// Classifies the result of an ffmpeg transcode-to-null probe of a partial
// (or finalized) media file. Used by the audio-checked download flow to
// detect mid-stream corruption that would render the final transcode useless.
//
// The probe runs a real transcode (e.g. libmp3lame) to /dev/null. ffmpeg's
// exit code is one signal, but it is unreliable on its own: a stream can
// fail to decode on hundreds of packets and ffmpeg still exits 0 because
// the encoder kept producing output from the packets that did decode. So
// we also count per-packet decoder failures in stderr — above a small
// threshold, the stream is malformed regardless of exit code or whether
// the container is also truncated.
//
// Pure-function module: no side effects, no IO. Keeps test specs from having
// to pull execa/etc. into the test process.

export type StreamVerdict = "clean" | "partial" | "malformed";

// Stderr line that ffmpeg emits when the input container is cut off
// mid-frame but the encoder still got enough decoded audio to produce a
// valid output. Presence of this line means the file is partial-but-clean.
export const PARTIAL_STDERR_PATTERN = /partial file/i;

// Per-packet decoder failure ffmpeg prints (at -v error) when the av_codec
// layer rejects a packet as undecodable. A genuinely truncated container
// produces at most one or two of these at the tail; mid-stream corruption
// produces hundreds.
export const DECODER_ERROR_PATTERN =
  /Error submitting packet to decoder: Invalid data found when processing input/g;

// Above this many decoder failures we classify as malformed. Tuned from
// observed logs: clean/partial probes show 0–2 trailing errors; corrupt
// streams show 100+.
export const DECODER_ERROR_THRESHOLD = 5;

export function classifyFfmpegProbe(
  exitCode: number | null,
  stderr: string,
): StreamVerdict {
  if (exitCode !== 0) return "malformed";
  const decoderErrors = stderr.match(DECODER_ERROR_PATTERN)?.length ?? 0;
  if (decoderErrors > DECODER_ERROR_THRESHOLD) return "malformed";
  return PARTIAL_STDERR_PATTERN.test(stderr) ? "partial" : "clean";
}