Simplify and improve solver heuristic (#44893)

When we changed how to deal with errors in November,
we didn't realize that for an unconstrained choice
rule it is more important in the heuristic to guess
what is NOT in the answer set, since it will be the
majority of options.

Previously this was following automatically from what
was in the answer set, via `1 { ... } 1` cardinality
constraints.

Here we improve the heuristic and the solve time for specs.

3 files changed, 23 insertions, 39 deletions

diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index 2701ea25fd..f53ae1c124 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -846,8 +846,6 @@ class PyclingoDriver:
         parent_dir = os.path.dirname(__file__)
         self.control.load(os.path.join(parent_dir, "concretize.lp"))
         self.control.load(os.path.join(parent_dir, "heuristic.lp"))
-        if spack.config.CONFIG.get("concretizer:duplicates:strategy", "none") != "none":
-            self.control.load(os.path.join(parent_dir, "heuristic_separate.lp"))
         self.control.load(os.path.join(parent_dir, "display.lp"))
         if not setup.concretize_everything:
             self.control.load(os.path.join(parent_dir, "when_possible.lp"))
diff --git a/lib/spack/spack/solver/heuristic.lp b/lib/spack/spack/solver/heuristic.lp
index 71ed64b3f8..1bace84312 100644
--- a/lib/spack/spack/solver/heuristic.lp
+++ b/lib/spack/spack/solver/heuristic.lp
@@ -4,21 +4,31 @@
 % SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 %=============================================================================
-% Heuristic to speed-up solves (node with ID 0)
+% Heuristic to speed-up solves
 %=============================================================================
 
+% No duplicates by default (most of them will be true)
+#heuristic attr("node",         node(1..X-1, Package)) : max_dupes(Package, X), not virtual(Package), X > 1. [50, false]
+#heuristic attr("virtual_node", node(1..X-1, Package)) : max_dupes(Package, X), virtual(Package)    , X > 1. [50, false]
 
-%-----------------
-% Domain heuristic
-%-----------------
+% Pick preferred version
+#heuristic attr("version", node(PackageID, Package), Version) : pkg_fact(Package, version_declared(Version, 0     )), attr("node", node(PackageID, Package)).             [40, true]
+#heuristic attr("version", node(PackageID, Package), Version) : pkg_fact(Package, version_declared(Version, Weight)), attr("node", node(PackageID, Package)), Weight > 0. [40, false]
+#heuristic version_weight(node(PackageID, Package), 0)        : pkg_fact(Package, version_declared(Version, 0     )), attr("node", node(PackageID, Package)).             [40, true]
 
-% Root node
-#heuristic attr("version", node(0, Package), Version) : pkg_fact(Package, version_declared(Version, 0)), attr("root", node(0, Package)). [35, true]
-#heuristic version_weight(node(0, Package), 0) : pkg_fact(Package, version_declared(Version, 0)), attr("root", node(0, Package)). [35, true]
-#heuristic attr("variant_value", node(0, Package), Variant, Value) : variant_default_value(Package, Variant, Value), attr("root", node(0, Package)). [35, true]
-#heuristic attr("node_target", node(0, Package), Target) : target_weight(Target, 0), attr("root", node(0, Package)). [35, true]
-#heuristic node_target_weight(node(0, Package), 0) : attr("root", node(0, Package)). [35, true]
-#heuristic node_compiler(node(0, Package), CompilerID) : compiler_weight(ID, 0), compiler_id(ID), attr("root", node(0, Package)). [35, true]
+% Use default variants
+#heuristic attr("variant_value", node(PackageID, Package), Variant, Value) : variant_default_value(Package, Variant, Value),     attr("node", node(PackageID, Package)). [40, true]
+#heuristic attr("variant_value", node(PackageID, Package), Variant, Value) : not variant_default_value(Package, Variant, Value), attr("node", node(PackageID, Package)). [40, false]
 
-% Providers
-#heuristic attr("node", node(0, Package)) : default_provider_preference(Virtual, Package, 0), possible_in_link_run(Package). [30, true]
+% Use default operating system and platform
+#heuristic attr("node_os", node(PackageID, Package), OS) : os(OS, 0), attr("root", node(PackageID, Package)). [40, true]
+#heuristic attr("node_platform", node(PackageID, Package), Platform) : allowed_platform(Platform), attr("root", node(PackageID, Package)). [40, true]
+
+% Use default targets
+#heuristic attr("node_target", node(PackageID, Package), Target) : target_weight(Target, 0), attr("node", node(PackageID, Package)). [30, true]
+#heuristic attr("node_target", node(PackageID, Package), Target) : target_weight(Target, Weight), attr("node", node(PackageID, Package)), Weight > 0. [30, false]
+#heuristic node_target_weight(node(PackageID, Package), 0) : attr("node", node(PackageID, Package)). [30, true]
+
+% Use the default compilers
+#heuristic node_compiler(node(0, Package), ID) : compiler_weight(ID, 0)     , compiler_id(ID), attr("node", node(0, Package)). [30, true]
+#heuristic node_compiler(node(0, Package), ID) : compiler_weight(ID, Weight), compiler_id(ID), attr("node", node(0, Package)), Weight > 0. [30, false]
diff --git a/lib/spack/spack/solver/heuristic_separate.lp b/lib/spack/spack/solver/heuristic_separate.lp
deleted file mode 100644
index 2b499d0798..0000000000
--- a/lib/spack/spack/solver/heuristic_separate.lp
+++ /dev/null
@@ -1,24 +0,0 @@
-% Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
-% Spack Project Developers. See the top-level COPYRIGHT file for details.
-%
-% SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-%=============================================================================
-% Heuristic to speed-up solves (node with ID > 0)
-%=============================================================================
-
-% node(ID, _)
-#heuristic attr("version", node(ID, Package), Version) : pkg_fact(Package, version_declared(Version, 0)), attr("node", node(ID, Package)), ID > 0. [25-5*ID, true]
-#heuristic version_weight(node(ID, Package), 0) : pkg_fact(Package, version_declared(Version, 0)), attr("node", node(ID, Package)), ID > 0. [25-5*ID, true]
-#heuristic attr("variant_value", node(ID, Package), Variant, Value) : variant_default_value(Package, Variant, Value), attr("node", node(ID, Package)), ID > 0. [25-5*ID, true]
-#heuristic attr("node_target", node(ID, Package), Target) : pkg_fact(Package, target_weight(Target, 0)), attr("node", node(ID, Package)), ID > 0. [25-5*ID, true]
-#heuristic node_target_weight(node(ID, Package), 0) : attr("node", node(ID, Package)), ID > 0. [25-5*ID, true]
-#heuristic node_compiler(node(ID, Package), CompilerID) : compiler_weight(CompilerID, 0), compiler_id(CompilerID), attr("node", node(ID, Package)), ID > 0. [25-5*ID, true]
-
-% node(ID, _), split build dependencies
-#heuristic attr("version", node(ID, Package), Version) : pkg_fact(Package, version_declared(Version, 0)), attr("node", node(ID, Package)), multiple_unification_sets(Package), ID > 0. [25, true]
-#heuristic version_weight(node(ID, Package), 0) : pkg_fact(Package, version_declared(Version, 0)), attr("node", node(ID, Package)), multiple_unification_sets(Package), ID > 0. [25, true]
-#heuristic attr("variant_value", node(ID, Package), Variant, Value) : variant_default_value(Package, Variant, Value), attr("node", node(ID, Package)), multiple_unification_sets(Package), ID > 0. [25, true]
-#heuristic attr("node_target", node(ID, Package), Target) : pkg_fact(Package, target_weight(Target, 0)), attr("node", node(ID, Package)), multiple_unification_sets(Package), ID > 0. [25, true]
-#heuristic node_target_weight(node(ID, Package), 0) : attr("node", node(ID, Package)), multiple_unification_sets(Package), ID > 0. [25, true]
-#heuristic node_compiler(node(ID, Package), CompilerID) : compiler_weight(CompilerID, 0), compiler_id(CompilerID), attr("node", node(ID, Package)), multiple_unification_sets(Package), ID > 0. [25, true]