codecrafters-shell-go/app/main.go

package main

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

func getenv(key string, fallback string) string {
	value, ok := os.LookupEnv(key)
	if ok {
		return value
	} else {
		return fallback
	}
}

type Context struct {
	stdout io.Writer
	stderr io.Writer
	stdin  io.Reader
	paths  []string
}

func makeContext() Context {
	path_str := getenv("PATH", "")
	paths := make([]string, 0)
	for elem := range strings.SplitSeq(path_str, ":") {
		if elem == "" {
			continue
		}
		if !strings.HasPrefix(elem, "/") {
			continue
		}
		paths = append(paths, elem)
	}
	return Context{
		stdout: os.Stdout,
		stderr: os.Stderr,
		stdin:  os.Stdin,
		paths:  paths,
	}
}

type GetCommandResult int

const (
	GetCommandResultOk GetCommandResult = iota
	GetCommandResultError
)

type Command struct {
	command_type CommandType
	tokens       []string
	exec_path    string
}

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

func makeCommandBuiltin(command_type CommandType, tokens []string) Command {
	switch command_type {
	case CommandTypeNone:
		panic("invalid usage")
	case CommandTypeExternal:
		panic("invalid usage")
	default:
		return Command{
			command_type: command_type,
			tokens:       tokens,
		}
	}
}

func makeCommandExternal(tokens []string, exec_path string) Command {
	return Command{
		command_type: CommandTypeExternal,
		tokens:       tokens,
		exec_path:    exec_path,
	}
}

type CommandType int

const (
	CommandTypeNone CommandType = iota
	CommandTypeExternal
	CommandTypeBuiltinExit
	CommandTypeBuiltinEcho
	CommandTypeBuiltinType
)

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, ctx.paths)
	// 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, paths []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] == "echo" {
		return makeCommandBuiltin(CommandTypeBuiltinEcho, tokens), nil
	}
	if tokens[0] == "type" {
		return makeCommandBuiltin(CommandTypeBuiltinType, tokens), nil
	}
	for _, path := range paths {
		full_path := fmt.Sprintf("%s/%s", path, tokens[0])
		// fmt.Printf("parseCommand():full_path=%#v\n", full_path)
		stat, err := os.Stat(full_path)
		if err != nil {
			continue
		}
		if stat.IsDir() {
			continue
		}
		if stat.Mode()&0100 == 0 {
			continue
		}
		return makeCommandExternal(tokens, full_path), nil
	}
	return makeCommandNone(), ParseCommandError{ParseCommandErrorCodeNotFound, tokens[0]}
}

func handleCommand(ctx Context, command Command) {
	// fmt.Printf("handleCommand():command=%#v\n", command)
	switch command.command_type {
	case CommandTypeNone:
		return
	case CommandTypeBuiltinExit:
		os.Exit(0)
	case CommandTypeBuiltinEcho:
		fmt.Fprintln(ctx.stdout, strings.Join(command.tokens[1:], " "))
	case CommandTypeBuiltinType:
		if len(command.tokens) != 2 {
			fmt.Fprintln(ctx.stderr, "usage: type COMMAND")
			return
		}
		parsed, err := parseCommand(command.tokens[1], ctx.paths)
		if err != nil {
			fmt.Fprintln(ctx.stderr, err)
			return
		}
		switch parsed.command_type {
		case CommandTypeNone:
			panic("impossible parseCommand() result")
		case CommandTypeExternal:
			fmt.Fprintf(ctx.stdout, "%s is %s\n", parsed.tokens[0], parsed.exec_path)
		default:
			fmt.Fprintf(ctx.stdout, "%s is a shell builtin\n", parsed.tokens[0])
		}
	}
}

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)
	}
}