react-virtualized/dist/commonjs/Grid/utils/getOverscanIndices.js

React components for efficiently rendering large, scrollable lists and tabular data

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = getOverscanIndices;
var SCROLL_DIRECTION_BACKWARD = exports.SCROLL_DIRECTION_BACKWARD = -1;
var SCROLL_DIRECTION_FIXED = exports.SCROLL_DIRECTION_FIXED = 0;
var SCROLL_DIRECTION_FORWARD = exports.SCROLL_DIRECTION_FORWARD = 1;

/**
 * Calculates the number of cells to overscan before and after a specified range.
 * This function ensures that overscanning doesn't exceed the available cells.
 *
 * @param cellCount Number of rows or columns in the current axis
 * @param scrollDirection One of SCROLL_DIRECTION_BACKWARD
 * @param overscanCellsCount Maximum number of cells to over-render in either direction
 * @param startIndex Begin of range of visible cells
 * @param stopIndex End of range of visible cells
 */
function getOverscanIndices(_ref) {
  var cellCount = _ref.cellCount;
  var overscanCellsCount = _ref.overscanCellsCount;
  var scrollDirection = _ref.scrollDirection;
  var startIndex = _ref.startIndex;
  var stopIndex = _ref.stopIndex;

  var overscanStartIndex = void 0;
  var overscanStopIndex = void 0;

  if (scrollDirection === SCROLL_DIRECTION_FORWARD) {
    overscanStartIndex = startIndex;
    overscanStopIndex = stopIndex + overscanCellsCount * 2;
  } else if (scrollDirection === SCROLL_DIRECTION_BACKWARD) {
    overscanStartIndex = startIndex - overscanCellsCount * 2;
    overscanStopIndex = stopIndex;
  } else {
    overscanStartIndex = startIndex - overscanCellsCount;
    overscanStopIndex = stopIndex + overscanCellsCount;
  }

  return {
    overscanStartIndex: Math.max(0, overscanStartIndex),
    overscanStopIndex: Math.min(cellCount - 1, overscanStopIndex)
  };
}