From 35d7fa1128757f2adc6c14be810a64a1b668410e Mon Sep 17 00:00:00 2001
From: Thomas Breuer <sam@math.rwth-aachen.de>
Date: Wed, 4 Sep 2024 13:41:55 +0200
Subject: [PATCH] add `GetJuliaScratchspace` to `JuliaInterface` (#1030)

---
 pkg/JuliaInterface/gap/JuliaInterface.gd | 15 +++++++++++++++
 pkg/JuliaInterface/gap/JuliaInterface.gi | 10 ++++++++++
 pkg/JuliaInterface/tst/utils.tst         |  9 ++++++++-
 src/utils.jl                             |  8 ++++++++
 4 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/pkg/JuliaInterface/gap/JuliaInterface.gd b/pkg/JuliaInterface/gap/JuliaInterface.gd
index bdd5c6cc4..5772cb116 100644
--- a/pkg/JuliaInterface/gap/JuliaInterface.gd
+++ b/pkg/JuliaInterface/gap/JuliaInterface.gd
@@ -535,3 +535,18 @@ DeclareGlobalFunction( "CallJuliaFunctionWithKeywordArguments" );
 #TODO: add the cross-references to MatElm, SetMatElm when they are documented
 
 #! @InsertChunk JuliaHelpInGAP
+
+#! @Section Utilities
+
+#! @Arguments key
+#! @Returns a string
+#! @Description
+#!  Returns the path of a &Julia; scratchspace associated to the given key.
+#!  This scratchspace gets created if it did not exist already,
+#!  one can rely on the fact that the returned path describes a writable
+#!  directory.
+#!  Subsequent calls with the same key yield the same result,
+#!  and calls with different keys yield different results.
+#!  The directory may be removed by &Julia; as soon as the &Julia; package
+#!  <C>GAP.jl</C> gets uninstalled.
+DeclareGlobalFunction( "GetJuliaScratchspace" );
diff --git a/pkg/JuliaInterface/gap/JuliaInterface.gi b/pkg/JuliaInterface/gap/JuliaInterface.gi
index 169bab92c..5da6d7dc2 100644
--- a/pkg/JuliaInterface/gap/JuliaInterface.gi
+++ b/pkg/JuliaInterface/gap/JuliaInterface.gi
@@ -166,3 +166,13 @@ InstallGlobalFunction( JuliaModule,
     return JuliaPointer( Julia.(name) );
 end );
 
+
+InstallGlobalFunction( GetJuliaScratchspace,
+  function( key )
+    if not IsString( key ) then
+        Error( "GetJuliaScratchspace: <key> must be a string" );
+    fi;
+    key:= Julia.Base.string( key );
+    return JuliaToGAP( IsString,
+             Julia.GAP.get_scratch_helper\!( key ) );
+end );
diff --git a/pkg/JuliaInterface/tst/utils.tst b/pkg/JuliaInterface/tst/utils.tst
index 702a4d8c2..6100358d6 100644
--- a/pkg/JuliaInterface/tst/utils.tst
+++ b/pkg/JuliaInterface/tst/utils.tst
@@ -97,4 +97,11 @@ gap> Unbind(Julia.Main.f00bar);
 Error, cannot unbind Julia variables
 
 ##
-gap> STOP_TEST( "utils.tst", 1 );
+gap> GetJuliaScratchspace( true);
+Error, GetJuliaScratchspace: <key> must be a string
+gap> path:= GetJuliaScratchspace( "test_scratch" );;
+gap> IsDirectoryPath( path );
+true
+
+##
+gap> STOP_TEST( "utils.tst" );
diff --git a/src/utils.jl b/src/utils.jl
index c37a2c413..be9769cdc 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -106,6 +106,14 @@ function Base.functionloc(f::GapObj)
     return (String(file), ln)
 end
 
+import Scratch: get_scratch!
+
+# Return the path of a Julia scratchspace with name `key`.
+# This scratchspace gets created if it did not exist already.
+# The scratchspace may be removed by Julia as soon as the Julia package GAP.jl
+# gets uninstalled.
+get_scratch_helper!(key::String) = get_scratch!(GAP, key)
+
 # Allow `InteractiveUtils.edit(GAP.Globals.Group)` and similar for any global
 # function. Not (yet?) useful for GAP operations, but better than nothing.
 import InteractiveUtils: edit