Implement SubFunctionPattern

core/src/main/java/it/cavallium/warppi/math/rules/dsl/patterns/SubFunctionPattern.java

@@ -0,0 +1,31 @@
+package it.cavallium.warppi.math.rules.dsl.patterns;
+
+import it.cavallium.warppi.math.Function;
+import it.cavallium.warppi.math.rules.dsl.Pattern;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * Matches and generates any function as a named sub-function.
+ */
+public class SubFunctionPattern implements Pattern {
+    private final String name;
+
+    public SubFunctionPattern(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public Optional<Map<String, Function>> match(Function function) {
+        HashMap<String, Function> subFunctions = new HashMap<>();
+        subFunctions.put(name, function);
+        return Optional.of(subFunctions);
+    }
+
+    @Override
+    public Function replace(Map<String, Function> subFunctions) {
+        return subFunctions.get(name);
+    }
+}

core/src/test/java/it/cavallium/warppi/math/rules/dsl/PatternTest.java

@@ -1,8 +1,30 @@
 package it.cavallium.warppi.math.rules.dsl;
 
+import it.cavallium.warppi.math.Function;
+import it.cavallium.warppi.math.functions.Number;
+import it.cavallium.warppi.math.functions.Sum;
+import it.cavallium.warppi.math.rules.dsl.patterns.SubFunctionPattern;
 import org.junit.Test;
 
+import java.util.Map;
+import java.util.Optional;
+
 import static org.junit.Assert.*;
 
 public class PatternTest {
+    @Test
+    public void subFunctionPattern() {
+        final Pattern pattern = new SubFunctionPattern("x");
+
+        final Function func = new Sum(
+                null,
+                new Number(null, 1),
+                new Number(null, 2)
+        );
+
+        final Optional<Map<String, Function>> subFunctions = pattern.match(func);
+        assertTrue(subFunctions.isPresent());
+
+        assertEquals(func, pattern.replace(subFunctions.get()));
+    }
 }