package code

// Author: simon
// Author: ynwdlxm@163.com
// Date: 2022/10/19 13:12
// Desc: bytecode

import (
	"bytes"
	"encoding/binary"
	"fmt"
)

type Instructions []byte

// String MiniDisassembler
func (ins Instructions) String() string {
	var out bytes.Buffer

	i := 0
	for i < len(ins) {
		def, err := Lookup(ins[i])
		if err != nil {
			_, err := fmt.Fprintf(&out, "ERROR: %s\n", err)
			if err != nil {
				return ""
			}
			continue
		}

		operands, read := ReadOperands(def, ins[i+1:])

		_, err = fmt.Fprintf(&out, "%04d %s\n", i, ins.fmtInstructions(def, operands))
		if err != nil {
			return ""
		}

		i += 1 + read
	}

	return out.String()
}

func (ins Instructions) fmtInstructions(def *Definition, operands []int) string {
	operandCount := len(def.OperandWidths)

	if len(operands) != operandCount {
		return fmt.Sprintf("ERROR: operand len %d does not match defined %d\n", len(operands), operandCount)
	}

	switch operandCount {
	case 0:
		return def.Name
	case 1:
		return fmt.Sprintf("%s %d", def.Name, operands[0])
	}

	return fmt.Sprintf("ERROR: unhandled operandCount for %s\n", def.Name)
}

type Opcode byte

const (
	OpConstant Opcode = iota
	OpAdd             // +
	OpSub             // -
	OpMul             // *
	OpDiv             // /
	OpPop

	OpTrue  // true
	OpFalse // false

	OpEqual       // ==
	OpNotEqual    // !=
	OpGreaterThan // >

	OpMinus
	OpBang
)

// Definition For debugging and testing purposes
//
// it’s handy being able to look up how many operands an opcode has and what its human-readable name is.
// In order to achieve that, we’ll add proper definitions and some tooling
//
// Name helps to make an opcode readable
// OperandWidths contains the number of bytes each operand takes up
type Definition struct {
	Name          string
	OperandWidths []int
}

var definitions = map[Opcode]*Definition{
	OpConstant:    {"OpConstant", []int{2}},
	OpAdd:         {"OpAdd", []int{}}, // doesn't have any operands
	OpSub:         {"OpSub", []int{}},
	OpMul:         {"OpMul", []int{}},
	OpDiv:         {"OpDiv", []int{}},
	OpPop:         {"OpPop", []int{}},
	OpTrue:        {"OpTrue", []int{}},
	OpFalse:       {"OpFalse", []int{}},
	OpEqual:       {"OpEqual", []int{}},
	OpNotEqual:    {"OpNotEqual", []int{}},
	OpGreaterThan: {"OpGreaterThan", []int{}},
	OpMinus:       {"OpMinus", []int{}},
	OpBang:        {"OpBang", []int{}},
}

func Lookup(op byte) (*Definition, error) {
	def, ok := definitions[Opcode(op)]
	if !ok {
		return nil, fmt.Errorf("opcode %d undefined", op)
	}
	return def, nil
}

func Make(op Opcode, operands ...int) []byte {
	def, ok := definitions[op]
	if !ok {
		return []byte{}
	}

	instructionLen := 1
	for _, w := range def.OperandWidths {
		instructionLen += w
	}

	instruction := make([]byte, instructionLen) // allocate the byte slice
	instruction[0] = byte(op)
	offset := 1
	for i, o := range operands {
		width := def.OperandWidths[i]
		switch width {
		case 2:
			binary.BigEndian.PutUint16(instruction[offset:], uint16(o))
		}
		offset += width
	}

	return instruction
}

// ReadOperands reverses everything Make did
func ReadOperands(def *Definition, ins Instructions) ([]int, int) {
	operands := make([]int, len(def.OperandWidths))
	offset := 0

	for i, width := range def.OperandWidths {
		switch width {
		case 2:
			operands[i] = int(ReadUint16(ins[offset:]))
		}

		offset += width
	}

	return operands, offset
}

func ReadUint16(ins Instructions) uint16 {
	return binary.BigEndian.Uint16(ins)
}