add ability to extend validation tester with functional validations

src/main/java/it/aboutbits/springboot/testing/validation/core/BaseRuleBuilder.java

@@ -46,6 +46,9 @@ public abstract class BaseRuleBuilder<R extends BaseRuleBuilder<R>> implements
     @Getter(AccessLevel.PACKAGE)
     private final List<Rule> rules = new ArrayList<>();
 
+    @Getter(AccessLevel.PACKAGE)
+    private final List<CustomValidationFunction> validationFunctions = new ArrayList<>();
+
     @Setter(AccessLevel.PACKAGE)
     private Runnable triggerValidation;
 
@@ -54,6 +57,11 @@ public void addRule(@NonNull Rule rule) {
         rules.add(rule);
     }
 
+    @Override
+    public void addValidationFunction(@NonNull CustomValidationFunction function) {
+        validationFunctions.add(function);
+    }
+
     public <T extends BaseRuleBuilder<T>> T withAdditionalRules(Consumer<T> registrar) {
         var self = (T) this;
         registrar.accept(self);

src/main/java/it/aboutbits/springboot/testing/validation/core/BaseValidationAssert.java

@@ -133,6 +133,7 @@ private void assertValidation() {
                         parameterUnderTest,
                         functionToCallWithParameter,
                         ruleBuilder.getRules(),
+                        ruleBuilder.getValidationFunctions(),
                         NON_BEAN_TYPES
                 )
         );

src/main/java/it/aboutbits/springboot/testing/validation/core/CustomValidationFunction.java

@@ -0,0 +1,8 @@
+package it.aboutbits.springboot.testing.validation.core;
+
+import java.util.function.Function;
+
+public interface CustomValidationFunction extends Function<Object, CustomValidationFunction.Result> {
+    record Result(boolean valid, String message) {
+    }
+}

src/main/java/it/aboutbits/springboot/testing/validation/core/RuleValidator.java

@@ -50,6 +50,8 @@ record AssertionParameter<P>(
             @NonNull
             List<Rule> rules,
             @NonNull
+            List<CustomValidationFunction> functions,
+            @NonNull
             Set<Class<?>> nonBeanTypes
     ) {
     }
@@ -59,11 +61,22 @@ void assertValidation(AssertionParameter<P> assertionParameter) {
         @SuppressWarnings("unchecked")
         var functionToCallWithParameter = (Consumer<P>) assertionParameter.functionToCallWithParameter();
         var rules = assertionParameter.rules();
+        var functions = assertionParameter.functions();
         var nonBeanTypes = assertionParameter.nonBeanTypes();
 
-        assertThat(rules)
+        var rulesAndFunctions = rules.size() + functions.size();
+
+        assertThat(rulesAndFunctions)
                 .withFailMessage("Validation failed: no rules were defined.")
-                .isNotEmpty();
+                .isNotZero();
+
+        for (var function : functions) {
+            var result = function.apply(parameterUnderTest);
+            assertThat(result.valid())
+                    .withFailMessage(
+                            "Validation failed: %s", result.message())
+                    .isTrue();
+        }
 
         var validator = VALIDATOR_FACTORY.getValidator();
 
@@ -126,14 +139,18 @@ private static <P> void assertThatValidationIsCompliantForEachProperty(
                                          )
                             );
 
-                    assertThat(violations)
+                    var violatingProperties = violations.stream().map(
+                            f -> f.getPropertyPath().toString()
+                    ).collect(Collectors.toSet());
+
+                    assertThat(violatingProperties)
                             .withFailMessage(
                                     "More than one property failed to validate during mutation. The supplied parameter possibly contains invalid values: %s",
                                     violatingFieldMessages.collect(Collectors.joining(" | "))
                             )
                             .hasSizeLessThan(2);
 
-                    assertThat(violations)
+                    assertThat(violatingProperties)
                             .withFailMessage("Validation failed for property: " + rule.getProperty() + " [" + alteredValue + "]")
                             .hasSize(1);
                 }

src/main/java/it/aboutbits/springboot/testing/validation/core/ValidationRulesData.java

@@ -4,4 +4,6 @@
 
 public interface ValidationRulesData {
     void addRule(@NonNull Rule rule);
+
+    void addValidationFunction(@NonNull CustomValidationFunction function);
 }