/*
Copyright 2012 Google Inc. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

/*
Package shlex implements a simple lexer which splits input in to tokens using
shell-style rules for quoting.

The basic use case uses the default ASCII lexer to split a string into sub-strings:

	shlex.Split("one \"two three\" four") -> []string{"one", "two three", "four"}

To process a stream of strings:

	l := NewLexer(os.Stdin)
	for ; token, err := l.Next(); err != nil {
		// process token
	}

To access the raw token stream (which includes tokens for spaces):

	  t := NewTokenizer(os.Stdin)
	  for ; token, err := t.Next(); err != nil {
		// process token
	  }
*/
package shlex

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

// TokenType is a top-level token classification: A word, space, unknown.
type TokenType int

// runeTokenClass is the type of a UTF-8 character classification: A quote, space, escape.
type runeTokenClass int

// the internal state used by the lexer state machine
type lexerState int

// Token is a (type, value) pair representing a lexographical token.
type Token struct {
	Type  TokenType
	Value string
	Pos   int64
}

// Named classes of UTF-8 runes
const (
	spaceRunes            = " \t\r\n"
	escapingQuoteRunes    = `"`
	nonEscapingQuoteRunes = "'"
	escapeRunes           = `\`
)

// Classes of rune token
const (
	unknownRuneClass runeTokenClass = iota
	spaceRuneClass
	escapingQuoteRuneClass
	nonEscapingQuoteRuneClass
	escapeRuneClass
	eofRuneClass
)

// Classes of lexographic token
const (
	UnknownToken TokenType = iota
	WordToken
	SpaceToken
)

func (t TokenType) String() string {
	switch t {
	default:
		return "UnknownToken"
	case WordToken:
		return "WordToken"
	case SpaceToken:
		return "SpaceToken"
	}
}

// Lexer state machine states
const (
	startState           lexerState = iota // no runes have been seen
	inWordState                            // processing regular runes in a word
	inSpaceState                           // processing runes in a space
	escapingState                          // we have just consumed an escape rune; the next rune is literal
	escapingQuotedState                    // we have just consumed an escape rune within a quoted string
	quotingEscapingState                   // we are within a quoted string that supports escaping ("...")
	quotingState                           // we are within a string that does not support escaping ('...')
)

// tokenClassifier is used for classifying rune characters.
type tokenClassifier map[rune]runeTokenClass

func (typeMap tokenClassifier) addRuneClass(runes string, tokenType runeTokenClass) {
	for _, runeChar := range runes {
		typeMap[runeChar] = tokenType
	}
}

// newDefaultClassifier creates a new classifier for ASCII characters.
func newDefaultClassifier() tokenClassifier {
	t := tokenClassifier{}
	t.addRuneClass(spaceRunes, spaceRuneClass)
	t.addRuneClass(escapingQuoteRunes, escapingQuoteRuneClass)
	t.addRuneClass(nonEscapingQuoteRunes, nonEscapingQuoteRuneClass)
	t.addRuneClass(escapeRunes, escapeRuneClass)
	return t
}

// ClassifyRune classifiees a rune
func (t tokenClassifier) ClassifyRune(runeVal rune) runeTokenClass {
	return t[runeVal]
}

// Lexer turns an input stream into a sequence of tokens. Whitespace is skipped.
type Lexer Tokenizer

// NewLexer creates a new lexer from an input stream.
func NewLexer(x io.RuneReader) *Lexer {

	return (*Lexer)(NewTokenizer(x))
}

// Next returns the next word, or an error. If there are no more words,
// the error will be io.EOF.
func (l *Lexer) Next() (string, error) {
	for {
		token, err := (*Tokenizer)(l).Next()
		if err != nil {
			return "", err
		}
		switch token.Type {
		case WordToken:
			return token.Value, nil
		case SpaceToken:
			// skip spaces
		default:
			return "", fmt.Errorf("Unknown token type: %s", token.Type)
		}
	}
}

// Tokenizer turns an input stream into a sequence of typed tokens
type Tokenizer struct {
	input      io.RuneReader
	classifier tokenClassifier
	pos        int64
	redo_rune  struct {
		char      rune
		sz        int
		rune_type runeTokenClass
	}
}

// NewTokenizer creates a new tokenizer from an input stream.
func NewTokenizer(input io.RuneReader) *Tokenizer {
	classifier := newDefaultClassifier()
	return &Tokenizer{
		input:      input,
		classifier: classifier}
}

var ErrTrailingEscape error = errors.New("EOF found after escape character")
var ErrTrailingQuoteEscape error = errors.New("EOF found after escape character for double quote")
var ErrUnclosedDoubleQuote error = errors.New("EOF found when expecting closing double quote")
var ErrUnclosedSingleQuote error = errors.New("EOF found when expecting closing single quote")

// scanStream scans the stream for the next token using the internal state machine.
// It will panic if it encounters a rune which it does not know how to handle.
func (t *Tokenizer) scanStream() (*Token, error) {
	state := startState
	var tokenType TokenType
	var nextRune rune
	var nextRuneType runeTokenClass
	var err error
	var sz int
	value := strings.Builder{}
	pos_at_start := t.pos

	unread_rune := func() {
		t.redo_rune.sz = sz
		t.redo_rune.char = nextRune
		t.redo_rune.rune_type = nextRuneType
		t.pos -= int64(sz)
	}

	token := func() *Token {
		return &Token{tokenType, value.String(), pos_at_start}
	}

	for {
		if t.redo_rune.sz > 0 {
			nextRune, sz = t.redo_rune.char, t.redo_rune.sz
			nextRuneType = t.redo_rune.rune_type
			t.redo_rune.sz = 0
		} else {
			nextRune, sz, err = t.input.ReadRune()
			nextRuneType = t.classifier.ClassifyRune(nextRune)
		}

		if err == io.EOF {
			nextRuneType = eofRuneClass
			err = nil
		} else if err != nil {
			return nil, err
		}
		t.pos += int64(sz)

		switch state {
		case startState: // no runes read yet
			{
				switch nextRuneType {
				case eofRuneClass:
					{
						return nil, io.EOF
					}
				case spaceRuneClass:
					{
						tokenType = SpaceToken
						value.WriteRune(nextRune)
						state = inSpaceState
					}
				case escapingQuoteRuneClass:
					{
						tokenType = WordToken
						state = quotingEscapingState
					}
				case nonEscapingQuoteRuneClass:
					{
						tokenType = WordToken
						state = quotingState
					}
				case escapeRuneClass:
					{
						tokenType = WordToken
						state = escapingState
					}
				default:
					{
						tokenType = WordToken
						value.WriteRune(nextRune)
						state = inWordState
					}
				}
			}
		case inSpaceState: // in a sequence of spaces separating words
			{
				switch nextRuneType {
				case spaceRuneClass:
					{
						value.WriteRune(nextRune)
					}
				default:
					{
						unread_rune()
						return token(), err
					}
				}
			}
		case inWordState: // in a regular word
			{
				switch nextRuneType {
				case eofRuneClass:
					{
						return token(), err
					}
				case spaceRuneClass:
					{
						unread_rune()
						return token(), err
					}
				case escapingQuoteRuneClass:
					{
						state = quotingEscapingState
					}
				case nonEscapingQuoteRuneClass:
					{
						state = quotingState
					}
				case escapeRuneClass:
					{
						state = escapingState
					}
				default:
					{
						value.WriteRune(nextRune)
					}
				}
			}
		case escapingState: // the rune after an escape character
			{
				switch nextRuneType {
				case eofRuneClass:
					{
						err = ErrTrailingEscape
						return token(), err
					}
				default:
					{
						state = inWordState
						value.WriteRune(nextRune)
					}
				}
			}
		case escapingQuotedState: // the next rune after an escape character, in double quotes
			{
				switch nextRuneType {
				case eofRuneClass:
					{
						err = ErrTrailingQuoteEscape
						return token(), err
					}
				default:
					{
						state = quotingEscapingState
						value.WriteRune(nextRune)
					}
				}
			}
		case quotingEscapingState: // in escaping double quotes
			{
				switch nextRuneType {
				case eofRuneClass:
					{
						err = ErrUnclosedDoubleQuote
						return token(), err
					}
				case escapingQuoteRuneClass:
					{
						state = inWordState
					}
				case escapeRuneClass:
					{
						state = escapingQuotedState
					}
				default:
					{
						value.WriteRune(nextRune)
					}
				}
			}
		case quotingState: // in non-escaping single quotes
			{
				switch nextRuneType {
				case eofRuneClass:
					{
						err = ErrUnclosedSingleQuote
						return token(), err
					}
				case nonEscapingQuoteRuneClass:
					{
						state = inWordState
					}
				default:
					{
						value.WriteRune(nextRune)
					}
				}
			}
		default:
			{
				return nil, fmt.Errorf("Unexpected state: %v", state)
			}
		}
	}
}

// Next returns the next token in the stream.
func (t *Tokenizer) Next() (*Token, error) {
	return t.scanStream()
}

// Pos returns the current position in the string as a byte offset
func (t *Tokenizer) Pos() int64 {
	return t.pos
}

// Split partitions a string into a slice of strings.
func Split(s string) ([]string, error) {
	l := NewLexer(strings.NewReader(s))
	subStrings := make([]string, 0)
	for {
		word, err := l.Next()
		if err != nil {
			if err == io.EOF {
				return subStrings, nil
			}
			return subStrings, err
		}
		subStrings = append(subStrings, word)
	}
}