Function findNearestPoint

src/interaction/findNearestPoint.ts:476–874

(
  series: ReadonlyArray<ResolvedSeriesConfig>,
  x: number,
  y: number,
  xScale: LinearScale,
  yScale: LinearScale,
  maxDistance: number = DEFAULT_MAX_DISTANCE_PX,
)

 * - Skips non-finite points and any points whose scaled coordinates are NaN.
 */
export function findNearestPoint(
  series: ReadonlyArray<ResolvedSeriesConfig>,
  x: number,
  y: number,
  xScale: LinearScale,
  yScale: LinearScale,
  maxDistance: number = DEFAULT_MAX_DISTANCE_PX,
): NearestPointMatch | null {
  if (!Number.isFinite(x) || !Number.isFinite(y)) return null;

  const md = Number.isFinite(maxDistance)
    ? Math.max(0, maxDistance)
    : DEFAULT_MAX_DISTANCE_PX;
  const maxDistSq = md * md;

  const xTarget = xScale.invert(x);
  if (!Number.isFinite(xTarget)) return null;

  let bestSeriesIndex = -1;
  let bestDataIndex = -1;
  let bestPoint: DataPoint | null = null;
  let bestDistSq = Number.POSITIVE_INFINITY;

  // Story 4.6: Bar hit-testing (range-space bounds).
  // - Only counts as a match when cursor is inside a bar rect.
  // - For stacked bars, uses the same stacking bucket logic as the bar renderer (xKey bucketing).
  // - If multiple segments match (shared edges), prefer visually topmost (smallest `top` in CSS px).
  //   If still tied, prefer larger `seriesIndex` for determinism.
  const barSeriesConfigs: ResolvedBarSeriesConfig[] = [];
  const barSeriesIndexByBar: number[] = [];
  for (let s = 0; s < series.length; s++) {
    const cfg = series[s];
    if (cfg?.type === 'bar') {
      barSeriesConfigs.push(cfg);
      barSeriesIndexByBar.push(s);
    }
  }

  if (barSeriesConfigs.length > 0) {
    const layoutPx = computeBarLayoutPx(barSeriesConfigs, xScale);
    if (layoutPx.barWidthPx > 0 && layoutPx.clusterWidthPx >= 0) {
      const plotHeightPx = inferPlotHeightPxForBarHitTesting(barSeriesConfigs, yScale);
      const { baselineDomain, baselinePx } = computeBaselineDomainAndPx(barSeriesConfigs, yScale, plotHeightPx);

      const { clusterSlots, barWidthPx, gapPx, clusterWidthPx, categoryWidthPx, categoryStep } = layoutPx;
      const stackSumsByStackId = new Map<string, Map<number, { posSum: number; negSum: number }>>();

      let bestBarHit:
        | {
            readonly seriesIndex: number;
            readonly dataIndex: number;
            readonly top: number;
          }
        | null = null;

      for (let b = 0; b < barSeriesConfigs.length; b++) {
        const seriesCfg = barSeriesConfigs[b];
        const originalSeriesIndex = barSeriesIndexByBar[b] ?? -1;