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