[libmath-prime-util-perl] 02/18: Don't use last bit of drand48

[Partha P. Mukherjee]

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ppm-guest pushed a commit to annotated tag v0.17
in repository libmath-prime-util-perl.

commit 113f73591ab0b2fd243907a9cea02ff39da50dc5
Author: Dana Jacobsen <dana at acm.org>
Date:   Wed Dec 12 08:47:56 2012 -0800

    Don't use last bit of drand48
---
 TODO                   | 3 +++
 lib/Math/Prime/Util.pm | 5 ++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/TODO b/TODO
index c97cc81..76f5081 100644
--- a/TODO
+++ b/TODO
@@ -40,3 +40,6 @@
 
 - Dynamically use a mulmodadd in PP aks, just like the new C code does.
   This will mean it'll work for full-size native ints.
+
+- Add configuration options for rand and randbits (maybe irand and irandrange).
+  This will help when being used as part of a library.
diff --git a/lib/Math/Prime/Util.pm b/lib/Math/Prime/Util.pm
index 66c3fdb..62cc59a 100644
--- a/lib/Math/Prime/Util.pm
+++ b/lib/Math/Prime/Util.pm
@@ -98,10 +98,13 @@ BEGIN {
   }
 
   # Try to figure out a system rand configuration that works for us.
+  # Using something other than the craptastic system rand would be best.
   use Config;
   $_Config{'system_randbits'} = $Config{'randbits'};
   # Keep things in integer range.
   $_Config{'system_randbits'} = $_Config{'maxbits'}-1 if $_Config{'system_randbits'} >= $_Config{'maxbits'};
+  # drand48 has an alternating last bit on almost every system.
+  $_Config{'system_randbits'}-- if $_Config{'system_randbits'} == 48;
   no Config;
 
 }
@@ -830,7 +833,7 @@ sub primes {
     # I've seen +0, +1, and +2 here.  Maurer uses +0.  Menezes uses +1.
     my $q = random_maurer_prime( ($r * $k)->bfloor + 1 );
     $q = Math::BigInt->new("$q") unless ref($q) eq 'Math::BigInt';
-    my $I = Math::BigInt->new(2)->bpow($k-1)->bdiv(2 * $q)->bfloor;
+    my $I = Math::BigInt->new(2)->bpow($k-2)->bdiv($q)->bfloor;
     print "B = $B  r = $r  k = $k  q = $q  I = $I\n" if $verbose;
 
     # Big GCD's are hugely fast with GMP or Pari, but super slow with Calc.

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