gson-compile/gson-compile-processor/src/main/java/io/gitlab/jfronny/gson/compile/processor/util/valueprocessor/ConstructionSource.java

package io.gitlab.jfronny.gson.compile.processor.util.valueprocessor;

import org.jetbrains.annotations.ApiStatus;

import javax.lang.model.element.*;
import javax.lang.model.type.TypeKind;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.util.ElementFilter;
import javax.lang.model.util.Types;
import java.util.Locale;
import java.util.Objects;

/**
 * How a [Value] can be constructed. Either a constructor, factory method, or builder.
 */
public sealed interface ConstructionSource {
    /**
     * The target [Value] class to construct.
     */
    TypeElement getTargetClass();
    /**
     * The executable element to construct the [Value]. This may be a constructor, factory method, or builder.
     */
    ExecutableElement getConstructionElement();
    /**
     * If this source is a constructor (either of the value or the builder).
     */
    boolean isConstructor();
    /**
     * If this source is a builder.
     */
    boolean isBuilder();

    final class Constructor implements ConstructionSource {
        private final ExecutableElement constructor;
        private TypeElement targetClass;

        public Constructor(ExecutableElement constructor) {
            this.constructor = Objects.requireNonNull(constructor);
        }

        @Override
        public TypeElement getTargetClass() {
            return targetClass != null ? targetClass : (targetClass = (TypeElement) constructor.enclosingElement);
        }

        @Override
        public ExecutableElement getConstructionElement() {
            return constructor;
        }

        @Override
        public boolean isConstructor() {
            return true;
        }

        @Override
        public boolean isBuilder() {
            return false;
        }
    }

    final class Factory implements ConstructionSource {
        private final Types types;
        private final ExecutableElement method;
        private TypeElement targetClass;

        public Factory(Types types, ExecutableElement method) {
            this.types = types;
            this.method = method;
        }

        @Override
        public TypeElement getTargetClass() {
            return targetClass != null ? targetClass : (targetClass = (TypeElement) types.asElement(method.returnType));
        }

        @Override
        public ExecutableElement getConstructionElement() {
            return method;
        }

        @Override
        public boolean isConstructor() {
            return false;
        }

        @Override
        public boolean isBuilder() {
            return false;
        }
    }

    sealed abstract class Builder implements ConstructionSource {
        public abstract TypeElement getBuilderClass();
        public abstract ExecutableElement getBuildMethod();

        @Override
        public boolean isBuilder() {
            return true;
        }
    }

    final class BuilderConstructor extends Builder {
        private final Types types;
        private final ExecutableElement constructor;
        private TypeElement targetClass;
        private TypeElement builderClass;
        private ExecutableElement buildMethod;

        public BuilderConstructor(Types types, ExecutableElement constructor) {
            this.types = types;
            this.constructor = constructor;
        }

        @Override
        public TypeElement getTargetClass() {
            return targetClass != null ? targetClass : (targetClass = (TypeElement) types.asElement(getBuildMethod().returnType));
        }

        @Override
        public ExecutableElement getConstructionElement() {
            return constructor;
        }

        @Override
        public boolean isConstructor() {
            return true;
        }

        @Override
        public TypeElement getBuilderClass() {
            return builderClass != null ? builderClass : (builderClass = (TypeElement) constructor.enclosingElement);
        }

        @Override
        public ExecutableElement getBuildMethod() {
            return buildMethod != null ? buildMethod : (buildMethod = findBuildMethod((TypeElement) constructor.enclosingElement));
        }
    }

    final class BuilderFactory extends Builder {
        private final Types types;
        private final ExecutableElement method;
        private TypeElement targetClass;
        private TypeElement builderClass;
        private ExecutableElement buildMethod;

        public BuilderFactory(Types types, ExecutableElement method) {
            this.types = types;
            this.method = method;
        }

        @Override
        public TypeElement getTargetClass() {
            return targetClass != null ? targetClass : (targetClass = (TypeElement) types.asElement(getBuildMethod().returnType));
        }

        @Override
        public ExecutableElement getConstructionElement() {
            return method;
        }

        @Override
        public boolean isConstructor() {
            return false;
        }

        @Override
        public TypeElement getBuilderClass() {
            return builderClass != null ? builderClass : (builderClass = (TypeElement) types.asElement(method.returnType));
        }

        @Override
        public ExecutableElement getBuildMethod() {
            return buildMethod != null ? buildMethod : (buildMethod = findBuildMethod((TypeElement) types.asElement(method.returnType)));
        }
    }

    @ApiStatus.Internal
    static ExecutableElement findBuildMethod(TypeElement builderClass) {
        // Ok, maybe there is just one possible builder method.
        {
            ExecutableElement candidate = null;
            boolean foundMultipleCandidates = false;
            boolean isCandidateReasonableBuilderMethodName = false;
            for (ExecutableElement method : ElementFilter.methodsIn(builderClass.enclosedElements)) {
                if (isPossibleBuilderMethod(method, builderClass)) {
                    if (candidate == null) {
                        candidate = method;
                    } else {
                        // Multiple possible methods, keep the one with a reasonable builder name if possible.
                        foundMultipleCandidates = true;
                        isCandidateReasonableBuilderMethodName = isCandidateReasonableBuilderMethodName || isReasonableBuilderMethodName(candidate);
                        if (isCandidateReasonableBuilderMethodName) {
                            if (isReasonableBuilderMethodName(method)) {
                                // both reasonable, too ambiguous.
                                candidate = null;
                                break;
                            }
                        } else {
                            candidate = method;
                        }
                    }
                }
            }
            if (candidate != null && (!foundMultipleCandidates || isCandidateReasonableBuilderMethodName)) {
                return candidate;
            }
        }
        // Last try, check to see if the immediate parent class makes sense.
        {
            Element candidate = builderClass.enclosingElement;
            if (candidate.kind == ElementKind.CLASS) {
                for (ExecutableElement method : ElementFilter.methodsIn(builderClass.enclosedElements)) {
                    if (method.returnType.equals(candidate.asType()) && method.parameters.isEmpty) {
                        return method;
                    }
                }
            }
        }
        // Well, I give up.
        return null;
    }

    /**
     * A possible builder method has no parameters and a return type of the class we want to
     * construct. Therefore, the return type is not going to be void, primitive, or a platform
     * class.
     */
    @ApiStatus.Internal
    static boolean isPossibleBuilderMethod(ExecutableElement method, TypeElement builderClass) {
        if (!method.parameters.isEmpty) return false;
        TypeMirror returnType = method.returnType;
        if (returnType.kind == TypeKind.VOID) return false;
        if (returnType.kind.isPrimitive) return false;
        if (returnType.equals(builderClass.asType())) return false;
        String returnTypeName = returnType.toString();
        return !(returnTypeName.startsWith("java.") || returnTypeName.startsWith("javax.") || returnTypeName.startsWith("android."));
    }

    @ApiStatus.Internal
    static boolean isReasonableBuilderMethodName(ExecutableElement method) {
        String methodName = method.simpleName.toString().toLowerCase(Locale.ROOT);
        return methodName.startsWith("build") || methodName.startsWith("create");
    }
}