@indielayer/ui/src/virtual/core/getOffsetForIndex.ts

Indielayer UI Components with Tailwind CSS build for Vue 3

import type { Align } from '../types'
import { getEstimatedSize } from './getEstimatedSize'
import type { CachedBounds, SizeFunction } from './types'

export function getOffsetForIndex<Props extends object>({
  align,
  cachedBounds,
  index,
  itemCount,
  itemSize,
  containerScrollOffset,
  containerSize,
}: {
  align: Align;
  cachedBounds: CachedBounds;
  index: number;
  itemCount: number;
  itemSize: number | SizeFunction<Props>;
  containerScrollOffset: number;
  containerSize: number;
}) {
  if (index < 0 || index >= itemCount) {
    throw new RangeError(
      `Invalid index specified: ${index}. Index ${index} is not within the range of 0 - ${itemCount - 1}`,
    )
  }

  const estimatedTotalSize = getEstimatedSize({
    cachedBounds,
    itemCount,
    itemSize,
  })

  const bounds = cachedBounds.get(index)
  const maxOffset = Math.max(
    0,
    Math.min(estimatedTotalSize - containerSize, bounds.scrollOffset),
  )
  const minOffset = Math.max(
    0,
    bounds.scrollOffset - containerSize + bounds.size,
  )

  if (align === 'smart') {
    if (
      containerScrollOffset >= minOffset &&
      containerScrollOffset <= maxOffset
    ) {
      align = 'auto'
    } else {
      align = 'center'
    }
  }

  switch (align) {
    case 'start': {
      return maxOffset
    }
    case 'end': {
      return minOffset
    }
    case 'center': {
      if (bounds.scrollOffset <= containerSize / 2) {
        // Too near the beginning to center-align
        return 0
      } else if (
        bounds.scrollOffset + bounds.size / 2 >=
        estimatedTotalSize - containerSize / 2
      ) {
        // Too near the end to center-align
        return estimatedTotalSize - containerSize
      } else {
        return bounds.scrollOffset + bounds.size / 2 - containerSize / 2
      }
    }
    case 'auto':
    default: {
      if (
        containerScrollOffset >= minOffset &&
        containerScrollOffset <= maxOffset
      ) {
        return containerScrollOffset
      } else if (containerScrollOffset < minOffset) {
        return minOffset
      } else {
        return maxOffset
      }
    }
  }
}