68 lines
2.1 KiB
Markdown
68 lines
2.1 KiB
Markdown
# LZ4 API Example : Dictionary Random Access
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`dictionaryRandomAccess.c` is LZ4 API example which implements dictionary compression and random access decompression.
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Please note that the output file is not compatible with lz4frame and is platform dependent.
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## What's the point of this example ?
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- Dictionary based compression for homogenous files.
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- Random access to compressed blocks.
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## How the compression works
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Reads the dictionary from a file, and uses it as the history for each block.
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This allows each block to be independent, but maintains compression ratio.
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```
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Dictionary
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+
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v
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+---------+
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| Block#1 |
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+----+----+
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v
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{Out#1}
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Dictionary
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+
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v
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+---------+
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| Block#2 |
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+----+----+
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v
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{Out#2}
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```
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After writing the magic bytes `TEST` and then the compressed blocks, write out the jump table.
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The last 4 bytes is an integer containing the number of blocks in the stream.
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If there are `N` blocks, then just before the last 4 bytes is `N + 1` 4 byte integers containing the offsets at the beginning and end of each block.
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Let `Offset#K` be the total number of bytes written after writing out `Block#K` *including* the magic bytes for simplicity.
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```
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+------+---------+ +---------+---+----------+ +----------+-----+
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| TEST | Block#1 | ... | Block#N | 4 | Offset#1 | ... | Offset#N | N+1 |
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+------+---------+ +---------+---+----------+ +----------+-----+
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```
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## How the decompression works
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Decompression will do reverse order.
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- Seek to the last 4 bytes of the file and read the number of offsets.
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- Read each offset into an array.
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- Seek to the first block containing data we want to read.
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We know where to look because we know each block contains a fixed amount of uncompressed data, except possibly the last.
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- Decompress it and write what data we need from it to the file.
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- Read the next block.
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- Decompress it and write that page to the file.
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Continue these procedure until all the required data has been read.
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