package vm

import "bytes"

import "github.com/lunixbochs/struc"
import "github.com/pkg/errors"

type Instruction struct {
	OpCode   OpCodeType
	Reserved [4]byte
	Operand1 Operand
	Operand2 Operand

	opHandler opHandler `struc:"skip"`
}

const instructionBytes = 32

func NewInstructionFromByteCode(byteCode []byte) (*Instruction, error) {
	instr := &Instruction{}

	reader := bytes.NewReader(byteCode)

	err := struc.Unpack(reader, instr)
	if err != nil {
		return nil, errors.Wrap(err, "Error decoding instruction")
	}

	return instr, nil
}

func NewInstructionsFromByteCode(byteCode []byte) ([]*Instruction, error) {
	instrs := []*Instruction{}

	for start := 0; start < len(byteCode); start += instructionBytes {
		chunk := byteCode[start : start+instructionBytes]
		instr, err := NewInstructionFromByteCode(chunk)
		if err != nil {
			return nil, errors.Wrapf(err, "At byte offset %d", start)
		}
		instrs = append(instrs, instr)
	}

	if len(instrs) == 0 || instrs[len(instrs)-1].OpCode != OpReturn {
		// Add implicit return at the end of each function
		instrs = append(instrs, &Instruction{
			OpCode: OpReturn,
		})
	}

	return instrs, nil
}