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multicool 1.0.2

New features

• allPerm() has a new argument nThreads to generate large results (at least 100,000 permutations) with several threads. It is about 1.4 to 1.9 times faster with 2 to 4 threads, and close to the speed at which the result can be written to memory. Character and factor multisets always use one thread. The default nThreads = 1 works as before.
• nextPerm() has a new argument n to return the next n permutations as the rows of a matrix (fewer at the end, then FALSE). This makes iterating through permutations much faster, while only holding n of them in memory. For example, nextPerm(m, 1000) costs about 0.016 microseconds per permutation, the same as allPerm(). The default n = 1 works as before.
• multinom(), Stirling2()/S2() and Bell()/B() are now computed exactly using the ‘gmp’ package, and have a new argument exact. By default they still return doubles, now correct to double precision rather than silently overflowing. With exact = TRUE they return exact big integers of class bigz. The useDouble argument of multinom() is kept for backwards compatibility but no longer has any effect.

Changes in behaviour

• initMC() now gives an error, rather than a warning, for input with fewer than two elements, and an error for input containing missing values.
• initMC() keeps the type of its input, so logical multisets are permuted as logicals rather than as the strings "TRUE" and "FALSE".

Bug fixes

• initMC() with non-whole numbers (e.g. c(1.5, 1.5, 2.5)) permuted the counts of each value rather than the values themselves.
• Stirling2() and Bell() overflowed for moderate arguments, and much sooner on Windows (e.g. Stirling2(20, 5) and Bell(16) were wrong).
• multinom() overflowed silently for large coefficients.
• genComp() crashed R when n was zero or negative, and returned NULL when len was greater than n.
• Calling allPerm() no longer resets the position of nextPerm().
• Fixed a memory leak in the permutation object.

Improvements

• genComp() is much faster for larger n. The time taken previously grew quadratically with the number of partitions (about 100 seconds for n = 50).
• allPerm() is 10 to 30 times faster and uses much less memory. It writes the elements straight into the result matrix in C++. In the permutation benchmarks from https://stackoverflow.com/questions/22569176, it now takes 1.4 to 1.9 times as long as single-threaded ‘RcppAlgos’ (before, 15 to 54 times as long), and 1.0 to 1.5 times as long as ‘arrangements’.
• initMC() keeps integer input as integer, so allPerm() returns an integer matrix for integer input.
• nextPerm() is about 9 times faster per call (about 0.9 rather than 8.4 microseconds), because the permutation object is now reached through an external pointer rather than an Rcpp Module.
• nextPerm() checks that its argument is an mc object.
• Using an mc object after it has been saved and reloaded gives a clear error asking you to call initMC() again.
• Native routines are now registered, and documentation and NAMESPACE are generated with roxygen2.

Acknowledgements

• Earlier versions of this package used C++ code by Dave Barber (http://tamivox.org/dave/multinomial/code.html) to compute multinomial coefficients. It has been replaced by exact computation with the ‘gmp’ package, but we thank Dave for the code that the package relied on for many years.

These binaries (installable software) and packages are in development.
They may not be fully stable and should be used with caution. We make no claims about them.