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/**
* Calculate virtual scroll stuffs
*
* @param {?Object} opts Options for calculation
*
* @returns {Object} result
*
* __opts__ should have following properties:
* - {number} itemCount
* - {number} rowHeight
* - {number} viewportTop
* - {number} viewportHeight
* - {Array<?number>} [itemHeights]
*
* __result__ have following properties:
* - {number} start
* - {number} end
* - {number} paddingTop
* - {number} paddingBottom
*/
export function calcVScroll(opts) {
if (!opts) {
return { start: 0, end: 0, paddingTop: 0, paddingBottom: 0 };
}
const { itemCount, rowHeight, viewportTop, viewportHeight, itemHeights } = opts;
const viewportBottom = viewportTop + viewportHeight;
let start = 0;
let end = 0;
let paddingTop = 0;
let paddingBottom = 0;
if (itemHeights) {
for (let i = 0, pos = 0; i < itemCount; i++) {
const height = itemHeights[i] || rowHeight;
if (pos <= viewportTop && i % 2 === 0) {
paddingTop = pos;
start = i;
}
if (pos <= viewportBottom) {
end = i + 1;
} else {
paddingBottom += height;
}
pos += height;
}
} else {
// Make sure that we start at an even row so that CSS `:nth-child(even)` is preserved
start = Math.max(0, Math.floor(viewportTop / rowHeight) - 1) & ~1;
end = Math.min(
itemCount,
start + Math.ceil(viewportHeight / rowHeight) + 1
);
// When a large trunk of elements is removed from the button, start may be far off the viewport.
// To make this issue less severe, limit the top placeholder to the total number of rows.
paddingTop = Math.min(start, itemCount) * rowHeight;
paddingBottom = Math.max(0, itemCount - end) * rowHeight;
}
return { start, end, paddingTop, paddingBottom };
}
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