JCIL/JavaClass/TypeLoader.cs

using System.Collections.Concurrent;
using System.Runtime.CompilerServices;

namespace JCIL.Java.Class;

public sealed class TypeLoader
{
    private readonly ConcurrentDictionary<ReadOnlyMemory<char>, string> _files =
        new(ReadOnlyMemoryCharComparer.Instance);

    private readonly ConcurrentDictionary<ReadOnlyMemory<char>, Class> _classes =
        new(ReadOnlyMemoryCharComparer.Instance);

    public Class Object
    {
        get
        {
            field ??= LoadClass("java/lang/Object".AsMemory());
            return field;
        }
    }

    public void AddPath(string path)
    {
        var basePath = path.AsSpan().TrimEnd(Path.DirectorySeparatorChar);
        foreach (var file in Directory.EnumerateFiles(path, "*.class", SearchOption.AllDirectories))
        {
            var name = file[(basePath.Length + 1)..^6];
            _files.AddOrUpdate(name.AsMemory(), file, (_, v) => v);
        }
    }

    public Class GetClass(ReadOnlyMemory<char> className)
    {
        return _classes.TryGetValue(className, out var existing) ? existing : GetClass(className, out _);
    }

    public Class GetClass(ReadOnlyMemory<char> className, out ReadOnlyMemory<char> rest)
    {
        var firstChar = className.Span[0];
        rest = className[1..];

        switch (firstChar)
        {
            // Init intrinsics
            case 'Z':
                return _classes.GetOrAdd("Z".AsMemory(), _ => new Class(this)
                {
                    Name = "boolean".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Boolean)
                });
            case 'B':
                return _classes.GetOrAdd("B".AsMemory(), _ => new Class(this)
                {
                    Name = "byte".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Byte)
                });
            case 'C':
                return _classes.GetOrAdd("C".AsMemory(), _ => new Class(this)
                {
                    Name = "char".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Char)
                });
            case 'S':
                return _classes.GetOrAdd("S".AsMemory(), _ => new Class(this)
                {
                    Name = "short".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Short)
                });
            case 'I':
                return _classes.GetOrAdd("I".AsMemory(), _ => new Class(this)
                {
                    Name = "int".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Int)
                });
            case 'J':
                return _classes.GetOrAdd("J".AsMemory(), _ => new Class(this)
                {
                    Name = "long".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Long)
                });
            case 'F':
                return _classes.GetOrAdd("F".AsMemory(), _ => new Class(this)
                {
                    Name = "float".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Float)
                });
            case 'D':
                return _classes.GetOrAdd("D".AsMemory(), _ => new Class(this)
                {
                    Name = "double".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Double)
                });
            case 'V':
                return _classes.GetOrAdd("V".AsMemory(), _ => new Class(this)
                {
                    Name = "void".AsMemory(),
                    Namespace = "java/lang".AsMemory(),
                    SpecialClassMetadata = new IntrinsicMetadata(IntrinsicType.Void)
                });

            case 'L' when className.Span.IndexOf('<') is var genericsStart and > 0:
            {
                var theName = className.Span;
                var semicolon = theName.IndexOf(';');
                if (semicolon < genericsStart)
                {
                    className = className[..(semicolon + 1)];
                    rest = rest[semicolon..];
                    return GetClass(className);
                }

                className = rest[..(genericsStart - 1)];
                rest = rest[genericsStart..];

                var loop = true;
                var generics = new List<Class>();
                while (loop)
                {
                    switch (rest.Span[0])
                    {
                        case '>': loop = false; break;
                        case '*':
                            generics.Add(Object);
                            rest = rest[1..];
                            break;
                        default: generics.Add(GetClass(rest, out rest)); break;
                    }
                }

                rest = rest[2..];

                return LoadClass(className).WithGenerics(generics);
            }

            case 'L':
            {
                className = rest[..rest.Span.IndexOf(';')];
                rest = rest[(className.Length + 1)..];
                return LoadClass(className);
            }

            case 'T':
            {
                className = rest[..rest.Span.IndexOf(';')];
                rest = rest[(className.Length + 1)..];
                // TODO Actually load a type from context
                return Object;
            }

            case '[':
            {
                var arrayName = rest;
                var arrayElem = GetClass(arrayName, out rest);
                arrayName = arrayName[..^rest.Length];
                arrayName = className[..(arrayName.Length + 1)];
                return _classes.GetOrAdd(arrayName, _ =>
                {
                    var arrClass = new Class(this)
                    {
                        Name = arrayName,
                        SuperClass = Object,
                        SpecialClassMetadata = new ArrayMetadata(arrayElem),
                    };
                    return arrClass;
                });
            }

            case '(':
            {
                var args = new List<Class>();
                while (rest.Span[0] != ')') args.Add(GetClass(rest, out rest));
                rest = rest[1..];
                var retClass = GetClass(rest, out rest);

                className = className[..^rest.Length];
                return _classes.GetOrAdd(className, _ => new Class(this)
                {
                    SpecialClassMetadata = new MethodSignature(retClass, args)
                });
            }

            // TODO Handle this somehow
            case '+': return GetClass(rest, out rest);
            case '-': return GetClass(rest, out rest);

            default:
            {
                rest = default;
                return LoadClass(className);
            }
        }
    }

    public Class LoadClass(ReadOnlyMemory<char> className)
    {
        var mustLoad = false;
        var javaClass = _classes.GetOrAdd(className, _ =>
        {
            mustLoad = true;
            var javaClass = new Class(this);
            Monitor.Enter(javaClass);
            return javaClass;
        });

        if (mustLoad)
        {
            if (!_files.TryGetValue(className, out var classFile))
                throw new FileNotFoundException($"{className}.class was not found");

            // Console.WriteLine($"Loading class {className}");
            using var file = File.OpenRead(classFile);
            using var reader = new BufferedStream(file);
            javaClass.Read(reader);
            Monitor.Exit(javaClass);
        }
        else
        {
            lock (javaClass)
            {
                // Wait for it to be loaded
            }
        }

        return javaClass;
    }

    private static ReadOnlyMemory<char> SliceFromSpan(ReadOnlyMemory<char> memory, ReadOnlySpan<char> span)
    {
        var mem = memory.Span;
        var start = Unsafe.ByteOffset(in mem[0], in span[0]) / sizeof(char);
        return memory.Slice((int)start, span.Length);
    }
}

internal class ReadOnlyMemoryCharComparer : IEqualityComparer<ReadOnlyMemory<char>>
{
    public static readonly ReadOnlyMemoryCharComparer Instance = new();

    public bool Equals(ReadOnlyMemory<char> x, ReadOnlyMemory<char> y)
        => x.Span.SequenceEqual(y.Span);

    public int GetHashCode(ReadOnlyMemory<char> obj)
        => string.GetHashCode(obj.Span);
}