solver3: Introduce new Assume() and Enqueue() helpers and use them

These are taken roughly from the MiniSAT paper. We still have a bit
to go in actually encoding all clauses so the reasons are still
variables, and Assume() isn't fully working yet.

Adjust the existing Install()/Reject() code to use these functions,
we already see additional lines in the log that we failed to log
before, and this ensures more consistency.

This is sort of still the wrong direction: Install()/Reject() do the
propagation too; but that is tbd.

2 files changed, 55 insertions, 23 deletions

diff --git a/apt-pkg/solver3.cc b/apt-pkg/solver3.cc
index 5065b9a87..a21af12f9 100644
--- a/apt-pkg/solver3.cc
+++ b/apt-pkg/solver3.cc
@@ -304,6 +304,35 @@ bool APT::Solver::Obsolete(pkgCache::PkgIterator pkg) const
    pkgObsolete[pkg->ID] = 2;
    return true;
 }
+bool APT::Solver::Assume(Var var, bool decision, Var reason)
+{
+   choices.push_back(solved.size());
+   return Enqueue(var, decision, std::move(reason));
+}
+
+bool APT::Solver::Enqueue(Var var, bool decision, Var reason)
+{
+   auto &state = (*this)[var];
+   auto decisionCast = decision ? Decision::MUST : Decision::MUSTNOT;
+
+   if (state.decision != Decision::NONE)
+   {
+      if (state.decision != decisionCast)
+	 return _error->Error("Conflict: %s -> %s%s but %s", WhyStr(reason).c_str(), decision ? "" : "not ", var.toString(cache).c_str(), WhyStr(var).c_str());
+      return true;
+   }
+
+   state.decision = decisionCast;
+   state.depth = depth();
+   state.reason = reason;
+
+   if (unlikely(debug >= 1))
+      std::cerr << "[" << depth() << "] " << (decision ? "Install" : "Reject") << ":" << var.toString(cache) << " (" << WhyStr(state.reason) << ")\n";
+
+   solved.push_back(Solved{var, std::nullopt});
+
+   return true;
+}
 
 bool APT::Solver::Install(pkgCache::PkgIterator Pkg, Var reason, Group group)
 {
@@ -329,10 +358,8 @@ bool APT::Solver::Install(pkgCache::PkgIterator Pkg, Var reason, Group group)
    }
 
    // Note decision
-   if (unlikely(debug >= 1))
-      std::cerr << "[" << depth() << "] Install:" << Pkg.FullName() << " (" << WhyStr(reason) << ")\n";
-   (*this)[Pkg] = {reason, depth(), Decision::MUST};
-   solved.push_back(Solved{Var(Pkg), std::nullopt});
+   if (not Enqueue(Var(Pkg), true, reason))
+      return false;
 
    // Insert the work item.
    Work workItem{Var(Pkg), depth(), group};
@@ -380,15 +407,10 @@ bool APT::Solver::Install(pkgCache::VerIterator Ver, Var reason, Group group)
 			      WhyStr(Var(otherVer)).c_str());
 
    // Note decision
-   if (unlikely(debug >= 1))
-      std::cerr << "[" << depth() << "] Install:" << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " (" << WhyStr(reason) << ")\n";
-   (*this)[Ver] = {reason, depth(), Decision::MUST};
-   solved.push_back(Solved{Var(Ver), std::nullopt});
-   if ((*this)[Ver.ParentPkg()].decision != Decision::MUST)
-   {
-      (*this)[Ver.ParentPkg()] = {Var(Ver), depth(), Decision::MUST};
-      solved.push_back(Solved{Var(Ver.ParentPkg()), std::nullopt});
-   }
+   if (not Enqueue(Var(Ver), true, reason))
+      return false;
+   if (not Enqueue(Var(Ver.ParentPkg()), true, Var(Ver)))
+      return false;
 
    for (auto OV = Ver.ParentPkg().VersionList(); not OV.end(); ++OV)
    {
@@ -423,10 +445,8 @@ bool APT::Solver::Reject(pkgCache::PkgIterator Pkg, Var reason, Group group)
       return _error->Error("Conflict: %s -> not %s but %s", WhyStr(reason).c_str(), Pkg.FullName().c_str(), WhyStr(Var(Pkg)).c_str());
 
    // Reject the package and its versions.
-   if (unlikely(debug >= 1))
-      std::cerr << "[" << depth() << "] Reject:" << Pkg.FullName() << " (" << WhyStr(reason) << ")\n";
-   (*this)[Pkg] = {reason, depth(), Decision::MUSTNOT};
-   solved.push_back(Solved{Var(Pkg), std::nullopt});
+   if (not Enqueue(Var(Pkg), false, reason))
+      return false;
    for (auto ver = Pkg.VersionList(); not ver.end(); ver++)
       if (not Reject(ver, Var(Pkg), group))
 	 return false;
@@ -452,10 +472,8 @@ bool APT::Solver::Reject(pkgCache::VerIterator Ver, Var reason, Group group)
 			   WhyStr(Var(Ver)).c_str());
 
    // Mark the package as rejected and propagate up as needed.
-   if (unlikely(debug >= 1))
-      std::cerr << "[" << depth() << "] Reject:" << Ver.ParentPkg().FullName() << "=" << Ver.VerStr() << " (" << WhyStr(reason) << ")\n";
-   (*this)[Ver] = {reason, depth(), Decision::MUSTNOT};
-   solved.push_back(Solved{Var(Ver), std::nullopt});
+   if (not Enqueue(Var(Ver), false, reason))
+      return false;
    if (auto pkg = Ver.ParentPkg(); (*this)[pkg].decision != Decision::MUSTNOT)
    {
       bool anyInstallable = false;
@@ -478,8 +496,8 @@ bool APT::Solver::Reject(pkgCache::VerIterator Ver, Var reason, Group group)
       }
       else if ((*this)[Ver.ParentPkg()].decision != Decision::MUSTNOT) // Last installable invalidated
       {
-	 (*this)[Ver.ParentPkg()] = {Var(Ver), depth(), Decision::MUSTNOT};
-	 solved.push_back(Solved{Var(Ver), std::nullopt});
+	 if (not Enqueue(Var(Ver.ParentPkg()), false, Var(Ver)))
+	    return false;
       }
    }
 
diff --git a/apt-pkg/solver3.h b/apt-pkg/solver3.h
index b0ced904a..527e6fc22 100644
--- a/apt-pkg/solver3.h
+++ b/apt-pkg/solver3.h
@@ -176,6 +176,11 @@ class Solver
    // \brief Basic solver initializer. This cannot fail.
    Solver(pkgCache &Cache, pkgDepCache::Policy &Policy);
 
+   // Assume that the variable is decided as specified.
+   bool Assume(Var var, bool decision, Var reason);
+   // Enqueue a decision fact
+   bool Enqueue(Var var, bool decision, Var reason);
+
    // \brief Mark the package for install. This is annoying as it incurs a decision
    bool Install(pkgCache::PkgIterator Pkg, Var reason, Group group);
    // \brief Install a version.
@@ -241,6 +246,15 @@ struct APT::Solver::Var
    {
       return IsVersion == 0 && MapPtr == 0;
    }
+
+   std::string toString(pkgCache &cache) const
+   {
+      if (auto P = Pkg(cache); not P.end())
+	 return P.FullName();
+      if (auto V = Ver(cache); not V.end())
+	 return V.ParentPkg().FullName() + "=" + V.VerStr();
+      return "(root)";
+   }
 };
 
 /**