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AddToIgs improvements 3: skip the adjust step when it is not needed #237

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Merged
fingolfin merged 3 commits into from
Oct 17, 2026
Merged

AddToIgs improvements 3: skip the adjust step when it is not needed #237

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3521dbe
Another small optimisation for AddToIgs code
stertooy Oct 12, 2026
ca5c88b
Apply batched suggestions from code review
stertooy Oct 17, 2026
c2b56c9
Merge branch 'master' into igs-improv-3
stertooy Oct 17, 2026
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16 changes: 14 additions & 2 deletions gap/basic/pcppara.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -112,8 +112,8 @@ end );
##
InstallGlobalFunction( AddToIgsParallel,
function( pcs, gens, ppcs, pgens )
local coll, rels, n, inf, todo, tododo, ind, indd, g, gg, d, h, hh, a, b, q, k,
c, i, r, sub, keep, val, j, f, nrmd, pairs;
local coll, rels, n, inf, todo, tododo, ind, indd, g, gg, d, h, hh, a, b,
q, k, c, i, r, sub, keep, val, j, f, nrmd, pairs, added;

if Length( gens ) = 0 then return [pcs, ppcs]; fi;

Expand All @@ -133,6 +133,9 @@ function( pcs, gens, ppcs, pgens )
indd[d] := ppcs[i];
od;

# initial gens still need to be reduced
added := true;

# do a reduction step
c := TailLimit(ind, c);
sub := Filtered([1..Length(gens)], i -> Depth(gens[i]) < c);
Expand All @@ -150,6 +153,7 @@ function( pcs, gens, ppcs, pgens )
j := PositionMinimum(val);
g := Remove(todo, j);
gg := Remove(tododo, j);
Remove(val, j);
d := Depth( g );
f := [];

Expand Down Expand Up @@ -196,11 +200,16 @@ function( pcs, gens, ppcs, pgens )
d := Depth(g);
od;

# ind unchanged and every todo entry already reduced, so ReduceExpo,
# TailLimit and the filter would be no-ops
if Length(f) = 0 and not added then continue; fi;

# adjust
c := TailLimit(ind, c);
if inf then ReduceExpoPara(ind, todo, indd, tododo, rels); fi;

# add powers and commutators
added := false;
for d in f do
g := ind[d];
gg := indd[d];
Expand All @@ -210,6 +219,7 @@ function( pcs, gens, ppcs, pgens )
if Depth(k) < c then
Add(todo, k);
Add(tododo, gg^r);
added := true;
fi;
fi;
for j in [1..n] do
Expand All @@ -221,12 +231,14 @@ function( pcs, gens, ppcs, pgens )
if Depth(k) < c then
Add(todo, k);
Add(tododo, Comm(gg, indd[j]));
added := true;
fi;
if rels[j] = 0 then
k := Comm(g, ind[j]^-1);
if Depth(k) < c then
Add(todo, k);
Add(tododo, Comm(gg, indd[j]^-1));
added := true;
fi;
fi;
fi;
Expand Down
27 changes: 23 additions & 4 deletions gap/basic/pcppcps.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -136,7 +136,8 @@ end );
#F AddToIgs( <igs>, <gens> )
##
InstallGlobalFunction(AddToIgs, function(igs, gens)
local coll, rels, n, inf, c, ind, g, d, todo, val, j, f, h, a, b, k, pair, t;
local coll, rels, n, inf, c, ind, g, d, todo, val, j, f, h, a, b, k,
pair, t, added;

if Length(gens) = 0 then return igs; fi;

Expand All @@ -151,6 +152,9 @@ InstallGlobalFunction(AddToIgs, function(igs, gens)
ind := ListWithIdenticalEntries(n, false);
for g in igs do ind[Depth(g)] := g; od;

# initial gens still need to be reduced
added := true;

# do a reduction step
c := TailLimit(ind, c);
todo := Set(Filtered(gens, x -> Depth(x) < c));
Expand All @@ -160,6 +164,7 @@ InstallGlobalFunction(AddToIgs, function(igs, gens)
while Length(todo) > 0 and c > 1 do
j := PositionMinimum(val);
g := Remove(todo, j);
Remove(val, j);
d := Depth(g);
f := [];

Expand Down Expand Up @@ -194,17 +199,25 @@ InstallGlobalFunction(AddToIgs, function(igs, gens)
d := Depth(g);
od;

# ind unchanged and every todo entry already reduced, so ReduceExpo,
# TailLimit and the filter would be no-ops
if Length(f) = 0 and not added then continue; fi;

# adjust
c := TailLimit(ind, c);
if inf then ReduceExpo(ind, todo, rels); fi;

# add powers and commutators
added := false;
for d in f do
g := ind[d];
# skip infinite factors and powers in the tail
if d < c-1 and rels[d] > 0 then
k := g ^ RelativeOrderPcp(g);
if Depth(k) < c then Add(todo, k); fi;
if Depth(k) < c then
Add(todo, k);
added := true;
fi;
fi;
for j in [1..n] do
# skip trivial commutators and those in the tail
Expand All @@ -213,10 +226,16 @@ InstallGlobalFunction(AddToIgs, function(igs, gens)
fi;
if not IsBool(ind[j]) then
k := Comm(g, ind[j]);
if Depth(k) < c then Add(todo, k); fi;
if Depth(k) < c then
Add(todo, k);
added := true;
fi;
if rels[j] = 0 then
k := Comm(g, ind[j]^-1);
if Depth(k) < c then Add(todo, k); fi;
if Depth(k) < c then
Add(todo, k);
added := true;
fi;
fi;
fi;
od;
Expand Down
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