(algorithm)

**Definition:**
An *algorithm* to produce a sequence of insert and copy commands (an *edit script*) which creates a new version file from a reference file. To begin, *hash* every block of the reference file and store every hash in a hash value *array*. Build a *suffix array* and three other data structures for quick access. Beginning at the first location in the version file, hash a block and look for the longest match in the reference file. Upon a match, encode an insert back to the previous match and a copy of the match. If no match, look at the next location. At the end, encode an insert for remaining unmatched characters.

**Aggregate parent** (I am a part of or used in ...)

*longest common subsequence*.

**Aggregate child** (... is a part of or used in me.)

*Bloom filter*, *hash table*, *binary search*.

*Note:
There are many differential compression algorithms, such as vcdiff, xdelta, and zdelta. See Agarwal et. al. I happened to find this one, and it uses neat data structures. *

The three other data structures for quick access to the suffix array are: the quick index array, a *Bloom filter* using one *hash function*, the block hash table, and the pointer array, an array of pointers into the block hash table. The quick index array allows most (97%) version file block hashes to be rejected as having no match. The block hash table has pointers into the suffix array. The pointer array can be viewed as a table of the *hash function* into the block hash table, which uses *open addressing* in case of *collisions*.

* When a match is found, the suffix array is searched, via binary search in the block hash table, to find the longest matching sequence of blocks. Then the match is extended character by character as far as possible.*

Author: PEB

**Ramesh C. Agarwal**, **Karan Gupta**, **Shaili Jain**, and **Suchitra Amalapurapu**, *An approximation to the greedy algorithm for differential compression*, IBM Journal of Research & Development, 50(1):149-166, January 2006.

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Entry modified 14 April 2009.

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Paul E. Black, "hsadelta", in
*Dictionary of Algorithms and Data Structures* [online], Paul E. Black, ed. 14 April 2009. (accessed TODAY)
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