JCIL/JavaClass/Constants.cs

using System.Text;

namespace JCIL.Java.Class;

public enum ConstantType
{
    Class = 7,
    FieldRef = 9,
    MethodRef = 10,
    InterfaceMethodRef = 11,
    String = 8,
    Integer = 3,
    Float = 4,
    Long = 5,
    Double = 6,
    NameAndType = 12,
    Utf8 = 1,
    MethodHandle = 15,
    MethodType = 16,
    InvokeDynamic = 18,
}

public sealed record NameAndType(string Name, string Descriptor);

public sealed record MethodRef(Class Class, NameAndType Method);
public sealed record FieldRef(Class Class, NameAndType Field);

public record struct Constant(ConstantType Type, object Value)
{
    public static Constant Read(Class self, TypeLoader loader, Reader reader)
    {
        object value;
        var type = (ConstantType)reader.ReadUInt8();
        switch (type)
        {

            case ConstantType.Class:
            case ConstantType.String:
            case ConstantType.MethodType:
            {
                value = reader.ReadUInt16();
                break;
            }

            case ConstantType.Integer:
            {
                value = reader.ReadInt32();
                break;
            }

            case ConstantType.Float:
            {
                value = reader.ReadFloat();
                break;
            }

            case ConstantType.Long:
            {
                value = reader.ReadInt64();
                break;
            }

            case ConstantType.Double:
            {
                value = reader.ReadDouble();
                break;
            }

            case ConstantType.FieldRef:
            case ConstantType.MethodRef:
            case ConstantType.NameAndType:
            case ConstantType.InvokeDynamic:
            case ConstantType.InterfaceMethodRef:
            {
                value = (reader.ReadUInt16(), reader.ReadUInt16());
                break;
            }

            case ConstantType.Utf8:
            {
                var length = reader.ReadUInt16();
                var buffer = new byte[length];
                reader.Stream.ReadExactly(buffer);
                value = Encoding.UTF8.GetString(buffer);
                break;
            }

            case ConstantType.MethodHandle:
            {
                value = (reader.ReadUInt8(), reader.ReadUInt16());
                break;
            }

        default:
            {
                value = null!;
                break;
            }
        }
        return new Constant(type, value);
    }
    
    public static object? Resolve(TypeLoader loader, Constant[] constants, ushort i)
    {
        if (i == 0 || i > constants.Length) return null;
        i -= 1;
        switch (constants[i])
        {
            case { Type: ConstantType.Class, Value: ushort index }:
            {
                var name = (string) Resolve(loader, constants, index)!;
                constants[i] = new Constant(ConstantType.Class, loader.GetClass(name.AsMemory()));
                break;
            }

            case { Type: ConstantType.MethodRef, Value: (ushort classIndex, ushort nameTypeIndex) }:
            {
                var name = (Class)Resolve(loader, constants, classIndex)!;
                var descriptor = (NameAndType)Resolve(loader, constants, nameTypeIndex)!;
                constants[i] = new Constant(ConstantType.MethodRef, new MethodRef(name, descriptor));
                break;
            }
            
            case { Type: ConstantType.FieldRef, Value: (ushort classIndex, ushort nameTypeIndex) }:
            {
                var name = (Class)Resolve(loader, constants, classIndex)!;
                var descriptor = (NameAndType)Resolve(loader, constants, nameTypeIndex)!;
                constants[i] = new Constant(ConstantType.FieldRef, new FieldRef(name, descriptor));
                break;
            }
            
            case { Type: ConstantType.InterfaceMethodRef, Value: (ushort classIndex, ushort nameTypeIndex) }:
            {
                var name = (Class)Resolve(loader, constants, classIndex)!;
                var descriptor = (NameAndType)Resolve(loader, constants, nameTypeIndex)!;
                constants[i] = new Constant(ConstantType.InterfaceMethodRef, new MethodRef(name, descriptor));
                break;
            }

            case { Type: ConstantType.String, Value: ushort index }:
            {
                var text = (string)Resolve(loader, constants, index)!;
                constants[i] = new Constant(ConstantType.String, text);
                break;
            }

            case { Type: ConstantType.NameAndType, Value: (ushort nameIndex, ushort descriptorIndex) }:
            {
                var name = (string)Resolve(loader, constants, nameIndex)!;
                var descriptor = (string)Resolve(loader, constants, descriptorIndex)!;
                constants[i] = new Constant(ConstantType.NameAndType, new NameAndType(name, descriptor));
                break;
            }
        }

        return constants[i].Value;
    }
}