package cli

import "os"
import "io"
import "fmt"
import "sort"
import "errors"
import "strings"
import "strconv"

// Cli represents a command line interface. It is not tied to os.Args, and it is
// possible to create multiple Cli's to have so many sub-commands in one
// program.
type Cli struct {
	Logger

	// Description describes the application.
	Description string

	// The syntax description of the command. It appears in Usage after the
	// name of the program (os.Args[0]). If not specified, "[OPTIONS]..."
	// will be used instead.
	Syntax string

	// Flag is a set of flags to parse.
	Flags []*Flag

	// Args is a list of leftover arguments that were not parsed.
	Args []string

	// Sub contains sub-commands.
	Sub map[string] *Cli

	// Super contains the string of commands used to invoke this Cli.
	// For example, a command invoked using
	//   <program> foo bar
	// Would have Super set to
	//   "foo bar"
	// This is automatically filled out by Cli.AddSub.
	Super string
}

// New creates a new Cli from a command description and a set of flags. These
// flags should be created as variables before passing them to New(), so that
// their values can be extracted after Parse is called.
func New (description string, flags ...*Flag) *Cli {
	return &Cli {
		Description: description,
		Flags:       flags,
		Sub:         make(map[string] *Cli),
	}
}

// AddSub adds a sub-command to this command. It automatically fills out the
// sub-command's Super field, and alters the prefix of its Logger to match.
func (this *Cli) AddSub (name string, sub *Cli) {
	super := this.Super
	if super != "" { super += " " }
	super += name
	sub.Super  = super
	sub.Prefix = os.Args[0] + " " + super
	this.Sub[name] = sub
}

// Parse parses the given set of command line arguments according to the flags
// defined within the Cli. The first element of the argument slice is always
// dropped, because it is assumed that it just contains the command name. If the
// second element matches up with the name of a sub-command, parsing is
// deferred to that sub-command instead. The sub-command will be given all but
// the first element of args. This method will return the address of the
// command that did the parsing.
func (this *Cli) Parse (args []string) (*Cli, error) {
	// try to find a sub-command
	if this.Sub != nil && len(args) > 1 {
		if sub, ok := this.Sub[args[1]]; ok {
			return sub.Parse(args[1:])
		}
	}

	// strip out program name
	if len(args) > 0 { args = args[1:] }

	next := func () string {
		if len(args) < 1 { return "" }
		args = args[1:]
		if len(args) > 0 {
			return args[0]
		} else {
			return ""
		}
	}

	processFlag := func (flag *Flag, allowInput bool) error {
		if flag.Validate == nil {
			flag.Value = "true"
		} else {
			if len(args) < 1 || !allowInput {
				return errors.New(
					flag.String() +
					" requires a value")
			}
			value := next()
			err   := flag.Validate(value)
			if err != nil {
				return errors.New(fmt.Sprint (
					"bad value for ", flag.String(), ": ",
					err))
			}
			flag.Value = value
		}
		if flag.Found != nil { flag.Found(this, flag.Value) }
		return nil
	}

	// process args one by one
	for ; len(args) > 0; next() {
		arg := args[0]
	
		switch {
		case arg == "--":
			// halt parsing
			this.Args = append(this.Args, args[1:]...)
			return nil, nil
		
		case strings.HasPrefix(arg, "--"):
			// long flag
			flag, ok := this.LongFlag(arg[2:])
			if !ok {
				return nil, errors.New("unknown flag: " + arg)
			}
			err := processFlag(flag, true)
			if err != nil { return nil, err }
			
		case strings.HasPrefix(arg, "-") && len(arg) > 1:
			// one or more short flags
			arg := arg[1:]
			for _, part := range arg {
				flag, ok := this.ShortFlag(part)
				if !ok {
					return nil, errors.New (
						"unknown flag: -" +
						string(part))
				}
				err := processFlag(flag, len(arg) == 1)
				if err != nil { return nil, err }
			}
			
		default:
			// not a flag
			this.Args = append(this.Args, arg)
		}
	}
	return this, nil
}

// ParseOrExit is like Parse, but if an error occurs while parsing the argument
// list, it prints out Usage and exits with error status 2.
func (this *Cli) ParseOrExit (args []string) *Cli {
	command, err := this.Parse(args)
	if err != nil {
		fmt.Fprintf(this, "%s: %v\n\n", os.Args[0], err)
		this.Usage()
		os.Exit(2)
	}
	return command
}

// ShortFlag searches for and returns the flag with the given short form. If it
// was found, it returns true. If it was not found, it returns nil, false.
func (this *Cli) ShortFlag (short rune) (*Flag, bool) {
	if short == 0 { return nil, false }
	for index := len(this.Flags) - 1; index >= 0; index -- {
		flag := this.Flags[index]
		if flag.Short == short { return flag, true }
	}
	return nil, false
}

// LongFlag searches for and returns the flag with the given long form. If it
// was found, it returns true. If it was not found, it returns nil, false.
func (this *Cli) LongFlag (long string) (*Flag, bool) {
	if long == "" { return nil, false }
	for index := len(this.Flags) - 1; index >= 0; index -- {
		flag := this.Flags[index]
		if flag.Long == long { return flag, true }
	}
	return nil, false
}

// Usage prints out usage/help information.
func (this *Cli) Usage () {
	// syntax
	fmt.Fprint(this, "Usage:")
	fmt.Fprint(this, " ", os.Args[0])
	if this.Super != "" {
		fmt.Fprint(this, " ", this.Super)
	}
	hasSubCommands := this.Sub != nil && len(this.Sub) > 0
	if hasSubCommands {
		fmt.Fprint(this, " [COMMAND]")
	}
	if this.Syntax == "" {
		fmt.Fprint(this, " [OPTION]...")
	} else {
		fmt.Fprint(this, " ", this.Syntax)
	}
	fmt.Fprint(this, "\n\n")

	// description
	if this.Description != "" {
		fmt.Fprintf(this, "%s\n\n", this.Description)
	}

	// fit the longest flag
	longest := 0
	for _, flag := range this.Flags {
		if len(flag.Long) > longest {
			longest = len(flag.Long)
		}
	}
	format := fmt.Sprint("\t%-", longest + 8, "s%s\n")

	// flags
	for _, flag := range this.Flags {
		shortLong := ""
		if flag.Short == 0 {
			shortLong += "    "
		} else {
			shortLong += "-" + string(flag.Short)
		}
		if flag.Short != 0 && flag.Long != "" {
			shortLong += ", "
		}
		if flag.Long != "" {
			shortLong += "--" + flag.Long
		}

		fmt.Fprintf(this, format, shortLong, flag.Help)
	}

	// commands
	if hasSubCommands {
		fmt.Fprint(this, "\nCommands:\n\n")
		names := sortMapKeys(this.Sub)

		// fit the longest command
		longest := 0
		for _, name := range names {
			if len(name) > longest {
				longest = len(name)
			}
		}
		format := fmt.Sprint("\t%-", longest + 2, "s%s\n")
		
		for _, name := range names {
			fmt.Fprintf(this, format, name, this.Sub[name].Description)
		}
	}
}

// Logger prints messages to an output writer.
type Logger struct {
	// Writer specifies the writer to output to. If it is nil, the Logger
	// will output to os.Stderr. If you wish to silence the output, set it
	// to io.Discard.
	io.Writer

	// Debug determines whether or not debug messages will be logged.
	Debug bool

	// Prefix is printed before all messages. If this is an empty string,
	// os.Args[0] will be used.
	Prefix string
}

// Println logs a normal message.
func (this *Logger) Println (v ...any) {
	fmt.Fprintf(this, "%s: %s", this.prefix(), fmt.Sprintln(v...))
}

// Errorln logs an error message.
func (this *Logger) Errorln (v ...any) {
	fmt.Fprintf(this, "%s: %s", this.prefix(), fmt.Sprintln(v...))
}

// Warnln logs a warning.
func (this *Logger) Warnln (v ...any) {
	fmt.Fprintf(this, "%s: warning: %s", this.prefix(), fmt.Sprintln(v...))
}

// Debugln logs debugging information. It will only print the message if the
// Debug field is set to true.
func (this *Logger) Debugln (v ...any) {
	if !this.Debug { return }
	fmt.Fprintf(this, "%s: debug: %s", this.prefix(), fmt.Sprintln(v...))
}

func (this *Logger) prefix () string {
	if this.Prefix == "" {
		return os.Args[0]
	} else {
		return this.Prefix
	}
}

// Write writes to the Logger's writer. If it is nil, it writes to os.Stderr.
func (this *Logger) Write (data []byte) (n int, err error) {
	if this.Writer == nil {
		return os.Stderr.Write(data)
	} else {
		return this.Writer.Write(data)
	}
}

// Flag is a command line option.
type Flag struct {
	Short rune   // The short form of the flag (-l)
	Long  string // Long form of the flag (--long-form)
	Help  string // Help text to display by the flag

	// Validate is an optional function that is called when an input is
	// passed to the flag. It checks whether or not the input is valid, and
	// returns an error if it isn't. If this function is nil, the flag is
	// assumed to have no input.
	Validate func (string) error

	// Value contains the input given to the flag, and is filled out when
	// the flags are parsed. If this is a non-input flag (if Validate is
	// nil), this will be set to true. If the flag was not specified, Value
	// will be unchanged.
	Value string

	// Found is an optional function that is called each time the flag is
	// found, and has a valid argument (if applicable). The value is passed
	// to the function, as well as the command that it was found in.
	Found func (*Cli, string)
}

// String returns --<LongForm> if specified, and if not returns -<ShortForm>
func (this *Flag) String () string {
	if this.Long == "" {
		return fmt.Sprintf("-%c", this.Short)
	} else {
		return fmt.Sprintf("--%s", this.Long)
	}
}

// NewFlag creates a new flag that does not take in a value.
func NewFlag (short rune, long string, help string) *Flag {
	return &Flag {
		Short: short,
		Long:  long,
		Help:  help,
	}
}

// NewInputFlag creates a new flag that does take in a value. This function will
// panic if the given validation function is nil.
func NewInputFlag (short rune, long string, help string, defaul string, validate func (string) error) *Flag {
	if validate == nil {
		panic("validate must be non-nil for a flag to take in a value")
	}
	return &Flag {
		Short:    short,
		Long:     long,
		Help:     help,
		Value:    defaul,
		Validate: validate,
	}
}

// NewHelp creates a new help flag activated by --help or -h. It shows help
// information for the command that it is a part of, and causes the program to
// exit with a status of 2.
func NewHelp () *Flag {
	flag := NewFlag('h', "help", "Display usage information and exit")
	flag.Found = func (command *Cli, value string) {
		command.Usage()
		os.Exit(2)
	}
	return flag
}

// ValString is a validation function that always returns nil, accepting any
// string.
func ValString (value string) error {
	return nil
}

// ValInt is a validation function that returns an error if the value is not an
// integer.
func ValInt (value string) error {
	_, err := strconv.Atoi(value)
	return err
}

// NewValSet returns a validation function that accepts a set of strings.
func NewValSet (allowed ...string) func (value string) error {
	return func (value string) error {
		allowedFmt := ""
		for index, test := range allowed {
			if test == value { return nil }

			if index > 0 { allowedFmt += ", " }
			allowedFmt += test
		}
		return errors.New (
			"value must be one of (" + allowedFmt + ")")
	}
}

func sortMapKeys[T any] (unsorted map[string] T) []string {
	keys := make([]string, len(unsorted))
	index := 0
	for key := range unsorted {
		keys[index] = key
		index ++
	}
	sort.Strings(keys)
	return keys
}