/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.expression.spel;

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

import org.springframework.asm.ClassWriter;
import org.springframework.asm.MethodVisitor;
import org.springframework.asm.Opcodes;
import org.springframework.util.Assert;

/**
 * Manages the class being generated by the compilation process.
 *
 * <p>Records intermediate compilation state as the bytecode is generated.
 * Also includes various bytecode generation helper functions.
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 4.1
 */
public class CodeFlow implements Opcodes {

        /**
         * Name of the class being generated. Typically used when generating code
         * that accesses freshly generated fields on the generated type.
         */
        private final String className;

        /**
         * The current class being generated.
         */
        private final ClassWriter classWriter;

        /**
         * Record the type of what is on top of the bytecode stack (i.e. the type of the
         * output from the previous expression component). New scopes are used to evaluate
         * sub-expressions like the expressions for the argument values in a method invocation
         * expression.
         */
        private final Stack<ArrayList<String>> compilationScopes;

        /**
         * As SpEL ast nodes are called to generate code for the main evaluation method
         * they can register to add a field to this class. Any registered FieldAdders
         * will be called after the main evaluation function has finished being generated.
         */
        private List<FieldAdder> fieldAdders;

        /**
         * As SpEL ast nodes are called to generate code for the main evaluation method
         * they can register to add code to a static initializer in the class. Any
         * registered ClinitAdders will be called after the main evaluation function
         * has finished being generated.
         */
        private List<ClinitAdder> clinitAdders;

        /**
         * When code generation requires holding a value in a class level field, this
         * is used to track the next available field id (used as a name suffix).
         */
        private int nextFieldId = 1;

        /**
         * When code generation requires an intermediate variable within a method,
         * this method records the next available variable (variable 0 is 'this').
         */
        private int nextFreeVariableId = 1;


        /**
         * Construct a new {@code CodeFlow} for the given class.
         * @param className the name of the class
         * @param classWriter the corresponding ASM {@code ClassWriter}
         */
        public CodeFlow(String className, ClassWriter classWriter) {
                this.className = className;
                this.classWriter = classWriter;
                this.compilationScopes = new Stack<ArrayList<String>>();
                this.compilationScopes.add(new ArrayList<String>());
        }


        /**
         * Push the byte code to load the target (i.e. what was passed as the first argument
         * to CompiledExpression.getValue(target, context))
         * @param mv the visitor into which the load instruction should be inserted
         */
        public void loadTarget(MethodVisitor mv) {
                mv.visitVarInsn(ALOAD, 1);
        }

        /**
         * Push the bytecode to load the EvaluationContext (the second parameter passed to
         * the compiled expression method).
         * @param mv the visitor into which the load instruction should be inserted
         * @since 4.3.4
         */
        public void loadEvaluationContext(MethodVisitor mv) {
                mv.visitVarInsn(ALOAD, 2);
        }

        /**
         * Record the descriptor for the most recently evaluated expression element.
         * @param descriptor type descriptor for most recently evaluated element
         */
        public void pushDescriptor(String descriptor) {
                Assert.notNull(descriptor, "Descriptor must not be null");
                this.compilationScopes.peek().add(descriptor);
        }

        /**
         * Enter a new compilation scope, usually due to nested expression evaluation. For
         * example when the arguments for a method invocation expression are being evaluated,
         * each argument will be evaluated in a new scope.
         */
        public void enterCompilationScope() {
                this.compilationScopes.push(new ArrayList<String>());
        }

        /**
         * Exit a compilation scope, usually after a nested expression has been evaluated. For
         * example after an argument for a method invocation has been evaluated this method
         * returns us to the previous (outer) scope.
         */
        public void exitCompilationScope() {
                this.compilationScopes.pop();
        }

        /**
         * Return the descriptor for the item currently on top of the stack (in the current scope).
         */
        public String lastDescriptor() {
                ArrayList<String> scopes = this.compilationScopes.peek();
                return (!scopes.isEmpty() ? scopes.get(scopes.size() - 1) : null);
        }

        /**
         * If the codeflow shows the last expression evaluated to java.lang.Boolean then
         * insert the necessary instructions to unbox that to a boolean primitive.
         * @param mv the visitor into which new instructions should be inserted
         */
        public void unboxBooleanIfNecessary(MethodVisitor mv) {
                if ("Ljava/lang/Boolean".equals(lastDescriptor())) {
                        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Boolean", "booleanValue", "()Z", false);
                }
        }

        /**
         * Called after the main expression evaluation method has been generated, this
         * method will callback any registered FieldAdders or ClinitAdders to add any
         * extra information to the class representing the compiled expression.
         */
        public void finish() {
                if (this.fieldAdders != null) {
                        for (FieldAdder fieldAdder : this.fieldAdders) {
                                fieldAdder.generateField(this.classWriter, this);
                        }
                }
                if (this.clinitAdders != null) {
                        MethodVisitor mv = this.classWriter.visitMethod(ACC_PUBLIC | ACC_STATIC, "<clinit>", "()V", null, null);
                        mv.visitCode();
                        this.nextFreeVariableId = 0;  // to 0 because there is no 'this' in a clinit
                        for (ClinitAdder clinitAdder : this.clinitAdders) {
                                clinitAdder.generateCode(mv, this);
                        }
                        mv.visitInsn(RETURN);
                        mv.visitMaxs(0,0);  // not supplied due to COMPUTE_MAXS
                        mv.visitEnd();
                }
        }

        /**
         * Register a FieldAdder which will add a new field to the generated
         * class to support the code produced by an ast nodes primary
         * generateCode() method.
         */
        public void registerNewField(FieldAdder fieldAdder) {
                if (this.fieldAdders == null) {
                        this.fieldAdders = new ArrayList<FieldAdder>();
                }
                this.fieldAdders.add(fieldAdder);
        }

        /**
         * Register a ClinitAdder which will add code to the static
         * initializer in the generated class to support the code
         * produced by an ast nodes primary generateCode() method.
         */
        public void registerNewClinit(ClinitAdder clinitAdder) {
                if (this.clinitAdders == null) {
                        this.clinitAdders = new ArrayList<ClinitAdder>();
                }
                this.clinitAdders.add(clinitAdder);
        }

        public int nextFieldId() {
                return this.nextFieldId++;
        }

        public int nextFreeVariableId() {
                return this.nextFreeVariableId++;
        }

        public String getClassName() {
                return this.className;
        }

        @Deprecated
        public String getClassname() {
                return this.className;
        }


        /**
         * Insert any necessary cast and value call to convert from a boxed type to a
         * primitive value.
         * @param mv the method visitor into which instructions should be inserted
         * @param ch the primitive type desired as output
         * @param stackDescriptor the descriptor of the type on top of the stack
         */
        public static void insertUnboxInsns(MethodVisitor mv, char ch, String stackDescriptor) {
                switch (ch) {
                        case 'Z':
                                if (!stackDescriptor.equals("Ljava/lang/Boolean")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Boolean");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Boolean", "booleanValue", "()Z", false);
                                break;
                        case 'B':
                                if (!stackDescriptor.equals("Ljava/lang/Byte")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Byte");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Byte", "byteValue", "()B", false);
                                break;
                        case 'C':
                                if (!stackDescriptor.equals("Ljava/lang/Character")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Character");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Character", "charValue", "()C", false);
                                break;
                        case 'D':
                                if (!stackDescriptor.equals("Ljava/lang/Double")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "doubleValue", "()D", false);
                                break;
                        case 'F':
                                if (!stackDescriptor.equals("Ljava/lang/Float")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Float");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Float", "floatValue", "()F", false);
                                break;
                        case 'I':
                                if (!stackDescriptor.equals("Ljava/lang/Integer")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Integer");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Integer", "intValue", "()I", false);
                                break;
                        case 'J':
                                if (!stackDescriptor.equals("Ljava/lang/Long")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Long");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Long", "longValue", "()J", false);
                                break;
                        case 'S':
                                if (!stackDescriptor.equals("Ljava/lang/Short")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Short");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Short", "shortValue", "()S", false);
                                break;
                        default:
                                throw new IllegalArgumentException("Unboxing should not be attempted for descriptor '" + ch + "'");
                }
        }

        /**
         * For numbers, use the appropriate method on the number to convert it to the primitive type requested.
         * @param mv the method visitor into which instructions should be inserted
         * @param targetDescriptor the primitive type desired as output
         * @param stackDescriptor the descriptor of the type on top of the stack
         */
        public static void insertUnboxNumberInsns(MethodVisitor mv, char targetDescriptor, String stackDescriptor) {
                switch (targetDescriptor) {
                        case 'D':
                                if (stackDescriptor.equals("Ljava/lang/Object")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Number");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Number", "doubleValue", "()D", false);
                                break;
                        case 'F':
                                if (stackDescriptor.equals("Ljava/lang/Object")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Number");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Number", "floatValue", "()F", false);
                                break;
                        case 'J':
                                if (stackDescriptor.equals("Ljava/lang/Object")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Number");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Number", "longValue", "()J", false);
                                break;
                        case 'I':
                                if (stackDescriptor.equals("Ljava/lang/Object")) {
                                        mv.visitTypeInsn(CHECKCAST, "java/lang/Number");
                                }
                                mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Number", "intValue", "()I", false);
                                break;
                        // does not handle Z, B, C, S
                        default:
                                throw new IllegalArgumentException("Unboxing should not be attempted for descriptor '" + targetDescriptor + "'");
                }
        }

        /**
         * Insert any necessary numeric conversion bytecodes based upon what is on the stack and the desired target type.
         * @param mv the method visitor into which instructions should be placed
         * @param targetDescriptor the (primitive) descriptor of the target type
         * @param stackDescriptor the descriptor of the operand on top of the stack
         */
        public static void insertAnyNecessaryTypeConversionBytecodes(MethodVisitor mv, char targetDescriptor, String stackDescriptor) {
                if (CodeFlow.isPrimitive(stackDescriptor)) {
                        char stackTop = stackDescriptor.charAt(0);
                        if (stackTop == 'I' || stackTop == 'B' || stackTop == 'S' || stackTop == 'C') {
                                if (targetDescriptor == 'D') {
                                        mv.visitInsn(I2D);
                                }
                                else if (targetDescriptor == 'F') {
                                        mv.visitInsn(I2F);
                                }
                                else if (targetDescriptor == 'J') {
                                        mv.visitInsn(I2L);
                                }
                                else if (targetDescriptor == 'I') {
                                        // nop
                                }
                                else {
                                        throw new IllegalStateException("Cannot get from " + stackTop + " to " + targetDescriptor);
                                }
                        }
                        else if (stackTop == 'J') {
                                if (targetDescriptor == 'D') {
                                        mv.visitInsn(L2D);
                                }
                                else if (targetDescriptor == 'F') {
                                        mv.visitInsn(L2F);
                                }
                                else if (targetDescriptor == 'J') {
                                        // nop
                                }
                                else if (targetDescriptor == 'I') {
                                        mv.visitInsn(L2I);
                                }
                                else {
                                        throw new IllegalStateException("Cannot get from " + stackTop + " to " + targetDescriptor);
                                }
                        }
                        else if (stackTop == 'F') {
                                if (targetDescriptor == 'D') {
                                        mv.visitInsn(F2D);
                                }
                                else if (targetDescriptor == 'F') {
                                        // nop
                                }
                                else if (targetDescriptor == 'J') {
                                        mv.visitInsn(F2L);
                                }
                                else if (targetDescriptor == 'I') {
                                        mv.visitInsn(F2I);
                                }
                                else {
                                        throw new IllegalStateException("Cannot get from " + stackTop + " to " + targetDescriptor);
                                }
                        }
                        else if (stackTop == 'D') {
                                if (targetDescriptor == 'D') {
                                        // nop
                                }
                                else if (targetDescriptor == 'F') {
                                        mv.visitInsn(D2F);
                                }
                                else if (targetDescriptor == 'J') {
                                        mv.visitInsn(D2L);
                                }
                                else if (targetDescriptor == 'I') {
                                        mv.visitInsn(D2I);
                                }
                                else {
                                        throw new IllegalStateException("Cannot get from " + stackDescriptor + " to " + targetDescriptor);
                                }
                        }
                }
        }


        /**
         * Create the JVM signature descriptor for a method. This consists of the descriptors
         * for the method parameters surrounded with parentheses, followed by the
         * descriptor for the return type. Note the descriptors here are JVM descriptors,
         * unlike the other descriptor forms the compiler is using which do not include the
         * trailing semicolon.
         * @param method the method
         * @return a String signature descriptor (e.g. "(ILjava/lang/String;)V")
         */
        public static String createSignatureDescriptor(Method method) {
                Class<?>[] params = method.getParameterTypes();
                StringBuilder sb = new StringBuilder();
                sb.append("(");
                for (Class<?> param : params) {
                        sb.append(toJvmDescriptor(param));
                }
                sb.append(")");
                sb.append(toJvmDescriptor(method.getReturnType()));
                return sb.toString();
        }

        /**
         * Create the JVM signature descriptor for a constructor. This consists of the
         * descriptors for the constructor parameters surrounded with parentheses, followed by
         * the descriptor for the return type, which is always "V". Note the
         * descriptors here are JVM descriptors, unlike the other descriptor forms the
         * compiler is using which do not include the trailing semicolon.
         * @param ctor the constructor
         * @return a String signature descriptor (e.g. "(ILjava/lang/String;)V")
         */
        public static String createSignatureDescriptor(Constructor<?> ctor) {
                Class<?>[] params = ctor.getParameterTypes();
                StringBuilder sb = new StringBuilder();
                sb.append("(");
                for (Class<?> param : params) {
                        sb.append(toJvmDescriptor(param));
                }
                sb.append(")V");
                return sb.toString();
        }

        /**
         * Determine the JVM descriptor for a specified class. Unlike the other descriptors
         * used in the compilation process, this is the one the JVM wants, so this one
         * includes any necessary trailing semicolon (e.g. Ljava/lang/String; rather than
         * Ljava/lang/String)
         * @param clazz a class
         * @return the JVM descriptor for the class
         */
        public static String toJvmDescriptor(Class<?> clazz) {
                StringBuilder sb = new StringBuilder();
                if (clazz.isArray()) {
                        while (clazz.isArray()) {
                                sb.append("[");
                                clazz = clazz.getComponentType();
                        }
                }
                if (clazz.isPrimitive()) {
                        if (clazz == Boolean.TYPE) {
                                sb.append('Z');
                        }
                        else if (clazz == Byte.TYPE) {
                                sb.append('B');
                        }
                        else if (clazz == Character.TYPE) {
                                sb.append('C');
                        }
                        else if (clazz == Double.TYPE) {
                                sb.append('D');
                        }
                        else if (clazz == Float.TYPE) {
                                sb.append('F');
                        }
                        else if (clazz == Integer.TYPE) {
                                sb.append('I');
                        }
                        else if (clazz == Long.TYPE) {
                                sb.append('J');
                        }
                        else if (clazz == Short.TYPE) {
                                sb.append('S');
                        }
                        else if (clazz == Void.TYPE) {
                                sb.append('V');
                        }
                }
                else {
                        sb.append("L");
                        sb.append(clazz.getName().replace('.', '/'));
                        sb.append(";");
                }
                return sb.toString();
        }

        /**
         * Determine the descriptor for an object instance (or {@code null}).
         * @param value an object (possibly {@code null})
         * @return the type descriptor for the object
         * (descriptor is "Ljava/lang/Object" for {@code null} value)
         */
        public static String toDescriptorFromObject(Object value) {
                if (value == null) {
                        return "Ljava/lang/Object";
                }
                else {
                        return toDescriptor(value.getClass());
                }
        }

        /**
         * Determine whether the descriptor is for a boolean primitive or boolean reference type.
         * @param descriptor type descriptor
         * @return {@code true} if the descriptor is boolean compatible
         */
        public static boolean isBooleanCompatible(String descriptor) {
                return (descriptor != null && (descriptor.equals("Z") || descriptor.equals("Ljava/lang/Boolean")));
        }

        /**
         * Determine whether the descriptor is for a primitive type.
         * @param descriptor type descriptor
         * @return {@code true} if a primitive type
         */
        public static boolean isPrimitive(String descriptor) {
                return (descriptor != null && descriptor.length() == 1);
        }

        /**
         * Determine whether the descriptor is for a primitive array (e.g. "[[I").
         * @param descriptor the descriptor for a possible primitive array
         * @return {@code true} if the descriptor a primitive array
         */
        public static boolean isPrimitiveArray(String descriptor) {
                boolean primitive = true;
                for (int i = 0, max = descriptor.length(); i < max; i++) {
                        char ch = descriptor.charAt(i);
                        if (ch == '[') {
                                continue;
                        }
                        primitive = (ch != 'L');
                        break;
                }
                return primitive;
        }

        /**
         * Determine whether boxing/unboxing can get from one type to the other.
         * Assumes at least one of the types is in boxed form (i.e. single char descriptor).
         * @return {@code true} if it is possible to get (via boxing) from one descriptor to the other
         */
        public static boolean areBoxingCompatible(String desc1, String desc2) {
                if (desc1.equals(desc2)) {
                        return true;
                }
                if (desc1.length() == 1) {
                        if (desc1.equals("Z")) {
                                return desc2.equals("Ljava/lang/Boolean");
                        }
                        else if (desc1.equals("D")) {
                                return desc2.equals("Ljava/lang/Double");
                        }
                        else if (desc1.equals("F")) {
                                return desc2.equals("Ljava/lang/Float");
                        }
                        else if (desc1.equals("I")) {
                                return desc2.equals("Ljava/lang/Integer");
                        }
                        else if (desc1.equals("J")) {
                                return desc2.equals("Ljava/lang/Long");
                        }
                }
                else if (desc2.length() == 1) {
                        if (desc2.equals("Z")) {
                                return desc1.equals("Ljava/lang/Boolean");
                        }
                        else if (desc2.equals("D")) {
                                return desc1.equals("Ljava/lang/Double");
                        }
                        else if (desc2.equals("F")) {
                                return desc1.equals("Ljava/lang/Float");
                        }
                        else if (desc2.equals("I")) {
                                return desc1.equals("Ljava/lang/Integer");
                        }
                        else if (desc2.equals("J")) {
                                return desc1.equals("Ljava/lang/Long");
                        }
                }
                return false;
        }

        /**
         * Determine if the supplied descriptor is for a supported number type or boolean. The
         * compilation process only (currently) supports certain number types. These are
         * double, float, long and int.
         * @param descriptor the descriptor for a type
         * @return {@code true} if the descriptor is for a supported numeric type or boolean
         */
        public static boolean isPrimitiveOrUnboxableSupportedNumberOrBoolean(String descriptor) {
                if (descriptor == null) {
                        return false;
                }
                if (isPrimitiveOrUnboxableSupportedNumber(descriptor)) {
                        return true;
                }
                return ("Z".equals(descriptor) || descriptor.equals("Ljava/lang/Boolean"));
        }

        /**
         * Determine if the supplied descriptor is for a supported number. The compilation
         * process only (currently) supports certain number types. These are double, float,
         * long and int.
         * @param descriptor the descriptor for a type
         * @return {@code true} if the descriptor is for a supported numeric type
         */
        public static boolean isPrimitiveOrUnboxableSupportedNumber(String descriptor) {
                if (descriptor == null) {
                        return false;
                }
                if (descriptor.length() == 1) {
                        return "DFIJ".contains(descriptor);
                }
                if (descriptor.startsWith("Ljava/lang/")) {
                        String name = descriptor.substring("Ljava/lang/".length());
                        if (name.equals("Double") || name.equals("Float") || name.equals("Integer") || name.equals("Long")) {
                                return true;
                        }
                }
                return false;
        }

        /**
         * Determine whether the given number is to be considered as an integer
         * for the purposes of a numeric operation at the bytecode level.
         * @param number the number to check
         * @return {@code true} if it is an {@link Integer}, {@link Short} or {@link Byte}
         */
        public static boolean isIntegerForNumericOp(Number number) {
                return (number instanceof Integer || number instanceof Short || number instanceof Byte);
        }

        /**
         * Convert a type descriptor to the single character primitive descriptor.
         * @param descriptor a descriptor for a type that should have a primitive representation
         * @return the single character descriptor for a primitive input descriptor
         */
        public static char toPrimitiveTargetDesc(String descriptor) {
                if (descriptor.length() == 1) {
                        return descriptor.charAt(0);
                }
                else if (descriptor.equals("Ljava/lang/Boolean")) {
                        return 'Z';
                }
                else if (descriptor.equals("Ljava/lang/Byte")) {
                        return 'B';
                }
                else if (descriptor.equals("Ljava/lang/Character")) {
                        return 'C';
                }
                else if (descriptor.equals("Ljava/lang/Double")) {
                        return 'D';
                }
                else if (descriptor.equals("Ljava/lang/Float")) {
                        return 'F';
                }
                else if (descriptor.equals("Ljava/lang/Integer")) {
                        return 'I';
                }
                else if (descriptor.equals("Ljava/lang/Long")) {
                        return 'J';
                }
                else if (descriptor.equals("Ljava/lang/Short")) {
                        return 'S';
                }
                else {
                        throw new IllegalStateException("No primitive for '" + descriptor + "'");
                }
        }

        /**
         * Insert the appropriate CHECKCAST instruction for the supplied descriptor.
         * @param mv the target visitor into which the instruction should be inserted
         * @param descriptor the descriptor of the type to cast to
         */
        public static void insertCheckCast(MethodVisitor mv, String descriptor) {
                if (descriptor.length() != 1) {
                        if (descriptor.charAt(0) == '[') {
                                if (isPrimitiveArray(descriptor)) {
                                        mv.visitTypeInsn(CHECKCAST, descriptor);
                                }
                                else {
                                        mv.visitTypeInsn(CHECKCAST, descriptor + ";");
                                }
                        }
                        else {
                                if (!descriptor.equals("Ljava/lang/Object")) {
                                        // This is chopping off the 'L' to leave us with "java/lang/String"
                                        mv.visitTypeInsn(CHECKCAST, descriptor.substring(1));
                                }
                        }
                }
        }

        /**
         * Determine the appropriate boxing instruction for a specific type (if it needs
         * boxing) and insert the instruction into the supplied visitor.
         * @param mv the target visitor for the new instructions
         * @param descriptor the descriptor of a type that may or may not need boxing
         */
        public static void insertBoxIfNecessary(MethodVisitor mv, String descriptor) {
                if (descriptor.length() == 1) {
                        insertBoxIfNecessary(mv, descriptor.charAt(0));
                }
        }

        /**
         * Determine the appropriate boxing instruction for a specific type (if it needs
         * boxing) and insert the instruction into the supplied visitor.
         * @param mv the target visitor for the new instructions
         * @param ch the descriptor of the type that might need boxing
         */
        public static void insertBoxIfNecessary(MethodVisitor mv, char ch) {
                switch (ch) {
                        case 'Z':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Boolean", "valueOf", "(Z)Ljava/lang/Boolean;", false);
                                break;
                        case 'B':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Byte", "valueOf", "(B)Ljava/lang/Byte;", false);
                                break;
                        case 'C':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Character", "valueOf", "(C)Ljava/lang/Character;", false);
                                break;
                        case 'D':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Double", "valueOf", "(D)Ljava/lang/Double;", false);
                                break;
                        case 'F':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Float", "valueOf", "(F)Ljava/lang/Float;", false);
                                break;
                        case 'I':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Integer", "valueOf", "(I)Ljava/lang/Integer;", false);
                                break;
                        case 'J':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Long", "valueOf", "(J)Ljava/lang/Long;", false);
                                break;
                        case 'S':
                                mv.visitMethodInsn(INVOKESTATIC, "java/lang/Short", "valueOf", "(S)Ljava/lang/Short;", false);
                                break;
                        case 'L':
                        case 'V':
                        case '[':
                                // no box needed
                                break;
                        default:
                                throw new IllegalArgumentException("Boxing should not be attempted for descriptor '" + ch + "'");
                }
        }

        /**
         * Deduce the descriptor for a type. Descriptors are like JVM type names but missing the
         * trailing ';' so for Object the descriptor is "Ljava/lang/Object" for int it is "I".
         * @param type the type (may be primitive) for which to determine the descriptor
         * @return the descriptor
         */
        public static String toDescriptor(Class<?> type) {
                String name = type.getName();
                if (type.isPrimitive()) {
                        switch (name.length()) {
                                case 3:
                                        return "I";
                                case 4:
                                        if (name.equals("byte")) {
                                                return "B";
                                        }
                                        else if (name.equals("char")) {
                                                return "C";
                                        }
                                        else if (name.equals("long")) {
                                                return "J";
                                        }
                                        else if (name.equals("void")) {
                                                return "V";
                                        }
                                        break;
                                case 5:
                                        if (name.equals("float")) {
                                                return "F";
                                        }
                                        else if (name.equals("short")) {
                                                return "S";
                                        }
                                        break;
                                case 6:
                                        if (name.equals("double")) {
                                                return "D";
                                        }
                                        break;
                                case 7:
                                        if (name.equals("boolean")) {
                                                return "Z";
                                        }
                                        break;
                        }
                }
                else {
                        if (name.charAt(0) != '[') {
                                return "L" + type.getName().replace('.', '/');
                        }
                        else {
                                if (name.endsWith(";")) {
                                        return name.substring(0, name.length() - 1).replace('.', '/');
                                }
                                else {
                                        return name;  // array has primitive component type
                                }
                        }
                }
                return null;
        }

        /**
         * Create an array of descriptors representing the parameter types for the supplied
         * method. Returns a zero sized array if there are no parameters.
         * @param method a Method
         * @return a String array of descriptors, one entry for each method parameter
         */
        public static String[] toParamDescriptors(Method method) {
                return toDescriptors(method.getParameterTypes());
        }

        /**
         * Create an array of descriptors representing the parameter types for the supplied
         * constructor. Returns a zero sized array if there are no parameters.
         * @param ctor a Constructor
         * @return a String array of descriptors, one entry for each constructor parameter
         */
        public static String[] toParamDescriptors(Constructor<?> ctor) {
                return toDescriptors(ctor.getParameterTypes());
        }

        /**
         * Create an array of descriptors from an array of classes.
         * @param types the input array of classes
         * @return an array of descriptors
         */
        public static String[] toDescriptors(Class<?>[] types) {
                int typesCount = types.length;
                String[] descriptors = new String[typesCount];
                for (int p = 0; p < typesCount; p++) {
                        descriptors[p] = toDescriptor(types[p]);
                }
                return descriptors;
        }

        /**
         * Create the optimal instruction for loading a number on the stack.
         * @param mv where to insert the bytecode
         * @param value the value to be loaded
         */
        public static void insertOptimalLoad(MethodVisitor mv, int value) {
                if (value < 6) {
                        mv.visitInsn(ICONST_0+value);
                }
                else if (value < Byte.MAX_VALUE) {
                        mv.visitIntInsn(BIPUSH, value);
                }
                else if (value < Short.MAX_VALUE) {
                        mv.visitIntInsn(SIPUSH, value);
                }
                else {
                        mv.visitLdcInsn(value);
                }
        }

        /**
         * Produce appropriate bytecode to store a stack item in an array. The
         * instruction to use varies depending on whether the type
         * is a primitive or reference type.
         * @param mv where to insert the bytecode
         * @param arrayElementType the type of the array elements
         */
        public static void insertArrayStore(MethodVisitor mv, String arrayElementType) {
                if (arrayElementType.length()==1) {
                        switch (arrayElementType.charAt(0)) {
                                case 'I':
                                        mv.visitInsn(IASTORE);
                                        break;
                                case 'J':
                                        mv.visitInsn(LASTORE);
                                        break;
                                case 'F':
                                        mv.visitInsn(FASTORE);
                                        break;
                                case 'D':
                                        mv.visitInsn(DASTORE);
                                        break;
                                case 'B':
                                        mv.visitInsn(BASTORE);
                                        break;
                                case 'C':
                                        mv.visitInsn(CASTORE);
                                        break;
                                case 'S':
                                        mv.visitInsn(SASTORE);
                                        break;
                                case 'Z':
                                        mv.visitInsn(BASTORE);
                                        break;
                                default:
                                        throw new IllegalArgumentException(
                                                        "Unexpected arraytype " + arrayElementType.charAt(0));
                        }
                }
                else {
                        mv.visitInsn(AASTORE);
                }
        }

        /**
         * Determine the appropriate T tag to use for the NEWARRAY bytecode.
         * @param arraytype the array primitive component type
         * @return the T tag to use for NEWARRAY
         */
        public static int arrayCodeFor(String arraytype) {
                switch (arraytype.charAt(0)) {
                        case 'I': return T_INT;
                        case 'J': return T_LONG;
                        case 'F': return T_FLOAT;
                        case 'D': return T_DOUBLE;
                        case 'B': return T_BYTE;
                        case 'C': return T_CHAR;
                        case 'S': return T_SHORT;
                        case 'Z': return T_BOOLEAN;
                        default:
                                throw new IllegalArgumentException("Unexpected arraytype " + arraytype.charAt(0));
                }
        }

        /**
         * Return if the supplied array type has a core component reference type.
         */
        public static boolean isReferenceTypeArray(String arraytype) {
                int length = arraytype.length();
                for (int i = 0; i < length; i++) {
                        char ch = arraytype.charAt(i);
                        if (ch == '[') {
                                continue;
                        }
                        return (ch == 'L');
                }
                return false;
        }

        /**
         * Produce the correct bytecode to build an array. The opcode to use and the
         * signature to pass along with the opcode can vary depending on the signature
         * of the array type.
         * @param mv the methodvisitor into which code should be inserted
         * @param size the size of the array
         * @param arraytype the type of the array
         */
        public static void insertNewArrayCode(MethodVisitor mv, int size, String arraytype) {
                insertOptimalLoad(mv, size);
                if (arraytype.length() == 1) {
                        mv.visitIntInsn(NEWARRAY, CodeFlow.arrayCodeFor(arraytype));
                }
                else {
                        if (arraytype.charAt(0) == '[') {
                                // Handling the nested array case here.
                                // If vararg is [[I then we want [I and not [I;
                                if (CodeFlow.isReferenceTypeArray(arraytype)) {
                                        mv.visitTypeInsn(ANEWARRAY, arraytype + ";");
                                }
                                else {
                                        mv.visitTypeInsn(ANEWARRAY, arraytype);
                                }
                        }
                        else {
                                mv.visitTypeInsn(ANEWARRAY, arraytype.substring(1));
                        }
                }
        }

        /**
         * For use in mathematical operators, handles converting from a (possibly boxed)
         * number on the stack to a primitive numeric type.
         * <p>For example, from a Integer to a double, just need to call 'Number.doubleValue()'
         * but from an int to a double, need to use the bytecode 'i2d'.
         * @param mv the method visitor when instructions should be appended
         * @param stackDescriptor a descriptor of the operand on the stack
         * @param targetDescriptor a primitive type descriptor
         */
        public static void insertNumericUnboxOrPrimitiveTypeCoercion(
                        MethodVisitor mv, String stackDescriptor, char targetDescriptor) {

                if (!CodeFlow.isPrimitive(stackDescriptor)) {
                        CodeFlow.insertUnboxNumberInsns(mv, targetDescriptor, stackDescriptor);
                }
                else {
                        CodeFlow.insertAnyNecessaryTypeConversionBytecodes(mv, targetDescriptor, stackDescriptor);
                }
        }

        public static String toBoxedDescriptor(String primitiveDescriptor) {
                switch (primitiveDescriptor.charAt(0)) {
                        case 'I': return "Ljava/lang/Integer";
                        case 'J': return "Ljava/lang/Long";
                        case 'F': return "Ljava/lang/Float";
                        case 'D': return "Ljava/lang/Double";
                        case 'B': return "Ljava/lang/Byte";
                        case 'C': return "Ljava/lang/Character";
                        case 'S': return "Ljava/lang/Short";
                        case 'Z': return "Ljava/lang/Boolean";
                        default:
                                throw new IllegalArgumentException("Unexpected non primitive descriptor " + primitiveDescriptor);
                }
        }


        /**
         * Interface used to generate fields.
         */
        @FunctionalInterface
        public interface FieldAdder {

                void generateField(ClassWriter cw, CodeFlow codeflow);
        }


        /**
         * Interface used to generate {@code clinit} static initializer blocks.
         */
        @FunctionalInterface
        public interface ClinitAdder {

                void generateCode(MethodVisitor mv, CodeFlow codeflow);
        }

}