using System;
using System.Reflection;
using System.Reflection.Emit;

namespace MonoMod.Utils
{
    public delegate object? FastReflectionDelegate(object? target, params object?[] args);

    public static class FastReflectionHelper
    {
        private static readonly Type[] _dynamicMethodDelegateArgs = { typeof(object), typeof(object?[]) };

        public static FastReflectionDelegate CreateFastDelegate(MethodBase method, bool directBoxValueAccess = true)
        {
            var dm = new DynamicMethod($"FastReflection<{method}>", typeof(object), _dynamicMethodDelegateArgs);
            var il = dm.GetILGenerator();

            var args = method.GetParameters();

            var generateLocalBoxValuePtr = true;

            if (!method.IsStatic)
            {
                il.Emit(OpCodes.Ldarg_0);

                if (method.DeclaringType?.IsValueType ?? false)
                {
                    il.Emit(OpCodes.Unbox_Any, method.DeclaringType);
                }
            }

            for (var i = 0; i < args.Length; i++)
            {
                var argType = args[i].ParameterType;
                var argIsByRef = argType.IsByRef;

                if (argIsByRef)
                {
                    argType = argType.GetElementType()!;
                }

                var argIsValueType = argType.IsValueType;

                if (argIsByRef && argIsValueType && !directBoxValueAccess)
                {
                    // Used later when storing back the reference to the new box in the array.
                    il.Emit(OpCodes.Ldarg_1);
                    il.Emit(OpCodes.Ldc_I4, i);
                }

                il.Emit(OpCodes.Ldarg_1);
                il.Emit(OpCodes.Ldc_I4, i);

                if (argIsByRef && !argIsValueType)
                {
                    il.Emit(OpCodes.Ldelema, typeof(object));
                }
                else
                {
                    il.Emit(OpCodes.Ldelem_Ref);

                    if (argIsValueType)
                    {
                        if (!argIsByRef || !directBoxValueAccess)
                        {
                            // if !directBoxValueAccess, create a new box if required
                            il.Emit(OpCodes.Unbox_Any, argType);

                            if (argIsByRef)
                            {
                                // box back
                                il.Emit(OpCodes.Box, argType);

                                // store new box value address to local 0
                                il.Emit(OpCodes.Dup);
                                il.Emit(OpCodes.Unbox, argType);

                                if (generateLocalBoxValuePtr)
                                {
                                    generateLocalBoxValuePtr = false;
                                    il.DeclareLocal(typeof(void*));
                                }

                                il.Emit(OpCodes.Stloc_0);

                                // arr and index set up already
                                il.Emit(OpCodes.Stelem_Ref);

                                // load address back to stack
                                il.Emit(OpCodes.Ldloc_0);
                            }
                        }
                        else
                        {
                            // if directBoxValueAccess, emit unbox (get value address)
                            il.Emit(OpCodes.Unbox, argType);
                        }
                    }
                }
            }

            if (method.IsConstructor)
            {
                il.Emit(OpCodes.Newobj, (ConstructorInfo) method);
            }
            else if (method.IsFinal || !method.IsVirtual)
            {
                il.Emit(OpCodes.Call, (MethodInfo) method);
            }
            else
            {
                il.Emit(OpCodes.Callvirt, (MethodInfo) method);
            }

            var returnType = method.IsConstructor ? method.DeclaringType! : ((MethodInfo) method).ReturnType;

            if (returnType != typeof(void))
            {
                if (returnType.IsValueType)
                {
                    il.Emit(OpCodes.Box, returnType);
                }
            }
            else
            {
                il.Emit(OpCodes.Ldnull);
            }

            il.Emit(OpCodes.Ret);

            return (FastReflectionDelegate) dm.CreateDelegate(typeof(FastReflectionDelegate));
        }

    }
}