diff options
Diffstat (limited to 'lib/util/rand.c')
| -rw-r--r-- | lib/util/rand.c | 43 |
1 files changed, 38 insertions, 5 deletions
diff --git a/lib/util/rand.c b/lib/util/rand.c index 2f60de83..6a0a2cdb 100644 --- a/lib/util/rand.c +++ b/lib/util/rand.c @@ -72,8 +72,8 @@ https://wnww.gnu.org/software/gsl/doc/html/rng.html Twister method and does not rely on any randomness facility of the operating system, but it is easy to compile an alternative version that uses others. - The Mersenne Twister method was new to Netpbm in Netpbm 10.94 - (March 2021). Before that, Netpbm used standard OS-provided facilities. + The Mersenne Twister method was new to Netpbm in Netpbm 10.94 (March 2021). + Before that, Netpbm used standard OS-provided facilities. Programs that use random numbers have existed in Netpbm since PBMPlus days. The system rand() function was used in instances randomness was required; @@ -87,15 +87,15 @@ https://wnww.gnu.org/software/gsl/doc/html/rng.html This was not considered a problem in the early days. Deterministic operation was not a feature users requested and it was impossible regardless - of the random number generation method on most programs because they did - not allow a user to specify a seed for the generator. + of the random number generation method on most programs because they did not + allow a user to specify a seed for the generator. This state of affairs changed as Netpbm got firmly established as a base-level system package. Security became critical for many users. A crucial component of quality control is automated regression tests (="make check"). Unpredictable behavior gets in the way of testing. One by one programs were given the -randomseed (or -seed) option to ensure reproducible - results. Often this was done as new tests cases were written. However, + results. Often this was done as new test cases were written. However, inconsistent output caused by system-level differences in rand() implementation remained a major obstacle. @@ -219,6 +219,39 @@ pm_gaussrand(struct pm_randSt * const randStP) { +uint32_t +pm_rand32(struct pm_randSt * const randStP) { +/*----------------------------------------------------------------------------- + Generate a 32-bit random number. + + This is a provision for users who select a non-default random number + generator which returns less than 32 bits per call. Many system generators + are known to return 31 bits (max = 2147483647 or 0x7FFFFFFF). + + This does not work with generators that return less than 11 bits per call. + The least we know of is the archaic RANDU, which generates 15 bits (max = + 32767 or 0x7FFF). +-----------------------------------------------------------------------------*/ + unsigned int const randMax = randStP->max; + + uint32_t retval; + + if (randMax >= 0xFFFFFFFF) + retval = pm_rand(randStP); + else { + uint32_t scale; + + retval = 0; /* Initial value */ + + for (scale = 0xFFFFFFFF; scale > 0; scale /= (randMax +1)) + retval *= (randMax + 1) + pm_rand(randStP); + } + + return retval;; +} + + + void pm_randinit(struct pm_randSt * const randStP) { /*---------------------------------------------------------------------------- |