Delta compression
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In software configuration management (SCM), a "delta" is a type of file in which the difference (or diff) between two successive versions, is recorded. This term suggests that differences should be small (2% different in average). So, using it can vastly reduce the amount of storage required.
A delta can be defined in 2 ways,symmetric delta and directed delta.
A symmetric delta can be expressed as <math>\Delta(v_1, v_2) = (v_1 \backslash v_2) \cup (v_2 \backslash v_1)<math>, where <math>v_1<math> and <math>v_2<math> represent two successive versions.
A directed delta, also called a change, is a sequence of (elementary) change operations which, when applied to one version <math>v_1<math>, yields another version <math>v_2<math>. (note the correspondence to transaction logs in datbases).