package vm

import (
	"fmt"
	"monkey/internal/builtins"
	"monkey/internal/code"
	"monkey/internal/compiler"
	"monkey/internal/context"
	"monkey/internal/lexer"
	"monkey/internal/object"
	"monkey/internal/parser"
	"monkey/internal/utils"
	"os"
	"path/filepath"
	"unicode"
)

const (
	maxStackSize = 2048
	maxFrames    = 1024
	maxGlobals   = 65536
)

func isTruthy(obj object.Object) bool {
	switch obj := obj.(type) {

	case object.Boolean:
		return obj.Value

	case object.Null:
		return false

	default:
		return true
	}
}

func executeModule(name string, state *State) (object.Object, error) {
	filename := utils.FindModule(name)
	if filename == "" {
		return nil, fmt.Errorf("ImportError: no module named '%s'", name)
	}

	b, err := os.ReadFile(filename)
	if err != nil {
		return nil, fmt.Errorf("IOError: error reading module '%s': %s", name, err)
	}

	l := lexer.New(string(b))
	p := parser.New(fmt.Sprintf("<module %s>", name), l)

	module := p.ParseProgram()
	if len(p.Errors()) != 0 {
		return nil, fmt.Errorf("ParseError: %s", p.Errors())
	}

	c := compiler.NewWithState(state.Symbols, &state.Constants)
	err = c.Compile(module)
	if err != nil {
		return nil, fmt.Errorf("CompileError: %s", err)
	}

	code := c.Bytecode()
	state.Constants = code.Constants

	machine := New(fmt.Sprintf("<module %s>", name), code, WithState(state))
	err = machine.Run()
	if err != nil {
		return nil, fmt.Errorf("RuntimeError: error loading module '%s'", err)
	}

	return state.ExportedHash(), nil
}

// State is the state of the virtual machine.
type State struct {
	Constants []object.Object
	Globals   []object.Object
	Symbols   *compiler.SymbolTable
}

func NewState() *State {
	symbolTable := compiler.NewSymbolTable()
	for i, builtin := range builtins.BuiltinsIndex {
		symbolTable.DefineBuiltin(i, builtin.Name)
	}

	return &State{
		Constants: []object.Object{},
		Globals:   make([]object.Object, maxGlobals),
		Symbols:   symbolTable,
	}
}

// exported binding in the vm state. That is every binding that starts with a
// capital letter. This is used by the module import system to wrap up the
// compiled and evaulated module into an object.
func (s *State) ExportedHash() *object.Hash {
	pairs := make(map[object.HashKey]object.HashPair)
	for name, symbol := range s.Symbols.Store {
		if unicode.IsUpper(rune(name[0])) {
			if symbol.Scope == compiler.GlobalScope {
				obj := s.Globals[symbol.Index]
				s := object.String{Value: name}
				pairs[s.Hash()] = object.HashPair{Key: s, Value: obj}
			}
		}
	}
	return &object.Hash{Pairs: pairs}
}

type VM struct {
	debug bool
	trace bool

	ctx   context.Context
	state *State

	dir  string
	file string

	stack []object.Object
	sp    int // Always points to the next value. Top of stack is stack[sp-1]

	frames []frame
	fp     int // Always points to the current frame. Current frame is frames[fp-1]
}

func (vm *VM) currentFrame() *frame {
	return &vm.frames[vm.fp-1]
}

func (vm *VM) pushFrame(f frame) {
	if vm.fp >= maxFrames {
		panic("frame overflow")
	}

	vm.frames[vm.fp] = f
	vm.fp++
}

func (vm *VM) popFrame() frame {
	if vm.fp == 0 {
		panic("fame underflow")
	}

	vm.fp--
	return vm.frames[vm.fp]
}

// Option defines a function option for the virtual machine.
type Option func(*VM)

// WithContext defines an option to set the context for the virtual machine.
func WithContext(ctx context.Context) Option {
	return func(vm *VM) { vm.ctx = ctx }
}

// WithDebug enables debug mode in the VM.
func WithDebug(debug bool) Option {
	return func(vm *VM) { vm.debug = debug }
}

// WithState sets the state of the VM.
func WithState(state *State) Option {
	return func(vm *VM) { vm.state = state }
}

// WithTrace sets the trace flag for the VM.
func WithTrace(trace bool) Option {
	return func(vm *VM) { vm.trace = trace }
}

// New constructs a new monkey-lang bytecode virtual machine
func New(fn string, bytecode *compiler.Bytecode, options ...Option) *VM {
	mainFn := object.CompiledFunction{Instructions: bytecode.Instructions}
	mainClosure := object.Closure{Fn: &mainFn}
	mainFrame := newFrame(&mainClosure, 0)

	frames := make([]frame, maxFrames)
	frames[0] = mainFrame

	ctx := context.New()

	state := NewState()
	state.Constants = bytecode.Constants

	vm := &VM{
		ctx:   ctx,
		state: state,

		stack: make([]object.Object, maxStackSize),
		sp:    0,

		frames: frames,
		fp:     1,
	}

	for _, option := range options {
		option(vm)
	}

	vm.dir, vm.file = filepath.Split(fn)

	return vm
}

func (vm *VM) push(o object.Object) error {
	if vm.sp >= maxStackSize {
		return fmt.Errorf("stack overflow")
	}

	vm.stack[vm.sp] = o
	vm.sp++

	return nil
}

func (vm *VM) pop() object.Object {
	if vm.sp == 0 {
		panic("stack underflow")
	}

	o := vm.stack[vm.sp-1]
	vm.sp--
	return o
}

func (vm *VM) pop2() (object.Object, object.Object) {
	if vm.sp == 1 {
		panic("stack underflow")
	}

	o1 := vm.stack[vm.sp-1]
	o2 := vm.stack[vm.sp-2]
	vm.sp -= 2
	return o1, o2
}

func (vm *VM) pop3() (object.Object, object.Object, object.Object) {
	if vm.sp == 2 {
		panic("stack underflow")
	}

	o1 := vm.stack[vm.sp-1]
	o2 := vm.stack[vm.sp-2]
	o3 := vm.stack[vm.sp-3]
	vm.sp -= 3
	return o1, o2, o3
}

func (vm *VM) executeSetGlobal() error {
	globalIndex := vm.currentFrame().ReadUint16()

	ref := vm.pop()
	if obj, ok := ref.(object.Copyable); ok {
		vm.state.Globals[globalIndex] = obj.Copy()
	} else {
		vm.state.Globals[globalIndex] = ref
	}

	return vm.push(object.NULL)
}

func (vm *VM) executeSetLocal() error {
	localIndex := vm.currentFrame().ReadUint8()

	ref := vm.pop()
	if obj, ok := ref.(object.Copyable); ok {
		vm.stack[vm.currentFrame().basePointer+int(localIndex)] = obj.Copy()
	} else {
		vm.stack[vm.currentFrame().basePointer+int(localIndex)] = ref
	}

	return vm.push(object.NULL)
}

func (vm *VM) buildHash(startIndex, endIndex int) (object.Object, error) {
	hashedPairs := make(map[object.HashKey]object.HashPair)

	for i := startIndex; i < endIndex; i += 2 {
		key := vm.stack[i]
		value := vm.stack[i+1]

		pair := object.HashPair{Key: key, Value: value}

		hashKey, ok := key.(object.Hasher)
		if !ok {
			return nil, fmt.Errorf("unusable as hash key: %s", key.Type())
		}

		hashedPairs[hashKey.Hash()] = pair
	}

	return &object.Hash{Pairs: hashedPairs}, nil
}

func (vm *VM) executeMakeHash() error {
	startIndex := vm.sp - int(vm.currentFrame().ReadUint16())
	hash, err := vm.buildHash(startIndex, vm.sp)
	if err != nil {
		return err
	}
	vm.sp = startIndex
	return vm.push(hash)
}

func (vm *VM) buildArray(startIndex, endIndex int) (object.Object, error) {
	elements := make([]object.Object, endIndex-startIndex)

	for i := startIndex; i < endIndex; i++ {
		elements[i-startIndex] = vm.stack[i]
	}

	return &object.Array{Elements: elements}, nil
}

func (vm *VM) executeMakeArray() error {
	startIndex := vm.sp - int(vm.currentFrame().ReadUint16())
	hash, err := vm.buildArray(startIndex, vm.sp)
	if err != nil {
		return err
	}
	vm.sp = startIndex
	return vm.push(hash)
}

func (vm *VM) executeAdd() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.Add(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	case object.String:
		val, err := obj.Add(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	case *object.Array:
		val, err := obj.Add(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	case *object.Hash:
		val, err := obj.Add(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "+")
	}
}

func (vm *VM) executeSub() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.Sub(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return fmt.Errorf("unsupported types for unary operation: -%s", left.Type())
	}
}

func (vm *VM) executeMul() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case *object.Array:
		val, err := obj.Mul(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	case object.Integer:
		val, err := obj.Mul(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	case object.String:
		val, err := obj.Mul(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "*")
	}
}

func (vm *VM) executeDiv() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.Div(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "/")
	}

}

func (vm *VM) executeMod() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.Mod(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "%")
	}
}

func (vm *VM) executeOr() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Boolean:
		val, err := obj.LogicalOr(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "||")
	}
}

func (vm *VM) executeAnd() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Boolean:
		val, err := obj.LogicalAnd(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "&&")
	}
}

func (vm *VM) executeBitwiseOr() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.BitwiseOr(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "|")
	}
}

func (vm *VM) executeBitwiseXor() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.BitwiseXor(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "^")
	}
}

func (vm *VM) executeBitwiseAnd() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.BitwiseAnd(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "&")
	}
}

func (vm *VM) executeBitwiseNot() error {
	left := vm.pop()

	switch obj := left.(type) {
	case object.Integer:
		return vm.push(obj.BitwiseNot())
	default:
		return fmt.Errorf("unsupported types for unary operation: ~%s", left.Type())
	}
}

func (vm *VM) executeLeftShift() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.LeftShift(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, "<<")
	}
}

func (vm *VM) executeRightShift() error {
	right, left := vm.pop2()

	switch obj := left.(type) {
	case object.Integer:
		val, err := obj.RightShift(right)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return object.NewBinaryOpError(left, right, ">>")
	}
}

func (vm *VM) executeEqual() error {
	right, left := vm.pop2()

	if left.Compare(right) == 0 {
		return vm.push(object.TRUE)
	}
	return vm.push(object.FALSE)
}

func (vm *VM) executeNotEqual() error {
	right, left := vm.pop2()

	if left.Compare(right) != 0 {
		return vm.push(object.TRUE)
	}
	return vm.push(object.FALSE)
}

func (vm *VM) executeGreaterThan() error {
	right, left := vm.pop2()

	if left.Compare(right) == 1 {
		return vm.push(object.TRUE)
	}
	return vm.push(object.FALSE)
}

func (vm *VM) executeGreaterThanOrEqual() error {
	right, left := vm.pop2()

	if left.Compare(right) == 1 {
		return vm.push(object.TRUE)
	}
	return vm.push(object.FALSE)
}

func (vm *VM) executeNot() error {
	left := vm.pop()

	switch obj := left.(type) {
	case object.Boolean:
		return vm.push(obj.LogicalNot())
	case object.Integer:
		return vm.push(obj.LogicalNot())
	case object.Null:
		return vm.push(obj.LogicalNot())
	default:
		return fmt.Errorf("unsupported types for unary operation: !%s", left.Type())
	}
}

func (vm *VM) executeMinus() error {
	left := vm.pop()

	switch obj := left.(type) {
	case object.Integer:
		return vm.push(obj.Negate())
	default:
		return fmt.Errorf("unsupported types for unary operation: -%s", left.Type())
	}
}

func (vm *VM) executeSetItem() error {
	right, index, left := vm.pop3()

	switch obj := left.(type) {
	case *object.Array:
		err := obj.Set(index, right)
		if err != nil {
			return err
		}
		return vm.push(object.NULL)
	case *object.Hash:
		err := obj.Set(index, right)
		if err != nil {
			return err
		}
		return vm.push(object.NULL)
	default:
		return fmt.Errorf(
			"set item operation not supported: left=%s index=%s",
			left.Type(), index.Type(),
		)
	}
}

func (vm *VM) executeGetItem() error {
	index, left := vm.pop2()

	switch obj := left.(type) {
	case object.String:
		val, err := obj.Get(index)
		if err != nil {
			return err
		}
		return vm.push(val)
	case *object.Array:
		val, err := obj.Get(index)
		if err != nil {
			return err
		}
		return vm.push(val)
	case *object.Hash:
		val, err := obj.Get(index)
		if err != nil {
			return err
		}
		return vm.push(val)
	case object.Module:
		val, err := obj.Get(index)
		if err != nil {
			return err
		}
		return vm.push(val)
	default:
		return fmt.Errorf(
			"get item operation not supported: left=%s index=%s",
			left.Type(), index.Type(),
		)
	}
}

func (vm *VM) executeCall() error {
	args := int(vm.currentFrame().ReadUint8())

	callee := vm.stack[vm.sp-1-args]
	switch callee := callee.(type) {
	case *object.Closure:
		return vm.callClosure(callee, args)
	case object.Builtin:
		return vm.callBuiltin(callee, args)
	default:
		return fmt.Errorf(
			"calling non-closure and non-builtin: %T %v",
			callee, callee,
		)
	}
}

func (vm *VM) callClosure(cl *object.Closure, numArgs int) error {
	if numArgs != cl.Fn.NumParameters {
		return fmt.Errorf("wrong number of arguments: want=%d, got=%d", cl.Fn.NumParameters, numArgs)
	}

	// Optimize tail calls and avoid a new frame
	if cl.Fn == vm.currentFrame().cl.Fn {
		nextOP := vm.currentFrame().PeekNextOp()
		if nextOP == code.OpReturn {
			for p := 0; p < numArgs; p++ {
				vm.stack[vm.currentFrame().basePointer+p] = vm.stack[vm.sp-numArgs+p]
			}
			vm.sp -= numArgs + 1
			vm.currentFrame().SetIP(0) // reset IP to the beginning of the frame
			return nil
		}
	}

	frame := newFrame(cl, vm.sp-numArgs)
	vm.pushFrame(frame)
	vm.sp = frame.basePointer + cl.Fn.NumLocals

	return nil
}

func (vm *VM) callBuiltin(builtin object.Builtin, numArgs int) error {
	args := vm.stack[vm.sp-numArgs : vm.sp]

	result := builtin.Fn(vm.ctx, args...)
	vm.sp = vm.sp - numArgs - 1

	if result != nil {
		return vm.push(result)
	}

	return vm.push(object.NULL)
}

func (vm *VM) pushClosure(constIndex, numFree int) error {
	constant := vm.state.Constants[constIndex]
	function, ok := constant.(*object.CompiledFunction)
	if !ok {
		return fmt.Errorf("not a function %+v", constant)
	}

	free := make([]object.Object, numFree)
	for i := 0; i < numFree; i++ {
		free[i] = vm.stack[vm.sp-numFree+i]
	}
	vm.sp -= numFree

	closure := object.Closure{Fn: function, Free: free}
	return vm.push(&closure)
}

func (vm *VM) loadModule(name object.Object) error {
	s, ok := name.(object.String)
	if !ok {
		return fmt.Errorf(
			"TypeError: import() expected argument #1 to be `str` got `%s`",
			name.Type(),
		)
	}

	attrs, err := executeModule(s.Value, vm.state)
	if err != nil {
		return err
	}

	module := object.Module{Name: s.Value, Attrs: attrs}
	return vm.push(module)
}

func (vm *VM) LastPoppedStackElem() object.Object {
	return vm.stack[vm.sp]
}

func (vm *VM) Run() (err error) {

	for err == nil {
		op := vm.currentFrame().ReadNextOp()

		switch op {
		case code.OpConstant:
			constIndex := vm.currentFrame().ReadUint16()
			err = vm.push(vm.state.Constants[constIndex])

		case code.OpPop:
			vm.pop()

		case code.OpTrue:
			err = vm.push(object.TRUE)

		case code.OpFalse:
			err = vm.push(object.FALSE)

		case code.OpJump:
			pos := vm.currentFrame().ReadUint16()
			vm.currentFrame().SetIP(pos)

		case code.OpJumpNotTruthy:
			pos := vm.currentFrame().ReadUint16()
			if !isTruthy(vm.pop()) {
				vm.currentFrame().SetIP(pos)
			}

		case code.OpNull:
			err = vm.push(object.NULL)

		case code.OpSetGlobal:
			err = vm.executeSetGlobal()

		case code.OpAssignGlobal:
			globalIndex := vm.currentFrame().ReadUint16()
			vm.state.Globals[globalIndex] = vm.pop()
			err = vm.push(object.NULL)

		case code.OpAssignLocal:
			localIndex := vm.currentFrame().ReadUint8()
			vm.stack[vm.currentFrame().basePointer+int(localIndex)] = vm.pop()
			err = vm.push(object.NULL)

		case code.OpGetGlobal:
			globalIndex := vm.currentFrame().ReadUint16()
			err = vm.push(vm.state.Globals[globalIndex])

		case code.OpArray:
			err = vm.executeMakeArray()

		case code.OpHash:
			err = vm.executeMakeHash()

		case code.OpSetItem:
			err = vm.executeSetItem()

		case code.OpGetItem:
			err = vm.executeGetItem()

		case code.OpCall:
			err = vm.executeCall()

		case code.OpReturn:
			returnValue := vm.pop()
			frame := vm.popFrame()
			vm.sp = frame.basePointer - 1
			err = vm.push(returnValue)

		case code.OpSetLocal:
			err = vm.executeSetLocal()

		case code.OpGetLocal:
			localIndex := vm.currentFrame().ReadUint8()
			err = vm.push(vm.stack[vm.currentFrame().basePointer+int(localIndex)])

		case code.OpGetBuiltin:
			builtinIndex := vm.currentFrame().ReadUint8()
			builtin := builtins.BuiltinsIndex[builtinIndex]
			err = vm.push(builtin)

		case code.OpClosure:
			constIndex := vm.currentFrame().ReadUint16()
			numFree := vm.currentFrame().ReadUint8()
			err = vm.pushClosure(int(constIndex), int(numFree))

		case code.OpGetFree:
			freeIndex := vm.currentFrame().ReadUint8()
			err = vm.push(vm.currentFrame().GetFree(freeIndex))

		case code.OpCurrentClosure:
			currentClosure := vm.currentFrame().cl
			err = vm.push(currentClosure)

		case code.OpLoadModule:
			name := vm.pop()
			err = vm.loadModule(name)

		case code.OpAdd:
			err = vm.executeAdd()

		case code.OpSub:
			err = vm.executeSub()

		case code.OpMul:
			err = vm.executeMul()

		case code.OpDiv:
			err = vm.executeDiv()

		case code.OpMod:
			err = vm.executeMod()

		case code.OpOr:
			err = vm.executeOr()

		case code.OpAnd:
			err = vm.executeAnd()

		case code.OpBitwiseOR:
			err = vm.executeBitwiseOr()

		case code.OpBitwiseXOR:
			err = vm.executeBitwiseXor()

		case code.OpBitwiseAND:
			err = vm.executeBitwiseAnd()

		case code.OpLeftShift:
			err = vm.executeLeftShift()

		case code.OpRightShift:
			err = vm.executeRightShift()

		case code.OpEqual:
			err = vm.executeEqual()

		case code.OpNotEqual:
			err = vm.executeNotEqual()

		case code.OpGreaterThan:
			err = vm.executeGreaterThan()

		case code.OpGreaterThanEqual:
			err = vm.executeGreaterThanOrEqual()

		case code.OpNot:
			err = vm.executeNot()

		case code.OpBitwiseNOT:
			err = vm.executeBitwiseNot()

		case code.OpMinus:
			err = vm.executeMinus()

		case code.OpHalt:
			return

		default:
			err = fmt.Errorf("unhandled opcode: %s", op)
		}
	}

	return
}