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// 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 provides a scanner and tokenizer for UTF-8-encoded text.
// It takes an io.Reader providing the source, which then can be tokenized
// through repeated calls to the Scan function.  For compatibility with
// existing tools, the NUL character is not allowed (implementation
// restriction).
//
// By default, a Scanner skips white space and Go comments and recognizes all
// literals as defined by the Go language specification.  It may be
// customized to recognize only a subset of those literals and to recognize
// different white space characters.
//
// Basic usage pattern:
//
//      var s scanner.Scanner
//      s.Init(src)
//      tok := s.Scan()
//      for tok != scanner.EOF {
//              // do something with tok
//              tok = s.Scan()
//      }
//
package scanner

import (
        "bytes"
        "fmt"
        "io"
        "os"
        "unicode"
        "unicode/utf8"
)

// TODO(gri): Consider changing this to use the new (token) Position package.

// A source position is represented by a Position value.
// A position is valid if Line > 0.
type Position struct {
        Filename string // filename, if any
        Offset   int    // byte offset, starting at 0
        Line     int    // line number, starting at 1
        Column   int    // column number, starting at 1 (character count per line)
}

// IsValid returns true if the position is valid.
func (pos *Position) IsValid() bool { return pos.Line > 0 }

func (pos Position) String() string {
        s := pos.Filename
        if pos.IsValid() {
                if s != "" {
                        s += ":"
                }
                s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
        }
        if s == "" {
                s = "???"
        }
        return s
}

// Predefined mode bits to control recognition of tokens. For instance,
// to configure a Scanner such that it only recognizes (Go) identifiers,
// integers, and skips comments, set the Scanner's Mode field to:
//
//      ScanIdents | ScanInts | SkipComments
//
const (
        ScanIdents     = 1 << -Ident
        ScanInts       = 1 << -Int
        ScanFloats     = 1 << -Float // includes Ints
        ScanChars      = 1 << -Char
        ScanStrings    = 1 << -String
        ScanRawStrings = 1 << -RawString
        ScanComments   = 1 << -Comment
        SkipComments   = 1 << -skipComment // if set with ScanComments, comments become white space
        GoTokens       = ScanIdents | ScanFloats | ScanChars | ScanStrings | ScanRawStrings | ScanComments | SkipComments
)

// The result of Scan is one of the following tokens or a Unicode character.
const (
        EOF = -(iota + 1)
        Ident
        Int
        Float
        Char
        String
        RawString
        Comment
        skipComment
)

var tokenString = map[rune]string{
        EOF:       "EOF",
        Ident:     "Ident",
        Int:       "Int",
        Float:     "Float",
        Char:      "Char",
        String:    "String",
        RawString: "RawString",
        Comment:   "Comment",
}

// TokenString returns a (visible) string for a token or Unicode character.
func TokenString(tok rune) string {
        if s, found := tokenString[tok]; found {
                return s
        }
        return fmt.Sprintf("%q", string(tok))
}

// GoWhitespace is the default value for the Scanner's Whitespace field.
// Its value selects Go's white space characters.
const GoWhitespace = 1<<'\t' | 1<<'\n' | 1<<'\r' | 1<<' '

const bufLen = 1024 // at least utf8.UTFMax

// A Scanner implements reading of Unicode characters and tokens from an io.Reader.
type Scanner struct {
        // Input
        src io.Reader

        // Source buffer
        srcBuf [bufLen + 1]byte // +1 for sentinel for common case of s.next()
        srcPos int              // reading position (srcBuf index)
        srcEnd int              // source end (srcBuf index)

        // Source position
        srcBufOffset int // byte offset of srcBuf[0] in source
        line         int // line count
        column       int // character count
        lastLineLen  int // length of last line in characters (for correct column reporting)
        lastCharLen  int // length of last character in bytes

        // Token text buffer
        // Typically, token text is stored completely in srcBuf, but in general
        // the token text's head may be buffered in tokBuf while the token text's
        // tail is stored in srcBuf.
        tokBuf bytes.Buffer // token text head that is not in srcBuf anymore
        tokPos int          // token text tail position (srcBuf index); valid if >= 0
        tokEnd int          // token text tail end (srcBuf index)

        // One character look-ahead
        ch rune // character before current srcPos

        // Error is called for each error encountered. If no Error
        // function is set, the error is reported to os.Stderr.
        Error func(s *Scanner, msg string)

        // ErrorCount is incremented by one for each error encountered.
        ErrorCount int

        // The Mode field controls which tokens are recognized. For instance,
        // to recognize Ints, set the ScanInts bit in Mode. The field may be
        // changed at any time.
        Mode uint

        // The Whitespace field controls which characters are recognized
        // as white space. To recognize a character ch <= ' ' as white space,
        // set the ch'th bit in Whitespace (the Scanner's behavior is undefined
        // for values ch > ' '). The field may be changed at any time.
        Whitespace uint64

        // Start position of most recently scanned token; set by Scan.
        // Calling Init or Next invalidates the position (Line == 0).
        // The Filename field is always left untouched by the Scanner.
        // If an error is reported (via Error) and Position is invalid,
        // the scanner is not inside a token. Call Pos to obtain an error
        // position in that case.
        Position
}

// Init initializes a Scanner with a new source and returns s.
// Error is set to nil, ErrorCount is set to 0, Mode is set to GoTokens,
// and Whitespace is set to GoWhitespace.
func (s *Scanner) Init(src io.Reader) *Scanner {
        s.src = src

        // initialize source buffer
        // (the first call to next() will fill it by calling src.Read)
        s.srcBuf[0] = utf8.RuneSelf // sentinel
        s.srcPos = 0
        s.srcEnd = 0

        // initialize source position
        s.srcBufOffset = 0
        s.line = 1
        s.column = 0
        s.lastLineLen = 0
        s.lastCharLen = 0

        // initialize token text buffer
        // (required for first call to next()).
        s.tokPos = -1

        // initialize one character look-ahead
        s.ch = -1 // no char read yet

        // initialize public fields
        s.Error = nil
        s.ErrorCount = 0
        s.Mode = GoTokens
        s.Whitespace = GoWhitespace
        s.Line = 0 // invalidate token position

        return s
}

// TODO(gri): The code for next() and the internal scanner state could benefit
//            from a rethink. While next() is optimized for the common ASCII
//            case, the "corrections" needed for proper position tracking undo
//            some of the attempts for fast-path optimization.

// next reads and returns the next Unicode character. It is designed such
// that only a minimal amount of work needs to be done in the common ASCII
// case (one test to check for both ASCII and end-of-buffer, and one test
// to check for newlines).
func (s *Scanner) next() rune {
        ch, width := rune(s.srcBuf[s.srcPos]), 1

        if ch >= utf8.RuneSelf {
                // uncommon case: not ASCII or not enough bytes
                for s.srcPos+utf8.UTFMax > s.srcEnd && !utf8.FullRune(s.srcBuf[s.srcPos:s.srcEnd]) {
                        // not enough bytes: read some more, but first
                        // save away token text if any
                        if s.tokPos >= 0 {
                                s.tokBuf.Write(s.srcBuf[s.tokPos:s.srcPos])
                                s.tokPos = 0
                                // s.tokEnd is set by Scan()
                        }
                        // move unread bytes to beginning of buffer
                        copy(s.srcBuf[0:], s.srcBuf[s.srcPos:s.srcEnd])
                        s.srcBufOffset += s.srcPos
                        // read more bytes
                        // (an io.Reader must return io.EOF when it reaches
                        // the end of what it is reading - simply returning
                        // n == 0 will make this loop retry forever; but the
                        // error is in the reader implementation in that case)
                        i := s.srcEnd - s.srcPos
                        n, err := s.src.Read(s.srcBuf[i:bufLen])
                        s.srcPos = 0
                        s.srcEnd = i + n
                        s.srcBuf[s.srcEnd] = utf8.RuneSelf // sentinel
                        if err != nil {
                                if s.srcEnd == 0 {
                                        if s.lastCharLen > 0 {
                                                // previous character was not EOF
                                                s.column++
                                        }
                                        s.lastCharLen = 0
                                        return EOF
                                }
                                if err != io.EOF {
                                        s.error(err.Error())
                                }
                                // If err == EOF, we won't be getting more
                                // bytes; break to avoid infinite loop. If
                                // err is something else, we don't know if
                                // we can get more bytes; thus also break.
                                break
                        }
                }
                // at least one byte
                ch = rune(s.srcBuf[s.srcPos])
                if ch >= utf8.RuneSelf {
                        // uncommon case: not ASCII
                        ch, width = utf8.DecodeRune(s.srcBuf[s.srcPos:s.srcEnd])
                        if ch == utf8.RuneError && width == 1 {
                                // advance for correct error position
                                s.srcPos += width
                                s.lastCharLen = width
                                s.column++
                                s.error("illegal UTF-8 encoding")
                                return ch
                        }
                }
        }

        // advance
        s.srcPos += width
        s.lastCharLen = width
        s.column++

        // special situations
        switch ch {
        case 0:
                // implementation restriction for compatibility with other tools
                s.error("illegal character NUL")
        case '\n':
                s.line++
                s.lastLineLen = s.column
                s.column = 0
        }

        return ch
}

// Next reads and returns the next Unicode character.
// It returns EOF at the end of the source. It reports
// a read error by calling s.Error, if not nil; otherwise
// it prints an error message to os.Stderr. Next does not
// update the Scanner's Position field; use Pos() to
// get the current position.
func (s *Scanner) Next() rune {
        s.tokPos = -1 // don't collect token text
        s.Line = 0    // invalidate token position
        ch := s.Peek()
        s.ch = s.next()
        return ch
}

// Peek returns the next Unicode character in the source without advancing
// the scanner. It returns EOF if the scanner's position is at the last
// character of the source.
func (s *Scanner) Peek() rune {
        if s.ch < 0 {
                s.ch = s.next()
        }
        return s.ch
}

func (s *Scanner) error(msg string) {
        s.ErrorCount++
        if s.Error != nil {
                s.Error(s, msg)
                return
        }
        pos := s.Position
        if !pos.IsValid() {
                pos = s.Pos()
        }
        fmt.Fprintf(os.Stderr, "%s: %s\n", pos, msg)
}

func (s *Scanner) scanIdentifier() rune {
        ch := s.next() // read character after first '_' or letter
        for ch == '_' || unicode.IsLetter(ch) || unicode.IsDigit(ch) {
                ch = s.next()
        }
        return ch
}

func digitVal(ch rune) int {
        switch {
        case '0' <= ch && ch <= '9':
                return int(ch - '0')
        case 'a' <= ch && ch <= 'f':
                return int(ch - 'a' + 10)
        case 'A' <= ch && ch <= 'F':
                return int(ch - 'A' + 10)
        }
        return 16 // larger than any legal digit val
}

func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }

func (s *Scanner) scanMantissa(ch rune) rune {
        for isDecimal(ch) {
                ch = s.next()
        }
        return ch
}

func (s *Scanner) scanFraction(ch rune) rune {
        if ch == '.' {
                ch = s.scanMantissa(s.next())
        }
        return ch
}

func (s *Scanner) scanExponent(ch rune) rune {
        if ch == 'e' || ch == 'E' {
                ch = s.next()
                if ch == '-' || ch == '+' {
                        ch = s.next()
                }
                ch = s.scanMantissa(ch)
        }
        return ch
}

func (s *Scanner) scanNumber(ch rune) (rune, rune) {
        // isDecimal(ch)
        if ch == '0' {
                // int or float
                ch = s.next()
                if ch == 'x' || ch == 'X' {
                        // hexadecimal int
                        ch = s.next()
                        for digitVal(ch) < 16 {
                                ch = s.next()
                        }
                } else {
                        // octal int or float
                        seenDecimalDigit := false
                        for isDecimal(ch) {
                                if ch > '7' {
                                        seenDecimalDigit = true
                                }
                                ch = s.next()
                        }
                        if s.Mode&ScanFloats != 0 && (ch == '.' || ch == 'e' || ch == 'E') {
                                // float
                                ch = s.scanFraction(ch)
                                ch = s.scanExponent(ch)
                                return Float, ch
                        }
                        // octal int
                        if seenDecimalDigit {
                                s.error("illegal octal number")
                        }
                }
                return Int, ch
        }
        // decimal int or float
        ch = s.scanMantissa(ch)
        if s.Mode&ScanFloats != 0 && (ch == '.' || ch == 'e' || ch == 'E') {
                // float
                ch = s.scanFraction(ch)
                ch = s.scanExponent(ch)
                return Float, ch
        }
        return Int, ch
}

func (s *Scanner) scanDigits(ch rune, base, n int) rune {
        for n > 0 && digitVal(ch) < base {
                ch = s.next()
                n--
        }
        if n > 0 {
                s.error("illegal char escape")
        }
        return ch
}

func (s *Scanner) scanEscape(quote rune) rune {
        ch := s.next() // read character after '/'
        switch ch {
        case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
                // nothing to do
                ch = s.next()
        case '0', '1', '2', '3', '4', '5', '6', '7':
                ch = s.scanDigits(ch, 8, 3)
        case 'x':
                ch = s.scanDigits(s.next(), 16, 2)
        case 'u':
                ch = s.scanDigits(s.next(), 16, 4)
        case 'U':
                ch = s.scanDigits(s.next(), 16, 8)
        default:
                s.error("illegal char escape")
        }
        return ch
}

func (s *Scanner) scanString(quote rune) (n int) {
        ch := s.next() // read character after quote
        for ch != quote {
                if ch == '\n' || ch < 0 {
                        s.error("literal not terminated")
                        return
                }
                if ch == '\\' {
                        ch = s.scanEscape(quote)
                } else {
                        ch = s.next()
                }
                n++
        }
        return
}

func (s *Scanner) scanRawString() {
        ch := s.next() // read character after '`'
        for ch != '`' {
                if ch < 0 {
                        s.error("literal not terminated")
                        return
                }
                ch = s.next()
        }
}

func (s *Scanner) scanChar() {
        if s.scanString('\'') != 1 {
                s.error("illegal char literal")
        }
}

func (s *Scanner) scanComment(ch rune) rune {
        // ch == '/' || ch == '*'
        if ch == '/' {
                // line comment
                ch = s.next() // read character after "//"
                for ch != '\n' && ch >= 0 {
                        ch = s.next()
                }
                return ch
        }

        // general comment
        ch = s.next() // read character after "/*"
        for {
                if ch < 0 {
                        s.error("comment not terminated")
                        break
                }
                ch0 := ch
                ch = s.next()
                if ch0 == '*' && ch == '/' {
                        ch = s.next()
                        break
                }
        }
        return ch
}

// Scan reads the next token or Unicode character from source and returns it.
// It only recognizes tokens t for which the respective Mode bit (1<<-t) is set.
// It returns EOF at the end of the source. It reports scanner errors (read and
// token errors) by calling s.Error, if not nil; otherwise it prints an error
// message to os.Stderr.
func (s *Scanner) Scan() rune {
        ch := s.Peek()

        // reset token text position
        s.tokPos = -1
        s.Line = 0

redo:
        // skip white space
        for s.Whitespace&(1<<uint(ch)) != 0 {
                ch = s.next()
        }

        // start collecting token text
        s.tokBuf.Reset()
        s.tokPos = s.srcPos - s.lastCharLen

        // set token position
        // (this is a slightly optimized version of the code in Pos())
        s.Offset = s.srcBufOffset + s.tokPos
        if s.column > 0 {
                // common case: last character was not a '\n'
                s.Line = s.line
                s.Column = s.column
        } else {
                // last character was a '\n'
                // (we cannot be at the beginning of the source
                // since we have called next() at least once)
                s.Line = s.line - 1
                s.Column = s.lastLineLen
        }

        // determine token value
        tok := ch
        switch {
        case unicode.IsLetter(ch) || ch == '_':
                if s.Mode&ScanIdents != 0 {
                        tok = Ident
                        ch = s.scanIdentifier()
                } else {
                        ch = s.next()
                }
        case isDecimal(ch):
                if s.Mode&(ScanInts|ScanFloats) != 0 {
                        tok, ch = s.scanNumber(ch)
                } else {
                        ch = s.next()
                }
        default:
                switch ch {
                case '"':
                        if s.Mode&ScanStrings != 0 {
                                s.scanString('"')
                                tok = String
                        }
                        ch = s.next()
                case '\'':
                        if s.Mode&ScanChars != 0 {
                                s.scanChar()
                                tok = Char
                        }
                        ch = s.next()
                case '.':
                        ch = s.next()
                        if isDecimal(ch) && s.Mode&ScanFloats != 0 {
                                tok = Float
                                ch = s.scanMantissa(ch)
                                ch = s.scanExponent(ch)
                        }
                case '/':
                        ch = s.next()
                        if (ch == '/' || ch == '*') && s.Mode&ScanComments != 0 {
                                if s.Mode&SkipComments != 0 {
                                        s.tokPos = -1 // don't collect token text
                                        ch = s.scanComment(ch)
                                        goto redo
                                }
                                ch = s.scanComment(ch)
                                tok = Comment
                        }
                case '`':
                        if s.Mode&ScanRawStrings != 0 {
                                s.scanRawString()
                                tok = String
                        }
                        ch = s.next()
                default:
                        ch = s.next()
                }
        }

        // end of token text
        s.tokEnd = s.srcPos - s.lastCharLen

        s.ch = ch
        return tok
}

// Pos returns the position of the character immediately after
// the character or token returned by the last call to Next or Scan.
func (s *Scanner) Pos() (pos Position) {
        pos.Filename = s.Filename
        pos.Offset = s.srcBufOffset + s.srcPos - s.lastCharLen
        switch {
        case s.column > 0:
                // common case: last character was not a '\n'
                pos.Line = s.line
                pos.Column = s.column
        case s.lastLineLen > 0:
                // last character was a '\n'
                pos.Line = s.line - 1
                pos.Column = s.lastLineLen
        default:
                // at the beginning of the source
                pos.Line = 1
                pos.Column = 1
        }
        return
}

// TokenText returns the string corresponding to the most recently scanned token.
// Valid after calling Scan().
func (s *Scanner) TokenText() string {
        if s.tokPos < 0 {
                // no token text
                return ""
        }

        if s.tokEnd < 0 {
                // if EOF was reached, s.tokEnd is set to -1 (s.srcPos == 0)
                s.tokEnd = s.tokPos
        }

        if s.tokBuf.Len() == 0 {
                // common case: the entire token text is still in srcBuf
                return string(s.srcBuf[s.tokPos:s.tokEnd])
        }

        // part of the token text was saved in tokBuf: save the rest in
        // tokBuf as well and return its content
        s.tokBuf.Write(s.srcBuf[s.tokPos:s.tokEnd])
        s.tokPos = s.tokEnd // ensure idempotency of TokenText() call
        return s.tokBuf.String()
}