Decoding PNG Huffman Tables
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0:00Recap and set the stage for the day decoding Huffman
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0:00Recap and set the stage for the day decoding Huffman
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0:00Recap and set the stage for the day decoding Huffman
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0:48Bring us up to speed with ParsePNG()
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0:48Bring us up to speed with ParsePNG()
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0:48Bring us up to speed with ParsePNG()
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2:09Return to the PNG1 and DEFLATE2 specifications with a few words on compressor creation
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2:09Return to the PNG1 and DEFLATE2 specifications with a few words on compressor creation
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2:09Return to the PNG1 and DEFLATE2 specifications with a few words on compressor creation
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6:17Set up to handle DEFLATE's use of Huffman coding3
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6:17Set up to handle DEFLATE's use of Huffman coding3
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6:17Set up to handle DEFLATE's use of Huffman coding3
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10:24Slightly organise handmade_png.h and let ComputeHuffman() process the entire HCLENSwizzle table
10:24Slightly organise handmade_png.h and let ComputeHuffman() process the entire HCLENSwizzle table
10:24Slightly organise handmade_png.h and let ComputeHuffman() process the entire HCLENSwizzle table
15:37Stupid Huffman Decoder
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15:37Stupid Huffman Decoder
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15:37Stupid Huffman Decoder
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21:09LUT+Shift Algorithm, Patent Pending 2018®™
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21:09LUT+Shift Algorithm, Patent Pending 2018®™
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21:09LUT+Shift Algorithm, Patent Pending 2018®™
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23:01Consult the DEFLATE spec for the maximum code length4
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23:01Consult the DEFLATE spec for the maximum code length4
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23:01Consult the DEFLATE spec for the maximum code length4
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28:10Consider the feasibility of building a 32k lookup table
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28:10Consider the feasibility of building a 32k lookup table
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28:10Consider the feasibility of building a 32k lookup table
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29:46Introduce AllocateHuffman() and png_huffman_entry struct
29:46Introduce AllocateHuffman() and png_huffman_entry struct
29:46Introduce AllocateHuffman() and png_huffman_entry struct
36:40Determine to implement HuffmanDecode()
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36:40Determine to implement HuffmanDecode()
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36:40Determine to implement HuffmanDecode()
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37:56Encoding symbols: "Numerical" vs "Bit"
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37:56Encoding symbols: "Numerical" vs "Bit"
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37:56Encoding symbols: "Numerical" vs "Bit"
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39:22Relieve png_huffman_entry of containing SymbolLength
39:22Relieve png_huffman_entry of containing SymbolLength
39:22Relieve png_huffman_entry of containing SymbolLength
40:08Introduce PeekBits(), the same as ConsumeBits() just without shifting the bits, and DiscardBits()
40:08Introduce PeekBits(), the same as ConsumeBits() just without shifting the bits, and DiscardBits()
40:08Introduce PeekBits(), the same as ConsumeBits() just without shifting the bits, and DiscardBits()
46:13Implement HuffmanDecode(), augmenting png_huffman with MaxCodeLengthInBits
46:13Implement HuffmanDecode(), augmenting png_huffman with MaxCodeLengthInBits
46:13Implement HuffmanDecode(), augmenting png_huffman with MaxCodeLengthInBits
49:30Dive into implementing ComputeHuffman()
49:30Dive into implementing ComputeHuffman()
49:30Dive into implementing ComputeHuffman()
53:25Determining symbol code locations
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53:25Determining symbol code locations
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53:25Determining symbol code locations
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55:16Enable ComputeHuffman() to correctly place symbol codes in the table
55:16Enable ComputeHuffman() to correctly place symbol codes in the table
55:16Enable ComputeHuffman() to correctly place symbol codes in the table
59:20Start to enable ComputeHuffman() to build those Huffman codes, in conjunction with the DEFLATE specification5
59:20Start to enable ComputeHuffman() to build those Huffman codes, in conjunction with the DEFLATE specification5
59:20Start to enable ComputeHuffman() to build those Huffman codes, in conjunction with the DEFLATE specification5
1:03:12Come to understand DEFLATE's use of Huffman coding6 specifications with a few words on compressor creation
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1:03:12Come to understand DEFLATE's use of Huffman coding6 specifications with a few words on compressor creation
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1:03:12Come to understand DEFLATE's use of Huffman coding6 specifications with a few words on compressor creation
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1:08:27Enable ComputeHuffman() to find the numerical value of the smallest code for each code length, and assign numerical values to all the codes
1:08:27Enable ComputeHuffman() to find the numerical value of the smallest code for each code length, and assign numerical values to all the codes
1:08:27Enable ComputeHuffman() to find the numerical value of the smallest code for each code length, and assign numerical values to all the codes
1:15:13Consider ourselves done with that part of it
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1:15:13Consider ourselves done with that part of it
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1:15:13Consider ourselves done with that part of it
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1:15:46Make ParsePNG() allocate our Huffman tables
1:15:46Make ParsePNG() allocate our Huffman tables
1:15:46Make ParsePNG() allocate our Huffman tables
1:18:54Determine to go over the LitLenDistTable construction routine and implement the filters
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1:18:54Determine to go over the LitLenDistTable construction routine and implement the filters
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1:18:54Determine to go over the LitLenDistTable construction routine and implement the filters
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1:19:58Step through ParsePNG() and inspect our data, in conjunction with the DEFLATE specification7
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1:19:58Step through ParsePNG() and inspect our data, in conjunction with the DEFLATE specification7
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1:19:58Step through ParsePNG() and inspect our data, in conjunction with the DEFLATE specification7
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1:25:15Fix AllocateHuffman() to set the MaxCodeLengthInBits
1:25:15Fix AllocateHuffman() to set the MaxCodeLengthInBits
1:25:15Fix AllocateHuffman() to set the MaxCodeLengthInBits
1:25:40Continue to step through ComputeHuffman() and consider the use of MaxCodeLengthInBits in the ArbitraryBits computation
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1:25:40Continue to step through ComputeHuffman() and consider the use of MaxCodeLengthInBits in the ArbitraryBits computation
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1:25:40Continue to step through ComputeHuffman() and consider the use of MaxCodeLengthInBits in the ArbitraryBits computation
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1:29:35Change AllocateHuffman() to set the MaxCodeLengthInBits according to the length of that length it was passed in, i.e. 2^Length
1:29:35Change AllocateHuffman() to set the MaxCodeLengthInBits according to the length of that length it was passed in, i.e. 2^Length
1:29:35Change AllocateHuffman() to set the MaxCodeLengthInBits according to the length of that length it was passed in, i.e. 2^Length
1:31:30Run it and hit our assertion in AllocateHuffman()
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1:31:30Run it and hit our assertion in AllocateHuffman()
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1:31:30Run it and hit our assertion in AllocateHuffman()
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1:31:44Revert AllocateHuffman() and instead make ParsePNG() pass 8 to it for the DictHuffman allocation
1:31:44Revert AllocateHuffman() and instead make ParsePNG() pass 8 to it for the DictHuffman allocation
1:31:44Revert AllocateHuffman() and instead make ParsePNG() pass 8 to it for the DictHuffman allocation
1:32:16Continue to step through ComputeHuffman() and inspect our data
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1:32:16Continue to step through ComputeHuffman() and inspect our data
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1:32:16Continue to step through ComputeHuffman() and inspect our data
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1:33:38Assert in HuffmanDecode() that BitsUsed != 0
1:33:38Assert in HuffmanDecode() that BitsUsed != 0
1:33:38Assert in HuffmanDecode() that BitsUsed != 0
1:34:20Continue to step through ParsePNG(), hit our assertion in HuffmanDecode() and investigate why
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1:34:20Continue to step through ParsePNG(), hit our assertion in HuffmanDecode() and investigate why
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1:34:20Continue to step through ParsePNG(), hit our assertion in HuffmanDecode() and investigate why
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1:35:26Carefully read 3.1.1 Packing into bytes8 to discover that Huffman codes are packed in the opposite direction to everything else
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1:35:26Carefully read 3.1.1 Packing into bytes8 to discover that Huffman codes are packed in the opposite direction to everything else
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1:35:26Carefully read 3.1.1 Packing into bytes8 to discover that Huffman codes are packed in the opposite direction to everything else
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1:45:30Make ComputeHuffman() flip the bits of Huffman codes
1:45:30Make ComputeHuffman() flip the bits of Huffman codes
1:45:30Make ComputeHuffman() flip the bits of Huffman codes
1:48:12Step through ComputeHuffman() to watch our bits flip
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1:48:12Step through ComputeHuffman() to watch our bits flip
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1:48:12Step through ComputeHuffman() to watch our bits flip
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1:49:07Fix ComputeHuffman() to flip all our bits correctly
1:49:07Fix ComputeHuffman() to flip all our bits correctly
1:49:07Fix ComputeHuffman() to flip all our bits correctly
1:50:13Step through ComputeHuffman() to see our correctly flipping bits
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1:50:13Step through ComputeHuffman() to see our correctly flipping bits
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1:50:13Step through ComputeHuffman() to see our correctly flipping bits
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1:51:17Consider packing the Huffman codes backwards
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1:51:17Consider packing the Huffman codes backwards
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1:51:17Consider packing the Huffman codes backwards
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1:53:12Step through HuffmanDecode() until we assert
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1:53:12Step through HuffmanDecode() until we assert
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1:53:12Step through HuffmanDecode() until we assert
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1:55:57Make ParsePNG() increment LitLenCount in the LitLenDistTable construction routine
1:55:57Make ParsePNG() increment LitLenCount in the LitLenDistTable construction routine
1:55:57Make ParsePNG() increment LitLenCount in the LitLenDistTable construction routine
1:56:50Run until we crash in HuffmanDecode()
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1:56:50Run until we crash in HuffmanDecode()
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1:56:50Run until we crash in HuffmanDecode()
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1:57:09Temporarily prevent ComputeHuffman() from flipping the Huffman bits
1:57:09Temporarily prevent ComputeHuffman() from flipping the Huffman bits
1:57:09Temporarily prevent ComputeHuffman() from flipping the Huffman bits
1:57:26Run it and crash earlier
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1:57:26Run it and crash earlier
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1:57:26Run it and crash earlier
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1:58:05Q&A
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1:58:05Q&A
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1:58:05Q&A
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1:58:17Fix typo in the LitLenDistTable construction in ParsePNG()9
1:58:17Fix typo in the LitLenDistTable construction in ParsePNG()9
1:58:17Fix typo in the LitLenDistTable construction in ParsePNG()9
1:59:30Close that issue10
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1:59:30Close that issue10
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1:59:30Close that issue10
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2:00:15ratchetfreak Q: Don't the bits when reading the table need to be reversed as well?
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2:00:15ratchetfreak Q: Don't the bits when reading the table need to be reversed as well?
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2:00:15ratchetfreak Q: Don't the bits when reading the table need to be reversed as well?
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2:00:53spensasaurusrex Q: How many hours long is this series now? I've dabbled in the first dozen or so episodes, but trying to catch up on so much content is discouraging
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2:00:53spensasaurusrex Q: How many hours long is this series now? I've dabbled in the first dozen or so episodes, but trying to catch up on so much content is discouraging
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2:00:53spensasaurusrex Q: How many hours long is this series now? I've dabbled in the first dozen or so episodes, but trying to catch up on so much content is discouraging
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2:01:12mmozeiko Q: You said multiple times that PNG spec does not allow fixed Huffman codes, but this is not true: PNG spec explicitly says that both fixed and dynamic Huffmans are allowed (section 10.1)
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2:01:12mmozeiko Q: You said multiple times that PNG spec does not allow fixed Huffman codes, but this is not true: PNG spec explicitly says that both fixed and dynamic Huffmans are allowed (section 10.1)
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2:01:12mmozeiko Q: You said multiple times that PNG spec does not allow fixed Huffman codes, but this is not true: PNG spec explicitly says that both fixed and dynamic Huffmans are allowed (section 10.1)
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2:01:28somebody_took_my_name Q: In AllocateHuffman() you use the sizeof the huffman table not the entries to malloc data
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2:01:28somebody_took_my_name Q: In AllocateHuffman() you use the sizeof the huffman table not the entries to malloc data
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2:01:28somebody_took_my_name Q: In AllocateHuffman() you use the sizeof the huffman table not the entries to malloc data
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2:01:38Fix typo in AllocateHuffman()
2:01:38Fix typo in AllocateHuffman()
2:01:38Fix typo in AllocateHuffman()
2:02:11Read 10.1 Compression method 011
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2:02:11Read 10.1 Compression method 011
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2:02:11Read 10.1 Compression method 011
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2:03:04ratchetfreak Q: When reading the 3 bit lengths you'd read, e.g. 0b011, but I believe it should be 0b11012
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2:03:04ratchetfreak Q: When reading the 3 bit lengths you'd read, e.g. 0b011, but I believe it should be 0b11012
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2:03:04ratchetfreak Q: When reading the 3 bit lengths you'd read, e.g. 0b011, but I believe it should be 0b11012
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2:05:51ratchetfreak Q: Yeah, you are correct
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2:05:51ratchetfreak Q: Yeah, you are correct
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2:05:51ratchetfreak Q: Yeah, you are correct
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2:08:29ratchetfreak Q: Only Huffman codes are bit-reversed when looking at the bit from first byte to last, and most significant bit to least significant
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2:08:29ratchetfreak Q: Only Huffman codes are bit-reversed when looking at the bit from first byte to last, and most significant bit to least significant
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2:08:29ratchetfreak Q: Only Huffman codes are bit-reversed when looking at the bit from first byte to last, and most significant bit to least significant
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2:09:37Close down the stream
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2:09:37Close down the stream
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2:09:37Close down the stream
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