Support overnight blocks in schedule validation

src/lib/schedule-utils.ts

@@ -49,14 +49,60 @@ export function snapMinutes(minutes: number): number {
 	return Math.max(0, Math.min(MINUTES_PER_DAY, snapped));
 }
 
-/** First sibling whose half-open range [start, end) intersects the input. */
+/** A Block whose end clock time is at or before its start wraps past midnight. */
+export function isOvernight(block: { startMinutes: number; endMinutes: number }): boolean {
+	return block.endMinutes <= block.startMinutes;
+}
+
+/** Block length in minutes, counting a wrap past midnight as the next day. */
+export function blockDuration(block: { startMinutes: number; endMinutes: number }): number {
+	if (block.endMinutes > block.startMinutes) {
+		return block.endMinutes - block.startMinutes;
+	}
+	return block.endMinutes - block.startMinutes + MINUTES_PER_DAY;
+}
+
+/**
+ * Split a Block into the linear half-open `[start, end)` segments it occupies
+ * within the 24h day. An overnight block produces two segments (start→midnight
+ * and midnight→end); a zero-length block produces none.
+ */
+export function blockSegments(block: {
+	startMinutes: number;
+	endMinutes: number;
+}): [number, number][] {
+	if (block.startMinutes < block.endMinutes) {
+		return [[block.startMinutes, block.endMinutes]];
+	}
+	if (block.startMinutes > block.endMinutes) {
+		return [
+			[block.startMinutes, MINUTES_PER_DAY],
+			[0, block.endMinutes]
+		];
+	}
+	return [];
+}
+
+function segmentsIntersect(a: [number, number], b: [number, number]): boolean {
+	return a[0] < b[1] && b[0] < a[1];
+}
+
+/**
+ * First sibling whose occupied time intersects the input. Works for overnight
+ * (midnight-crossing) blocks by comparing their linear segments; ranges that
+ * only touch at an endpoint (e.g. 9PM–9AM vs 9AM–9PM) do not overlap.
+ */
 export function findOverlap(
 	input: { startMinutes: number; endMinutes: number },
 	siblings: BlockSibling[]
 ): BlockSibling | null {
+	const inputSegments = blockSegments(input);
 	for (const sibling of siblings) {
-		if (input.startMinutes < sibling.endMinutes && sibling.startMinutes < input.endMinutes) {
-			return sibling;
+		const siblingSegments = blockSegments(sibling);
+		for (const segment of inputSegments) {
+			for (const other of siblingSegments) {
+				if (segmentsIntersect(segment, other)) return sibling;
+			}
 		}
 	}
 	return null;
@@ -78,13 +124,17 @@ export function validateBlock(
 		!Number.isInteger(startMinutes) ||
 		!Number.isInteger(endMinutes) ||
 		startMinutes < 0 ||
-		endMinutes > MINUTES_PER_DAY ||
-		startMinutes >= endMinutes
+		startMinutes > MINUTES_PER_DAY ||
+		endMinutes < 0 ||
+		endMinutes > MINUTES_PER_DAY
 	) {
-		return {
-			ok: false,
-			error: "Start time must be before end time. Overnight blocks aren't supported."
-		};
+		return { ok: false, error: 'Enter a valid start and end time.' };
+	}
+
+	// Equal clock times would be a zero- or full-length block; both are
+	// ambiguous, so reject them. Overnight ranges (end before start) are valid.
+	if (startMinutes === endMinutes) {
+		return { ok: false, error: "Start and end time can't be the same." };
 	}
 
 	if (!Array.isArray(proclamationIds) || proclamationIds.length === 0) {