finish refactor pre merge

lib/timeline.ts

@@ -0,0 +1,25 @@
+import type { TimelineRange } from "@/lib/editor/session/sessionTypes";
+
+// Single source of truth for the app-wide timeline range.
+export const FIXED_TIMELINE_START_YEAR = -2000;
+export const FIXED_TIMELINE_END_YEAR = 2000;
+
+export const FIXED_TIMELINE_RANGE: TimelineRange = {
+    min: FIXED_TIMELINE_START_YEAR,
+    max: FIXED_TIMELINE_END_YEAR,
+};
+
+// UI debounce when user drags timeline before triggering data fetch.
+export const TIMELINE_DEBOUNCE_MS = 180;
+
+export function clampYearValue(year: number, minYear: number, maxYear: number): number {
+    const lower = Math.min(minYear, maxYear);
+    const upper = Math.max(minYear, maxYear);
+    if (year < lower) return lower;
+    if (year > upper) return upper;
+    return year;
+}
+
+export function clampYearToFixedRange(year: number): number {
+    return clampYearValue(year, FIXED_TIMELINE_START_YEAR, FIXED_TIMELINE_END_YEAR);
+}