## ziggurat algorithm

A ziggurat (Akkadian: ziqqurat, D-stem of zaqāru “to build on a raised area”) is a type of massive stone structure built in ancient Mesopotamia. It has the form of a terraced compound of successively receding stories or levels. Wikipedia

In a recent arXival, Jalalvand and Charsooghi revisit the ziggurat algorithm that simulates from a univariate distribution by finding horizontal strips that pile up on top of the target as in a ziggurat or a pyramid, hence the name. Which George Marsaglia introduced in 1963. When finely tuned the method is quite efficient. Maybe because it designs an accept-reject move for each strip of the ziggurat rather than globally. For instance, versions constructed for a Normal target are more efficient [3½ times faster] than the Box-Muller algorithm. The generalisation found in the paper divides the target into strips of equal area, rather than dominating rectangular strips of equal area, which requires some work when the target density is non-standard. For targets with unbounded support or unbounded values, a function g transforming the tail into (0,1) has to be selected. A further constraint is that the inverse cdf of the transformed g(X) has to be known. And a large part of the paper examines several scenarii towards simulating from the tail region. For unbounded densities, a similarly minute analysis is undertaken, again with requests about the target like its algebraic order.

“…the result of division of a random integer by its range is a fixed-point number which unlike a floating-point number does not enjoy increased precision near 0. When such random numbers are used in the tail algorithm they cause premature termination of the tail and large gaps between produced random numbers near the termination point.”

The paper further discusses the correction of an error common to earlier ziggurat algorithms, due to the  conversion from fixed-point to floating-point numbers, as indicated in the above quote. Although this had already been addressed by George Marsaglia in the early 1990’s.

“Ziggurat algorithm has a high setup time, so it’s not suitable for applications that require variates with frequently changing shape parameters.”

When testing the algorithm against different methods (in STL and Boost), and different distributions, the gains are between two and seven times faster, except for the Exponential target where the original ziggurat algorithm performs better. Interestingly, the gains (and the computing time) increase with the degrees of freedom for the Gamma target, in relation with Devroye’s (1986) remark on the absence of uniformly bounded execution times for this distribution. Same thing for the Weibull variates, obviously. Reflecting upon the usually costly computation of cdfs and inverse cdfs on machines and software, the inverse cdf method is systematically left behind! In conclusion, a good Sunday morning read if not of direct consequences for MCMC implementation, as warned by the authors.

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