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frontend/src/utils/stats.ts

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+import type { MonthStat } from '@/api/stats'
+
+/**
+ * Pure derived-stat helpers for the Stats page (Task 6).
+ *
+ * Extracted from {@link Stats.vue} so the two pieces of trivia that are easy
+ * to get wrong — (1) finding the current month's count in a DESC-ordered list
+ * and (2) flipping that list to chronological ASC for the line chart — are
+ * unit-testable without mounting the component or stubbing ECharts.
+ */
+
+/**
+ * Look up the inquiry count for a given `YYYY-MM` month in the `monthly` list.
+ *
+ * Used by Stats.vue for the "本月新增" card: it passes
+ * `dayjs().format('YYYY-MM')` and we return the matching count, or `0` when
+ * there's no data for the current month yet (the backend omits zero-count
+ * months entirely, and a fresh month may not have appeared in the trailing
+ * 12-month window at all).
+ *
+ * The lookup is by value equality on `ym`, so input order (DESC vs ASC) is
+ * irrelevant. Returns `0` for an empty list — graceful fallback for an empty
+ * DB / new install.
+ */
+export function currentMonthCount(
+  monthly: readonly MonthStat[],
+  ym: string,
+): number {
+  // `find` rather than indexing `[0]` — the current month is not guaranteed to
+  // be the first entry (DESC newest-first only covers months that have data,
+  // and a brand-new month may be ABSENT from the trailing 12-month window).
+  const hit = monthly.find((m) => m.ym === ym)
+  return hit?.count ?? 0
+}
+
+/**
+ * Return a new array of monthly stats sorted **ascending** by `ym`
+ * (chronological order — what the trend line chart needs).
+ *
+ * The backend returns `monthly` **DESC** (newest first). The line chart's X
+ * axis wants oldest → newest, so this flips the order. A fresh copy is always
+ * returned (the input is never mutated), even when the input is already ASC —
+ * callers can safely pass a `readonly` slice from the API response.
+ *
+ * String comparison on `YYYY-MM` is lexicographically correct (zero-padded
+ * fixed-width format), so no Date parsing is needed.
+ */
+export function sortMonthlyAsc(monthly: readonly MonthStat[]): MonthStat[] {
+  // Slice first so `.sort` cannot reorder the caller's array — important
+  // because `monthly` may be the live API response object.
+  return monthly.slice().sort((a, b) => {
+    if (a.ym !== b.ym) return a.ym < b.ym ? -1 : 1
+    return 0
+  })
+}