aacdec: add a decoder for AAC USAC (xHE-AAC)

This commit adds a decoder for the frequency-domain part of USAC.

What works:
 - Mono
 - Stereo (no prediction)
 - Stereo (mid/side coding)
 - Stereo (complex prediction)

What's left:
 - SBR
 - Speech coding

Known issues:
 - Desync with certain sequences
 - Preroll crossover missing (shouldn't matter, bitrate adaptation only)

1 files changed, 37 insertions, 0 deletions

diff --git a/libavcodec/aac/aacdec_usac.h b/libavcodec/aac/aacdec_usac.h
new file mode 100644
index 0000000000..4116a2073a
--- /dev/null
+++ b/libavcodec/aac/aacdec_usac.h
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2024 Lynne <dev@lynne.ee>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVCODEC_AAC_AACDEC_USAC_H
+#define AVCODEC_AAC_AACDEC_USAC_H
+
+#include "aacdec.h"
+
+#include "libavcodec/get_bits.h"
+
+int ff_aac_usac_config_decode(AACDecContext *ac, AVCodecContext *avctx,
+                              GetBitContext *gb, OutputConfiguration *oc,
+                              int channel_config);
+
+int ff_aac_usac_reset_state(AACDecContext *ac, OutputConfiguration *oc);
+
+int ff_aac_usac_decode_frame(AVCodecContext *avctx, AACDecContext *ac,
+                             GetBitContext *gb, int *got_frame_ptr);
+
+#endif /* AVCODEC_AAC_AACDEC_USAC_H */