WIP: track which direct deps induced which constraints.

Completely untested. Moving in the direction of error reporting.

packages/constraint-solver/constraint-solver.js

@@ -294,7 +294,7 @@ ConstraintSolver.PackagesResolver.prototype._getResolverOptions =
           var latestFitting = null;
 
           for (var i = versions.length - 1; i >= 0; i--) {
-            if (! constraints.violated(versions[i])) {
+            if (_.isEmpty(constraints.violatedConstraints(versions[i]))) {
               latestFitting = versions[i];
               break;
             }
@@ -349,4 +349,3 @@ ConstraintSolver.PackagesResolver.prototype._getResolverOptions =
 
   return resolverOptions;
 };
-

packages/constraint-solver/constraints-list.js

@@ -111,17 +111,28 @@ ConstraintSolver.ConstraintsList.prototype.union = function (anotherList) {
   return newList;
 };
 
-// Checks if the passed unit version violates any of the constraints
-ConstraintSolver.ConstraintsList.prototype.violated = function (uv) {
+// Checks if the passed unit version violates any of the constraints.
+// Returns a list of constraints that are violated (empty if the unit
+// version does not violate any constraints).
+// XXX Returns a regular array, not a ConstraintsList.
+ConstraintSolver.ConstraintsList.prototype.violatedConstraints = function (uv) {
   var self = this;
   var forPackage = mori.get(self.byName, uv.name);
   var exact = mori.get(forPackage, "exact");
   var inexact = mori.get(forPackage, "inexact");
 
-  if (! mori.every(function (c) { return mori.last(c).isSatisfied(uv); }, exact))
-    return true;
-
-  return ! mori.every(function (c) { return mori.last(c).isSatisfied(uv); }, inexact);
+  var violated = [];
+  mori.each(exact, function (c) {
+    if (! mori.last(c).isSatisified(uv)) {
+      violated.push(c);
+    }
+  });
+  mori.each(inexact, function (c) {
+    if (! mori.last(c).isSatisified(uv)) {
+      violated.push(c);
+    }
+  });
+  return violated;
 };
 
 // a weird method that returns a list of exact constraints those correspond to
@@ -179,4 +190,3 @@ ConstraintSolver.ConstraintsList.fromArray = function (arr) {
 
   return list;
 };
-

packages/constraint-solver/resolver.js

@@ -154,6 +154,11 @@ ConstraintSolver.Resolver.prototype.resolve =
 // - dependencies: DependenciesList - remaining dependencies
 // - constraints: ConstraintsList - constraints to satisfy
 // - choices: array of UnitVersion - current fixed set of choices
+// - constraintAncestors: Constraint (string representation) ->
+//   DependencyList. Used for error reporting to indicate which direct
+//   dependencies have caused a failure. For every constraint, this is
+//   the list of direct dependencies which led to this constraint being
+//   present.
 //
 // returns {
 //   success: Boolean,
@@ -169,19 +174,44 @@ ConstraintSolver.Resolver.prototype._stateNeighbors =
   var dependencies = state.dependencies;
   var constraints = state.constraints;
   var choices = state.choices;
+  var constraintAncestors = state.constraintAncestors;
 
   var candidateName = dependencies.peek();
   dependencies = dependencies.remove(candidateName);
 
   var candidateVersions =
     _.filter(self.unitsVersions[candidateName], function (uv) {
-      return !constraints.violated(uv);
+      return _.isEmpty(constraints.violatedConstraints(uv));
     });
 
-  if (_.isEmpty(candidateVersions))
+  if (_.isEmpty(candidateVersions)) {
+    var uv = self.unitsVersions[candidateName][0];
+    if (! uv) {
+      // XXX Some stupid error message, this should not happen
+    } else {
+      var violatedConstraint = _.filter(
+        constraints.violatedConstraints(uv)[0],
+        function (constraint) {
+          return constraint.name === uv.name;
+        }
+      );
+      // XXX Should we return information about all the violated
+      // constraints instead of just one?
+      var directDeps = constraintAncestors[violatedConstraint.toString()];
+      return {
+        success: false,
+        // XXX We really want to say "directDep1 depends on X@1.0 and
+        // directDep2 depends on X@2.0"
+        failureMsg: "Direct dependencies " + directDeps.toString() +
+          " conflict on " + uv.name + ". " + violatedConstraint.toString()
+      };
+    }
+
+
     return { success: false,
              failureMsg: "Cannot choose satisfying versions of package -- "
                          + candidateName };
+  }
 
   var lastInvalidNeighbor = null;
 
@@ -291,7 +321,7 @@ ConstraintSolver.Resolver.prototype._propagateExactTransDeps =
 
 var choicesDontViolateConstraints = function (choices, constraints) {
   return _.all(choices, function (uv) {
-    return !constraints.violated(uv);
+    return _.isEmpty(constraints.violatedConstraints(uv));
   });
 };
 
@@ -446,4 +476,3 @@ ConstraintSolver.Constraint.prototype.getSatisfyingUnitVersion =
                            _.bind(self.isSatisfied, self));
   return unitVersion;
 };
-