using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Mono.Cecil;
using Mono.Cecil.Cil;

namespace zzzUnity.Burst.CodeGen
{
    /// <summary>
    /// Main class for post processing assemblies. The post processing is currently performing:
    /// - Replace C# call from C# to Burst functions with attributes [BurstCompile] to a call to the compiled Burst function
    ///   In both editor and standalone scenarios. For DOTS Runtime, this is done differently at BclApp level by patching
    ///   DllImport.
    /// - Replace calls to `SharedStatic.GetOrCreate` with `SharedStatic.GetOrCreateUnsafe`, and calculate the hashes during ILPP time
    ///   rather than in static constructors at runtime.
    /// </summary>
    internal class ILPostProcessing
    {
        private AssemblyDefinition _burstAssembly;
        private MethodReference _burstCompilerIsEnabledMethodDefinition;
        private MethodReference _burstCompilerCompileFunctionPointer;
        private FieldReference _burstCompilerOptionsField;
        private TypeReference _burstCompilerOptionsType;
        private TypeReference _functionPointerType;
        private MethodReference _functionPointerGetValue;
        private MethodReference _burstDiscardAttributeConstructor;
        private TypeSystem _typeSystem;
        private TypeReference _systemDelegateType;
        private TypeReference _systemASyncCallbackType;
        private TypeReference _systemIASyncResultType;
        private AssemblyDefinition _assemblyDefinition;
        private bool _modified;
#if !UNITY_DOTSPLAYER
        private bool _containsDirectCall;
#endif
        private readonly StringBuilder _builder = new StringBuilder(1024);
        private readonly List<Instruction> _instructionsToReplace = new List<Instruction>(4);

        public const string PostfixManaged = "$BurstManaged";
        private const string PostfixBurstDirectCall = "$BurstDirectCall";
        private const string PostfixBurstDelegate = "$PostfixBurstDelegate";
        private const string GetFunctionPointerName = "GetFunctionPointer";
        private const string GetFunctionPointerDiscardName = "GetFunctionPointerDiscard";
        private const string InvokeName = "Invoke";

        public ILPostProcessing(AssemblyResolver loader, bool isForEditor, ErrorDiagnosticDelegate error, LogDelegate log = null, int logLevel = 0, bool skipInitializeOnLoad = false)
        {
            _skipInitializeOnLoad = skipInitializeOnLoad;
            Loader = loader;
            IsForEditor = isForEditor;
        }

        public bool _skipInitializeOnLoad;

        public bool IsForEditor { get; private set; }

        private AssemblyResolver Loader { get; }

        public bool Run(AssemblyDefinition assemblyDefinition)
        {
            _assemblyDefinition = assemblyDefinition;
            _typeSystem = assemblyDefinition.MainModule.TypeSystem;

            _modified = false;
            var types = assemblyDefinition.MainModule.GetTypes().ToArray();
            foreach (var type in types)
            {
                ProcessType(type);
            }

#if !UNITY_DOTSPLAYER
            if (_containsDirectCall)
            {
                GenerateInitializeOnLoadMethod();
            }
#endif

            return _modified;
        }

        private void GenerateInitializeOnLoadMethod()
        {
            // This method is needed to ensure that BurstCompiler.Options is initialized on the main thread,
            // before any direct call methods are called on a background thread.

            // [UnityEngine.RuntimeInitializeOnLoadMethod(UnityEngine.RuntimeInitializeLoadType.AfterAssembliesLoaded)]
            // [UnityEditor.InitializeOnLoadMethod] // When its an editor assembly
            // private static void Initialize()
            // {
            //     var _ = BurstCompiler.Options;
            // }
            const string initializeOnLoadClassName = "$BurstDirectCallInitializer";
            var initializeOnLoadClass = _assemblyDefinition.MainModule.Types.FirstOrDefault(x => x.Name == initializeOnLoadClassName);
            if (initializeOnLoadClass != null)
            {
                // If there's already a class with this name, remove it,
                // This would mean that we're postprocessing an already-postprocessed assembly;
                // I don't think that ever happens, but no sense in breaking if it does.
                _assemblyDefinition.MainModule.Types.Remove(initializeOnLoadClass);
            }
            initializeOnLoadClass = new TypeDefinition(
                "",
                initializeOnLoadClassName,
                TypeAttributes.NotPublic |
                TypeAttributes.AutoLayout |
                TypeAttributes.AnsiClass |
                TypeAttributes.Abstract |
                TypeAttributes.Sealed |
                TypeAttributes.BeforeFieldInit)
            {
                BaseType = _typeSystem.Object
            };
            _assemblyDefinition.MainModule.Types.Add(initializeOnLoadClass);
            var initializeOnLoadMethod = new MethodDefinition("Initialize", MethodAttributes.Private | MethodAttributes.HideBySig | MethodAttributes.Static, _typeSystem.Void)
            {
                ImplAttributes = MethodImplAttributes.IL | MethodImplAttributes.Managed,
                DeclaringType = initializeOnLoadClass
            };

            initializeOnLoadMethod.Body.Variables.Add(new VariableDefinition(_burstCompilerOptionsType));

            var processor = initializeOnLoadMethod.Body.GetILProcessor();
            processor.Emit(OpCodes.Ldsfld, _burstCompilerOptionsField);
            processor.Emit(OpCodes.Stloc_0);
            processor.Emit(OpCodes.Ret);
            initializeOnLoadClass.Methods.Add(FixDebugInformation(initializeOnLoadMethod));

            var attribute = new CustomAttribute(_unityEngineInitializeOnLoadAttributeCtor);
            attribute.ConstructorArguments.Add(new CustomAttributeArgument(_unityEngineRuntimeInitializeLoadType, _unityEngineRuntimeInitializeLoadAfterAssemblies.Constant));
            initializeOnLoadMethod.CustomAttributes.Add(attribute);

            if (IsForEditor && !_skipInitializeOnLoad)
            {
                // Need to ensure the editor tag for initialize on load is present, otherwise edit mode tests will not call Initialize
                attribute = new CustomAttribute(_unityEditorInitilizeOnLoadAttributeCtor);
                initializeOnLoadMethod.CustomAttributes.Add(attribute);
            }
        }

        private static bool CanComputeCompileTimeHash(TypeReference typeRef)
        {
            if (typeRef.ContainsGenericParameter)
            {
                return false;
            }

            var assemblyNameReference = typeRef.Scope as AssemblyNameReference ?? typeRef.Module.Assembly?.Name;

            if (assemblyNameReference == null)
            {
                return false;
            }

            switch (assemblyNameReference.Name)
            {
                case "netstandard":
                case "mscorlib":
                    return false;
            }

            return true;
        }

        private void ProcessType(TypeDefinition type)
        {
            if (!type.HasGenericParameters && TryGetBurstCompileAttribute(type, out _))
            {
                // Make a copy because we are going to modify it
                var methodCount = type.Methods.Count;
                for (var j = 0; j < methodCount; j++)
                {
                    var method = type.Methods[j];
                    if (!method.IsStatic || method.HasGenericParameters || !TryGetBurstCompileAttribute(method, out var methodBurstCompileAttribute)) continue;

                    bool isDirectCallDisabled = false;
                    bool foundProperty = false;
                    if (methodBurstCompileAttribute.HasProperties)
                    {
                        foreach (var property in methodBurstCompileAttribute.Properties)
                        {
                            if (property.Name == "DisableDirectCall")
                            {
                                isDirectCallDisabled = (bool)property.Argument.Value;
                                foundProperty = true;
                                break;
                            }
                        }
                    }

                    // If the method doesn't have a direct call specified, try the assembly level, do one last check for any assembly level [BurstCompile] instead.
                    if (foundProperty == false && TryGetBurstCompileAttribute(method.Module.Assembly, out var assemblyBurstCompileAttribute))
                    {
                        if (assemblyBurstCompileAttribute.HasProperties)
                        {
                            foreach (var property in assemblyBurstCompileAttribute.Properties)
                            {
                                if (property.Name == "DisableDirectCall")
                                {
                                    isDirectCallDisabled = (bool)property.Argument.Value;
                                    break;
                                }
                            }
                        }
                    }

                    foreach (var customAttribute in method.CustomAttributes)
                    {
                        if (customAttribute.AttributeType.FullName == "System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute")
                        {
                            // Can't / shouldn't enable direct call for [UnmanagedCallersOnly] methods -
                            // these can't be called from managed code.
                            isDirectCallDisabled = true;
                            break;
                        }
                    }

#if !UNITY_DOTSPLAYER       // Direct call is not Supported for dots runtime via this pre-processor, its handled elsewhere, this code assumes a Unity Editor based burst
                    if (!isDirectCallDisabled)
                    {
                        if (_burstAssembly == null)
                        {
                            var resolved = methodBurstCompileAttribute.Constructor.DeclaringType.Resolve();
                            InitializeBurstAssembly(resolved.Module.Assembly);
                        }

                        ProcessMethodForDirectCall(method);
                        _modified = true;
                        _containsDirectCall = true;
                    }
#endif
                }
            }

            if (TypeHasSharedStaticInIt(type))
            {
                foreach (var method in type.Methods)
                {
                    // Skip anything that isn't the static constructor.
                    if (method.Name != ".cctor")
                    {
                        continue;
                    }

                    try
                    {
#if DEBUG
                        if (_instructionsToReplace.Count != 0)
                        {
                            throw new InvalidOperationException("Instructions to replace wasn't cleared properly!");
                        }
#endif

                        foreach (var instruction in method.Body.Instructions)
                        {
                            // Skip anything that isn't a call.
                            if (instruction.OpCode != OpCodes.Call)
                            {
                                continue;
                            }

                            var calledMethod = (MethodReference)instruction.Operand;

                            if (calledMethod.Name != "GetOrCreate")
                            {
                                continue;
                            }

                            // Skip anything that isn't member of the `SharedStatic` class.
                            if (!TypeIsSharedStatic(calledMethod.DeclaringType))
                            {
                                continue;
                            }

                            // We only handle the `GetOrCreate` calls with a single parameter (the alignment).
                            if (calledMethod.Parameters.Count != 1)
                            {
                                continue;
                            }

                            // We only post-process the generic versions of `GetOrCreate`.
                            if (!(calledMethod is GenericInstanceMethod genericInstanceMethod))
                            {
                                continue;
                            }

                            var atLeastOneArgumentCanBeComputed = false;

                            foreach (var genericArgument in genericInstanceMethod.GenericArguments)
                            {
                                if (CanComputeCompileTimeHash(genericArgument))
                                {
                                    atLeastOneArgumentCanBeComputed = true;
                                }
                            }

                            // We cannot post-process a shared static with all arguments being open generic.
                            // We cannot post-process a shared static where all of its types are in core libraries.
                            if (!atLeastOneArgumentCanBeComputed)
                            {
                                continue;
                            }

                            _instructionsToReplace.Add(instruction);
                        }

                        if (_instructionsToReplace.Count > 0)
                        {
                            _modified = true;
                        }

                        foreach (var instruction in _instructionsToReplace)
                        {
                            var calledMethod = (GenericInstanceMethod)instruction.Operand;

                            var hashCode64 = CalculateHashCode64(calledMethod.GenericArguments[0]);

                            long subHashCode64 = 0;

                            var useCalculatedHashCode = true;
                            var useCalculatedSubHashCode = true;

                            if (calledMethod.GenericArguments.Count == 2)
                            {
                                subHashCode64 = CalculateHashCode64(calledMethod.GenericArguments[1]);

                                useCalculatedHashCode = CanComputeCompileTimeHash(calledMethod.GenericArguments[0]);
                                useCalculatedSubHashCode = CanComputeCompileTimeHash(calledMethod.GenericArguments[1]);
                            }

#if DEBUG
                            if (!useCalculatedHashCode && !useCalculatedSubHashCode)
                            {
                                throw new InvalidOperationException("Cannot replace when both hashes are invalid!");
                            }
#endif

                            var methodToCall = "GetOrCreateUnsafe";
                            TypeReference genericArgument = null;

                            if (!useCalculatedHashCode)
                            {
                                methodToCall = "GetOrCreatePartiallyUnsafeWithSubHashCode";
                                genericArgument = calledMethod.GenericArguments[0];
                            }
                            else if (!useCalculatedSubHashCode)
                            {
                                methodToCall = "GetOrCreatePartiallyUnsafeWithHashCode";
                                genericArgument = calledMethod.GenericArguments[1];
                            }

                            var getOrCreateUnsafe = _assemblyDefinition.MainModule.ImportReference(
                                calledMethod.DeclaringType.Resolve().Methods.First(m => m.Name == methodToCall));

                            getOrCreateUnsafe.DeclaringType = calledMethod.DeclaringType;

                            if (genericArgument != null)
                            {
                                var genericInstanceMethod = new GenericInstanceMethod(getOrCreateUnsafe);

                                genericInstanceMethod.GenericArguments.Add(genericArgument);

                                getOrCreateUnsafe = genericInstanceMethod;
                            }

                            var processor = method.Body.GetILProcessor();

                            if (useCalculatedHashCode)
                            {
                                processor.InsertBefore(instruction, processor.Create(OpCodes.Ldc_I8, hashCode64));
                            }

                            if (useCalculatedSubHashCode)
                            {
                                processor.InsertBefore(instruction, processor.Create(OpCodes.Ldc_I8, subHashCode64));
                            }

                            processor.Replace(instruction, processor.Create(OpCodes.Call, getOrCreateUnsafe));
                        }
                    }
                    finally
                    {
                        _instructionsToReplace.Clear();
                    }
                }
            }
        }

        // WARNING: This **must** be kept in sync with the definition in BurstRuntime.cs!
        private static long HashStringWithFNV1A64(string text)
        {
            // Using http://www.isthe.com/chongo/tech/comp/fnv/index.html#FNV-1a
            // with basis and prime:
            const ulong offsetBasis = 14695981039346656037;
            const ulong prime = 1099511628211;

            ulong result = offsetBasis;

            foreach (var c in text)
            {
                result = prime * (result ^ (byte)(c & 255));
                result = prime * (result ^ (byte)(c >> 8));
            }

            return (long)result;
        }

        private long CalculateHashCode64(TypeReference type)
        {
            try
            {
#if DEBUG
                if (_builder.Length != 0)
                {
                    throw new InvalidOperationException("StringBuilder wasn't cleared properly!");
                }
#endif

                type.BuildAssemblyQualifiedName(_builder);
                return HashStringWithFNV1A64(_builder.ToString());
            }
            finally
            {
                _builder.Clear();
            }
        }

        private static bool TypeIsSharedStatic(TypeReference typeRef)
        {
            if (typeRef.Namespace != "Unity.Burst")
            {
                return false;
            }

            if (typeRef.Name != "SharedStatic`1")
            {
                return false;
            }

            return true;
        }

        private static bool TypeHasSharedStaticInIt(TypeDefinition typeDef)
        {
            foreach (var field in typeDef.Fields)
            {
                if (TypeIsSharedStatic(field.FieldType))
                {
                    return true;
                }
            }

            return false;
        }

        private TypeDefinition InjectDelegate(TypeDefinition declaringType, string originalName, MethodDefinition managed, string uniqueSuffix)
        {
            var injectedDelegateType = new TypeDefinition(declaringType.Namespace, $"{originalName}{uniqueSuffix}{PostfixBurstDelegate}",
                TypeAttributes.NestedPublic |
                TypeAttributes.AutoLayout |
                TypeAttributes.AnsiClass |
                TypeAttributes.Sealed
            )
            {
                DeclaringType = declaringType,
                BaseType = _systemDelegateType
            };

            declaringType.NestedTypes.Add(injectedDelegateType);

            {
                var constructor = new MethodDefinition(".ctor",
                    MethodAttributes.Public |
                    MethodAttributes.HideBySig |
                    MethodAttributes.SpecialName |
                    MethodAttributes.RTSpecialName,
                    _typeSystem.Void)
                {
                    HasThis = true,
                    IsManaged = true,
                    IsRuntime = true,
                    DeclaringType = injectedDelegateType
                };

                constructor.Parameters.Add(new ParameterDefinition(_typeSystem.Object));
                constructor.Parameters.Add(new ParameterDefinition(_typeSystem.IntPtr));
                injectedDelegateType.Methods.Add(constructor);
            }

            {
                var invoke = new MethodDefinition("Invoke",
                    MethodAttributes.Public |
                    MethodAttributes.HideBySig |
                    MethodAttributes.NewSlot |
                    MethodAttributes.Virtual,
                    managed.ReturnType)
                {
                    HasThis = true,
                    IsManaged = true,
                    IsRuntime = true,
                    DeclaringType = injectedDelegateType
                };

                foreach (var parameter in managed.Parameters)
                {
                    invoke.Parameters.Add(parameter);
                }

                injectedDelegateType.Methods.Add(invoke);
            }

            {
                var beginInvoke = new MethodDefinition("BeginInvoke",
                    MethodAttributes.Public |
                    MethodAttributes.HideBySig |
                    MethodAttributes.NewSlot |
                    MethodAttributes.Virtual,
                    _systemIASyncResultType)
                {
                    HasThis = true,
                    IsManaged = true,
                    IsRuntime = true,
                    DeclaringType = injectedDelegateType
                };

                foreach (var parameter in managed.Parameters)
                {
                    beginInvoke.Parameters.Add(parameter);
                }

                beginInvoke.Parameters.Add(new ParameterDefinition(_systemASyncCallbackType));
                beginInvoke.Parameters.Add(new ParameterDefinition(_typeSystem.Object));

                injectedDelegateType.Methods.Add(beginInvoke);
            }

            {
                var endInvoke = new MethodDefinition("EndInvoke",
                    MethodAttributes.Public |
                    MethodAttributes.HideBySig |
                    MethodAttributes.NewSlot |
                    MethodAttributes.Virtual,
                    managed.ReturnType)
                {
                    HasThis = true,
                    IsManaged = true,
                    IsRuntime = true,
                    DeclaringType = injectedDelegateType
                };

                endInvoke.Parameters.Add(new ParameterDefinition(_systemIASyncResultType));

                injectedDelegateType.Methods.Add(endInvoke);
            }

            return injectedDelegateType;
        }

        private MethodDefinition CreateGetFunctionPointerDiscardMethod(TypeDefinition cls, FieldDefinition pointerField, MethodDefinition targetMethod, TypeDefinition injectedDelegate)
        {
            var genericCompileFunctionPointer = new GenericInstanceMethod(_burstCompilerCompileFunctionPointer);
            genericCompileFunctionPointer.GenericArguments.Add(injectedDelegate);

            var genericFunctionPointerType = new GenericInstanceType(_functionPointerType);
            genericFunctionPointerType.GenericArguments.Add(injectedDelegate);

            var genericGetValue = new MethodReference(_functionPointerGetValue.Name, _functionPointerGetValue.ReturnType, genericFunctionPointerType);

            foreach (var p in _functionPointerGetValue.Parameters)
            {
                genericGetValue.Parameters.Add(new ParameterDefinition(p.Name, p.Attributes, p.ParameterType));
            }

            genericGetValue.HasThis = _functionPointerGetValue.HasThis;
            genericGetValue.MetadataToken = _functionPointerGetValue.MetadataToken;

            /*var genericGetValue = new Mono.Cecil.GenericInstanceMethod(_functionPointerGetValue)
            {
                DeclaringType = genericFunctionPointerType
            };*/

            // Create GetFunctionPointerDiscard method:
            //
            // [BurstDiscard]
            // public static void GetFunctionPointerDiscard(ref IntPtr ptr) {
            //   if (Pointer == null) {
            //     Pointer = BurstCompiler.CompileFunctionPointer<InjectedDelegate>(d);
            //   }
            //
            //   ptr = Pointer
            // }
            var getFunctionPointerDiscardMethod = new MethodDefinition(GetFunctionPointerDiscardName, MethodAttributes.Private | MethodAttributes.HideBySig | MethodAttributes.Static, _typeSystem.Void)
            {
                ImplAttributes = MethodImplAttributes.IL | MethodImplAttributes.Managed,
                DeclaringType = cls
            };

            getFunctionPointerDiscardMethod.Body.Variables.Add(new VariableDefinition(genericFunctionPointerType));

            getFunctionPointerDiscardMethod.Parameters.Add(new ParameterDefinition(new ByReferenceType(_typeSystem.IntPtr)));

            var processor = getFunctionPointerDiscardMethod.Body.GetILProcessor();
            processor.Emit(OpCodes.Ldsfld, pointerField);
            var branchPosition = processor.Body.Instructions[processor.Body.Instructions.Count - 1];

            processor.Emit(OpCodes.Ldnull);
            processor.Emit(OpCodes.Ldftn, targetMethod);
            processor.Emit(OpCodes.Newobj, injectedDelegate.Methods.First(md => md.IsConstructor && md.Parameters.Count == 2));

            processor.Emit(OpCodes.Call, genericCompileFunctionPointer);
            processor.Emit(OpCodes.Stloc_0);

            processor.Emit(OpCodes.Ldloca, 0);
            processor.Emit(OpCodes.Call, genericGetValue);

            processor.Emit(OpCodes.Stsfld, pointerField);

            processor.Emit(OpCodes.Ldarg_0);
            processor.InsertAfter(branchPosition, Instruction.Create(OpCodes.Brtrue, processor.Body.Instructions[processor.Body.Instructions.Count - 1]));
            processor.Emit(OpCodes.Ldsfld, pointerField);
            processor.Emit(OpCodes.Stind_I);
            processor.Emit(OpCodes.Ret);

            cls.Methods.Add(FixDebugInformation(getFunctionPointerDiscardMethod));

            getFunctionPointerDiscardMethod.CustomAttributes.Add(new CustomAttribute(_burstDiscardAttributeConstructor));

            return getFunctionPointerDiscardMethod;
        }

        private MethodDefinition CreateGetFunctionPointerMethod(TypeDefinition cls, MethodDefinition getFunctionPointerDiscardMethod)
        {
            // Create GetFunctionPointer method:
            //
            // public static IntPtr GetFunctionPointer() {
            //   var ptr;
            //   GetFunctionPointerDiscard(ref ptr);
            //   return ptr;
            // }
            var getFunctionPointerMethod = new MethodDefinition(GetFunctionPointerName, MethodAttributes.Private | MethodAttributes.HideBySig | MethodAttributes.Static, _typeSystem.IntPtr)
            {
                ImplAttributes = MethodImplAttributes.IL | MethodImplAttributes.Managed,
                DeclaringType = cls
            };

            getFunctionPointerMethod.Body.Variables.Add(new VariableDefinition(_typeSystem.IntPtr));
            getFunctionPointerMethod.Body.InitLocals = true;

            var processor = getFunctionPointerMethod.Body.GetILProcessor();

            processor.Emit(OpCodes.Ldc_I4_0);
            processor.Emit(OpCodes.Conv_I);
            processor.Emit(OpCodes.Stloc_0);
            processor.Emit(OpCodes.Ldloca_S, (byte)0);
            processor.Emit(OpCodes.Call, getFunctionPointerDiscardMethod);
            processor.Emit(OpCodes.Ldloc_0);

            processor.Emit(OpCodes.Ret);

            cls.Methods.Add(FixDebugInformation(getFunctionPointerMethod));

            return getFunctionPointerMethod;
        }

        private void ProcessMethodForDirectCall(MethodDefinition burstCompileMethod)
        {
            var declaringType = burstCompileMethod.DeclaringType;

            var uniqueSuffix = $"_{burstCompileMethod.MetadataToken.RID:X8}";

            var injectedDelegate = InjectDelegate(declaringType, burstCompileMethod.Name, burstCompileMethod, uniqueSuffix);

            // Create a copy of the original method that will be the actual managed method
            // The original method is patched at the end of this method to call
            // the dispatcher that will go to the Burst implementation or the managed method (if in the editor and Burst is disabled)
            var managedFallbackMethod = new MethodDefinition($"{burstCompileMethod.Name}{PostfixManaged}", burstCompileMethod.Attributes, burstCompileMethod.ReturnType)
            {
                DeclaringType = declaringType,
                ImplAttributes = burstCompileMethod.ImplAttributes,
                MetadataToken = burstCompileMethod.MetadataToken,
            };

            // Ensure the CustomAttributes are the same
            managedFallbackMethod.CustomAttributes.Clear();
            foreach (var attr in burstCompileMethod.CustomAttributes)
            {
                managedFallbackMethod.CustomAttributes.Add(attr);
            }

            declaringType.Methods.Add(managedFallbackMethod);

            foreach (var parameter in burstCompileMethod.Parameters)
            {
                managedFallbackMethod.Parameters.Add(parameter);
            }

            // Copy the body from the original burst method to the managed fallback, we'll replace the burstCompileMethod body later.
            managedFallbackMethod.Body.InitLocals = burstCompileMethod.Body.InitLocals;
            managedFallbackMethod.Body.LocalVarToken = burstCompileMethod.Body.LocalVarToken;
            managedFallbackMethod.Body.MaxStackSize = burstCompileMethod.Body.MaxStackSize;

            foreach (var variable in burstCompileMethod.Body.Variables)
            {
                managedFallbackMethod.Body.Variables.Add(variable);
            }

            foreach (var instruction in burstCompileMethod.Body.Instructions)
            {
                managedFallbackMethod.Body.Instructions.Add(instruction);
            }

            foreach (var exceptionHandler in burstCompileMethod.Body.ExceptionHandlers)
            {
                managedFallbackMethod.Body.ExceptionHandlers.Add(exceptionHandler);
            }

            managedFallbackMethod.ImplAttributes &= MethodImplAttributes.NoInlining;
            // 0x0100 is AggressiveInlining
            managedFallbackMethod.ImplAttributes |= (MethodImplAttributes)0x0100;

            // The method needs to be public because we query for it in the ILPP code.
            managedFallbackMethod.Attributes &= ~MethodAttributes.Private;
            managedFallbackMethod.Attributes |= MethodAttributes.Public;

            // private static class (Name_RID.$Postfix)
            var cls = new TypeDefinition(declaringType.Namespace, $"{burstCompileMethod.Name}{uniqueSuffix}{PostfixBurstDirectCall}",
                TypeAttributes.NestedAssembly |
                TypeAttributes.AutoLayout |
                TypeAttributes.AnsiClass |
                TypeAttributes.Abstract |
                TypeAttributes.Sealed |
                TypeAttributes.BeforeFieldInit
            )
            {
                DeclaringType = declaringType,
                BaseType = _typeSystem.Object
            };

            declaringType.NestedTypes.Add(cls);

            // Create Field:
            //
            // private static IntPtr Pointer;
            var pointerField = new FieldDefinition("Pointer", FieldAttributes.Static | FieldAttributes.Private, _typeSystem.IntPtr)
            {
                DeclaringType = cls
            };
            cls.Fields.Add(pointerField);

            var getFunctionPointerDiscardMethod = CreateGetFunctionPointerDiscardMethod(
                cls, pointerField,
                // In the player the function pointer is looked up in a registry by name
                // so we can't request a `$BurstManaged` function (because it was never compiled, only the toplevel one)
                // But, it's safe *in the player* to request the toplevel function
                IsForEditor ? managedFallbackMethod : burstCompileMethod,
                injectedDelegate);
            var getFunctionPointerMethod = CreateGetFunctionPointerMethod(cls, getFunctionPointerDiscardMethod);

            // Create the Invoke method based on the original method (same signature)
            //
            // public static XXX Invoke(...args) {
            //    if (BurstCompiler.IsEnabled)
            //    {
            //        var funcPtr = GetFunctionPointer();
            //        if (funcPtr != null) return funcPtr(...args);
            //    }
            //    return OriginalMethod(...args);
            // }
            var invokeAttributes = managedFallbackMethod.Attributes;
            invokeAttributes &= ~MethodAttributes.Private;
            invokeAttributes |= MethodAttributes.Public;
            var invoke = new MethodDefinition(InvokeName, invokeAttributes, burstCompileMethod.ReturnType)
            {
                ImplAttributes = MethodImplAttributes.IL | MethodImplAttributes.Managed,
                DeclaringType = cls
            };

            var signature = new CallSite(burstCompileMethod.ReturnType)
            {
                CallingConvention = MethodCallingConvention.C
            };

            foreach (var parameter in burstCompileMethod.Parameters)
            {
                invoke.Parameters.Add(parameter);
                signature.Parameters.Add(parameter);
            }

            invoke.Body.Variables.Add(new VariableDefinition(_typeSystem.IntPtr));
            invoke.Body.InitLocals = true;

            var processor = invoke.Body.GetILProcessor();
            processor.Emit(OpCodes.Call, _burstCompilerIsEnabledMethodDefinition);
            var branchPosition0 = processor.Body.Instructions[processor.Body.Instructions.Count - 1];

            processor.Emit(OpCodes.Call, getFunctionPointerMethod);
            processor.Emit(OpCodes.Stloc_0);
            processor.Emit(OpCodes.Ldloc_0);
            var branchPosition1 = processor.Body.Instructions[processor.Body.Instructions.Count - 1];

            EmitArguments(processor, invoke);
            processor.Emit(OpCodes.Ldloc_0);
            processor.Emit(OpCodes.Calli, signature);
            processor.Emit(OpCodes.Ret);
            var previousRet = processor.Body.Instructions[processor.Body.Instructions.Count - 1];

            EmitArguments(processor, invoke);
            processor.Emit(OpCodes.Call, managedFallbackMethod);
            processor.Emit(OpCodes.Ret);

            // Insert the branch once we have emitted the instructions
            processor.InsertAfter(branchPosition0, Instruction.Create(OpCodes.Brfalse, previousRet.Next));
            processor.InsertAfter(branchPosition1, Instruction.Create(OpCodes.Brfalse, previousRet.Next));
            cls.Methods.Add(FixDebugInformation(invoke));

            // Final patching of the original method
            // public static XXX OriginalMethod(...args) {
            //      Name_RID.$Postfix.Invoke(...args);
            //      ret;
            // }
            burstCompileMethod.Body = new MethodBody(burstCompileMethod);
            processor = burstCompileMethod.Body.GetILProcessor();
            EmitArguments(processor, burstCompileMethod);
            processor.Emit(OpCodes.Call, invoke);
            processor.Emit(OpCodes.Ret);
            FixDebugInformation(burstCompileMethod);
        }

        private static MethodDefinition FixDebugInformation(MethodDefinition method)
        {
            method.DebugInformation.Scope = new ScopeDebugInformation(method.Body.Instructions.First(), method.Body.Instructions.Last());
            return method;
        }

        private AssemblyDefinition GetAsmDefinitionFromFile(AssemblyResolver loader, string assemblyName)
        {
            if (loader.TryResolve(AssemblyNameReference.Parse(assemblyName), out var result))
            {
                return result;
            }
            return null;
        }

        private MethodReference _unityEngineInitializeOnLoadAttributeCtor;
        private TypeReference _unityEngineRuntimeInitializeLoadType;
        private FieldDefinition _unityEngineRuntimeInitializeLoadAfterAssemblies;
        private MethodReference _unityEditorInitilizeOnLoadAttributeCtor;

        private void InitializeBurstAssembly(AssemblyDefinition burstAssembly)
        {
            _burstAssembly = burstAssembly;

            var burstCompilerTypeDefinition = burstAssembly.MainModule.GetType("Unity.Burst", "BurstCompiler");
            _burstCompilerIsEnabledMethodDefinition = _assemblyDefinition.MainModule.ImportReference(burstCompilerTypeDefinition.Methods.FirstOrDefault(x => x.Name == "get_IsEnabled"));
            _burstCompilerCompileFunctionPointer = _assemblyDefinition.MainModule.ImportReference(burstCompilerTypeDefinition.Methods.FirstOrDefault(x => x.Name == "CompileFunctionPointer"));
            _burstCompilerOptionsField = _assemblyDefinition.MainModule.ImportReference(burstCompilerTypeDefinition.Fields.FirstOrDefault(x => x.Name == "Options"));
            _burstCompilerOptionsType = _assemblyDefinition.MainModule.ImportReference(burstAssembly.MainModule.GetType("Unity.Burst", "BurstCompilerOptions"));

            var functionPointerTypeDefinition = burstAssembly.MainModule.GetType("Unity.Burst", "FunctionPointer`1");
            _functionPointerType = _assemblyDefinition.MainModule.ImportReference(functionPointerTypeDefinition);
            _functionPointerGetValue = _assemblyDefinition.MainModule.ImportReference(functionPointerTypeDefinition.Methods.FirstOrDefault(x => x.Name == "get_Value"));

            var corLibrary = Loader.Resolve((AssemblyNameReference)_typeSystem.CoreLibrary);
            _systemDelegateType = _assemblyDefinition.MainModule.ImportReference(corLibrary.MainModule.GetType("System.MulticastDelegate"));
            _systemASyncCallbackType = _assemblyDefinition.MainModule.ImportReference(corLibrary.MainModule.GetType("System.AsyncCallback"));
            _systemIASyncResultType = _assemblyDefinition.MainModule.ImportReference(corLibrary.MainModule.GetType("System.IAsyncResult"));

            var asmDef = GetAsmDefinitionFromFile(Loader, "UnityEngine.CoreModule");
            var runtimeInitializeOnLoadMethodAttribute = asmDef.MainModule.GetType("UnityEngine", "RuntimeInitializeOnLoadMethodAttribute");
            var runtimeInitializeLoadType = asmDef.MainModule.GetType("UnityEngine", "RuntimeInitializeLoadType");

            var burstDiscardType = asmDef.MainModule.GetType("Unity.Burst", "BurstDiscardAttribute");
            _burstDiscardAttributeConstructor = _assemblyDefinition.MainModule.ImportReference(burstDiscardType.Methods.First(method => method.Name == ".ctor"));

            _unityEngineInitializeOnLoadAttributeCtor = _assemblyDefinition.MainModule.ImportReference(runtimeInitializeOnLoadMethodAttribute.Methods.FirstOrDefault(x => x.Name == ".ctor" && x.HasParameters));
            _unityEngineRuntimeInitializeLoadType = _assemblyDefinition.MainModule.ImportReference(runtimeInitializeLoadType);
            _unityEngineRuntimeInitializeLoadAfterAssemblies = runtimeInitializeLoadType.Fields.FirstOrDefault(x => x.Name == "AfterAssembliesLoaded");

            if (IsForEditor && !_skipInitializeOnLoad)
            {
                asmDef = GetAsmDefinitionFromFile(Loader, "UnityEditor.CoreModule");
                if (asmDef == null)
                    asmDef = GetAsmDefinitionFromFile(Loader, "UnityEditor");
                var initializeOnLoadMethodAttribute = asmDef.MainModule.GetType("UnityEditor", "InitializeOnLoadMethodAttribute");

                _unityEditorInitilizeOnLoadAttributeCtor = _assemblyDefinition.MainModule.ImportReference(initializeOnLoadMethodAttribute.Methods.FirstOrDefault(x => x.Name == ".ctor" && !x.HasParameters));
            }
        }

        private static void EmitArguments(ILProcessor processor, MethodDefinition method)
        {
            for (var i = 0; i < method.Parameters.Count; i++)
            {
                switch (i)
                {
                    case 0:
                        processor.Emit(OpCodes.Ldarg_0);
                        break;
                    case 1:
                        processor.Emit(OpCodes.Ldarg_1);
                        break;
                    case 2:
                        processor.Emit(OpCodes.Ldarg_2);
                        break;
                    case 3:
                        processor.Emit(OpCodes.Ldarg_3);
                        break;
                    default:
                        if (i <= 255)
                        {
                            processor.Emit(OpCodes.Ldarg_S, (byte)i);
                        }
                        else
                        {
                            processor.Emit(OpCodes.Ldarg, i);
                        }
                        break;
                }
            }
        }

        private static bool TryGetBurstCompileAttribute(ICustomAttributeProvider provider, out CustomAttribute customAttribute)
        {
            if (provider.HasCustomAttributes)
            {
                foreach (var customAttr in provider.CustomAttributes)
                {
                    if (customAttr.Constructor.DeclaringType.Name == "BurstCompileAttribute")
                    {
                        customAttribute = customAttr;
                        return true;
                    }
                }
            }
            customAttribute = null;
            return false;
        }
    }
}