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cache.ts
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cache.ts
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import { InternalCacheSnapshot, ItemsRange } from "./types";
import { clamp, max, median, min } from "./utils";
type Writeable<T> = {
-readonly [key in keyof T]: Writeable<T[key]>;
};
/** @internal */
export const UNCACHED = -1;
/**
* @internal
*/
export type Cache = {
readonly _length: number;
// sizes
readonly _sizes: number[];
readonly _defaultItemSize: number;
// offsets
readonly _computedOffsetIndex: number;
readonly _offsets: number[];
};
const fill = (array: number[], length: number, prepend?: boolean): number[] => {
const key = prepend ? "unshift" : "push";
for (let i = 0; i < length; i++) {
array[key](UNCACHED);
}
return array;
};
/**
* @internal
*/
export const getItemSize = (cache: Cache, index: number): number => {
const size = cache._sizes[index]!;
return size === UNCACHED ? cache._defaultItemSize : size;
};
/**
* @internal
*/
export const setItemSize = (
cache: Writeable<Cache>,
index: number,
size: number
): boolean => {
const isInitialMeasurement = cache._sizes[index] === UNCACHED;
cache._sizes[index] = size;
// mark as dirty
cache._computedOffsetIndex = min(index, cache._computedOffsetIndex);
return isInitialMeasurement;
};
/**
* @internal
*/
export const computeOffset = (
cache: Writeable<Cache>,
index: number
): number => {
if (!cache._length) return 0;
if (cache._computedOffsetIndex >= index) {
return cache._offsets[index]!;
}
if (cache._computedOffsetIndex < 0) {
// first offset must be 0 to avoid returning NaN, which can cause infinite rerender.
// https://github.com/inokawa/virtua/pull/160
cache._offsets[0] = 0;
cache._computedOffsetIndex = 0;
}
let i = cache._computedOffsetIndex;
let top = cache._offsets[i]!;
while (i < index) {
top += getItemSize(cache, i);
cache._offsets[++i] = top;
}
// mark as measured
cache._computedOffsetIndex = index;
return top;
};
/**
* @internal
*/
export const computeTotalSize = (cache: Cache): number => {
if (!cache._length) return 0;
return (
computeOffset(cache, cache._length - 1) +
getItemSize(cache, cache._length - 1)
);
};
/**
* @internal
*/
export const findIndex = (cache: Cache, offset: number, i: number): number => {
while (i >= 0 && i < cache._length) {
const itemOffset = computeOffset(cache, i);
if (itemOffset <= offset) {
if (itemOffset + getItemSize(cache, i) > offset) {
break;
} else {
i++;
}
} else {
i--;
}
}
return clamp(i, 0, cache._length - 1);
};
/**
* @internal
*/
export const computeRange = (
cache: Cache,
scrollOffset: number,
prevStartIndex: number,
viewportSize: number
): ItemsRange => {
const start = findIndex(
cache,
scrollOffset,
// Clamp because prevStartIndex may exceed the limit when children decreased a lot after scrolling
min(prevStartIndex, cache._length - 1)
);
return [start, findIndex(cache, scrollOffset + viewportSize, start)];
};
/**
* @internal
*/
export const estimateDefaultItemSize = (
cache: Writeable<Cache>,
startIndex: number
): number => {
let measuredCountBeforeStart = 0;
// This function will be called after measurement so measured size array must be longer than 0
const measuredSizes = cache._sizes.filter((s, i) => {
const isMeasured = s !== UNCACHED;
if (isMeasured && i < startIndex) {
measuredCountBeforeStart++;
}
return isMeasured;
});
const prevDefaultItemSize = cache._defaultItemSize;
// Discard cache for now
cache._computedOffsetIndex = -1;
// Calculate diff of unmeasured items before start
return (
((cache._defaultItemSize = median(measuredSizes)) - prevDefaultItemSize) *
max(startIndex - measuredCountBeforeStart, 0)
);
};
/**
* @internal
*/
export const initCache = (
length: number,
itemSize: number,
snapshot?: InternalCacheSnapshot
): Cache => {
let sizes: number[] | undefined;
if (snapshot && snapshot[0]) {
// https://github.com/inokawa/virtua/issues/441
sizes = fill(
snapshot[0].slice(0, min(snapshot[0].length, length)),
max(0, length - snapshot[0].length)
);
} else {
sizes = fill([], length);
}
return {
_defaultItemSize: snapshot ? snapshot[1] : itemSize,
_sizes: sizes,
_length: length,
_computedOffsetIndex: -1,
_offsets: fill([], length),
};
};
/**
* @internal
*/
export const takeCacheSnapshot = (cache: Cache): InternalCacheSnapshot => {
return [[...cache._sizes], cache._defaultItemSize];
};
/**
* @internal
*/
export const updateCacheLength = (
cache: Writeable<Cache>,
length: number,
isShift?: boolean
): number => {
const diff = length - cache._length;
cache._computedOffsetIndex = isShift
? // Discard cache for now
-1
: min(length - 1, cache._computedOffsetIndex);
cache._length = length;
if (diff > 0) {
// Added
fill(cache._offsets, diff);
fill(cache._sizes, diff, isShift);
return cache._defaultItemSize * diff;
} else {
// Removed
cache._offsets.splice(diff);
return (
isShift ? cache._sizes.splice(0, -diff) : cache._sizes.splice(diff)
).reduce(
(acc, removed) =>
acc - (removed === UNCACHED ? cache._defaultItemSize : removed),
0
);
}
};