[Init] Initial commit - NetMesh terminal manager

components/ui/virtualListMath.ts

@@ -0,0 +1,68 @@
+/**
+ * Pure math for virtual lists and keyboard cursors.
+ * Kept free of React so tests drive the same functions the pickers use.
+ */
+
+export function clampScrollTop(
+  scrollTop: number,
+  totalHeight: number,
+  viewportHeight: number,
+): number {
+  const maxScroll = Math.max(0, totalHeight - Math.max(viewportHeight, 0));
+  return Math.min(Math.max(0, scrollTop), maxScroll);
+}
+
+export function getFixedSizeVirtualWindow({
+  itemCount,
+  itemHeight,
+  scrollTop,
+  viewportHeight,
+  overscan,
+}: {
+  itemCount: number;
+  itemHeight: number;
+  scrollTop: number;
+  viewportHeight: number;
+  overscan: number;
+}): { startIndex: number; endIndex: number; effectiveScrollTop: number; totalHeight: number } {
+  const totalHeight = Math.max(0, itemCount * itemHeight);
+  const effectiveScrollTop = clampScrollTop(scrollTop, totalHeight, viewportHeight);
+  if (itemCount <= 0 || itemHeight <= 0) {
+    return { startIndex: 0, endIndex: 0, effectiveScrollTop, totalHeight };
+  }
+  const startIndex = Math.max(0, Math.floor(effectiveScrollTop / itemHeight) - overscan);
+  const visibleCount = Math.ceil(Math.max(viewportHeight, 1) / itemHeight) + overscan * 2;
+  const endIndex = Math.min(itemCount, startIndex + visibleCount);
+  return { startIndex, endIndex, effectiveScrollTop, totalHeight };
+}
+
+/** Clamp a keyboard cursor into [0, length-1], or 0 when the list is empty. */
+export function clampListIndex(index: number, length: number): number {
+  if (length <= 0) return 0;
+  if (!Number.isFinite(index)) return 0;
+  return Math.max(0, Math.min(Math.trunc(index), length - 1));
+}
+
+/** Move a keyboard cursor by delta without ever landing on -1. */
+export function stepListIndex(index: number, length: number, delta: number): number {
+  if (length <= 0) return 0;
+  return clampListIndex(index + delta, length);
+}
+
+/**
+ * Map selectable item indices to visual row indices for lists that interleave
+ * non-selectable chrome (headers, hints, empty rows).
+ */
+export function buildItemIndexToVisualIndexMap(
+  visualRows: ReadonlyArray<{ kind: string }>,
+  itemKind = 'item',
+): Map<number, number> {
+  const map = new Map<number, number>();
+  let itemIndex = 0;
+  visualRows.forEach((row, visualIndex) => {
+    if (row.kind !== itemKind) return;
+    map.set(itemIndex, visualIndex);
+    itemIndex += 1;
+  });
+  return map;
+}