refactor(web): extract drag position helpers

apps/web/src/hooks/timelineDragPosition.ts

@@ -0,0 +1,74 @@
+import { computeDragDates, pixelsToDays } from "~/components/timeline/dragMath.js";
+
+type DragPositionLike = {
+  originalStartDate: Date | null;
+  originalEndDate: Date | null;
+  currentStartDate: Date | null;
+  currentEndDate: Date | null;
+  startMouseX: number;
+  pointerDeltaX: number;
+  daysDelta: number;
+};
+
+type DragMode = "move" | "resize-start" | "resize-end";
+
+export type TimelineDragPositionResult<State> =
+  | { handled: false }
+  | { handled: true; nextState: State; pointerDeltaX: number; daysDelta: number; shouldSyncState: boolean };
+
+function resolveDragPosition<State extends DragPositionLike>(
+  state: State,
+  clientX: number,
+  cellWidth: number,
+  mode: DragMode,
+): TimelineDragPositionResult<State> {
+  const pointerDeltaX = clientX - state.startMouseX;
+  const daysDelta = pixelsToDays(pointerDeltaX, cellWidth);
+
+  if (daysDelta === state.daysDelta) {
+    return {
+      handled: true,
+      nextState: pointerDeltaX === state.pointerDeltaX ? state : { ...state, pointerDeltaX },
+      pointerDeltaX,
+      daysDelta,
+      shouldSyncState: false,
+    };
+  }
+
+  const { start, end } = computeDragDates(mode, state.originalStartDate!, state.originalEndDate!, daysDelta);
+  return {
+    handled: true,
+    nextState: {
+      ...state,
+      currentStartDate: start,
+      currentEndDate: end,
+      pointerDeltaX,
+      daysDelta,
+    },
+    pointerDeltaX,
+    daysDelta,
+    shouldSyncState: true,
+  };
+}
+
+export function resolveProjectDragPosition<State extends DragPositionLike & { isDragging: boolean }>(
+  drag: State,
+  clientX: number,
+  cellWidth: number,
+): TimelineDragPositionResult<State> {
+  if (!drag.isDragging || !drag.originalStartDate || !drag.originalEndDate) {
+    return { handled: false };
+  }
+
+  return resolveDragPosition(drag, clientX, cellWidth, "move");
+}
+
+export function resolveAllocationDragPosition<
+  State extends DragPositionLike & { isActive: boolean; mode: DragMode },
+>(alloc: State, clientX: number, cellWidth: number): TimelineDragPositionResult<State> {
+  if (!alloc.isActive || !alloc.originalStartDate || !alloc.originalEndDate) {
+    return { handled: false };
+  }
+
+  return resolveDragPosition(alloc, clientX, cellWidth, alloc.mode);
+}