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Fix bug in BlistList for two ranges that could lead to wrong results #3689

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Oct 6, 2019
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Fix bug in BlistList for two ranges that could lead to wrong results #3689

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29 changes: 16 additions & 13 deletions src/blister.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1286,9 +1286,9 @@ static Obj FuncUNITE_BLIST_LIST(Obj self, Obj list, Obj blist, Obj sub)
UInt bit; /* one bit of block */
Int lenList; /* logical length of the list */
const Obj * ptrSub; /* pointer to the sublist */
UInt lenSub; /* logical length of sublist */
UInt i, j, k = 0, l; /* loop variables */
long s, t; /* elements of a range */
Int lenSub; /* logical length of sublist */
Int i, j, k, l; /* loop variables */
Int s, t; /* elements of a range */
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s, t were just of type long, so in 64 bit mode, with compilers where long is only 32 bit (basically anywhere except some Microsoft / Windows compilers), there could be overflow here. I didn't add tests for that, though.


RequireSmallList("UniteBlistList", list);
RequireBlist("UniteBlistList", blist);
Expand All @@ -1313,19 +1313,22 @@ static Obj FuncUNITE_BLIST_LIST(Obj self, Obj list, Obj blist, Obj sub)
/* get the bounds of the subset with respect to the boolean list */
s = INT_INTOBJ( GET_ELM_RANGE( list, 1 ) );
t = INT_INTOBJ( GET_ELM_RANGE( sub, 1 ) );
if ( s <= t ) i = t - s + 1;
else i = 1;

if ( i + lenSub - 1 <= lenList ) j = i + lenSub - 1;
else j = lenList;
// compute bounds
i = t - s;
j = lenSub + i;
if (i < 0)
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Note that it is crucial to compute i and j before capping the bounds; clearly the previous two lines do not commute; yet in the old code, they were in the wrong order.

i = 0;
if (j > lenList)
j = lenList;

/* set the corresponding entries to 'true' */
for ( k = i; k <= j && (k-1)%BIPEB != 0; k++ )
ptrBlist[(k-1)/BIPEB] |= (1UL << (k-1)%BIPEB);
for ( ; k+BIPEB <= j; k += BIPEB )
ptrBlist[(k-1)/BIPEB] = ~(UInt)0;
for ( ; k <= j; k++ )
ptrBlist[(k-1)/BIPEB] |= (1UL << (k-1)%BIPEB);
for ( k = i; k < j && k%BIPEB != 0; k++ )
ptrBlist[k/BIPEB] |= (1UL << k%BIPEB);
for ( ; k+BIPEB < j; k += BIPEB )
ptrBlist[k/BIPEB] = ~(UInt)0;
for ( ; k < j; k++ )
ptrBlist[k/BIPEB] |= (1UL << k%BIPEB);
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Note that k-1 changed to k by adjusting the bounds. I hope the optimizer would have produced identical results in both cases; but the result is certainly slightly easier to read.


}

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20 changes: 20 additions & 0 deletions tst/testinstall/kernel/blister.tst
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Expand Up @@ -149,15 +149,35 @@ gap> List(l, IsSSortedList);
gap> List(l, SizeBlist);
[ 1, 1, 2, 2 ]

#
# FuncBLIST_LIST
#
gap> BlistList(fail, fail);
Error, BlistList: <list> must be a small list (not the value 'fail')
gap> BlistList([1,2,3], fail);
Error, BlistList: <sub> must be a small list (not the value 'fail')
gap> BlistList([1,2,3], [1,3]);
[ true, false, true ]

# ranges with increment 1
gap> BlistList([0..3], [-3..-1]);
[ false, false, false, false ]
gap> BlistList([0..3], [-3..1]);
[ true, true, false, false ]
gap> BlistList([0..3], [-3..6]);
[ true, true, true, true ]
gap> BlistList([0..3], [0..1]);
[ true, true, false, false ]
gap> BlistList([0..3], [0..6]);
[ true, true, true, true ]
gap> BlistList([0..3], [2..6]);
[ false, false, true, true ]
gap> BlistList([0..3], [4..6]);
[ false, false, false, false ]

#
# FuncLIST_BLIST
#
gap> ListBlist(fail, fail);
Error, ListBlist: <list> must be a small list (not the value 'fail')
gap> ListBlist([1,2,3], fail);
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