bluez5: add AAC-ELD vendor codec used by Airpods

Add AAC-ELD vendor codec.

This is based on the documentation by Sa Xiao:

Link: https://github.com/wasdwasd0105/PicoW-usb2bt-audio/blob/main/aac-eld-apple.md

spa/plugins/bluez5/aac-bits.h

@@ -0,0 +1,190 @@
+/* Spa AAC bits */
+/* SPDX-FileCopyrightText: Copyright © 2025 Pauli Virtanen */
+/* SPDX-License-Identifier: MIT */
+
+#ifndef SPA_BLUEZ5_AAC_BITS_H
+#define SPA_BLUEZ5_AAC_BITS_H
+
+#include <spa/utils/defs.h>
+
+struct bits_out {
+	uint8_t *buf;
+	size_t size;
+	size_t pos;
+};
+
+#define BITS_OUT_INIT(buf, size)	((struct bits_out) { (buf), (size), 0 })
+
+static inline void bits_push(struct bits_out *b, uint8_t nbits, uint8_t value)
+{
+	size_t pos = b->pos;
+
+	/* Maximally simple, doesn't need to be fast... */
+
+	spa_assert(nbits <= 8);
+
+	value = ((uint16_t)value) << (8 - nbits);
+	b->pos += nbits;
+
+	while (nbits) {
+		size_t n = pos / 8;
+		size_t bit = pos % 8;
+
+		if (n >= b->size)
+			break;
+
+		if (bit == 0)
+			b->buf[n] = 0;
+		if (value & 0x80)
+			b->buf[n] |= 1 << (7 - bit);
+
+		pos++;
+		nbits--;
+		value = ((uint16_t)value) << 1;
+	}
+}
+
+enum aac_aot_type {
+	AAC_AOT_AAC_LC = 2,
+	AAC_AOT_ER_AAC_ELD = 39,
+};
+
+static inline int aac_frequency_index(int frequency)
+{
+	switch (frequency) {
+	case 96000: return  0x0;
+	case 88200: return  0x1;
+	case 64000: return  0x2;
+	case 48000: return  0x3;
+	case 44100: return  0x4;
+	case 32000: return  0x5;
+	case 24000: return  0x6;
+	case 22050: return  0x7;
+	case 16000: return  0x8;
+	case 12000: return  0x9;
+	case 11025: return  0xa;
+	case 8000: return  0xb;
+	case 7350: return  0xc;
+	default:
+		spa_assert_not_reached();
+		return -1;
+	}
+}
+
+static inline int aac_channel_index(int channels)
+{
+	switch (channels) {
+	case 1: return 0x1;
+	case 2: return 0x2;
+	default:
+		spa_assert_not_reached();
+		return -1;
+	}
+}
+
+/** Write AudioSpecificConfig to given buffer */
+static inline int aac_make_asc(void *buf, size_t buf_size, enum aac_aot_type aot,
+		int frequency, int downscale_frequency, int channels, bool sbr)
+{
+	int freq, down_freq, chan;
+	struct bits_out b = BITS_OUT_INIT(buf, buf_size);
+
+	if ((freq = aac_frequency_index(frequency)) < 0)
+		return -1;
+	if ((down_freq = aac_frequency_index(downscale_frequency)) < 0)
+		return -1;
+	if ((chan = aac_channel_index(channels)) < 0)
+		return -1;
+
+	switch (aot) {
+	case AAC_AOT_AAC_LC:
+	case AAC_AOT_ER_AAC_ELD:
+		break;
+	default:
+		spa_assert_not_reached();
+		return -EINVAL;
+	}
+
+	if (aot <= 31) {
+		bits_push(&b, 5, aot);
+	} else {
+		bits_push(&b, 5, 31);
+		bits_push(&b, 6, aot - 32);
+	}
+
+	bits_push(&b, 4, freq);	/* frequency index */
+	bits_push(&b, 4, chan);	/* channel configuration */
+
+	switch (aot) {
+	case AAC_AOT_AAC_LC:
+		/* GASpecificConfig */
+		bits_push(&b, 1, 0x0);	/* frame length flag (1024 length) */
+		bits_push(&b, 1, 0x0);	/* depends on core coder */
+		bits_push(&b, 1, 0x0);	/* extension flag */
+		break;
+	case AAC_AOT_ER_AAC_ELD:
+		/* ELDSpecificConfig */
+		bits_push(&b, 1, 0x1);	/* frame length flag (480 length) */
+		bits_push(&b, 1, 0x0);	/* SectionDataResilience? */
+		bits_push(&b, 1, 0x0);	/* ScalefactorDataResilience? */
+		bits_push(&b, 1, 0x0);	/* SpectralDataResilience? */
+		bits_push(&b, 1, sbr ? 0x1 : 0x0); /* SBR */
+
+		if (sbr) {
+			bits_push(&b, 1, 0x0); /* ldSbrSamplingRate */
+			bits_push(&b, 1, 0x0); /* ldSbrCrcFlag */
+
+			/* ld_sbr_header */
+			if (channels != 1 && channels != 2)
+				return -EINVAL;
+
+			/* sbr_header:
+			 * These are just the FDK-AAC default values for 48000/48000...
+			 */
+			if (freq != down_freq)
+				return -EINVAL;
+
+			bits_push(&b, 1, 1); /* bs_amp_res */
+			bits_push(&b, 4, 13); /* bs_start_freq */
+			bits_push(&b, 4, 7); /* bs_stop_freq */
+			bits_push(&b, 3, 0); /* bs_xover_band */
+
+			bits_push(&b, 2, 0x0); /* bs_reserved */
+			bits_push(&b, 1, 0x1); /* bs_header_extra_1 */
+			bits_push(&b, 1, 0x0); /* bs_header_extra_2 */
+
+			/* bs_header_extra_1 */
+			bits_push(&b, 2, 0x1); /* bs_freq_scale */
+			bits_push(&b, 1, 0x1); /* bs_alter_scale */
+			bits_push(&b, 2, 0x3); /* bs_noise_bands */
+
+		}
+
+		if (freq != down_freq) {
+			bits_push(&b, 4, 0x3);  /* ELDEXT_DOWNSCALEINFO */
+			bits_push(&b, 4, 0x1);
+			bits_push(&b, 4, down_freq);
+			bits_push(&b, 4, 0x0);
+		}
+
+                bits_push(&b, 4, 0x0);  /* ELDEXT_TERM */
+
+		/* epConfig */
+		bits_push(&b, 2, 0x0);
+		break;
+	}
+
+	/* byte align */
+	if (b.pos % 8)
+		bits_push(&b, 8 - (b.pos % 8), 0x0);
+	spa_assert(b.pos % 8 == 0);
+
+	if (b.pos / 8 >= buf_size) {
+		spa_assert_not_reached();
+		return -EINVAL;
+	}
+
+	return b.pos / 8;
+}
+
+#endif