Allow different decay values depending on the score function

docs/changelog/91195.yaml

@@ -0,0 +1,6 @@
+pr: 91195
+summary: Allow different decay values depending on the score function
+area: Search
+type: bug
+issues:
+  - 78887

server/src/main/java/org/elasticsearch/index/query/functionscore/DecayFunctionBuilder.java

@@ -79,9 +79,7 @@ protected DecayFunctionBuilder(String fieldName, Object origin, Object scale, Ob
         if (scale == null) {
             throw new IllegalArgumentException("decay function: scale must not be null");
         }
-        if (decay <= 0 || decay >= 1.0) {
-            throw new IllegalStateException("decay function: decay must be in range 0..1!");
-        }
+        validateDecay(decay);
         this.fieldName = fieldName;
         try {
             XContentBuilder builder = XContentFactory.jsonBuilder();
@@ -112,6 +110,15 @@ protected DecayFunctionBuilder(String fieldName, BytesReference functionBytes) {
         this.functionBytes = functionBytes;
     }
 
+    /**
+    * Override this function if you have different validation rules per score function
+    */
+    protected void validateDecay(double decay) {
+        if (decay <= 0 || decay >= 1.0) {
+            throw new IllegalStateException("decay function: decay must be in range (0..1)!");
+        }
+    }
+
     /**
      * Read from a stream.
      */

server/src/main/java/org/elasticsearch/index/query/functionscore/LinearDecayFunctionBuilder.java

@@ -39,6 +39,13 @@ public LinearDecayFunctionBuilder(StreamInput in) throws IOException {
         super(in);
     }
 
+    @Override
+    protected void validateDecay(double decay) {
+        if (decay < 0 || decay >= 1.0) {
+            throw new IllegalStateException("decay function: decay must be in range [0..1)!");
+        }
+    }
+
     @Override
     public String getName() {
         return NAME;

server/src/test/java/org/elasticsearch/index/query/functionscore/ScoreFunctionBuilderTests.java

@@ -40,6 +40,17 @@ public void testIllegalArguments() {
         expectThrows(IllegalArgumentException.class, () -> new ExponentialDecayFunctionBuilder("", "", null, "", randomDouble()));
     }
 
+    public void testDecayValues() {
+        expectThrows(IllegalStateException.class, () -> new ExponentialDecayFunctionBuilder("", "", "", "", 0.0));
+        expectThrows(IllegalStateException.class, () -> new ExponentialDecayFunctionBuilder("", "", "", "", 1.0));
+        expectThrows(IllegalStateException.class, () -> new GaussDecayFunctionBuilder("", "", "", "", 0.0));
+        expectThrows(IllegalStateException.class, () -> new GaussDecayFunctionBuilder("", "", "", "", 1.0));
+        expectThrows(IllegalStateException.class, () -> new LinearDecayFunctionBuilder("", "", "", "", 1.0));
+        expectThrows(IllegalStateException.class, () -> new LinearDecayFunctionBuilder("", "", "", "", -1.0));
+        // should not throw since the score formula allows it
+        new LinearDecayFunctionBuilder("", "", "", "", 0.0);
+    }
+
     public void testRandomScoreFunctionWithSeedNoField() throws Exception {
         RandomScoreFunctionBuilder builder = new RandomScoreFunctionBuilder();
         builder.seed(42);