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Diffstat (limited to 'media/libstagefright/codecs/aacdec/find_adts_syncword.cpp')
| -rw-r--r-- | media/libstagefright/codecs/aacdec/find_adts_syncword.cpp | 305 |
1 files changed, 305 insertions, 0 deletions
diff --git a/media/libstagefright/codecs/aacdec/find_adts_syncword.cpp b/media/libstagefright/codecs/aacdec/find_adts_syncword.cpp new file mode 100644 index 0000000..535f177 --- /dev/null +++ b/media/libstagefright/codecs/aacdec/find_adts_syncword.cpp @@ -0,0 +1,305 @@ +/* ------------------------------------------------------------------ + * Copyright (C) 1998-2009 PacketVideo + * + * Licensed under the Apache License, Version 2.0 (the "License"); + * you may not use this file except in compliance with the License. + * You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either + * express or implied. + * See the License for the specific language governing permissions + * and limitations under the License. + * ------------------------------------------------------------------- + */ +/* + + Pathname: ./src/find_adts_syncword.c + +------------------------------------------------------------------------------ + REVISION HISTORY + + Description: Fixed error in logic that determines whether there are enough + bits available to conduct a search for the syncword. The plus sign in + the following condition should be a minus. + + if (pInputStream->usedBits < + (pInputStream->availableBits + syncword_length) + + The length of the syncword should subtract from the number of available + bits, not add. + + Description: Fixed condition when the end of file was found, unsigned + comparison produced a undesired search. Fixed by casting comparison + if ((Int)pInputStream->usedBits < + ((Int)pInputStream->availableBits - syncword_length) ) + + Description: + +------------------------------------------------------------------------------ + INPUT AND OUTPUT DEFINITIONS + + Inputs: + pSyncword = Pointer to variable containing the syncword that the + function should be scanning for in the buffer. [ UInt32 * ] + + pInputStream = Pointer to a BITS structure, used by the function getbits + to retrieve data from the bitstream. [ BITS * ] + + syncword_length = The length of the syncword. [ Int ] + + syncword_mask = A mask to be applied to the bitstream before comparison + with the value pointed to by pSyncword. [ UInt32 ] + + Local Stores/Buffers/Pointers Needed: + None + + Global Stores/Buffers/Pointers Needed: + None + + Outputs: + None + + Pointers and Buffers Modified: + None + + Local Stores Modified: + None + + Global Stores Modified: + None + +------------------------------------------------------------------------------ + FUNCTION DESCRIPTION + + This module scans the bitstream for a syncword of any length between 1 and 32. + If certain bits in the syncword are to be ignored, that bit position should + be set to 0 in both parameters *(pSyncword) and syncword_mask. This allows + for a syncword to be constructed out of non-contiguous bits. + + Upon finding the syncword's position in the bitstream, a value denoting the + syncword's degree of deviance from being byte-aligned (byte_align_offset) + is set in the structure pointed to by pInputStream. + This is a value between 0 and 7. + + If no syncword is found, the function returns status == ERROR. + +------------------------------------------------------------------------------ + REQUIREMENTS + + "Don't care" bits must be set to '0' in both *(pSyncword) and syncword_mask. + + This function should not be called if there are less than + (8 + syncword_length) bits in the buffer. + +------------------------------------------------------------------------------ + REFERENCES + (1) ISO/IEC 13818-7:1997(E) + Part 7 + Subpart 6.2 (Audio_Data_Transport_Stream frame, ADTS) + + (2) ISO/IEC 11172-3:1993(E) + Part 3 + Subpart 2.4.3 The audio decoding process + +------------------------------------------------------------------------------ + PSEUDO-CODE + + IF (pInputStream->usedBits < + (pInputStream->availableBits + syncword_length) ) + + max_search_length = (pInputStream->availableBits - pInputStream->usedBits); + + max_search_length = max_search_length - syncword_length; + + search_length = 0; + + adts_header = + CALL getbits(syncword_length, pInputStream); + MODIFYING pInputStream->usedBits + RETURNING bits from bitstream of length (syncword_length) + + test_for_syncword = adts_header AND syncword_mask; + test_for_syncword = test_for_syncword XOR syncword; + + WHILE ( (test_for_syncword != 0) && (search_length > 0) ) + + search_length = search_length - 1; + + adts_header = adts_header << 1; + adts_header = adts_header OR ... + + CALL getbits(syncword_length, pInputStream); + MODIFYING pInputStream->usedBits + RETURNING 1 bit from the bitstream + + test_for_syncword = adts_header AND syncword_mask; + test_for_syncword = test_for_syncword XOR syncword; + + ENDWHILE + + IF (search_length == 0) + status = ERROR; + ENDIF + + *(pSyncword) = adts_header; + + pInputStream->byteAlignOffset = + (pInputStream->usedBits - syncwordlength) AND 0x7; + + ELSE + status = ERROR; + ENDIF + + return (status); + + +------------------------------------------------------------------------------ + RESOURCES USED + When the code is written for a specific target processor the + the resources used should be documented below. + + STACK USAGE: [stack count for this module] + [variable to represent + stack usage for each subroutine called] + + where: [stack usage variable] = stack usage for [subroutine + name] (see [filename].ext) + + DATA MEMORY USED: x words + + PROGRAM MEMORY USED: x words + + CLOCK CYCLES: [cycle count equation for this module] + [variable + used to represent cycle count for each subroutine + called] + + where: [cycle count variable] = cycle count for [subroutine + name] (see [filename].ext) + +------------------------------------------------------------------------------ +*/ + + +/*---------------------------------------------------------------------------- +; INCLUDES +----------------------------------------------------------------------------*/ +#include "pv_audio_type_defs.h" +#include "s_bits.h" +#include "ibstream.h" +#include "find_adts_syncword.h" + +/*---------------------------------------------------------------------------- +; MACROS +; Define module specific macros here +----------------------------------------------------------------------------*/ + +/*---------------------------------------------------------------------------- +; DEFINES +; Include all pre-processor statements here. Include conditional +; compile variables also. +----------------------------------------------------------------------------*/ +#define FIND_ADTS_ERROR -1 + +/*---------------------------------------------------------------------------- +; LOCAL FUNCTION DEFINITIONS +; Function Prototype declaration +----------------------------------------------------------------------------*/ + +/*---------------------------------------------------------------------------- +; LOCAL STORE/BUFFER/POINTER DEFINITIONS +; Variable declaration - defined here and used outside this module +----------------------------------------------------------------------------*/ + +/*---------------------------------------------------------------------------- +; EXTERNAL FUNCTION REFERENCES +; Declare functions defined elsewhere and referenced in this module +----------------------------------------------------------------------------*/ + +/*---------------------------------------------------------------------------- +; EXTERNAL GLOBAL STORE/BUFFER/POINTER REFERENCES +; Declare variables used in this module but defined elsewhere +----------------------------------------------------------------------------*/ + +/*---------------------------------------------------------------------------- +; FUNCTION CODE +----------------------------------------------------------------------------*/ +Int find_adts_syncword( + UInt32 *pSyncword, + BITS *pInputStream, + Int syncword_length, + UInt32 syncword_mask) +{ + + Int status = SUCCESS; + UInt search_length; + UInt32 adts_header = 0; + UInt32 test_for_syncword; + UInt32 syncword = *(pSyncword); + + /* + * Determine the maximum number of bits available to this function for + * the syncword search. + */ + if ((Int)pInputStream->usedBits < + ((Int)pInputStream->availableBits - syncword_length)) + { + search_length = (pInputStream->availableBits - pInputStream->usedBits); + + search_length -= syncword_length; + + adts_header = getbits(syncword_length, pInputStream); + + /* + * Mask the result in adts_header with the syncword_mask, so only the + * bits relevant to syncword detection are compared to *(pSyncword). + */ + test_for_syncword = adts_header & syncword_mask; + test_for_syncword ^= syncword; + + /* + * Scan bit-by-bit through the bitstream, until the function either + * runs out of bits, or finds the syncword. + */ + + while ((test_for_syncword != 0) && (search_length > 0)) + { + search_length--; + + adts_header <<= 1; + adts_header |= getbits(1, pInputStream); + + test_for_syncword = adts_header & syncword_mask; + test_for_syncword ^= syncword; + } + + if (search_length == 0) + { + status = FIND_ADTS_ERROR; + } + + /* + * Return the syncword's position in the bitstream. Correct placement + * of the syncword will result in byte_align_offset == 0. + * If the syncword is found not to be byte-aligned, then return + * the degree of disalignment, so further decoding can + * be shifted as necessary. + * + */ + pInputStream->byteAlignOffset = + (pInputStream->usedBits - syncword_length) & 0x7; + + } /* END if (pInputStream->usedBits < ...) */ + + else + { + status = FIND_ADTS_ERROR; + } + + *(pSyncword) = adts_header; + + return (status); + +} /* find_adts_syncword() */ |