Add compression-ratio and log-prob quality checks for decode fallback

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/worker/longform/quality.ts

@@ -0,0 +1,42 @@
+/**
+ * Per-chunk decode quality checks for the long-form pipeline's temperature
+ * fallback (D-WS3a) — faster-whisper's `generate_with_fallback` criteria.
+ */
+
+export interface FallbackOptions {
+  /** Decode temperatures in order; the first is the normal (greedy) pass. */
+  temperatures: number[];
+  /** Reject a decode whose text compresses better than this (a loop). */
+  compressionRatioThreshold: number;
+  /** Reject a decode whose average token log-probability is below this. */
+  logProbThreshold: number;
+}
+
+/** faster-whisper / OpenAI Whisper defaults (spec Goal 3). */
+export const DEFAULT_FALLBACK: FallbackOptions = {
+  temperatures: [0, 0.2, 0.4, 0.6, 0.8, 1.0],
+  compressionRatioThreshold: 2.4,
+  logProbThreshold: -1.0,
+};
+
+/**
+ * UTF-8 length over zlib-deflated length — `get_compression_ratio` in
+ * faster-whisper (`zlib.compress`). `CompressionStream("deflate")` is the
+ * same zlib format and exists in workers and Bun alike.
+ */
+export async function compressionRatio(text: string): Promise<number> {
+  const bytes = new TextEncoder().encode(text);
+  if (bytes.length === 0) return 0;
+  const stream = new Blob([bytes]).stream().pipeThrough(new CompressionStream("deflate"));
+  const compressed = await new Response(stream).arrayBuffer();
+  return bytes.length / compressed.byteLength;
+}
+
+/** Whether a decode should be retried at the next temperature. */
+export function needsFallback(
+  result: { compressionRatio: number; avgLogProb: number | null },
+  thresholds: Pick<FallbackOptions, "compressionRatioThreshold" | "logProbThreshold"> = DEFAULT_FALLBACK,
+): boolean {
+  if (result.compressionRatio > thresholds.compressionRatioThreshold) return true;
+  return result.avgLogProb !== null && result.avgLogProb < thresholds.logProbThreshold;
+}