defaultOverscanIndicesGetter.js

// @flow

import type {OverscanIndicesGetterParams, OverscanIndices} from './types';

export const SCROLL_DIRECTION_BACKWARD = -1;
export const SCROLL_DIRECTION_FORWARD = 1;

export const SCROLL_DIRECTION_HORIZONTAL = 'horizontal';
export const SCROLL_DIRECTION_VERTICAL = 'vertical';

/**
 * Calculates the number of cells to overscan before and after a specified range.
 * This function ensures that overscanning doesn't exceed the available cells.
 */

export default function defaultOverscanIndicesGetter({
  cellCount,
  overscanCellsCount,
  scrollDirection,
  startIndex,
  stopIndex,
}: OverscanIndicesGetterParams): OverscanIndices {
  if (scrollDirection === SCROLL_DIRECTION_FORWARD) {
    return {
      overscanStartIndex: Math.max(0, startIndex),
      overscanStopIndex: Math.min(
        cellCount - 1,
        stopIndex + overscanCellsCount,
      ),
    };
  } else {
    return {
      overscanStartIndex: Math.max(0, startIndex - overscanCellsCount),
      overscanStopIndex: Math.min(cellCount - 1, stopIndex),
    };
  }
}