// 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.

// Package scanner implements a scanner for Go source text.
// It takes a []byte as source which can then be tokenized
// through repeated calls to the Scan method.
//
package scanner

import (
	"bytes"
	"fmt"
	"path/filepath"
	"strconv"
	"unicode"
	"unicode/utf8"
)

import (
	"code.google.com/p/gcfg/token"
)

// An ErrorHandler may be provided to Scanner.Init. If a syntax error is
// encountered and a handler was installed, the handler is called with a
// position and an error message. The position points to the beginning of
// the offending token.
//
type ErrorHandler func(pos token.Position, msg string)

// A Scanner holds the scanner's internal state while processing
// a given text.  It can be allocated as part of another data
// structure but must be initialized via Init before use.
//
type Scanner struct {
	// immutable state
	file *token.File  // source file handle
	dir  string       // directory portion of file.Name()
	src  []byte       // source
	err  ErrorHandler // error reporting; or nil
	mode Mode         // scanning mode

	// scanning state
	ch         rune // current character
	offset     int  // character offset
	rdOffset   int  // reading offset (position after current character)
	lineOffset int  // current line offset

	// public state - ok to modify
	ErrorCount int // number of errors encountered
}

// Read the next Unicode char into s.ch.
// s.ch < 0 means end-of-file.
//
func (s *Scanner) next() {
	if s.rdOffset < len(s.src) {
		s.offset = s.rdOffset
		if s.ch == '\n' {
			s.lineOffset = s.offset
			s.file.AddLine(s.offset)
		}
		r, w := rune(s.src[s.rdOffset]), 1
		switch {
		case r == 0:
			s.error(s.offset, "illegal character NUL")
		case r >= 0x80:
			// not ASCII
			r, w = utf8.DecodeRune(s.src[s.rdOffset:])
			if r == utf8.RuneError && w == 1 {
				s.error(s.offset, "illegal UTF-8 encoding")
			}
		}
		s.rdOffset += w
		s.ch = r
	} else {
		s.offset = len(s.src)
		if s.ch == '\n' {
			s.lineOffset = s.offset
			s.file.AddLine(s.offset)
		}
		s.ch = -1 // eof
	}
}

// A mode value is a set of flags (or 0).
// They control scanner behavior.
//
type Mode uint

const (
	ScanComments Mode = 1 << iota // return comments as COMMENT tokens
)

// Init prepares the scanner s to tokenize the text src by setting the
// scanner at the beginning of src. The scanner uses the file set file
// for position information and it adds line information for each line.
// It is ok to re-use the same file when re-scanning the same file as
// line information which is already present is ignored. Init causes a
// panic if the file size does not match the src size.
//
// Calls to Scan will invoke the error handler err if they encounter a
// syntax error and err is not nil. Also, for each error encountered,
// the Scanner field ErrorCount is incremented by one. The mode parameter
// determines how comments are handled.
//
// Note that Init may call err if there is an error in the first character
// of the file.
//
func (s *Scanner) Init(file *token.File, src []byte, err ErrorHandler, mode Mode) {
	// Explicitly initialize all fields since a scanner may be reused.
	if file.Size() != len(src) {
		panic(fmt.Sprintf("file size (%d) does not match src len (%d)", file.Size(), len(src)))
	}
	s.file = file
	s.dir, _ = filepath.Split(file.Name())
	s.src = src
	s.err = err
	s.mode = mode

	s.ch = ' '
	s.offset = 0
	s.rdOffset = 0
	s.lineOffset = 0
	s.ErrorCount = 0

	s.next()
}

func (s *Scanner) error(offs int, msg string) {
	if s.err != nil {
		s.err(s.file.Position(s.file.Pos(offs)), msg)
	}
	s.ErrorCount++
}

var prefix = []byte("//line ")

func (s *Scanner) interpretLineComment(text []byte) {
	if bytes.HasPrefix(text, prefix) {
		// get filename and line number, if any
		if i := bytes.LastIndex(text, []byte{':'}); i > 0 {
			if line, err := strconv.Atoi(string(text[i+1:])); err == nil && line > 0 {
				// valid //line filename:line comment;
				filename := filepath.Clean(string(text[len(prefix):i]))
				if !filepath.IsAbs(filename) {
					// make filename relative to current directory
					filename = filepath.Join(s.dir, filename)
				}
				// update scanner position
				s.file.AddLineInfo(s.lineOffset+len(text)+1, filename, line) // +len(text)+1 since comment applies to next line
			}
		}
	}
}

func (s *Scanner) scanComment() string {
	// initial '/' already consumed; s.ch == '/' || s.ch == '*'
	offs := s.offset - 1 // position of initial '/'

	if s.ch == '/' {
		//-style comment
		s.next()
		for s.ch != '\n' && s.ch >= 0 {
			s.next()
		}
		if offs == s.lineOffset {
			// comment starts at the beginning of the current line
			s.interpretLineComment(s.src[offs:s.offset])
		}
		goto exit
	}

	/*-style comment */
	s.next()
	for s.ch >= 0 {
		ch := s.ch
		s.next()
		if ch == '*' && s.ch == '/' {
			s.next()
			goto exit
		}
	}

	s.error(offs, "comment not terminated")

exit:
	return string(s.src[offs:s.offset])
}

func (s *Scanner) findLineEnd() bool {
	// initial '/' already consumed

	defer func(offs int) {
		// reset scanner state to where it was upon calling findLineEnd
		s.ch = '/'
		s.offset = offs
		s.rdOffset = offs + 1
		s.next() // consume initial '/' again
	}(s.offset - 1)

	// read ahead until a newline, EOF, or non-comment token is found
	for s.ch == '/' || s.ch == '*' {
		if s.ch == '/' {
			//-style comment always contains a newline
			return true
		}
		/*-style comment: look for newline */
		s.next()
		for s.ch >= 0 {
			ch := s.ch
			if ch == '\n' {
				return true
			}
			s.next()
			if ch == '*' && s.ch == '/' {
				s.next()
				break
			}
		}
		s.skipWhitespace() // s.insertSemi is set
		if s.ch < 0 || s.ch == '\n' {
			return true
		}
		if s.ch != '/' {
			// non-comment token
			return false
		}
		s.next() // consume '/'
	}

	return false
}

func isLetter(ch rune) bool {
	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_' || ch >= 0x80 && unicode.IsLetter(ch)
}

func isDigit(ch rune) bool {
	return '0' <= ch && ch <= '9' || ch >= 0x80 && unicode.IsDigit(ch)
}

func (s *Scanner) scanIdentifier() string {
	offs := s.offset
	for isLetter(s.ch) || isDigit(s.ch) {
		s.next()
	}
	return string(s.src[offs:s.offset])
}

func (s *Scanner) scanEscape(quote rune) {
	offs := s.offset

	switch s.ch {
	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
		s.next()
		return
	}
	s.next() // always make progress
	s.error(offs, "unknown escape sequence")
	return
}

func (s *Scanner) scanString() string {
	// '"' opening already consumed
	offs := s.offset - 1

	for s.ch != '"' {
		ch := s.ch
		s.next()
		if ch == '\n' || ch < 0 {
			s.error(offs, "string not terminated")
			break
		}
		if ch == '\\' {
			s.scanEscape('"')
		}
	}

	s.next()

	return string(s.src[offs:s.offset])
}

func stripCR(b []byte) []byte {
	c := make([]byte, len(b))
	i := 0
	for _, ch := range b {
		if ch != '\r' {
			c[i] = ch
			i++
		}
	}
	return c[:i]
}

func (s *Scanner) scanRawString() string {
	// '`' opening already consumed
	offs := s.offset - 1

	hasCR := false
	for s.ch != '`' {
		ch := s.ch
		s.next()
		if ch == '\r' {
			hasCR = true
		}
		if ch < 0 {
			s.error(offs, "string not terminated")
			break
		}
	}

	s.next()

	lit := s.src[offs:s.offset]
	if hasCR {
		lit = stripCR(lit)
	}

	return string(lit)
}

func (s *Scanner) skipWhitespace() {
	for s.ch == ' ' || s.ch == '\t' || s.ch == '\n' || s.ch == '\r' {
		s.next()
	}
}

// Scan scans the next token and returns the token position, the token,
// and its literal string if applicable. The source end is indicated by
// token.EOF.
//
// If the returned token is a literal (token.IDENT, token.INT, token.FLOAT,
// token.IMAG, token.CHAR, token.STRING) or token.COMMENT, the literal string
// has the corresponding value.
//
// If the returned token is token.SEMICOLON, the corresponding
// literal string is ";" if the semicolon was present in the source,
// and "\n" if the semicolon was inserted because of a newline or
// at EOF.
//
// If the returned token is token.ILLEGAL, the literal string is the
// offending character.
//
// In all other cases, Scan returns an empty literal string.
//
// For more tolerant parsing, Scan will return a valid token if
// possible even if a syntax error was encountered. Thus, even
// if the resulting token sequence contains no illegal tokens,
// a client may not assume that no error occurred. Instead it
// must check the scanner's ErrorCount or the number of calls
// of the error handler, if there was one installed.
//
// Scan adds line information to the file added to the file
// set with Init. Token positions are relative to that file
// and thus relative to the file set.
//
func (s *Scanner) Scan() (pos token.Pos, tok token.Token, lit string) {
scanAgain:
	s.skipWhitespace()

	// current token start
	pos = s.file.Pos(s.offset)

	// determine token value
	switch ch := s.ch; {
	case isLetter(ch):
		lit = s.scanIdentifier()
		tok = token.IDENT
	default:
		s.next() // always make progress
		switch ch {
		case -1:
			tok = token.EOF
		case '\n':
			return pos, token.EOL, "\n"
		case '"':
			tok = token.STRING
			lit = s.scanString()
		case '`':
			tok = token.STRING
			lit = s.scanRawString()
		case '[':
			tok = token.LBRACK
		case ']':
			tok = token.RBRACK
		case ';', '#':
			// comment
			lit = s.scanComment()
			if s.mode&ScanComments == 0 {
				// skip comment
				goto scanAgain
			}
			tok = token.COMMENT
		case '=':
			tok = token.ASSIGN
		default:
			s.error(s.file.Offset(pos), fmt.Sprintf("illegal character %#U", ch))
			tok = token.ILLEGAL
			lit = string(ch)
		}
	}

	return
}