codecrafters-shell-go/app/builtin/builtin.go

package builtin

import "fmt"
import "os"
import "os/exec"
import "strings"
import "github.com/codecrafters-io/shell-starter-go/app/core"

type Builtin interface {
	Run(ctx *core.Context) core.ExitStatus
	ParseArgs(ctx *core.Context, args []string) error
}

func Parse(ctx *core.Context, name string, args []string) (Builtin, error) {
	selected, err := selectBuiltin(name)
	if err != nil {
		return nil, err
	}
	if err := selected.ParseArgs(ctx, args); err != nil {
		return nil, err
	}
	return selected, nil
}

func selectBuiltin(name string) (Builtin, error) {
	switch name {
	case "cd":
		return &CdBuiltin{}, nil
	case "echo":
		return &EchoBuiltin{}, nil
	case "exit":
		return &ExitBuiltin{}, nil
	case "pwd":
		return &PwdBuiltin{}, nil
	case "type":
		return &TypeBuiltin{}, nil
	default:
		return nil, ParseError{Code: ParseErrorCodeUnknownBuiltin, value: name}
	}
}

type ParseError struct {
	Code  ParseErrorCode
	value string
}
type ParseErrorCode int

func (self ParseError) Error() string {
	switch self.Code {
	case ParseErrorCodeUsage:
		return fmt.Sprintf("usage: %s", self.value)
	case ParseErrorCodeUnknownBuiltin:
		// this should not be seen in normal REPL (we fallback to external command)
		return fmt.Sprintf("unknown built-in: %s", self.value)
	default:
		panic(self.Code)
	}
}

const (
	ParseErrorCodeUsage ParseErrorCode = iota
	ParseErrorCodeUnknownBuiltin
)

func makeUsage(pattern string) ParseError {
	return ParseError{
		Code:  ParseErrorCodeUsage,
		value: pattern,
	}
}

//
// builtin: "cd"
//

type CdBuiltin struct {
	path string
}

func (self *CdBuiltin) ParseArgs(ctx *core.Context, args []string) error {
	usage := makeUsage("cd [PATH]")
	self.path = ""
	switch len(args) {
	case 0:
		return usage
	case 1:
		self.path = args[0]
		return nil
	default:
		return usage
	}
}

func (self *CdBuiltin) Run(ctx *core.Context) core.ExitStatus {
	chdir := func(path string) core.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
	}
	if self.path == "~" || self.path == "" {
		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 chdir(home_path)
	}
	return chdir(self.path)
}

//
// builtin: "echo"
//

type EchoBuiltin struct {
	args []string
}

func (self *EchoBuiltin) ParseArgs(ctx *core.Context, args []string) error {
	self.args = args
	return nil
}

func (self *EchoBuiltin) Run(ctx *core.Context) core.ExitStatus {
	fmt.Fprintln(ctx.Stdout, strings.Join(self.args, " "))
	return 0
}

//
// builtin: "exit"
//

type ExitBuiltin struct {
}

func (self *ExitBuiltin) ParseArgs(ctx *core.Context, args []string) error {
	if len(args) != 0 {
		return makeUsage("exit")
	}
	return nil
}

func (self *ExitBuiltin) Run(ctx *core.Context) core.ExitStatus {
	os.Exit(0)
	return 0
}

//
// builtin: "pwd"
//

type PwdBuiltin struct {
}

func (self *PwdBuiltin) ParseArgs(ctx *core.Context, args []string) error {
	if len(args) != 0 {
		return makeUsage("pwd")
	}
	return nil
}

func (self *PwdBuiltin) Run(ctx *core.Context) core.ExitStatus {
	cwd_path, err := os.Getwd()
	if err != nil {
		panic("error getting current working directory")
	}
	fmt.Fprintln(ctx.Stdout, cwd_path)
	return 0
}

//
// builtin: "type"
//

type TypeBuiltin struct {
	query string
}

func (self *TypeBuiltin) ParseArgs(ctx *core.Context, args []string) error {
	if len(args) != 1 {
		return makeUsage("type COMMAND")
	}
	self.query = args[0]
	return nil
}

func (self *TypeBuiltin) Run(ctx *core.Context) core.ExitStatus {
	_, err := selectBuiltin(self.query)
	if err == nil {
		fmt.Fprintf(ctx.Stdout, "%s is a shell builtin\n", self.query)
		return 0
	}
	exec_path, err := exec.LookPath(self.query)
	if err == nil {
		fmt.Fprintf(ctx.Stdout, "%s is %s\n", self.query, exec_path)
		return 0
	}
	fmt.Fprintf(ctx.Stderr, "%s: not found\n", self.query)
	return 0
}