Merge branch 'bellmanford' of https://github.com/baydrea/Digraphs int…

…o bellmanford

.github/workflows/stale.yml

@@ -0,0 +1,28 @@
+# This workflow warns and then closes issues and PRs that have had no activity for a specified amount of time.
+#
+# You can adjust the behavior by modifying this file.
+# For more information, see:
+# https://github.com/actions/stale
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: '42 0 * * *'
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+
+    steps:
+    - uses: actions/stale@v3
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-issue-message: 'Stale issue message'
+        stale-pr-message: 'Stale pull request message'
+        stale-issue-label: 'no-issue-activity'
+        stale-pr-label: 'no-pr-activity'
+        days-before-close: -1

doc/attr.xml

@@ -2430,3 +2430,40 @@ gap> Length(M);
   </Description>
 </ManSection>
 <#/GAPDoc>
+
+<#GAPDoc Label="NonUpperSemimodularPair">
+<ManSection>
+  <Attr Name="NonUpperSemimodularPair" Arg="D"/>
+  <Attr Name="NonLowerSemimodularPair" Arg="D"/>
+  <Returns>A pair of vertices or <K>fail</K>.</Returns>
+  <Description>
+    <C>NonUpperSemimodularPair</C> returns a pair of vertices in the digraph
+    <A>D</A> that witnesses the fact that <A>D</A> does not represent an upper
+    semimodular lattice, if such a pair exists. <P/> 
+
+    If the digraph <A>D</A> does not satisfy <Ref
+    Prop="IsLatticeDigraph"/>, then an error is given. Otherwise if the
+    digraph <A>D</A> does satisfy <Ref
+    Prop="IsLatticeDigraph"/>, then either a non-upper semimodular pair of
+    vertices is returned, or <K>fail</K> is returned if no such pair exists
+    (meaning that <A>D</A> is an upper semimodular lattice. <P/>
+
+    <C>NonLowerSemimodularPair</C> behaves in the analogous way to
+    <C>NonUpperSemimodularPair</C> with respect to lower semimodularity. <P/>
+
+    See <Ref Prop="IsUpperSemimodularDigraph"/> and <Ref
+    Prop="IsLowerSemimodularDigraph"/> for the definition of upper
+    semimodularity of a lattice. 
+
+    <Example><![CDATA[
+gap> D := DigraphFromDigraph6String(
+> "&M~~sc`lYUZO__KIBboC_@h?U_?_GL?A_?c");
+<immutable digraph with 14 vertices, 66 edges>
+gap> NonLowerSemimodularPair(D);
+[ 10, 9 ]
+gap> NonUpperSemimodularPair(D);
+fail
+]]></Example>
+  </Description>
+</ManSection>
+<#/GAPDoc>

doc/digraph.xml

@@ -421,33 +421,52 @@ gap> D := DigraphByInNeighbours(IsMutableDigraph,
 
 <#GAPDoc Label="AsDigraph">
 <ManSection>
-  <Oper Name="AsDigraph" Arg="[filt, ]f[, n]"
-    Label="for a transformation or perm"/>
+  <Oper Name="AsDigraph" Arg="[filt, ]f[, n]" Label="for a binary relation"/>
   <Returns>A digraph, or <K>fail</K>.</Returns>
   <Description>
-    If <A>f</A> is a transformation or permutation,
-    and <A>n</A> is a non-negative integer
-    such that the restriction of <A>f</A> to <C>[1..<A>n</A>]</C> defines
-    a function of <C>[1..<A>n</A>]</C>, then <C>AsDigraph</C>
-    returns the functional digraph with <A>n</A> vertices defined by
-    <A>f</A>. See <Ref Prop="IsFunctionalDigraph"/>.
-    <P/>
-
-    Specifically, the digraph returned by <C>AsDigraph</C> has  <A>n</A> edges:
-    for each vertex <C>v</C> in <C>[1..<A>n</A>]</C>, there is a unique edge
-    with source <C>v</C>; this edge has range <C>v^<A>f</A></C>.
-    <P/>
 
-    If the optional second argument <A>n</A> is not supplied, then
-    the degree of the transformation <A>f</A>,
-    or the largest moved point of the permutation <A>f</A>,
-    as applicable, is used by default.  If the restriction of
-    <A>f</A> to <C>[1..<A>n</A>]</C> does not define a function of
-    <C>[1..<A>n</A>]</C>, then <C>AsDigraph(<A>f</A>, <A>n</A>)</C>
-    returns <K>fail</K>.
+      If <A>f</A> is a binary relation represented as one of the following in
+        &GAP;:
+      <List>
+        <Mark>
+          a transformation
+        </Mark>
+        <Item>
+          satisfying <Ref Filt="IsTransformation" BookName="ref"/>;
+        </Item>
+        <Mark>
+          a permutation
+        </Mark>
+        <Item>
+          satisfying <Ref Filt="IsPerm" BookName="ref"/>;
+        </Item>
+        <Mark>
+          a partial perm
+        </Mark>
+        <Item>
+          satisfying <Ref Filt="IsPartialPerm" BookName="ref"/>;
+        </Item>
+        <Mark>
+          a binary relation
+        </Mark>
+        <Item>
+          satisfying <Ref Filt="IsBinaryRelation" BookName="ref"/>;
+        </Item>
+      </List>
+      and <A>n</A> is a non-negative integer, then <C>AsDigraph</C> attempts
+      to construct a digraph with <A>n</A> vertices whose edges are determined
+      by <A>f</A>.<P/>
+
+    The digraph returned by <C>AsDigraph</C> has for each vertex
+    <C>v</C> in <C>[1 .. <A>n</A>]</C>, an edge with source <C>v</C> and range
+    <C>v ^ <A>f</A></C>.  If <C>v ^ <A>f</A></C> is greater than <A>n</A> for any
+    <C>v</C>, then <K>fail</K> is returned.
     <P/>
 
-    See also <Ref Attr="AsTransformation"/>.
+    If the optional second argument <A>n</A> is not supplied, then the degree
+    of the transformation <A>f</A>, the largest moved point of the permutation
+    <A>f</A>, the maximum of the degree and the codegree of the partial perm
+    <A>f</A>, or as applicable, is used by default.  
     <P/>
 
     &STANDARD_FILT_TEXT;

doc/prop.xml

@@ -1459,3 +1459,39 @@ true
   </Description>
 </ManSection>
 <#/GAPDoc>
+
+<#GAPDoc Label="IsUpperSemimodularDigraph">
+<ManSection>
+  <Prop Name="IsUpperSemimodularDigraph" Arg="D"/>
+  <Prop Name="IsLowerSemimodularDigraph" Arg="D"/>
+  <Returns><K>true</K> or <K>false</K>.</Returns>
+  <Description>
+    <C>IsUpperSemimodularDigraph</C> returns <K>true</K> if the digraph
+    <A>D</A> represents an upper semimodular lattice and <K>false</K> if it
+    does not.  Similarly, <C>IsLowerSemimodularDigraph</C> returns <K>true</K>
+    if <A>D</A> represents a lower semimodular lattice and <K>false</K> if
+    it does not. <P/>
+
+    In a lattice we say that a vertex <C>a</C> <E>covers</E> a vertex <C>b</C>
+    if <C>a</C> is greater than <C>b</C>, and there are no further vertices
+    between <C>a</C> and <C>b</C>.  A lattice is <E>upper semimodular</E> if
+    whenever the meet of <C>a</C> and <C>b</C> is covered by <C>a</C>, the join
+    of <C>a</C> and <C>b</C> covers <C>b</C>.  <E>Lower semimodularity</E> is
+    defined analogously. <P/>
+
+    See also <Ref Prop="IsLatticeDigraph"/>, <Ref Oper="NonUpperSemimodularPair"/>, 
+    and <Ref Oper="NonLowerSemimodularPair"/>.
+
+    &MUTABLE_RECOMPUTED_PROP;
+
+    <Example><![CDATA[
+gap> D := DigraphFromDigraph6String(
+> "&M~~sc`lYUZO__KIBboC_@h?U_?_GL?A_?c");
+<immutable digraph with 14 vertices, 66 edges>
+gap> IsUpperSemimodularDigraph(D);
+true
+gap> IsLowerSemimodularDigraph(D);
+false]]></Example>
+</Description>
+</ManSection>
+<#/GAPDoc>

doc/z-chap4.xml

@@ -35,10 +35,14 @@
     <#Include Label="InDegreeOfVertex">
     <#Include Label="InNeighboursOfVertex">
     <#Include Label="DigraphLoops">
-    <#Include Label="PartialOrderDigraphMeetOfVertices">
     <#Include Label="DegreeMatrix">
     <#Include Label="LaplacianMatrix">
   </Section>
+
+  <Section><Heading>Orders</Heading>
+    <#Include Label="PartialOrderDigraphMeetOfVertices">
+    <#Include Label="NonUpperSemimodularPair">
+  </Section>
 
   <Section><Heading>Reachability and connectivity</Heading>
     <#Include Label="DigraphDiameter">

doc/z-chap5.xml

@@ -22,9 +22,13 @@
     <#Include Label="IsSymmetricDigraph">
     <#Include Label="IsTournament">
     <#Include Label="IsTransitiveDigraph">
+  </Section>
+
+  <Section><Heading>Orders</Heading>
     <#Include Label="IsPreorderDigraph">
     <#Include Label="IsPartialOrderDigraph">
     <#Include Label="IsMeetSemilatticeDigraph">
+    <#Include Label="IsUpperSemimodularDigraph">
   </Section>
 
   <Section><Heading>Regularity</Heading>

gap/attr.gd

@@ -121,3 +121,9 @@ DeclareAttribute("DigraphMaximumMatching", IsDigraph);
 
 DeclareAttribute("Bridges", IsDigraph);
 DeclareAttributeThatReturnsDigraph("StrongOrientation", IsDigraph);
+
+DeclareAttribute("NonUpperSemimodularPair", IsDigraph);
+DeclareAttribute("NonLowerSemimodularPair", IsDigraph);
+
+DeclareProperty("IsUpperSemimodularDigraph", IsDigraph);
+DeclareProperty("IsLowerSemimodularDigraph", IsDigraph);

gap/attr.gi

@@ -2655,3 +2655,73 @@ function(D)
   M := List(DigraphLoops(D), x -> [x, x]);
   return Union(M, DIGRAPHS_MateToMatching(D, mateD));
 end);
+
+# The following function is a transliteration from python to GAP of
+# the function find_nonsemimodular_pair
+# in sage/src/sage/combinat/posets/hasse_diagram.py
+
+BindGlobal("DIGRAPHS_NonSemimodularPair",
+function(nbs)
+  local n, covers, covers_len, a, covers_a, b, e, a_i, b_i;
+  n := Length(nbs);
+
+  for e in [1 .. n] do
+    covers := nbs[e];
+    covers_len := Length(covers);
+    if covers_len < 2 then
+        continue;
+    fi;
+    for a_i in [1 .. covers_len] do
+      a := covers[a_i];
+      covers_a := nbs[a];
+      for b_i in [1 .. a_i] do
+        b := covers[b_i];
+        if not ForAny(nbs[b], j -> j in covers_a) then
+          return [a, b];
+        fi;
+      od;
+    od;
+  od;
+
+  return fail;
+end);
+
+InstallMethod(NonUpperSemimodularPair, "for a digraph",
+[IsDigraphByOutNeighboursRep],
+function(D)
+  if not IsLatticeDigraph(D) then
+    ErrorNoReturn("the argument (a digraph) is not a lattice");
+  fi;
+  D := DigraphReflexiveTransitiveReduction(DigraphMutableCopyIfMutable(D));
+  return DIGRAPHS_NonSemimodularPair(OutNeighbours(D));
+end);
+
+InstallMethod(NonLowerSemimodularPair, "for a digraph",
+[IsDigraphByOutNeighboursRep],
+function(D)
+  if not IsLatticeDigraph(D) then
+    ErrorNoReturn("the argument (a digraph) is not a lattice");
+  fi;
+  D := DigraphReflexiveTransitiveReduction(DigraphMutableCopyIfMutable(D));
+  return DIGRAPHS_NonSemimodularPair(InNeighbours(D));
+end);
+
+InstallMethod(IsUpperSemimodularDigraph, "for a digraph",
+[IsDigraphByOutNeighboursRep],
+function(D)
+  if not IsLatticeDigraph(D) then
+    return false;
+  fi;
+  D := DigraphReflexiveTransitiveReduction(DigraphMutableCopyIfMutable(D));
+  return DIGRAPHS_NonSemimodularPair(OutNeighbours(D)) = fail;
+end);
+
+InstallMethod(IsLowerSemimodularDigraph, "for a digraph",
+[IsDigraphByOutNeighboursRep],
+function(D)
+  if not IsLatticeDigraph(D) then
+    return false;
+  fi;
+  D := DigraphReflexiveTransitiveReduction(DigraphMutableCopyIfMutable(D));
+  return DIGRAPHS_NonSemimodularPair(InNeighbours(D)) = fail;
+end);

gap/digraph.gd

@@ -106,18 +106,24 @@ DeclareSynonym("DigraphByInNeighbors", DigraphByInNeighbours);
 DeclareConstructor("AsDigraphCons", [IsDigraph, IsBinaryRelation]);
 DeclareConstructor("AsDigraphCons", [IsDigraph, IsTransformation]);
 DeclareConstructor("AsDigraphCons", [IsDigraph, IsTransformation, IsInt]);
+DeclareConstructor("AsDigraphCons", [IsDigraph, IsPartialPerm]);
+DeclareConstructor("AsDigraphCons", [IsDigraph, IsPartialPerm, IsInt]);
 
 DeclareOperation("AsDigraph", [IsFunction, IsBinaryRelation]);
 DeclareOperation("AsDigraph", [IsFunction, IsTransformation]);
 DeclareOperation("AsDigraph", [IsFunction, IsTransformation, IsInt]);
 DeclareOperation("AsDigraph", [IsFunction, IsPerm]);
 DeclareOperation("AsDigraph", [IsFunction, IsPerm, IsInt]);
+DeclareOperation("AsDigraph", [IsFunction, IsPartialPerm]);
+DeclareOperation("AsDigraph", [IsFunction, IsPartialPerm, IsInt]);
 
 DeclareOperation("AsDigraph", [IsBinaryRelation]);
 DeclareOperation("AsDigraph", [IsTransformation]);
 DeclareOperation("AsDigraph", [IsTransformation, IsInt]);
 DeclareOperation("AsDigraph", [IsPerm]);
 DeclareOperation("AsDigraph", [IsPerm, IsInt]);
+DeclareOperation("AsDigraph", [IsPartialPerm]);
+DeclareOperation("AsDigraph", [IsPartialPerm, IsInt]);
 
 DeclareOperation("AsBinaryRelation", [IsDigraph]);
 DeclareOperation("AsSemigroup", [IsFunction, IsDigraph]);

gap/digraph.gi

@@ -1062,6 +1062,58 @@ InstallMethod(AsDigraph, "for a function and a perm",
 InstallMethod(AsDigraph, "for a perm", [IsPerm],
 p -> AsDigraph(AsTransformation(p)));
 
+InstallMethod(AsDigraphCons,
+"for IsMutableDigraph, a partial perm, and an integer",
+[IsMutableDigraph, IsPartialPerm, IsInt],
+function(filt, f, n)
+  local list, x, i;
+  if n < 0 then
+    ErrorNoReturn("the 2nd argument <n> should be a non-negative integer,");
+  fi;
+
+  list := EmptyPlist(n);
+  for i in [1 .. n] do
+    x := i ^ f;
+    if x > n then
+      return fail;
+    elif x <> 0 then
+      list[i] := [x];
+    else
+      list[i] := [];
+    fi;
+  od;
+  return DigraphNC(IsMutableDigraph, list);
+end);
+
+InstallMethod(AsDigraphCons,
+"for IsImmutableDigraph, a partial perm, and an integer",
+[IsImmutableDigraph, IsPartialPerm, IsInt],
+function(filt, f, n)
+  local D;
+  D := AsDigraph(IsMutableDigraph, f, n);
+  if D <> fail then
+    D := MakeImmutable(D);
+    SetIsMultiDigraph(D, false);
+  fi;
+  return D;
+end);
+
+InstallMethod(AsDigraph, "for a function, a partial perm, and an integer",
+[IsFunction, IsPartialPerm, IsInt], AsDigraphCons);
+
+InstallMethod(AsDigraph, "for a partial perm and an integer",
+[IsPartialPerm, IsInt],
+{t, n} -> AsDigraphCons(IsImmutableDigraph, t, n));
+
+InstallMethod(AsDigraph, "for a function and a partial perm",
+[IsFunction, IsPartialPerm],
+{func, t} -> AsDigraphCons(func, t, Maximum(DegreeOfPartialPerm(t),
+                                            CodegreeOfPartialPerm(t))));
+
+InstallMethod(AsDigraph, "for a partial perm", [IsPartialPerm],
+t -> AsDigraphCons(IsImmutableDigraph, t, Maximum(DegreeOfPartialPerm(t),
+                                                  CodegreeOfPartialPerm(t))));
+
 InstallMethod(AsBinaryRelation, "for a digraph", [IsDigraphByOutNeighboursRep],
 function(D)
   local rel;