pipewire/spa/plugins/bluez5/aac-bits.h

/* 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