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

Indielayer UI Components with Tailwind CSS build for Vue 3

import type { CachedBounds } from './types'

export function getStartStopIndices({
  cachedBounds,
  containerScrollOffset,
  containerSize,
  itemCount,
  overscanCount,
}: {
  cachedBounds: CachedBounds;
  containerScrollOffset: number;
  containerSize: number;
  itemCount: number;
  overscanCount: number;
}): {
    startIndexVisible: number;
    stopIndexVisible: number;
    startIndexOverscan: number;
    stopIndexOverscan: number;
  } {
  const maxIndex = itemCount - 1

  let startIndexVisible = 0
  let stopIndexVisible = -1
  let startIndexOverscan = 0
  let stopIndexOverscan = -1
  let currentIndex = 0

  while (currentIndex < maxIndex) {
    const bounds = cachedBounds.get(currentIndex)

    if (bounds.scrollOffset + bounds.size > containerScrollOffset) {
      break
    }

    currentIndex++
  }

  startIndexVisible = currentIndex
  startIndexOverscan = Math.max(0, startIndexVisible - overscanCount)

  while (currentIndex < maxIndex) {
    const bounds = cachedBounds.get(currentIndex)

    if (
      bounds.scrollOffset + bounds.size >=
      containerScrollOffset + containerSize
    ) {
      break
    }

    currentIndex++
  }

  stopIndexVisible = Math.min(maxIndex, currentIndex)
  stopIndexOverscan = Math.min(itemCount - 1, stopIndexVisible + overscanCount)

  if (startIndexVisible < 0) {
    startIndexVisible = 0
    stopIndexVisible = -1
    startIndexOverscan = 0
    stopIndexOverscan = -1
  }

  return {
    startIndexVisible,
    stopIndexVisible,
    startIndexOverscan,
    stopIndexOverscan,
  }
}