first commit

src/core/rendering.ts

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+import { bisector } from 'd3'
+
+import type { WaveformPoint, WaveformRenderingOptions } from '../types'
+
+export interface ResolvedWaveformRenderingOptions {
+  downsample: boolean
+  downsampleThreshold: number
+  maxPointsPerPixel: number
+}
+
+export const DEFAULT_WAVEFORM_RENDERING_OPTIONS: ResolvedWaveformRenderingOptions = {
+  downsample: true,
+  downsampleThreshold: 2_000,
+  maxPointsPerPixel: 4,
+}
+
+const pointBisector = bisector((point: WaveformPoint) => point.x)
+
+export function resolveWaveformRenderingOptions(
+  options?: WaveformRenderingOptions,
+): ResolvedWaveformRenderingOptions {
+  const threshold = Number(options?.downsampleThreshold)
+  const pointsPerPixel = Number(options?.maxPointsPerPixel)
+  return {
+    downsample: options?.downsample ?? DEFAULT_WAVEFORM_RENDERING_OPTIONS.downsample,
+    downsampleThreshold:
+      Number.isFinite(threshold) && threshold >= 2
+        ? Math.floor(threshold)
+        : DEFAULT_WAVEFORM_RENDERING_OPTIONS.downsampleThreshold,
+    maxPointsPerPixel:
+      Number.isFinite(pointsPerPixel) && pointsPerPixel > 0
+        ? pointsPerPixel
+        : DEFAULT_WAVEFORM_RENDERING_OPTIONS.maxPointsPerPixel,
+  }
+}
+
+function pushUniquePoint(target: WaveformPoint[], point: WaveformPoint | undefined) {
+  if (point && target[target.length - 1] !== point) target.push(point)
+}
+
+/**
+ * Select the visible source range and preserve first/min/max/last values in each X bucket.
+ * Source points must be sorted by X.
+ */
+export function selectRenderablePoints(
+  points: WaveformPoint[],
+  domain: [number, number],
+  width: number,
+  options: ResolvedWaveformRenderingOptions,
+): WaveformPoint[] {
+  if (!points.length || width <= 0) return []
+
+  const domainStart = Math.min(domain[0], domain[1])
+  const domainEnd = Math.max(domain[0], domain[1])
+  const visibleStart = pointBisector.left(points, domainStart)
+  const visibleEnd = pointBisector.right(points, domainEnd)
+  const start = Math.max(0, visibleStart - 1)
+  const end = Math.min(points.length, visibleEnd + 1)
+  const visibleCount = end - start
+  if (visibleCount <= 0) return []
+  if (!options.downsample || visibleCount <= options.downsampleThreshold) {
+    return points.slice(start, end)
+  }
+
+  const maximumPointCount = Math.max(4, Math.floor(width * options.maxPointsPerPixel))
+  const bucketCount = Math.max(1, Math.floor(maximumPointCount / 4))
+  if (visibleCount <= maximumPointCount) return points.slice(start, end)
+
+  const result: WaveformPoint[] = []
+  const span = domainEnd - domainStart || 1
+  let activeBucket = -1
+  let firstIndex = -1
+  let lastIndex = -1
+  let minimumIndex = -1
+  let maximumIndex = -1
+
+  const flushBucket = () => {
+    if (firstIndex < 0) return
+    const indexes = [firstIndex, minimumIndex, maximumIndex, lastIndex]
+      .filter((index, position, source) => index >= 0 && source.indexOf(index) === position)
+      .sort((left, right) => left - right)
+    indexes.forEach((index) => pushUniquePoint(result, points[index]))
+  }
+
+  pushUniquePoint(result, points[start])
+  for (let index = Math.max(start, visibleStart); index < Math.min(end, visibleEnd); index += 1) {
+    const point = points[index]
+    const bucket = Math.min(
+      bucketCount - 1,
+      Math.max(0, Math.floor(((point.x - domainStart) / span) * bucketCount)),
+    )
+    if (bucket !== activeBucket) {
+      flushBucket()
+      activeBucket = bucket
+      firstIndex = index
+      lastIndex = index
+      minimumIndex = index
+      maximumIndex = index
+      continue
+    }
+    lastIndex = index
+    if (point.y < points[minimumIndex].y) minimumIndex = index
+    if (point.y > points[maximumIndex].y) maximumIndex = index
+  }
+  flushBucket()
+  pushUniquePoint(result, points[end - 1])
+  return result
+}