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