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Fix garbled result produced by CosetLeadersMatFFE #6325

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Fix garbled result produced by CosetLeadersMatFFE #6325

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24 changes: 19 additions & 5 deletions src/vec8bit.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2482,6 +2482,7 @@ static UInt CosetLeadersInner8Bits(Obj veclis,
UInt elts;
UInt1 * ptr, *ptrw;
const UInt1 * gettab;
const Obj * gapseq;
const UInt1 * feltffe;
Obj x;
Obj vp;
Expand All @@ -2491,20 +2492,29 @@ static UInt CosetLeadersInner8Bits(Obj veclis,
elts = ELS_BYTE_FIELDINFO_8BIT(info);
settab = SETELT_FIELDINFO_8BIT(info);
gettab = GETELT_FIELDINFO_8BIT(info);
gapseq = GAPSEQ_FELT_FIELDINFO_8BIT(info);
ptrw = BYTES_VEC8BIT(w);
if (weight == 1) {
for (i = pos; i <= len; i++) {
vp = ELM_PLIST(veclis, i);
u = ELM_PLIST(vp, 1);
AddVec8BitVec8BitInner(w, w, u, 1, lenw);
ptr = BYTES_VEC8BIT(v) + (i - 1) / elts;
*ptr = settab[*ptr + 256 * (elts + ((i - 1) % elts))];
// Keep the coefficient vector in the same sorted field-element
// order that NumberFFVector uses for list indices.
feltffe = FELT_FFE_FIELDINFO_8BIT(info);
x = ELM_PLIST(felts, 2);
*ptr = settab[*ptr + 256 * (elts * feltffe[VAL_FFE(x)] +
((i - 1) % elts))];
sy = 0;
for (j = 0; j < lenw; j++) {
UInt xxxx;
sy *= q;
xxxx = gettab[ptrw[j / elts] + 256 * (j % elts)];
sy += xxxx;
// The packed 8-bit representation uses a different internal
// field ordering, so translate back to GAP's sequence before
// using the syndrome number as a plain-list position.
sy += INT_INTOBJ(gapseq[xxxx]);
}
if ((Obj)0 == ELM_PLIST(leaders, sy + 1)) {
UInt k;
Expand All @@ -2517,22 +2527,26 @@ static UInt CosetLeadersInner8Bits(Obj veclis,
wc = ZeroVec8Bit(q, lenw, 1);
settab = SETELT_FIELDINFO_8BIT(info);
gettab = GETELT_FIELDINFO_8BIT(info);
gapseq = GAPSEQ_FELT_FIELDINFO_8BIT(info);
ptr = BYTES_VEC8BIT(v) + (i - 1) / elts;
ptrw = BYTES_VEC8BIT(w);
for (k = 2; k < q; k++) {
qk = FFE_FELT_FIELDINFO_8BIT(info, k);
for (k = 3; k <= q; k++) {
// Record scalar multiples in the same order as the GAP
// fallback, namely the sorted field elements in 'felts'.
qk = ELM_PLIST(felts, k);
MultVec8BitFFEInner(wc, w, qk, 1, lenw);
ptrw = BYTES_VEC8BIT(wc);
sy = 0;
for (j = 0; j < lenw; j++) {
UInt xxxx;
sy *= q;
xxxx = gettab[ptrw[j / elts] + 256 * (j % elts)];
sy += xxxx;
sy += INT_INTOBJ(gapseq[xxxx]);
}
vc = ZeroVec8Bit(q, len, 0);
settab = SETELT_FIELDINFO_8BIT(info);
gettab = GETELT_FIELDINFO_8BIT(info);
gapseq = GAPSEQ_FELT_FIELDINFO_8BIT(info);
ptr = BYTES_VEC8BIT(v) + (i - 1) / elts;
ptrw = BYTES_VEC8BIT(w);
MultVec8BitFFEInner(vc, v, qk, 1, len);
Expand Down
33 changes: 33 additions & 0 deletions tst/testbugfix/2026-04-15-issue-5923.tst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,33 @@
# Regression test for issue #5923: the 8-bit kernel path for
# CosetLeadersMatFFE must not leave holes in the result list.
gap> F := GF(4);;
gap> M := One(F)*[[1,0,Z(4)],[0,1,Z(4)^2]];;
gap> L := CosetLeadersMatFFE(M, F);;
gap> Length(L) = Size(F)^2;
true
gap> ForAll([1..Length(L)], i -> IsBound(L[i]));
true
gap> List(L, v -> NumberFFVector(M * v, Size(F))) = [0..Length(L)-1];
true

# some additional tests "because we can" for other cases
gap> F := GF(2);;
gap> M := One(F)*[[1,0,1,0,1,0],[1,1,1,0,0,0]];;
gap> L := CosetLeadersMatFFE(M, F);;
gap> Length(L) = Size(F)^2;
true
gap> ForAll([1..Length(L)], i -> IsBound(L[i]));
true
gap> List(L, v -> NumberFFVector(M * v, Size(F))) = [0..Length(L)-1];
true

# some additional tests "because we can" for other cases
gap> F := GF(257);;
gap> M := One(F)*[[1,0,1,0,1,0],[1,1,1,0,0,0]];;
gap> L := CosetLeadersMatFFE(M, F);;
gap> Length(L) = Size(F)^2;
true
gap> ForAll([1..Length(L)], i -> IsBound(L[i]));
true
gap> List(L, v -> NumberFFVector(M * v, Size(F))) = [0..Length(L)-1];
true
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