新增订阅额度风险预警计算

server/src/atRisk.js

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+/**
+ * 风险预警计算：算出未来一段时间内，有哪些提醒会因额度不足发不出去
+ *
+ * 首页拿这个结果给用户看「谁的提醒有风险」，而不是「还剩几次额度」——
+ * 用户理解不了额度这种抽象概念，但看得懂人名。
+ */
+
+const { getNextOccurrence, daysBetween } = require('./occurrence')
+
+// 预警窗口。太短则提示不及时，太长会拿几个月后的事惊扰用户，两个月是折中。
+const LOOKAHEAD_DAYS = 60
+
+/**
+ * 把一条纪念日展开成未来窗口内的提醒事件
+ * 每条纪念日最多产生两个事件：提前 N 天一个、当天一个
+ */
+function expandEvents(anniv, today = new Date()) {
+  const next = getNextOccurrence(anniv, today)
+  const onDayIn = daysBetween(next, today)
+  const remindDays = anniv.remindDays || 0
+
+  const events = [{
+    anniversaryId: anniv.id,
+    personName: anniv.personName,
+    kind: 'onDay',
+    fireInDays: onDayIn,
+    importance: anniv.importance
+  }]
+
+  if (remindDays > 0) {
+    events.push({
+      anniversaryId: anniv.id,
+      personName: anniv.personName,
+      kind: 'ahead',
+      fireInDays: onDayIn - remindDays,
+      importance: anniv.importance
+    })
+  }
+
+  // 已经过去的（如提前提醒的时点早已过）和超出窗口的都不算
+  return events.filter(e => e.fireInDays >= 0 && e.fireInDays <= LOOKAHEAD_DAYS)
+}
+
+/**
+ * 算出有风险的提醒
+ * 排序用「时间先后」而非重要程度——跨天的额度就是先到先消耗。
+ * 重要程度只在同一天内比较（那部分逻辑在 reminder.js）。
+ */
+function computeAtRisk(anniversaries, balance, today = new Date()) {
+  const events = (anniversaries || [])
+    .filter(a => a && a.remindEnabled)
+    .flatMap(a => expandEvents(a, today))
+    .sort((x, y) => x.fireInDays - y.fireInDays)
+
+  const safeCount = Math.max(0, balance)
+  const atRisk = events.slice(safeCount)
+
+  const names = []
+  for (const e of atRisk) {
+    if (e.personName && !names.includes(e.personName)) names.push(e.personName)
+  }
+
+  return { atRiskCount: atRisk.length, atRiskNames: names }
+}
+
+module.exports = { LOOKAHEAD_DAYS, expandEvents, computeAtRisk }