import type { CachedBounds, SizeFunction } from './types'
import { assert } from '../utils/assert'

export function getEstimatedSize<Props extends object>({
  cachedBounds,
  itemCount,
  itemSize,
}: {
  cachedBounds: CachedBounds;
  itemCount: number;
  itemSize: number | SizeFunction<Props>;
}) {
  if (itemCount === 0) {
    return 0
  } else if (typeof itemSize === 'number') {
    return itemCount * itemSize
  } else {
    const bounds = cachedBounds.get(
      cachedBounds.size === 0 ? 0 : cachedBounds.size - 1,
    )

    assert(bounds !== undefined, 'Unexpected bounds cache miss')

    const averageItemSize =
      (bounds.scrollOffset + bounds.size) / cachedBounds.size

    return itemCount * averageItemSize
  }
}