From 6283516d6af80485e34909a1445c6ce3852b63df Mon Sep 17 00:00:00 2001
From: Christopher Chianelli <christopher@timefold.ai>
Date: Thu, 29 May 2025 11:53:25 -0400
Subject: [PATCH] chore: fail fast if variable path ends on a fact

---
 .../variable/declarative/RootVariableSource.java | 16 +++++++++++-----
 .../declarative/RootVariableSourceTest.java      | 15 +++++++++++++++
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/core/src/main/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSource.java b/core/src/main/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSource.java
index 5423081c497..52064992abe 100644
--- a/core/src/main/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSource.java
+++ b/core/src/main/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSource.java
@@ -65,7 +65,7 @@ public static <Entity_, Value_> RootVariableSource<Entity_, Value_> from(
         List<List<MemberAccessor>> chainStartingFromSourceVariableList = new ArrayList<>();
         boolean isAfterVariable = false;
         Class<?> currentEntity = rootEntityClass;
-        var factCount = 0;
+        var factCountSinceLastVariable = 0;
 
         for (var iterator = pathIterator(rootEntityClass, variablePath); iterator.hasNext();) {
             var pathPart = iterator.next();
@@ -87,7 +87,7 @@ public static <Entity_, Value_> RootVariableSource<Entity_, Value_> from(
                                 descriptorPolicy);
                 listMemberAccessors.add(memberAccessor);
                 chainToVariable = new ArrayList<>();
-                factCount = 0;
+                factCountSinceLastVariable = 0;
 
                 currentEntity = ConfigUtils.extractGenericTypeParameterOrFail(ShadowSources.class.getSimpleName(),
                         currentEntity,
@@ -115,10 +115,10 @@ public static <Entity_, Value_> RootVariableSource<Entity_, Value_> from(
                     chainStartingFromSourceVariableList.add(chainStartingFromSourceVariable);
 
                     isAfterVariable = true;
-                    factCount = 0;
+                    factCountSinceLastVariable = 0;
                 } else {
-                    factCount++;
-                    if (factCount == 2) {
+                    factCountSinceLastVariable++;
+                    if (factCountSinceLastVariable == 2) {
                         throw new IllegalArgumentException(
                                 "The source path (%s) starting from root entity (%s) referencing multiple facts (%s, %s) in a row."
                                         .formatted(variablePath, rootEntityClass.getSimpleName(),
@@ -156,6 +156,12 @@ public static <Entity_, Value_> RootVariableSource<Entity_, Value_> from(
                             .formatted(variablePath, rootEntityClass.getSimpleName()));
         }
 
+        if (factCountSinceLastVariable != 0) {
+            throw new IllegalArgumentException(
+                    "The source path (%s) starting from root entity class (%s) does not end on a variable."
+                            .formatted(variablePath, rootEntityClass.getSimpleName()));
+        }
+
         for (var variableSourceReference : variableSourceReferences) {
             assertIsValidVariableReference(rootEntityClass, variablePath, variableSourceReference);
         }
diff --git a/core/src/test/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSourceTest.java b/core/src/test/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSourceTest.java
index 443e3a5f66a..79612e6524f 100644
--- a/core/src/test/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSourceTest.java
+++ b/core/src/test/java/ai/timefold/solver/core/impl/domain/variable/declarative/RootVariableSourceTest.java
@@ -428,6 +428,21 @@ void invalidPathNoVariables() {
                         " does not reference any variables.");
     }
 
+    @Test
+    void invalidPathEndOnFact() {
+        assertThatCode(() -> RootVariableSource.from(
+                planningSolutionMetaModel,
+                TestdataInvalidDeclarativeValue.class,
+                "shadow",
+                "previous.fact",
+                DEFAULT_MEMBER_ACCESSOR_FACTORY,
+                DEFAULT_DESCRIPTOR_POLICY))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("The source path (previous.fact)" +
+                        " starting from root entity class (TestdataInvalidDeclarativeValue)" +
+                        " does not end on a variable.");
+    }
+
     @Test
     void invalidPathMultipleFactsInARow() {
         assertThatCode(() -> RootVariableSource.from(