Yearly AC Rate — Design Spec
+
+**Date:** 2026-05-11  
+**Status:** Approved
+
+---
+
+## Overview
+
+Add a per-problem "historical AC rate by year" feature. A new backend API returns yearly submission statistics for a given problem; the frontend displays this as a line chart on the problem stats page, alongside the existing submission-result pie chart.
+
+---
+
+## Backend
+
+### Endpoint
+
+```
+GET /api/problem/yearly_ac?problem_id=<_id>
+```
+
+- Public (user-facing), no authentication required
+- `problem_id`: the problem's display ID (`Problem._id`, string)
+
+### Query Logic
+
+```python
+Submission.objects.filter(
+    problem_id=problem.id,
+    contest_id__isnull=True,          # exclude contest submissions
+    result__not_in=[JudgeStatus.PENDING, JudgeStatus.JUDGING],  # exclude in-flight submissions
+)
+.annotate(year=ExtractYear('create_time'))
+.values('year')
+.annotate(
+    total=Count('id'),
+    accepted=Count('id', filter=Q(result=JudgeStatus.ACCEPTED))
+)
+.order_by('year')
+```
+
+### Response Format
+
+```json
+{
+  "error": null,
+  "data": [
+    {"year": 2023, "total": 120, "accepted": 54, "ac_rate": 45.0},
+    {"year": 2024, "total": 88,  "accepted": 31, "ac_rate": 35.23}
+  ]
+}
+```
+
+- `ac_rate`: float, two decimal places, percentage (0–100)
+- If `total == 0` for a year: `ac_rate = 0` (guards against division by zero)
+- Empty list returned if no judged submissions exist
+
+### Error Cases
+
+| Condition | Response |
+|---|---|
+| `problem_id` missing | `error("problem_id is required")` |
+| Problem not found / not visible | `error("Problem does not exist")` |
+
+### Caching
+
+- Redis cache key: `problem_yearly_ac:{problem._id}`
+- TTL: 3600 seconds (1 hour), consistent with other problem caches
+
+### Implementation Location
+
+- View class: `ProblemYearlyACRateAPI` in `OnlineJudge/problem/views/oj.py`
+- URL: `path("problem/yearly_ac", ProblemYearlyACRateAPI.as_view())` in `OnlineJudge/problem/urls/oj.py`
+
+---
+
+## Frontend
+
+### New Component
+
+`ojnext/src/oj/problem/components/ProblemYearlyChart.vue`
+
+- Uses `vue-chartjs` Line chart (same library as `oj/contest/components/LineChart.vue`)
+- Registers: `CategoryScale`, `LinearScale`, `PointElement`, `LineElement`, `Title`, `Tooltip`, `Filler`
+- Props: `data: Array<{ year: number; total: number; accepted: number; ac_rate: number }>`
+- X-axis: year (string labels)
+- Y-axis: AC rate 0–100%
+- Tooltip per point: year, AC rate, accepted/total counts
+- Shows nothing (v-if) when data is empty or has only one point
+
+### Chart Options
+
+- `responsive: true`, `maintainAspectRatio: false`, height `250px`
+- Dataset: `fill: true` (area chart feel), single line
+- Y-axis: `min: 0`, `max: 100`, tick suffix `%`
+
+### API Call
+
+Add to `ojnext/src/oj/api.ts`:
+
+```ts
+export function getProblemYearlyAC(problemId: string) {
+  return http.get<Array<{ year: number; total: number; accepted: number; ac_rate: number }>>(
+    "/problem/yearly_ac",
+    { params: { problem_id: problemId } }
+  )
+}
+```
+
+### Integration
+
+In `ojnext/src/oj/problem/components/ProblemInfo.vue`:
+
+- Import `getProblemYearlyAC` and call it on mount (alongside existing `getProblemBeatRate`)
+- Render `<ProblemYearlyChart>` below the existing pie chart, with a section title "历年 AC 率"
+- No loading spinner needed (chart simply absent until data arrives)
+
+---
+
+## Assumptions
+
+- Contest submissions excluded; they represent a different access context
+- PENDING (6) and JUDGING (7) excluded; PARTIALLY_ACCEPTED (8) and SYSTEM_ERROR (5) are included as final verdicts
+- No migrations needed (query only, no model changes)