package main

import (
	"bufio"
	"errors"
	"fmt"
	"io/ioutil"
	"os"
	"strings"

	"github.com/fatih/color"
	"github.com/forestgiant/sliceutil"
	flag "github.com/spf13/pflag"
)

func main() {
	bf := BrainfuckEmulator{}
	var err error
	err = bf.New()
	if err != nil {
		if strings.HasSuffix(err.Error(), "::ignore") {
			return
		}
		panic(err)
	}
	err = bf.Run()
	if err != nil {
		if strings.HasSuffix(err.Error(), "::ignore") {
			return
		}
		panic(err)
	}
}

// BrainfuckEmulator is used to emulate brainfuck.
type BrainfuckEmulator struct {
	code      string
	pairs     [][]int
	memory    []int
	execution int
	pointer   int
	debug     bool
	filename  string
}

// New instance of BrainfuckEmulator.
func (bf *BrainfuckEmulator) New() error {
	var err error
	err = bf.args()
	if err != nil {
		return err
	}
	err = bf.readScript(bf.filename)
	if err != nil {
		return err
	}
	err = bf.getBracketPairs()
	if err != nil {
		return err
	}
	return nil
}

func (bf *BrainfuckEmulator) args() error {
	debugptr := flag.BoolP("debug", "d", false, "Enable debug mode")
	flag.Parse()
	args := flag.Args()
	if len(args) == 1 {
		bf.filename = args[0]
		bf.debug = *debugptr
	} else {
		fmt.Fprintln(os.Stdout, "Brainfuck help")
		fmt.Fprintln(os.Stdout, "usage: brainfuck <filename>")
		flag.PrintDefaults()
		return errors.New("bf: invalid arguments ::ignore")
	}
	return nil
}

func (bf *BrainfuckEmulator) fancyoutput() {
	for i := 0; i < len(bf.memory); i++ {
		if bf.pointer == i {
			color.Set(color.FgRed)
			fmt.Fprintf(os.Stdout, fmt.Sprint(bf.memory[i]))
			color.Unset()
		} else {
			fmt.Fprintf(os.Stdout, fmt.Sprint(bf.memory[i]))
		}
		if i < len(bf.memory)-1 {
			fmt.Fprintf(os.Stdout, ",")
		}
	}
	fmt.Fprintln(os.Stdout, "")
	for i := 0; i < len(bf.code); i++ {
		if bf.execution == i {
			color.Set(color.FgRed)
			fmt.Fprintf(os.Stdout, string(bf.code[i]))
			color.Unset()
		} else {
			fmt.Fprintf(os.Stdout, string(bf.code[i]))
		}
	}
	fmt.Fprintln(os.Stdout, "")
}

// Run the brainfuck program.
func (bf *BrainfuckEmulator) Run() error {
	bf.pointer = 0
	bf.execution = 0
	bf.memory = []int{0}

	for true {
		if bf.debug {
			bf.fancyoutput()
		}

		if bf.execution >= len(bf.code) {
			break
		}
		switch bf.code[bf.execution] {
		case '+':
			bf.memory[bf.pointer]++
			if bf.memory[bf.pointer] == 256 {
				bf.memory[bf.pointer] = 0
			}
		case '-':
			bf.memory[bf.pointer]--
			if bf.memory[bf.pointer] == -1 {
				bf.memory[bf.pointer] = 255
			}
		case '>':
			bf.pointer++
			if len(bf.memory) <= bf.pointer {
				bf.memory = append(bf.memory, 0)
			}
		case '<':
			bf.pointer--
			if bf.pointer < 0 {
				fmt.Fprintln(os.Stdout, "Memory underflow: pointer can't be less than 0")
				return errors.New("bf: memory underflow ::ignore")
			}
		case '[':
			if bf.memory[bf.pointer] == 0 {
				for i := 0; i < len(bf.pairs[0]); i++ {
					if bf.pairs[0][i] == bf.execution {
						bf.execution = bf.pairs[1][i]
						break
					}
				}
			}
		case ']':
			if bf.memory[bf.pointer] != 0 {
				for i := 0; i < len(bf.pairs[1]); i++ {
					if bf.pairs[1][i] == bf.execution {
						bf.execution = bf.pairs[0][i]
						break
					}
				}
			}
		case '@':
			bf.pointer = 0
		case '$':
			bf.fancyoutput()
		case ',':
			char := make([]byte, 1)
			os.Stdin.Read(char)
			bf.memory[bf.pointer] = int(char[0])
		case '.':
			b := byte(bf.memory[bf.pointer])
			os.Stdout.Write([]byte{b})
		}
		bf.execution++
	}
	return nil
}

func (bf *BrainfuckEmulator) readScript(filename string) error {
	filebuffer, err := ioutil.ReadFile(filename)
	if err != nil {
		if strings.HasSuffix(err.Error(), "no such file or directory") {
			fmt.Fprintln(os.Stdout, "IO Error: no such file or directory")
			return errors.New("bf: file missing ::ignore")
		}
		return err
	}
	inputdata := string(filebuffer)
	data := bufio.NewScanner(strings.NewReader(inputdata))
	data.Split(bufio.ScanRunes)

	allowed := []string{"+", "-", "<", ">", "[", "]", ",", ".", "@"}
	final := ""
	for data.Scan() {
		if sliceutil.Contains(allowed, data.Text()) {
			final += data.Text()
		}
	}

	bf.code = final
	return nil
}

func (bf *BrainfuckEmulator) getBracketPairs() error {
	pairs := [][]int{{}, {}}
	temp := []int{}
	var err error

	for i := 0; i < len(bf.code); i++ {
		if bf.code[i] == '[' {
			temp = append(temp, i)
		} else if bf.code[i] == ']' {
			if len(temp) > 0 {
				pairs[0] = append(pairs[0], temp[len(temp)-1])
				pairs[1] = append(pairs[1], i)
				temp = temp[:len(temp)-1]
			} else {
				fmt.Fprintln(os.Stdout, "Syntax error: missing opening bracket")
				return errors.New("bf: missing opening bracket ::ignore")
			}
		}
	}
	if len(temp) != 0 {
		fmt.Fprintln(os.Stdout, "Syntax error: spare opening bracket")
		return errors.New("bf: sqare opening brackets ::ignore")
	}
	bf.pairs = pairs
	return err
}