add apple compatibility service: codec checks, transcode target mapping

server/services/apple-compat.ts

@@ -0,0 +1,72 @@
+// Codec sets and transcode target mapping for Apple device compatibility.
+// Apple natively decodes: AAC, AC3, EAC3, ALAC, FLAC, MP3, PCM, Opus
+// Everything else (DTS family, TrueHD family) needs transcoding.
+
+const APPLE_COMPATIBLE_AUDIO = new Set([
+	'aac', 'ac3', 'eac3', 'alac', 'flac', 'mp3',
+	'pcm_s16le', 'pcm_s24le', 'pcm_s32le', 'pcm_f32le',
+	'pcm_s16be', 'pcm_s24be', 'pcm_s32be', 'pcm_f64le',
+	'opus',
+]);
+
+// Lossless source codecs — get FLAC in MKV, EAC3 in MP4
+const LOSSLESS_CODECS = new Set([
+	'dts',      // DTS-HD MA reports as 'dts' with profile
+	'truehd',
+]);
+
+// Codec strings Jellyfin may report for DTS variants
+const DTS_CODECS = new Set([
+	'dts', 'dca',
+]);
+
+const TRUEHD_CODECS = new Set([
+	'truehd',
+]);
+
+export function isAppleCompatible(codec: string): boolean {
+	return APPLE_COMPATIBLE_AUDIO.has(codec.toLowerCase());
+}
+
+/** Maps (codec, profile, container) → target codec for transcoding. */
+export function transcodeTarget(
+	codec: string,
+	profile: string | null,
+	container: string | null,
+): string | null {
+	const c = codec.toLowerCase();
+	const isMkv = !container || container.toLowerCase() === 'mkv' || container.toLowerCase() === 'matroska';
+
+	if (isAppleCompatible(c)) return null; // no transcode needed
+
+	// DTS-HD MA and DTS:X are lossless → FLAC in MKV, EAC3 in MP4
+	if (DTS_CODECS.has(c)) {
+		const p = (profile ?? '').toLowerCase();
+		const isLossless = p.includes('ma') || p.includes('hd ma') || p.includes('x');
+		if (isLossless) return isMkv ? 'flac' : 'eac3';
+		// Lossy DTS variants → EAC3
+		return 'eac3';
+	}
+
+	// TrueHD (including Atmos) → FLAC in MKV, EAC3 in MP4
+	if (TRUEHD_CODECS.has(c)) {
+		return isMkv ? 'flac' : 'eac3';
+	}
+
+	// Any other incompatible codec → EAC3 as safe fallback
+	return 'eac3';
+}
+
+/** Determine overall Apple compatibility for a set of kept audio streams. */
+export function computeAppleCompat(
+	keptAudioCodecs: string[],
+	container: string | null,
+): 'direct_play' | 'remux' | 'audio_transcode' {
+	const hasIncompatible = keptAudioCodecs.some(c => !isAppleCompatible(c));
+	if (hasIncompatible) return 'audio_transcode';
+
+	const isMkv = !container || container.toLowerCase() === 'mkv' || container.toLowerCase() === 'matroska';
+	if (isMkv) return 'remux';
+
+	return 'direct_play';
+}