Add an option to merge placeholder contexts

honey-common/src/dev/shiza/honey/placeholder/evaluator/PlaceholderContext.java

@@ -61,4 +61,13 @@ public Map<String, Object> getValues() {
   public Map<String, CompletableFuture<?>> getPromisedValues() {
     return promisedValues;
   }
+
+  public PlaceholderContext merge(final PlaceholderContext context) {
+    final PlaceholderContext newContext = new PlaceholderContext();
+    newContext.withValues(getValues());
+    newContext.withValues(context.getValues());
+    newContext.withPromisedValues(getPromisedValues());
+    newContext.withPromisedValues(context.getPromisedValues());
+    return newContext;
+  }
 }

honey-common/test/dev/shiza/honey/placeholder/evaluator/PlaceholderContextTests.java

@@ -0,0 +1,37 @@
+package dev.shiza.honey.placeholder.evaluator;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class PlaceholderContextTests {
+
+  @Test
+  void mergeContexts() {
+    final PlaceholderContext one =
+        PlaceholderContext.create().withValue("hello", "world").withPromisedValue("hello", "world");
+    final PlaceholderContext two =
+        PlaceholderContext.create().withValue("world", "hello").withPromisedValue("world", "hello");
+    final PlaceholderContext mergedContext = one.merge(two);
+    assertThat(mergedContext.getValues()).hasSize(2);
+    assertThat(mergedContext.getPromisedValues()).hasSize(2);
+  }
+
+  @Test
+  void mergeContextsWithDuplicates() {
+    final PlaceholderContext one = PlaceholderContext.create().withValue("hello", "world");
+    final PlaceholderContext two = PlaceholderContext.create().withValue("hello", "world2");
+    final PlaceholderContext mergedContext = one.merge(two);
+    assertThat(mergedContext.getValues()).hasSize(1);
+    assertThat(mergedContext.getPromisedValues()).isEmpty();
+  }
+
+  @Test
+  void mergeEmptyContexts() {
+    final PlaceholderContext one = PlaceholderContext.create();
+    final PlaceholderContext two = PlaceholderContext.create();
+    final PlaceholderContext mergedContext = one.merge(two);
+    assertThat(mergedContext.getValues()).isEmpty();
+    assertThat(mergedContext.getPromisedValues()).isEmpty();
+  }
+}