package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"os/exec"
	"strings"
)

type ExitStatus uint16

type Context struct {
	stdout io.Writer
	stderr io.Writer
	stdin  io.Reader
}

func makeContext() Context {
	return Context{
		stdout: os.Stdout,
		stderr: os.Stderr,
		stdin:  os.Stdin,
	}
}

type GetCommandResult int

const (
	GetCommandResultOk GetCommandResult = iota
	GetCommandResultError
)

type Command struct {
	command_type CommandType
	args         []string
	exec_path    string
	name         string
}

func makeCommandNone() Command {
	return Command{
		command_type: CommandTypeNone,
		args:         nil,
		exec_path:    "",
		name:         "",
	}
}

func makeCommandBuiltin(command_type CommandType, tokens []string) Command {
	switch command_type {
	case CommandTypeNone:
		panic("invalid usage")
	case CommandTypeExternal:
		panic("invalid usage")
	default:
		if len(tokens) < 1 {
			panic("assertion failed: token list must not be empty")
		}
		return Command{
			command_type: command_type,
			name:         tokens[0],
			args:         tokens[1:],
		}
	}
}

func makeCommandExternal(tokens []string, exec_path string) Command {
	if len(tokens) < 1 {
		panic("assertion failed: token list must not be empty")
	}
	return Command{
		command_type: CommandTypeExternal,
		name:         tokens[0],
		args:         tokens[1:],
		exec_path:    exec_path,
	}
}

type CommandType int

const (
	CommandTypeNone CommandType = iota
	CommandTypeExternal
	CommandTypeBuiltinExit
	CommandTypeBuiltinEcho
	CommandTypeBuiltinType
	CommandTypeBuiltinPwd
	CommandTypeBuiltinCd
)

func getCommand(ctx *Context) (Command, error) {
	fmt.Print("$ ")
	command_line, err := bufio.NewReader(ctx.stdin).ReadString('\n')
	if err != nil {
		return makeCommandNone(), err
	}
	command, err := parseCommand(command_line)
	// fmt.Printf("getCommand():command=%#v\n", command)
	if err != nil {
		fmt.Fprintln(ctx.stderr, err)
		return makeCommandNone(), nil
	}
	return command, nil
}

func tokenize(str string) []string {
	tokens := strings.Fields(str)
	return tokens
}

type ParseCommandError struct {
	code  ParseCommandErrorCode
	value string
}
type ParseCommandErrorCode int

const (
	ParseCommandErrorCodeNotFound ParseCommandErrorCode = iota
)

func (e ParseCommandError) Error() string {
	switch e.code {
	case ParseCommandErrorCodeNotFound:
		return fmt.Sprintf("%s: not found", e.value)
	default:
		return fmt.Sprintf("unknown ParseCommandErrorCode code: %d .value=%q", e.code, e.value)
	}
}

func parseCommand(command_line string) (Command, error) {
	tokens := tokenize(command_line)
	if len(tokens) == 0 {
		return makeCommandNone(), nil
	}
	if tokens[0] == "exit" {
		return makeCommandBuiltin(CommandTypeBuiltinExit, tokens), nil
	}
	if tokens[0] == "pwd" {
		return makeCommandBuiltin(CommandTypeBuiltinPwd, tokens), nil
	}
	if tokens[0] == "cd" {
		return makeCommandBuiltin(CommandTypeBuiltinCd, tokens), nil
	}
	if tokens[0] == "echo" {
		return makeCommandBuiltin(CommandTypeBuiltinEcho, tokens), nil
	}
	if tokens[0] == "type" {
		return makeCommandBuiltin(CommandTypeBuiltinType, tokens), nil
	}
	exec_path, err := exec.LookPath(tokens[0])
	if err != nil {
		return makeCommandNone(), ParseCommandError{ParseCommandErrorCodeNotFound, tokens[0]}
	}
	return makeCommandExternal(tokens, exec_path), nil
}

func bltnChdir(ctx Context, path string) ExitStatus {
	err := os.Chdir(path)
	if err != nil {
		fmt.Fprintf(ctx.stderr, "cd: %s: No such file or directory\n", path)
		return 4
	}
	return 0
}

func handleCommand(ctx Context, command Command) ExitStatus {
	// fmt.Printf("handleCommand():command=%#v\n", command)
	switch command.command_type {

	case CommandTypeNone:
		return 0

	case CommandTypeBuiltinExit:
		os.Exit(0)

	case CommandTypeBuiltinEcho:
		fmt.Fprintln(ctx.stdout, strings.Join(command.args, " "))
		return 0

	case CommandTypeBuiltinCd:
		switch len(command.args) {
		case 0:
			home_path := os.Getenv("HOME")
			if len(home_path) == 0 {
				fmt.Fprintln(ctx.stderr, "error: $HOME environment variable is empty or unset")
				return 4
			}
			return bltnChdir(ctx, home_path)
		case 1:
			if command.args[0] == "~" {
				home_path := os.Getenv("HOME")
				if len(home_path) == 0 {
					fmt.Fprintln(ctx.stderr, "error: $HOME environment variable is empty or unset")
					return 4
				}
				return bltnChdir(ctx, home_path)
			}
			return bltnChdir(ctx, command.args[0])
		default:
			fmt.Fprintln(ctx.stderr, "usage: cd PATH")
			return 2
		}

	case CommandTypeBuiltinPwd:
		if len(command.args) != 0 {
			fmt.Fprintln(ctx.stderr, "usage: pwd")
			return 2
		}
		cwd_path, err := os.Getwd()
		if err != nil {
			panic("error getting current working directory")
		}
		fmt.Fprintln(ctx.stdout, cwd_path)
		return 0

	case CommandTypeBuiltinType:
		if len(command.args) != 1 {
			fmt.Fprintln(ctx.stderr, "usage: type COMMAND")
			return 2
		}
		parsed, err := parseCommand(command.args[0])
		if err != nil {
			fmt.Fprintln(ctx.stderr, err)
			return 0
		}
		switch parsed.command_type {
		case CommandTypeNone:
			panic("impossible parseCommand() result")
		case CommandTypeExternal:
			fmt.Fprintf(ctx.stdout, "%s is %s\n", parsed.name, parsed.exec_path)
		default:
			fmt.Fprintf(ctx.stdout, "%s is a shell builtin\n", parsed.name)
		}
		return 0

	case CommandTypeExternal:
		cmd := exec.Command(command.exec_path, command.args...)
		cmd.Args[0] = command.name
		cmd.Stdout = ctx.stdout
		cmd.Stderr = ctx.stderr
		cmd.Stdin = ctx.stdin
		err := cmd.Run()
		if err != nil {
			if exiterr, ok := err.(*exec.ExitError); ok {
				return ExitStatus(exiterr.ExitCode())
				// fmt.Printf("handleCommand():exiterr=%#v\n", exiterr.ExitCode())
			} else {
				fmt.Printf("handleCommand():err=%#v\n", err)
				return ExitStatus(65535)
			}
		}
		return 0

	default:
		panic("unknown command type")
	}
	return ExitStatus(65535)
}

func main() {
	ctx := makeContext()
	for {
		command, err := getCommand(&ctx)
		if err != nil {
			fmt.Fprintln(os.Stderr, "Error reading input: ", err)
			os.Exit(1)
		}
		_ = handleCommand(ctx, command)
	}
}