// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
	opflag is a drop-in replacement for Go's flag package, implementing
	POSIX/GNU-style --flags.

	opflag is compatible with the GNU extensions to the POSIX recommendations
	for command-line options. See
	http://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html

	Usage:

	opflag is a drop-in replacement of Go's native flag package. If you import
	opflag under the name "flag" then all code should continue to function
	with no changes.

		import flag "github.com/ogier/opflag"

	There is one exception to this: if you directly instantiate the Flag struct
	there is one more field "Shorthand" that you will need to set.
	Most code never instantiates this struct directly, and instead uses
	functions such as String(), BoolVar(), and Var(), and is therefore
	unaffected.

	Define flags using flag.String(), Bool(), Int(), etc.

	This declares an integer flag, -flagname, stored in the pointer ip, with type *int.
		var ip = flag.Int("flagname", 1234, "help message for flagname")
	If you like, you can bind the flag to a variable using the Var() functions.
		var flagvar int
		func init() {
			flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
		}
	Or you can create custom flags that satisfy the Value interface (with
	pointer receivers) and couple them to flag parsing by
		flag.Var(&flagVal, "name", "help message for flagname")
	For such flags, the default value is just the initial value of the variable.

	After all flags are defined, call
		flag.Parse()
	to parse the command line into the defined flags.

	Flags may then be used directly. If you're using the flags themselves,
	they are all pointers; if you bind to variables, they're values.
		fmt.Println("ip has value ", *ip)
		fmt.Println("flagvar has value ", flagvar)

	After parsing, the arguments after the flag are available as the
	slice flag.Args() or individually as flag.Arg(i).
	The arguments are indexed from 0 through flag.NArg()-1.

	The opflag package also defines some new functions that are not in flag,
	that give one-letter shorthands for flags. You can use these by appending
	'P' to the name of any function that defines a flag.
		var ip = flag.IntP("flagname", "f", 1234, "help message")
		var flagvar bool
		func init() {
			flag.BoolVarP("boolname", "b", true, "help message")
		}
		flag.VarP(&flagVar, "varname", "v", 1234, "help message")
	Shorthand letters can be used with single dashes on the command line.
	Boolean shorthand flags can be combined with other shorthand flags.

	Command line flag syntax:
		--flag    // boolean flags only
		--flag=x

	Unlike the flag package, a single dash before an option means something
	different than a double dash. Single dashes signify a series of shorthand
	letters for flags. All but the last shorthand letter must be boolean flags.
		// boolean flags
		-f
		-abc
		// non-boolean flags
		-n 1234
		-Ifile
		// mixed
		-abcs "hello"
		-abcn1234

	Flag parsing stops after the terminator "--". Unlike the flag package,
	flags can be interspersed with arguments anywhere on the command line
	before this terminator.

	Integer flags accept 1234, 0664, 0x1234 and may be negative.
	Boolean flags (in their long form) accept 1, 0, t, f, true, false,
	TRUE, FALSE, True, False.
	Duration flags accept any input valid for time.ParseDuration.

	The default set of command-line flags is controlled by
	top-level functions.  The FlagSet type allows one to define
	independent sets of flags, such as to implement subcommands
	in a command-line interface. The methods of FlagSet are
	analogous to the top-level functions for the command-line
	flag set.
*/
package opflag

import (
	"errors"
	"fmt"
	"io"
	"os"
	"sort"
	"strings"
)

// ErrHelp is the error returned if the flag -help is invoked but no such flag is defined.
var ErrHelp = errors.New("opflag: help requested")

// SortFlags disables sorting, ensuring flags are printed in the order defined
var SortFlags bool = true

// ErrorHandling defines how to handle flag parsing errors.
type ErrorHandling int

const (
	ContinueOnError ErrorHandling = iota
	ExitOnError
	PanicOnError
)

// A FlagSet represents a set of defined flags.
type FlagSet struct {
	// Usage is the function called when an error occurs while parsing flags.
	// The field is a function (not a method) that may be changed to point to
	// a custom error handler.
	Usage func()

	name          string
	parsed        bool
	actual        map[string]*Flag
	formal        map[string]*Flag
	shorthands    map[byte]*Flag
	orderedFormal []*Flag
	args          []string // arguments after flags
	exitOnError   bool     // does the program exit if there's an error?
	errorHandling ErrorHandling
	output        io.Writer // nil means stderr; use out() accessor
	interspersed  bool      // allow interspersed option/non-option args
}

// A Flag represents the state of a flag.
type Flag struct {
	Name          string // name as it appears on command line
	Shorthand     string // one-letter abbreviated flag
	Usage         string // help message
	Value         Value  // value as set
	DefValue      string // default value (as text); for usage message
	shorthandOnly bool   // true if the user set only the shorthand option
}

// sortFlags returns the flags as a slice in lexicographical sorted order.
func sortFlags(flags map[string]*Flag) []*Flag {
	if !SortFlags {
		return CommandLine.orderedFormal
	}
	list := make(sort.StringSlice, len(flags))
	i := 0
	for _, f := range flags {
		list[i] = f.Name
		i++
	}
	list.Sort()
	result := make([]*Flag, len(list))
	for i, name := range list {
		result[i] = flags[name]
	}
	return result
}

func (f *FlagSet) out() io.Writer {
	if f.output == nil {
		return os.Stderr
	}
	return f.output
}

// SetOutput sets the destination for usage and error messages.
// If output is nil, os.Stderr is used.
func (f *FlagSet) SetOutput(output io.Writer) {
	f.output = output
}

// VisitAll visits the flags in lexicographical order, calling fn for each.
// It visits all flags, even those not set.
func (f *FlagSet) VisitAll(fn func(*Flag)) {
	for _, flag := range sortFlags(f.formal) {
		fn(flag)
	}
}

// VisitAll visits the command-line flags in lexicographical order, calling
// fn for each.  It visits all flags, even those not set.
func VisitAll(fn func(*Flag)) {
	CommandLine.VisitAll(fn)
}

// Visit visits the flags in lexicographical order, calling fn for each.
// It visits only those flags that have been set.
func (f *FlagSet) Visit(fn func(*Flag)) {
	for _, flag := range sortFlags(f.actual) {
		fn(flag)
	}
}

// Visit visits the command-line flags in lexicographical order, calling fn
// for each.  It visits only those flags that have been set.
func Visit(fn func(*Flag)) {
	CommandLine.Visit(fn)
}

// Lookup returns the Flag structure of the named flag, returning nil if none exists.
func (f *FlagSet) Lookup(name string) *Flag {
	return f.formal[name]
}

// Lookup returns the Flag structure of the named command-line flag,
// returning nil if none exists.
func Lookup(name string) *Flag {
	return CommandLine.formal[name]
}

// Set sets the value of the named flag.
func (f *FlagSet) Set(name, value string) error {
	flag, ok := f.formal[name]
	if !ok {
		return fmt.Errorf("no such flag -%v", name)
	}
	err := flag.Value.Set(value)
	if err != nil {
		return err
	}
	if f.actual == nil {
		f.actual = make(map[string]*Flag)
	}
	f.actual[name] = flag
	return nil
}

// Set sets the value of the named command-line flag.
func Set(name, value string) error {
	return CommandLine.Set(name, value)
}

// isZeroValue guesses whether the string represents the zero
// value for a flag. It is not accurate but in practice works OK.
func isZeroValue(value string) bool {
	switch value {
	case "false":
		return true
	case "":
		return true
	case "0":
		return true
	}
	return false
}

// UnquoteUsage extracts a back-quoted name from the usage
// string for a flag and returns it and the un-quoted usage.
// Given "a `name` to show" it returns ("name", "a name to show").
// If there are no back quotes, the name is an educated guess of the
// type of the flag's value, or the empty string if the flag is boolean.
func UnquoteUsage(flag *Flag) (name string, usage string) {
	// Look for a back-quoted name, but avoid the strings package.
	usage = flag.Usage
	for i := 0; i < len(usage); i++ {
		if usage[i] == '`' {
			for j := i + 1; j < len(usage); j++ {
				if usage[j] == '`' {
					name = usage[i+1 : j]
					usage = usage[:i] + name + usage[j+1:]
					return name, usage
				}
			}
			break // Only one back quote; use type name.
		}
	}
	// No explicit name, so use type if we can find one.
	name = "value"
	switch flag.Value.(type) {
	case boolFlag:
		name = ""
	case *durationValue:
		name = "duration"
	case *float64Value:
		name = "float"
	case *intValue, *int64Value:
		name = "int"
	case *stringValue:
		name = "string"
	case *uintValue, *uint64Value:
		name = "uint"
	}
	return
}

// PrintDefaults prints to standard error the default values of all
// defined command-line flags in the set. See the documentation for
// the global function PrintDefaults for more information.
func (f *FlagSet) PrintDefaults() {
	indent := 0
	f.VisitAll(func(flag *Flag) {
		s := ""
		if len(flag.Shorthand) > 0 {
			if flag.shorthandOnly {
				s = fmt.Sprintf("  -%s      ", flag.Shorthand)
			} else {
				s = fmt.Sprintf("  -%s, --%s", flag.Shorthand, flag.Name)
			}
		} else {
			s = fmt.Sprintf("  --%s", flag.Name)
		}

		name, _ := UnquoteUsage(flag)
		if len(name) > 0 {
			s += " " + name
		}
		s += " "
		if len(s) > indent {
			indent = len(s)
		}
	})
	f.VisitAll(func(flag *Flag) {
		s := ""
		if len(flag.Shorthand) > 0 {
			if flag.shorthandOnly {
				s = fmt.Sprintf("  -%s      ", flag.Shorthand)
			} else {
				s = fmt.Sprintf("  -%s, --%s", flag.Shorthand, flag.Name)
			}
		} else {
			s = fmt.Sprintf("  --%s", flag.Name)
		}

		name, usage := UnquoteUsage(flag)
		if len(name) > 0 {
			s += " " + name
		}

		for i := len(s); i < indent; i++ {
			s += " "
		}

		s += usage
		if !isZeroValue(flag.DefValue) {
			if _, ok := flag.Value.(*stringValue); ok {
				// put quotes on the value
				s += fmt.Sprintf(" (default %q)", flag.DefValue)
			} else {
				s += fmt.Sprintf(" (default %v)", flag.DefValue)
			}
		}
		fmt.Fprint(f.out(), s, "\n")
	})
}

// PrintDefaults prints to standard error the default values of all defined command-line flags.
func PrintDefaults() {
	CommandLine.PrintDefaults()
}

// defaultUsage is the default function to print a usage message.
func defaultUsage(f *FlagSet) {
	if f.name == "" {
		fmt.Fprintf(f.out(), "Usage:\n")
	} else {
		fmt.Fprintf(f.out(), "Usage of %s:\n", f.name)
	}
	f.PrintDefaults()
}

// NOTE: Usage is not just defaultUsage(CommandLine)
// because it serves (via godoc flag Usage) as the example
// for how to write your own usage function.

// Usage prints to standard error a usage message documenting all defined command-line flags.
// The function is a variable that may be changed to point to a custom function.
var Usage = func() {
	fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
	PrintDefaults()
}

// NFlag returns the number of flags that have been set.
func (f *FlagSet) NFlag() int { return len(f.actual) }

// NFlag returns the number of command-line flags that have been set.
func NFlag() int { return len(CommandLine.actual) }

// Arg returns the i'th argument.  Arg(0) is the first remaining argument
// after flags have been processed.
func (f *FlagSet) Arg(i int) string {
	if i < 0 || i >= len(f.args) {
		return ""
	}
	return f.args[i]
}

// Arg returns the i'th command-line argument.  Arg(0) is the first remaining argument
// after flags have been processed.
func Arg(i int) string {
	return CommandLine.Arg(i)
}

// NArg is the number of arguments remaining after flags have been processed.
func (f *FlagSet) NArg() int { return len(f.args) }

// NArg is the number of arguments remaining after flags have been processed.
func NArg() int { return len(CommandLine.args) }

// Args returns the non-flag arguments.
func (f *FlagSet) Args() []string { return f.args }

// Args returns the non-flag command-line arguments.
func Args() []string { return CommandLine.args }

// Var defines a flag with the specified name and usage string. The type and
// value of the flag are represented by the first argument, of type Value, which
// typically holds a user-defined implementation of Value. For instance, the
// caller could create a flag that turns a comma-separated string into a slice
// of strings by giving the slice the methods of Value; in particular, Set would
// decompose the comma-separated string into the slice.
func (f *FlagSet) Var(value Value, name string, usage string) {
	f.VarP(value, name, "", usage)
}

// Like Var, but accepts a shorthand letter that can be used after a single dash.
func (f *FlagSet) VarP(value Value, name, shorthand, usage string) {
	// Remember the default value as a string; it won't change.
	flag := &Flag{name, shorthand, usage, value, value.String(), false}
	if name == "" {
		flag.shorthandOnly = true
		flag.Name = flag.Shorthand
		name = shorthand
	}
	_, alreadythere := f.formal[name]
	if alreadythere {
		msg := fmt.Sprintf("%s flag redefined: %s", f.name, name)
		fmt.Fprintln(f.out(), msg)
		panic(msg) // Happens only if flags are declared with identical names
	}
	if f.formal == nil {
		f.formal = make(map[string]*Flag)
	}
	if f.orderedFormal == nil {
		f.orderedFormal = make([]*Flag, 0)
	}
	f.formal[name] = flag
	f.orderedFormal = append(f.orderedFormal, flag)

	if len(shorthand) == 0 {
		return
	}
	if len(shorthand) > 1 {
		fmt.Fprintf(f.out(), "%s shorthand more than ASCII character: %s\n", f.name, shorthand)
		panic("shorthand is more than one character")
	}
	if f.shorthands == nil {
		f.shorthands = make(map[byte]*Flag)
	}
	c := shorthand[0]
	old, alreadythere := f.shorthands[c]
	if alreadythere {
		fmt.Fprintf(f.out(), "%s shorthand reused: %q for %s already used for %s\n", f.name, c, name, old.Name)
		panic("shorthand redefinition")
	}
	f.shorthands[c] = flag
}

// Var defines a flag with the specified name and usage string. The type and
// value of the flag are represented by the first argument, of type Value, which
// typically holds a user-defined implementation of Value. For instance, the
// caller could create a flag that turns a comma-separated string into a slice
// of strings by giving the slice the methods of Value; in particular, Set would
// decompose the comma-separated string into the slice.
func Var(value Value, name string, usage string) {
	CommandLine.VarP(value, name, "", usage)
}

// Like Var, but accepts a shorthand letter that can be used after a single dash.
func VarP(value Value, name, shorthand, usage string) {
	CommandLine.VarP(value, name, shorthand, usage)
}

// failf prints to standard error a formatted error and usage message and
// returns the error.
func (f *FlagSet) failf(format string, a ...interface{}) error {
	err := fmt.Errorf(format, a...)
	fmt.Fprintln(f.out(), err)
	f.usage()
	return err
}

// usage calls the Usage method for the flag set, or the usage function if
// the flag set is CommandLine.
func (f *FlagSet) usage() {
	if f == CommandLine {
		Usage()
	} else if f.Usage == nil {
		defaultUsage(f)
	} else {
		f.Usage()
	}
}

func (f *FlagSet) setFlag(flag *Flag, value string, origArg string) error {
	if err := flag.Value.Set(value); err != nil {
		return f.failf("invalid argument %q for %s: %v", value, origArg, err)
	}
	// mark as visited for Visit()
	if f.actual == nil {
		f.actual = make(map[string]*Flag)
	}
	f.actual[flag.Name] = flag

	return nil
}

func (f *FlagSet) parseArgs(args []string) error {
	for len(args) > 0 {
		s := args[0]
		args = args[1:]
		if len(s) == 0 || s[0] != '-' || len(s) == 1 {
			if !f.interspersed {
				f.args = append(f.args, s)
				f.args = append(f.args, args...)
				return nil
			}
			f.args = append(f.args, s)
			continue
		}

		if s[1] == '-' {
			if len(s) == 2 { // "--" terminates the flags
				f.args = append(f.args, args...)
				return nil
			}
			name := s[2:]
			if len(name) == 0 || name[0] == '-' || name[0] == '=' {
				return f.failf("bad flag syntax: %s", s)
			}
			split := strings.SplitN(name, "=", 2)
			name = split[0]
			m := f.formal
			flag, alreadythere := m[name] // BUG
			if !alreadythere {
				if name == "help" { // special case for nice help message.
					f.usage()
					return ErrHelp
				}
				return f.failf("unknown flag: --%s", name)
			}
			if len(split) == 1 {
				if bv, ok := flag.Value.(boolFlag); !ok || !bv.IsBoolFlag() {
					if len(args) == 0 || args[0][0] == '-' {
						return f.failf("flag needs an argument: %s", s)
					}
					if err := f.setFlag(flag, args[0], s); err != nil {
						return err
					}
					args = args[1:]
				} else {
					f.setFlag(flag, "true", s)
				}
			} else {
				if err := f.setFlag(flag, split[1], s); err != nil {
					return err
				}
			}
		} else {
			shorthands := s[1:]
			for i := 0; i < len(shorthands); i++ {
				c := shorthands[i]
				flag, alreadythere := f.shorthands[c]
				if !alreadythere {
					if c == 'h' { // special case for nice help message.
						f.usage()
						return ErrHelp
					}
					return f.failf("unknown shorthand flag: %q in -%s", c, shorthands)
				}
				if bv, ok := flag.Value.(boolFlag); ok && bv.IsBoolFlag() {
					f.setFlag(flag, "true", s)
					continue
				}
				if i < len(shorthands)-1 {
					if err := f.setFlag(flag, shorthands[i+1:], s); err != nil {
						return err
					}
					break
				}
				if len(args) == 0 {
					return f.failf("flag needs an argument: %q in -%s", c, shorthands)
				}
				if err := f.setFlag(flag, args[0], s); err != nil {
					return err
				}
				args = args[1:]
				break // should be unnecessary
			}
		}
	}
	return nil
}

// Parse parses flag definitions from the argument list, which should not
// include the command name.  Must be called after all flags in the FlagSet
// are defined and before flags are accessed by the program.
// The return value will be ErrHelp if -help was set but not defined.
func (f *FlagSet) Parse(arguments []string) error {
	f.parsed = true
	f.args = make([]string, 0, len(arguments))
	err := f.parseArgs(arguments)
	if err != nil {
		switch f.errorHandling {
		case ContinueOnError:
			return err
		case ExitOnError:
			os.Exit(2)
		case PanicOnError:
			panic(err)
		}
	}
	return nil
}

// Parsed reports whether f.Parse has been called.
func (f *FlagSet) Parsed() bool {
	return f.parsed
}

// Parse parses the command-line flags from os.Args[1:].  Must be called
// after all flags are defined and before flags are accessed by the program.
func Parse() {
	// Ignore errors; CommandLine is set for ExitOnError.
	CommandLine.Parse(os.Args[1:])
}

// Whether to support interspersed option/non-option arguments.
func SetInterspersed(interspersed bool) {
	CommandLine.SetInterspersed(interspersed)
}

// Parsed returns true if the command-line flags have been parsed.
func Parsed() bool {
	return CommandLine.Parsed()
}

// The default set of command-line flags, parsed from os.Args.
var CommandLine = NewFlagSet(os.Args[0], ExitOnError)

// NewFlagSet returns a new, empty flag set with the specified name and
// error handling property.
func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
	f := &FlagSet{
		name:          name,
		errorHandling: errorHandling,
		interspersed:  true,
	}
	return f
}

// Whether to support interspersed option/non-option arguments.
func (f *FlagSet) SetInterspersed(interspersed bool) {
	f.interspersed = interspersed
}

// Init sets the name and error handling property for a flag set.
// By default, the zero FlagSet uses an empty name and the
// ContinueOnError error handling policy.
func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
	f.name = name
	f.errorHandling = errorHandling
}
