package creature

// Word is a type constraint defining possible word types for the Machine.
type Word interface {
	int8 | int16 | int32 | int64 |
		uint8 | uint16 | uint32 | uint64 |
		int | uint
}

// Machine is a stack machine. It contains an array of integers as its program
// data, and provides methods to run this program data, as well as interact with
// it.
type Machine[WORD Word] struct {
	program   []WORD
	stack     []WORD
	block     []WORD
	counter   WORD
	pointer   WORD
	functions map[WORD]MachineFunction[WORD]
}

// MachineFunction is a function that can extend the functionality of the stack
// machine. It is passed a pointer to the machine that is calling it, and the
// machine will halt execution if true is returned.
type MachineFunction[WORD Word] func(machine *Machine[WORD]) (stop bool)

// All supported opcodes
const (
	PUSH = 0x0
	POP  = 0x1
	PEEK = 0x2
	POKE = 0x3
	ADD  = 0x4
	SUB  = 0x5
	MUL  = 0x6
	DIV  = 0x7
	EQ   = 0x8
	GT   = 0x9
	LT   = 0xa
	NEQ  = 0xb
	MOD  = 0xc
	HALT = 0xd
	JMP  = 0xe
	CAL  = 0xf
)

// Error is an error type that can be returned from the machine's methods if
// they experience problems.
type Error int

const (
	// ErrorIDTaken is returned by Register when a new function is
	// registered with the same ID as another one.
	ErrorIDTaken Error = iota
	ErrorUnknownInstruction
	ErrorDivideByZero
)

// Error returns a textual description of the error.
func (err Error) Error() (description string) {
	switch err {
	case ErrorIDTaken:
		description = "this ID is taken by another function"
	case ErrorUnknownInstruction:
		description = "machine encountered unknown instruction"
	case ErrorDivideByZero:
		description = "cannot divide by zero"
	}

	return
}

// Reset resets the stack of the  machine. This should be called before Execute,
// since Execute does not reset the stack.
func (machine *Machine[WORD]) Reset() {
	machine.stack = nil
	machine.pointer = 0
}

// Execute starts execution at the address specified by offset. The machine will
// run until a HALT instruction is encountered, or the end of program memory is
// reached. If an unknown instruction is encountered, the machine halts and
// returns an error.
func (machine *Machine[WORD]) Execute(offset WORD) (err error) {
	machine.counter = offset

	for int(machine.counter) < len(machine.program) {
		switch machine.instruction() {
		case PUSH:
			machine.counter++
			machine.Push(machine.instruction())

		case POP:
			machine.Pop()

		case PEEK:
			machine.Push(machine.Peek(machine.Pop()))

		case POKE:
			address := machine.Pop()
			word := machine.Pop()
			machine.Poke(address, word)

		case ADD:
			right := machine.Pop()
			left := machine.Pop()
			machine.Push(left + right)

		case SUB:
			right := machine.Pop()
			left := machine.Pop()
			machine.Push(left - right)

		case MUL:
			right := machine.Pop()
			left := machine.Pop()
			machine.Push(left * right)

		case DIV:
			right := machine.Pop()
			left := machine.Pop()
			if right == 0 {
				err = ErrorDivideByZero
				return
			}
			machine.Push(left / right)

		case EQ:
			right := machine.Pop()
			left := machine.Pop()
			equal := 0
			if left == right {
				equal = 1
			}
			machine.Push(WORD(equal))

		case GT:
			right := machine.Pop()
			left := machine.Pop()
			greater := 0
			if left > right {
				greater = 1
			}
			machine.Push(WORD(greater))

		case LT:
			right := machine.Pop()
			left := machine.Pop()
			less := 0
			if left < right {
				less = 1
			}
			machine.Push(WORD(less))

		case NEQ:
			right := machine.Pop()
			left := machine.Pop()
			notEqual := 0
			if left != right {
				notEqual = 1
			}
			machine.Push(WORD(notEqual))

		case MOD:
			right := machine.Pop()
			left := machine.Pop()
			machine.Push(left % right)

		case HALT:
			return

		case JMP:
			jumpTo := machine.Pop()
			if machine.Pop() != 0 {
				machine.counter = jumpTo - 1
			}

		case CAL:
			id := machine.Pop()
			if machine.functions == nil {
				break
			}
			function, exists := machine.functions[id]
			if exists {
				if function(machine) {
					return
				}
			}

		default:
			err = ErrorUnknownInstruction
			return
		}
		machine.counter++
	}

	return
}

// Instruction returns the current instruction in program memory.
func (machine *Machine[WORD]) instruction() (instruction WORD) {
	instruction = machine.program[machine.counter]
	return
}

// reallocateStack changes the size of the stack to something reasonable. This
// should be called then the stack pointer is really small compared to the
// actual stack size, or the stack pointer is bigger than the stack.
func (machine *Machine[WORD]) reallocateStack() {
	reallocatedStack := make([]WORD, machine.pointer*2)
	copy(reallocatedStack, machine.stack)
	machine.stack = reallocatedStack
}

// Push pushes a word onto the stack, increasing the stack pointer and
// reallocating the stack if necessary.
func (machine *Machine[WORD]) Push(word WORD) {
	machine.pointer++
	if len(machine.stack) <= int(machine.pointer) {
		machine.reallocateStack()
	}
	machine.stack[machine.pointer] = word
}

// Pop pops the last word off of the stack, and returns it, decreasing the stack
// pointer and reallocating the stack if necessary.
func (machine *Machine[WORD]) Pop() (word WORD) {
	if int(machine.pointer) <= 0 || int(machine.pointer) >= len(machine.stack) {
		return
	}

	word = machine.stack[machine.pointer]
	machine.pointer--

	if int(machine.pointer) < len(machine.stack)/3 {
		machine.reallocateStack()
	}

	return
}

// Peek returns the word at address in the block.
func (machine *Machine[WORD]) Peek(address WORD) (word WORD) {
	if int(address) < len(machine.block) {
		word = machine.block[address]
	}
	return
}

// Poke sets the value at address in the block to word.
func (machine *Machine[WORD]) Poke(address WORD, word WORD) {
	if int(address) >= len(machine.block) {
		reallocatedBlock := make([]WORD, address*3/2+1)
		copy(reallocatedBlock, machine.block)
		machine.block = reallocatedBlock
	}
	machine.block[address] = word
}

// Register registers a function at the specified ID. If there is already a
// function registered at that ID, this method will return an error.
func (machine *Machine[WORD]) Register(
	id WORD,
	function MachineFunction[WORD],
) (
	err error,
) {
	if machine.functions == nil {
		machine.functions = make(map[WORD]MachineFunction[WORD])
	}

	_, exists := machine.functions[id]
	if exists {
		err = ErrorIDTaken
		return
	}

	machine.functions[id] = function

	return
}

// Unregister removes a function that is registered at the specified ID.
func (machine *Machine[WORD]) Unregister(id WORD) {
	if machine.functions == nil {
		return
	}
	delete(machine.functions, id)
}

// LoadProgram loads the contents of program into the machine's program memory.
func (machine *Machine[WORD]) LoadProgram(program []WORD) {
	machine.program = make([]WORD, len(program))
	copy(machine.program, program)
}

// LoadMemory loads the contents of block into the machine's memory.
func (machine *Machine[WORD]) LoadMemory(block []WORD) {
	machine.block = make([]WORD, len(block))
	copy(machine.block, block)
}

// Milk milks the stack machine.
func (machine *Machine[WORD]) Milk() {}