Fix Cron.prev month day rollover (#6472)

Author: fubhy

.changeset/fix-cron-prev-month-rollover.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix `Cron.prev` day-of-month rollover across shorter months and non-leap years.

packages/effect/src/Cron.ts

@@ -878,10 +878,20 @@ const stepCron = (cron: Cron, now: DateTime.DateTime.Input | undefined, directio
             const nextDay = table.day[currentDay]
             if (nextDay === undefined) {
               if (reverse) {
-                const prevMonthDays = daysInMonth(
-                  new Date(Date.UTC(current.getUTCFullYear(), current.getUTCMonth(), 0))
-                )
-                b = -(currentDay + (prevMonthDays - boundary.day))
+                const previous = new Date(current)
+                // Day zero is the previous month's last day. These two probes cover every
+                // valid day-of-month.
+                previous.setUTCDate(0)
+                let day = table.day[previous.getUTCDate()]
+                if (day === undefined) {
+                  previous.setUTCDate(0)
+                  day = table.day[previous.getUTCDate()]
+                }
+                if (day === undefined) {
+                  throw new Error("Unable to find cron date")
+                }
+                previous.setUTCDate(day)
+                b = (previous.getTime() - current.getTime()) / 86_400_000
               } else {
                 b = daysInMonth(current) - currentDay + boundary.day
               }
@@ -908,10 +918,10 @@ const stepCron = (cron: Cron, now: DateTime.DateTime.Input | undefined, directio
         const currentMonth = current.getUTCMonth() + 1
         const nextMonth = table.month[currentMonth]
         const clampBoundaryDay = (targetMonthIndex: number): number => {
+          const maxDayInMonth = daysInMonth(new Date(Date.UTC(current.getUTCFullYear(), targetMonthIndex + 1, 0)))
           if (cron.days.size !== 0 && cron.weekdays.size === 0) {
-            return boundary.day
+            return reverse ? table.day[maxDayInMonth] ?? maxDayInMonth : boundary.day
           }
-          const maxDayInMonth = daysInMonth(new Date(Date.UTC(current.getUTCFullYear(), targetMonthIndex + 1, 0)))
           return reverse ? maxDayInMonth : 1
         }
         if (nextMonth === undefined) {
@@ -933,7 +943,7 @@ const stepCron = (cron: Cron, now: DateTime.DateTime.Input | undefined, directio
       return
     }
 
-    throw new Error("Unable to find " + direction + " cron date")
+    throw new Error("Unable to find cron date")
   })
 
   return dateTime.toDateUtc(result)

packages/effect/test/Cron.test.ts

@@ -564,6 +564,40 @@ describe("Cron", () => {
     deepStrictEqual(prev(cron, from), new Date("2024-01-31T00:00:00.000Z"))
   })
 
+  it("prev skips an invalid day in the immediately preceding month", () => {
+    const tz = DateTime.zoneMakeNamedUnsafe("UTC")
+    const cron = Cron.parseUnsafe("0 0 31 * *", tz)
+    const from = new Date("2024-03-15T00:00:00.000Z")
+    deepStrictEqual(prev(cron, from), new Date("2024-01-31T00:00:00.000Z"))
+  })
+
+  it("prev finds the previous leap day", () => {
+    const tz = DateTime.zoneMakeNamedUnsafe("UTC")
+    const cron = Cron.parseUnsafe("0 0 29 2 *", tz)
+    const from = new Date("2024-01-01T00:00:00.000Z")
+    deepStrictEqual(prev(cron, from), new Date("2020-02-29T00:00:00.000Z"))
+  })
+
+  it("prev day-of-month rollovers preserve match and ordering invariants", () => {
+    const tz = DateTime.zoneMakeNamedUnsafe("UTC")
+    const cases = [
+      [Cron.parseUnsafe("0 0 31 * *", tz), new Date("2024-03-15T00:00:00.000Z")],
+      [Cron.parseUnsafe("0 0 29 2 *", tz), new Date("2024-01-01T00:00:00.000Z")]
+    ] as const
+
+    for (const [cron, from] of cases) {
+      const result = prev(cron, from)
+      assertTrue(Cron.match(cron, result))
+      assertTrue(result < from)
+    }
+  })
+
+  it("prev terminates for an impossible day and month combination", () => {
+    const tz = DateTime.zoneMakeNamedUnsafe("UTC")
+    const cron = Cron.parseUnsafe("0 0 30 2 *", tz)
+    throws(() => prev(cron, new Date("2024-03-01T00:00:00.000Z")))
+  })
+
   it("prev clamps to the last valid day when rolling back a month with only month constraints", () => {
     const tz = DateTime.zoneMakeNamedUnsafe("UTC")
     const cron = Cron.parseUnsafe("0 0 0 * FEB *", tz)