优化，添加滚动记忆

web/src/utils/playbackRate.test.ts

@@ -0,0 +1,22 @@
+import {
+  formatPlaybackRate,
+  normalizePlaybackRate,
+  stepPlaybackRate,
+} from './playbackRate.ts'
+
+function check(name: string, condition: boolean) {
+  if (!condition) throw new Error(`playbackRate: ${name}`)
+}
+
+check('invalid value falls back to 1x', normalizePlaybackRate(undefined) === 1)
+check('value snaps to nearest selectable rate', normalizePlaybackRate(1.2) === 1.25)
+check('value below range clamps to the minimum', normalizePlaybackRate(0.1) === 0.5)
+check('value above range clamps to the maximum', normalizePlaybackRate(9) === 3)
+check('up steps to the next rate', stepPlaybackRate(1, 1) === 1.25)
+check('down steps to the previous rate', stepPlaybackRate(1, -1) === 0.75)
+check('up stops at the maximum', stepPlaybackRate(3, 1) === 3)
+check('down stops at the minimum', stepPlaybackRate(0.5, -1) === 0.5)
+check('integer formatting', formatPlaybackRate(1) === '1x')
+check('decimal formatting', formatPlaybackRate(1.25) === '1.25x')
+
+console.log('playbackRate.test.ts ok')

web/src/utils/playbackRate.ts

@@ -0,0 +1,31 @@
+export const PLAYBACK_RATE_OPTIONS = [0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3] as const
+
+export const MIN_PLAYBACK_RATE = PLAYBACK_RATE_OPTIONS[0]
+export const MAX_PLAYBACK_RATE = PLAYBACK_RATE_OPTIONS[PLAYBACK_RATE_OPTIONS.length - 1]
+
+export function normalizePlaybackRate(value: unknown): number {
+  const parsed = Number(value)
+  if (!Number.isFinite(parsed)) return 1
+  let closest: number = PLAYBACK_RATE_OPTIONS[0]
+  let closestDistance = Number.POSITIVE_INFINITY
+  for (const option of PLAYBACK_RATE_OPTIONS) {
+    const distance = Math.abs(option - parsed)
+    if (distance < closestDistance) {
+      closest = option
+      closestDistance = distance
+    }
+  }
+  return closest
+}
+
+export function stepPlaybackRate(current: unknown, direction: 1 | -1): number {
+  const normalized = normalizePlaybackRate(current)
+  const index = PLAYBACK_RATE_OPTIONS.indexOf(normalized as (typeof PLAYBACK_RATE_OPTIONS)[number])
+  const next = Math.min(PLAYBACK_RATE_OPTIONS.length - 1, Math.max(0, index + direction))
+  return PLAYBACK_RATE_OPTIONS[next]
+}
+
+export function formatPlaybackRate(rate: unknown): string {
+  const normalized = normalizePlaybackRate(rate)
+  return `${Number.isInteger(normalized) ? normalized.toFixed(0) : String(normalized)}x`
+}