random
https://en.wikipedia.org/wiki/Linear_congruential_generator
A linear congruential generator (LCG) is an algorithm that yields a sequence of pseudo-randomized numbers calculated with a discontinuous piecewise linear equation. The method represents one of the oldest and best-known pseudorandom number generatoralgorithms.[1] The theory behind them is relatively easy to understand, and they are easily implemented and fast, especially on computer hardware which can provide modulo arithmetic by storage-bit truncation.
the "seed" or "start value"
The period of a general mixed congruential generator is at most m, and for some choices of factor a much less than that. The mixed congruential generator will have a full period for all seed values if and only if:
These three requirements are referred to as the Hull-Dobell Theorem.[3][4]
原文：http://www.cnblogs.com/yuanjiangw/p/5797894.html