From 20e698c6b0ac068cd1c33e3c1d8baebfb8a10537 Mon Sep 17 00:00:00 2001
From: Todd Gamblin <tgamblin@llnl.gov>
Date: Mon, 2 Jan 2023 00:34:32 -0800
Subject: [PATCH] concretizer: add depth calculation

---
 lib/spack/spack/solver/concretize.lp | 27 +++++++++++++++++++++++++++
 lib/spack/spack/solver/display.lp    |  1 +
 2 files changed, 28 insertions(+)

diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index 059bb5eb630..fc3c72d978a 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -1106,6 +1106,33 @@ build_priority(Package, 0)   :- attr("node", Package), not optimize_for_reuse().
 
 #defined installed_hash/2.
 
+%-----------------------------------------------------------------------------
+% Calculate min depth of nodes in the DAG
+% We use this to optimize nodes closer to roots with higher precedence.
+%-----------------------------------------------------------------------------
+#const max_depth = 10.
+
+% roots have depth 0
+depth(Package, 0) :- attr("root", Package).
+
+% possible optimization
+% adding this to the minimization below doesn't seem to help.
+%possible_dependency(Dependent, Package) :- dependency_condition(_, Dependent, Package).
+%possible_dependency(Dependent, Package) :-
+%    dependency_condition(_, Dependent, Virtual), possible_provider(Package, Virtual).
+
+% other nodes' depth is the minimum depth of any dependent plus one.
+depth(Package, N+1) :-
+    N = #min{
+        D: depends_on(Dependent, Package),
+           depth(Dependent, D),
+           0 <= D,
+           D < max_depth
+    },
+    0 <= N,
+    N <= max_depth,
+    attr("node", Package).
+
 %-----------------------------------------------------------------
 % Optimization to avoid errors
 %-----------------------------------------------------------------
diff --git a/lib/spack/spack/solver/display.lp b/lib/spack/spack/solver/display.lp
index 6e1cf6391ae..1f308f03648 100644
--- a/lib/spack/spack/solver/display.lp
+++ b/lib/spack/spack/solver/display.lp
@@ -26,3 +26,4 @@
 #show error/7.
 
 % debug
+#show depth/2.