From 89346928278269c1788452a63ec606810a7943db Mon Sep 17 00:00:00 2001
From: stertooy <5571903+stertooy@users.noreply.github.com>
Date: Mon, 12 Oct 2026 16:02:15 +0200
Subject: [PATCH] More mini-optimisations for AddToIgs code

Avoid calling GcdPcp(Para) when it will not do anything

Assisted-by: Claude Code (Opus 5.5, Fable 5.1)
---
 gap/basic/pcppara.gi | 31 ++++++++++++++++++++-----------
 gap/basic/pcppcps.gi | 21 ++++++++++++++-------
 2 files changed, 34 insertions(+), 18 deletions(-)

diff --git a/gap/basic/pcppara.gi b/gap/basic/pcppara.gi
index 66cd6b2e..4c38d0be 100644
--- a/gap/basic/pcppara.gi
+++ b/gap/basic/pcppara.gi
@@ -112,7 +112,7 @@ end );
 ##
 InstallGlobalFunction( AddToIgsParallel,
 function( pcs, gens, ppcs, pgens )
-    local coll, rels, n, todo, tododo, ind, indd, g, gg, d, h, hh, k,
+    local coll, rels, n, todo, tododo, ind, indd, g, gg, d, h, hh, a, b, q, k,
           c, i, r, sub, val, j, f, nrmd, pairs;
 
     if Length( gens ) = 0 then return [pcs, ppcs]; fi;
@@ -167,16 +167,25 @@ function( pcs, gens, ppcs, pgens )
                 g  := g^0;
                 gg := gg^0;
             else
-                pairs := GcdPcpPara(g, h, gg, hh);
-                h := pairs[1];
-                g := pairs[2];
-                hh := pairs[3];
-                gg := pairs[4];
-                if h <> ind[d] then
-                    nrmd := NormedPcpElementPara( h, hh );
-                    ind[d]  := nrmd[1];
-                    indd[d] := nrmd[2];
-                    AddSet(f, d);
+                a := LeadingExponent(g);
+                b := LeadingExponent(h);
+                if a > 0 and b > 0 and a mod b = 0 then
+                    # GcdPcpPara would leave h and hh unchanged.
+                    q  := a/b;
+                    g  := g  * h^-q;
+                    gg := gg * hh^-q;
+                else
+                    pairs := GcdPcpPara(g, h, gg, hh);
+                    h := pairs[1];
+                    g := pairs[2];
+                    hh := pairs[3];
+                    gg := pairs[4];
+                    if h <> ind[d] then
+                        nrmd := NormedPcpElementPara( h, hh );
+                        ind[d]  := nrmd[1];
+                        indd[d] := nrmd[2];
+                        AddSet(f, d);
+                    fi;
                 fi;
             fi;
             d := Depth(g);
diff --git a/gap/basic/pcppcps.gi b/gap/basic/pcppcps.gi
index 4f04b2f8..b6b4f49e 100644
--- a/gap/basic/pcppcps.gi
+++ b/gap/basic/pcppcps.gi
@@ -134,7 +134,7 @@ end );
 #F AddToIgs( <igs>, <gens> )
 ##
 InstallGlobalFunction(AddToIgs, function(igs, gens)
-    local coll, rels, n, c, ind, g, d, todo, val, j, f, h, k, pair, t;
+    local coll, rels, n, c, ind, g, d, todo, val, j, f, h, a, b, k, pair, t;
 
     if Length(gens) = 0 then return igs; fi;
 
@@ -173,12 +173,19 @@ InstallGlobalFunction(AddToIgs, function(igs, gens)
             if g = h then
                 g := g^0;
             else
-                pair := GcdPcp(g, h);
-                h := pair[1];
-                g := pair[2];
-                if h <> ind[d] then
-                    ind[d] := NormedPcpElement( h );
-                    AddSet(f, d);
+                a := LeadingExponent(g);
+                b := LeadingExponent(h);
+                if a > 0 and b > 0 and a mod b = 0 then
+                    # GcdPcp would leave h unchanged.
+                    g := g * h^(-a/b);
+                else
+                    pair := GcdPcp(g, h);
+                    h := pair[1];
+                    g := pair[2];
+                    if h <> ind[d] then
+                        ind[d] := NormedPcpElement( h );
+                        AddSet(f, d);
+                    fi;
                 fi;
             fi;
             d := Depth(g);