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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.validation;

import static java.util.Objects.isNull;
import static java.util.Objects.requireNonNull;
import static java.util.stream.Collectors.toUnmodifiableList;
import static org.apache.commons.lang3.StringUtils.isBlank;
import static org.kiwiproject.base.KiwiStrings.f;
import static org.kiwiproject.collect.KiwiLists.isNullOrEmpty;

import com.google.common.base.Strings;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
import java.util.List;
import java.util.Locale;

/**
 * Validates that a string value is in the list of enum constants, or a value derived from an enum constant.
 */
public class InEnumValidator implements ConstraintValidator {

    private InEnum inEnum;
    private List allowableValues;
    private String errorMessage;

    /**
     * @param constraintAnnotation the InEnum annotation
     * @throws IllegalStateException if the enum has no constants or if using a valueMethod which throws an exception
     *                               when called
     */
    @Override
    public void initialize(InEnum constraintAnnotation) {
        this.inEnum = constraintAnnotation;

        var constants = List.of(inEnum.enumClass().getEnumConstants());
        this.allowableValues = constants.stream()
                .map(anEnum -> enumConstantOrStringValue(inEnum, anEnum))
                .map(stringValue -> uppercaseIfIgnoringCase(inEnum, stringValue))
                .collect(toUnmodifiableList());
        checkAllowableValuesExist();

        this.errorMessage = inEnum.message() + " " + allowableValues;
    }

    private String enumConstantOrStringValue(InEnum inEnum, Enum enumValue) {
        var valueMethod = inEnum.valueMethod();
        if (isBlank(valueMethod)) {
            return enumValue.name();
        }

        var enumClass = enumValue.getClass();
        try {
            var method = enumClass.getDeclaredMethod(valueMethod);
            return String.valueOf(method.invoke(enumValue));
        } catch (Exception e) {
            var message = f("Unable to invoke valueMethod '{}' on class {}. Is it a public no-arg method?",
                    valueMethod, enumClass.getName());
            throw new IllegalStateException(message);
        }
    }

    private String uppercaseIfIgnoringCase(InEnum inEnum, String value) {
        if (inEnum.ignoreCase()) {
            return uppercase(value);
        }

        return value;
    }

    private String uppercase(String value) {
        return Strings.nullToEmpty(value).toUpperCase(Locale.getDefault());
    }

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        checkAllowableValuesExist();  // just in case

        if (isNull(value)) {
            if (inEnum.allowNull()) {
                return true;
            }

            KiwiValidations.addError(context, errorMessage);
            return false;
        }

        var nonNullValue = valueToCompare(value);
        if (allowableValues.contains(nonNullValue)) {
            return true;
        }

        KiwiValidations.addError(context, errorMessage);
        return false;
    }

    private void checkAllowableValuesExist() {
        if (isNullOrEmpty(allowableValues)) {
            var message = f("Enum {} has no values to validate against!", inEnum.enumClass().getSimpleName());
            throw new IllegalStateException(message);
        }
    }

    private String valueToCompare(String value) {
        requireNonNull(value);
        return inEnum.ignoreCase() ? uppercase(value) : value;
    }

}