Replace broken per-segment Opus decode with WebCodecs AudioDecoder streaming pipeline

DeepDrftPublic/Interop/audio/OpusSidecar.ts

@@ -123,6 +123,39 @@ export function presentationTimeSeconds(granulePosition: number, preSkip: number
     return Math.max(0, (granulePosition - preSkip) / OPUS_SAMPLE_RATE);
 }
 
+/**
+ * Resolve a seek time (seconds) to a file-absolute, page-start byte offset via the precomputed index —
+ * the accurate VBR-safe transfer function (§3.4a A/C). Binary-searches for the largest entry whose
+ * presentation time is <= `positionSeconds` and returns its exact page-start byte offset. NOT
+ * interpolation, NOT byteRate math. With an empty index it degrades to the start of audio (the offset
+ * of the first audio page == the setup-header length, since the server emits [setup pages][audio pages]).
+ *
+ * This is the single source of truth for Opus seek-offset math, shared by the seek-beyond-buffer path
+ * (AudioPlayer) and any byte-offset resolver. The Range fetch from this offset lands the decoder
+ * Ogg-sync-aligned because every indexed offset is a real page start.
+ */
+export function resolveOpusByteOffset(sidecar: OpusSeekData, positionSeconds: number): number {
+    const points = sidecar.points;
+    if (points.length === 0) {
+        return sidecar.setupHeaderBytes.length;
+    }
+
+    let lo = 0;
+    let hi = points.length - 1;
+    let best = 0;
+    while (lo <= hi) {
+        const mid = (lo + hi) >> 1;
+        const t = presentationTimeSeconds(points[mid].granulePosition, sidecar.preSkip);
+        if (t <= positionSeconds) {
+            best = mid;
+            lo = mid + 1;
+        } else {
+            hi = mid - 1;
+        }
+    }
+    return points[best].byteOffset;
+}
+
 function toUint8Array(input: Uint8Array | ArrayBuffer | ArrayBufferView): Uint8Array {
     if (input instanceof Uint8Array) return input;
     if (input instanceof ArrayBuffer) return new Uint8Array(input);