I bumped into this today and I've attached a patch with test-case. Cheers,
Rupert
From ca9fbed1a43ca5f6f4b07bab90c7aadf560ffea5 Mon Sep 17 00:00:00 2001
From: Rupert Swarbrick <rswarbr...@gmail.com>
Date: Thu, 21 Oct 2010 11:49:13 +0100
Subject: [PATCH] math.primes.erato: Fix off-by-one error
The sieve bit vector deals with numbers in chunks of 30. Therefore,
the number 90 (say) is the 91st 'element' of the vector. Each byte
deals with some range {0,1,...,29}+30n so to have the number 90, you
need four bytes.
Rather pleasingly, I bumped into this bug and it reduced to the
incantation:
2010 2010 sieve marked-prime?
---
basis/math/primes/erato/erato-tests.factor | 8 +++++++-
basis/math/primes/erato/erato.factor | 2 +-
2 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/basis/math/primes/erato/erato-tests.factor b/basis/math/primes/erato/erato-tests.factor
index e6f7765..ff44ec2 100644
--- a/basis/math/primes/erato/erato-tests.factor
+++ b/basis/math/primes/erato/erato-tests.factor
@@ -1,4 +1,5 @@
-USING: byte-arrays math math.bitwise math.primes.erato sequences tools.test ;
+USING: kernel byte-arrays sequences tools.test ;
+USING: math math.bitwise math.ranges math.primes.erato ;
[ B{ 255 251 247 126 } ] [ 100 sieve ] unit-test
[ 1 100 sieve marked-prime? ] [ bounds-error? ] must-fail-with
@@ -8,3 +9,8 @@ USING: byte-arrays math math.bitwise math.primes.erato sequences tools.test ;
! There are 25997 primes below 300000. 1 must be removed and 3 5 7 added.
[ 25997 ] [ 299999 sieve [ bit-count ] map-sum 2 + ] unit-test
+
+! Check sieve array length logic by making sure we get the right
+! end-point for numbers with all possibilities mod 30. If something
+! were to go wrong, we'd get a bounds-error.
+[ ] [ 2 100 [a,b] [ dup sieve marked-prime? drop ] each ] unit-test
diff --git a/basis/math/primes/erato/erato.factor b/basis/math/primes/erato/erato.factor
index fdc2f9f..4df724c 100644
--- a/basis/math/primes/erato/erato.factor
+++ b/basis/math/primes/erato/erato.factor
@@ -28,7 +28,7 @@ CONSTANT: masks B{ 0 128 0 0 0 0 0 64 0 0 0 32 0 16 0 0 0 8 0 4 0 0 0 2 0 0 0 0
2drop
] if ;
-: init-sieve ( n -- arr ) 29 + 30 /i 255 <array> >byte-array ;
+: init-sieve ( n -- arr ) 30 /i 1 + 255 <array> >byte-array ;
PRIVATE>
--
1.7.2.3
pgppWFEfwtHSv.pgp
Description: PGP signature
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