fix(chart): honor minimum visible zoom points

src/components/interaction/zoomConstraints.ts

@@ -0,0 +1,167 @@
+import { scaleLinear, zoomIdentity, type ZoomTransform } from 'd3'
+
+import type { WaveformPoint } from '../../types'
+import { ZOOM_CONSTRAINTS } from '../core/constants'
+
+interface PointSeriesSource {
+  points: WaveformPoint[]
+}
+
+export type ZoomSeriesGroup = readonly PointSeriesSource[]
+
+interface ZoomConstraintOptions {
+  minZoomSpan?: number
+  minVisiblePoints?: number
+}
+
+const xValueCache = new WeakMap<object, Map<string, number[]>>()
+
+function normalizedBoundary(boundary: [number, number]): [number, number] {
+  return boundary[0] <= boundary[1] ? [...boundary] : [boundary[1], boundary[0]]
+}
+
+function resolveRequiredPointCount(minimum: number | undefined): number {
+  return Number.isFinite(minimum) && (minimum ?? 0) > 0 ? Math.ceil(minimum as number) : 0
+}
+
+function uniqueXValues(group: ZoomSeriesGroup, boundary: [number, number]): number[] {
+  const boundaryKey = `${boundary[0]}\u0000${boundary[1]}`
+  const cached = xValueCache.get(group)?.get(boundaryKey)
+  if (cached) return cached
+  const values = new Set<number>()
+  for (const series of group) {
+    for (const point of series.points) {
+      if (point.x >= boundary[0] && point.x <= boundary[1]) values.add(point.x)
+    }
+  }
+  const sorted = Array.from(values).sort((left, right) => left - right)
+  const groupCache = xValueCache.get(group) ?? new Map<string, number[]>()
+  groupCache.set(boundaryKey, sorted)
+  xValueCache.set(group, groupCache)
+  return sorted
+}
+
+function minimumPointSpan(
+  groups: readonly ZoomSeriesGroup[],
+  boundary: [number, number],
+  required: number,
+): number {
+  if (required <= 1) return 0
+  let minimumSpan = 0
+  for (const group of groups) {
+    const values = uniqueXValues(group, boundary)
+    if (values.length < required) return boundary[1] - boundary[0]
+    let groupSpan = Number.POSITIVE_INFINITY
+    for (let index = 0; index + required <= values.length; index += 1) {
+      groupSpan = Math.min(groupSpan, values[index + required - 1] - values[index])
+    }
+    const endpointTolerance =
+      Number.EPSILON * Math.max(1, Math.abs(boundary[0]), Math.abs(boundary[1])) * 16
+    minimumSpan = Math.max(minimumSpan, groupSpan + endpointTolerance)
+  }
+  return minimumSpan
+}
+
+export function resolveMinimumZoomSpan(
+  boundary: [number, number],
+  groups: readonly ZoomSeriesGroup[],
+  options: ZoomConstraintOptions,
+): number {
+  const normalized = normalizedBoundary(boundary)
+  const boundarySpan = normalized[1] - normalized[0]
+  if (!Number.isFinite(boundarySpan) || boundarySpan <= 0) return 0
+  const configuredSpan =
+    Number.isFinite(options.minZoomSpan) && (options.minZoomSpan ?? 0) > 0
+      ? Math.min(boundarySpan, options.minZoomSpan as number)
+      : 0
+  const required = resolveRequiredPointCount(options.minVisiblePoints)
+  const pointSpan = minimumPointSpan(groups, normalized, required)
+  if (configuredSpan > 0 || required > 0) {
+    return Math.max(configuredSpan, pointSpan)
+  }
+  return boundarySpan / ZOOM_CONSTRAINTS.DEFAULT_MAX_SCALE
+}
+
+function expandToMinimumPoints(
+  domain: [number, number],
+  boundary: [number, number],
+  groups: readonly ZoomSeriesGroup[],
+  required: number,
+): [number, number] {
+  if (required <= 0) return domain
+  const endpointTolerance =
+    Number.EPSILON * Math.max(1, Math.abs(boundary[0]), Math.abs(boundary[1])) * 16
+  let expanded = domain
+  for (const group of groups) {
+    const values = uniqueXValues(group, boundary)
+    if (values.length < required) return [...boundary]
+    const visibleCount = values.filter(
+      (value) => value >= expanded[0] && value <= expanded[1],
+    ).length
+    if (visibleCount >= required) continue
+    let best: [number, number] | undefined
+    let bestSpan = Number.POSITIVE_INFINITY
+    let bestCenterDistance = Number.POSITIVE_INFINITY
+    const requestedCenter = (expanded[0] + expanded[1]) / 2
+    for (let index = 0; index + required <= values.length; index += 1) {
+      const candidate: [number, number] = [
+        Math.max(boundary[0], Math.min(expanded[0], values[index] - endpointTolerance)),
+        Math.min(
+          boundary[1],
+          Math.max(expanded[1], values[index + required - 1] + endpointTolerance),
+        ),
+      ]
+      const span = candidate[1] - candidate[0]
+      const centerDistance = Math.abs((candidate[0] + candidate[1]) / 2 - requestedCenter)
+      if (span < bestSpan || (span === bestSpan && centerDistance < bestCenterDistance)) {
+        best = candidate
+        bestSpan = span
+        bestCenterDistance = centerDistance
+      }
+    }
+    expanded = best ?? [...boundary]
+  }
+  return expanded
+}
+
+export function constrainZoomDomain(
+  domain: [number, number],
+  boundary: [number, number],
+  groups: readonly ZoomSeriesGroup[],
+  options: ZoomConstraintOptions,
+): [number, number] {
+  const normalized = normalizedBoundary(boundary)
+  const boundarySpan = normalized[1] - normalized[0]
+  if (!Number.isFinite(boundarySpan) || boundarySpan <= 0) return normalized
+  if (Math.abs(domain[1] - domain[0]) >= boundarySpan * (1 - 1e-12)) return normalized
+  const requested: [number, number] = [
+    Math.max(normalized[0], Math.min(domain[0], domain[1])),
+    Math.min(normalized[1], Math.max(domain[0], domain[1])),
+  ]
+  const expanded = expandToMinimumPoints(
+    requested,
+    normalized,
+    groups,
+    resolveRequiredPointCount(options.minVisiblePoints),
+  )
+  const minimumSpan = resolveMinimumZoomSpan(normalized, groups, options)
+  const span = Math.max(minimumSpan, Math.min(boundarySpan, expanded[1] - expanded[0]))
+  const center = (expanded[0] + expanded[1]) / 2
+  const start = Math.max(normalized[0], Math.min(center - span / 2, normalized[1] - span))
+  return [start, start + span]
+}
+
+export function transformForDomain(
+  domain: [number, number],
+  baseDomain: [number, number],
+  width: number,
+): ZoomTransform {
+  const baseSpan = baseDomain[1] - baseDomain[0]
+  const span = domain[1] - domain[0]
+  if (!Number.isFinite(baseSpan) || !Number.isFinite(span) || baseSpan <= 0 || span <= 0) {
+    return zoomIdentity
+  }
+  const scale = baseSpan / span
+  const baseScale = scaleLinear(baseDomain, [0, width])
+  return zoomIdentity.translate(-scale * baseScale(domain[0]), 0).scale(scale)
+}