src/pkg/text/template/parse/lex.go

// Copyright 2011 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 parse

import (
	"fmt"
	"strings"
	"unicode"
	"utf8"
)

// item represents a token or text string returned from the scanner.
type item struct {
	typ itemType
	val string
}

func (i item) String() string {
	switch {
	case i.typ == itemEOF:
		return "EOF"
	case i.typ == itemError:
		return i.val
	case i.typ > itemKeyword:
		return fmt.Sprintf("<%s>", i.val)
	case len(i.val) > 10:
		return fmt.Sprintf("%.10q...", i.val)
	}
	return fmt.Sprintf("%q", i.val)
}

// itemType identifies the type of lex items.
type itemType int

const (
	itemError        itemType = iota // error occurred; value is text of error
	itemBool                         // boolean constant
	itemChar                         // printable ASCII character; grab bag for comma etc.
	itemCharConstant                 // character constant
	itemComplex                      // complex constant (1+2i); imaginary is just a number
	itemColonEquals                  // colon-equals (':=') introducing a declaration
	itemEOF
	itemField      // alphanumeric identifier, starting with '.', possibly chained ('.x.y')
	itemIdentifier // alphanumeric identifier
	itemLeftDelim  // left action delimiter
	itemNumber     // simple number, including imaginary
	itemPipe       // pipe symbol
	itemRawString  // raw quoted string (includes quotes)
	itemRightDelim // right action delimiter
	itemString     // quoted string (includes quotes)
	itemText       // plain text
	itemVariable   // variable starting with '$', such as '$' or  '$1' or '$hello'.
	// Keywords appear after all the rest.
	itemKeyword  // used only to delimit the keywords
	itemDot      // the cursor, spelled '.'.
	itemDefine   // define keyword
	itemElse     // else keyword
	itemEnd      // end keyword
	itemIf       // if keyword
	itemRange    // range keyword
	itemTemplate // template keyword
	itemWith     // with keyword
)

// Make the types prettyprint.
var itemName = map[itemType]string{
	itemError:        "error",
	itemBool:         "bool",
	itemChar:         "char",
	itemCharConstant: "charconst",
	itemComplex:      "complex",
	itemColonEquals:  ":=",
	itemEOF:          "EOF",
	itemField:        "field",
	itemIdentifier:   "identifier",
	itemLeftDelim:    "left delim",
	itemNumber:       "number",
	itemPipe:         "pipe",
	itemRawString:    "raw string",
	itemRightDelim:   "right delim",
	itemString:       "string",
	itemVariable:     "variable",
	// keywords
	itemDot:      ".",
	itemDefine:   "define",
	itemElse:     "else",
	itemIf:       "if",
	itemEnd:      "end",
	itemRange:    "range",
	itemTemplate: "template",
	itemWith:     "with",
}

func (i itemType) String() string {
	s := itemName[i]
	if s == "" {
		return fmt.Sprintf("item%d", int(i))
	}
	return s
}

var key = map[string]itemType{
	".":        itemDot,
	"define":   itemDefine,
	"else":     itemElse,
	"end":      itemEnd,
	"if":       itemIf,
	"range":    itemRange,
	"template": itemTemplate,
	"with":     itemWith,
}

const eof = -1

// stateFn represents the state of the scanner as a function that returns the next state.
type stateFn func(*lexer) stateFn

// lexer holds the state of the scanner.
type lexer struct {
	name       string    // the name of the input; used only for error reports.
	input      string    // the string being scanned.
	leftDelim  string    // start of action.
	rightDelim string    // end of action.
	state      stateFn   // the next lexing function to enter.
	pos        int       // current position in the input.
	start      int       // start position of this item.
	width      int       // width of last rune read from input.
	items      chan item // channel of scanned items.
}

// next returns the next rune in the input.
func (l *lexer) next() (r rune) {
	if l.pos >= len(l.input) {
		l.width = 0
		return eof
	}
	r, l.width = utf8.DecodeRuneInString(l.input[l.pos:])
	l.pos += l.width
	return r
}

// peek returns but does not consume the next rune in the input.
func (l *lexer) peek() rune {
	r := l.next()
	l.backup()
	return r
}

// backup steps back one rune. Can only be called once per call of next.
func (l *lexer) backup() {
	l.pos -= l.width
}

// emit passes an item back to the client.
func (l *lexer) emit(t itemType) {
	l.items <- item{t, l.input[l.start:l.pos]}
	l.start = l.pos
}

// ignore skips over the pending input before this point.
func (l *lexer) ignore() {
	l.start = l.pos
}

// accept consumes the next rune if it's from the valid set.
func (l *lexer) accept(valid string) bool {
	if strings.IndexRune(valid, l.next()) >= 0 {
		return true
	}
	l.backup()
	return false
}

// acceptRun consumes a run of runes from the valid set.
func (l *lexer) acceptRun(valid string) {
	for strings.IndexRune(valid, l.next()) >= 0 {
	}
	l.backup()
}

// lineNumber reports which line we're on. Doing it this way
// means we don't have to worry about peek double counting.
func (l *lexer) lineNumber() int {
	return 1 + strings.Count(l.input[:l.pos], "\n")
}

// error returns an error token and terminates the scan by passing
// back a nil pointer that will be the next state, terminating l.run.
func (l *lexer) errorf(format string, args ...interface{}) stateFn {
	l.items <- item{itemError, fmt.Sprintf(format, args...)}
	return nil
}

// nextItem returns the next item from the input.
func (l *lexer) nextItem() item {
	for {
		select {
		case item := <-l.items:
			return item
		default:
			l.state = l.state(l)
		}
	}
	panic("not reached")
}

// lex creates a new scanner for the input string.
func lex(name, input, left, right string) *lexer {
	if left == "" {
		left = leftDelim
	}
	if right == "" {
		right = rightDelim
	}
	l := &lexer{
		name:       name,
		input:      input,
		leftDelim:  left,
		rightDelim: right,
		state:      lexText,
		items:      make(chan item, 2), // Two items of buffering is sufficient for all state functions
	}
	return l
}

// state functions

const (
	leftDelim    = "{{"
	rightDelim   = "}}"
	leftComment  = "/*"
	rightComment = "*/"
)

// lexText scans until an opening action delimiter, "{{".
func lexText(l *lexer) stateFn {
	for {
		if strings.HasPrefix(l.input[l.pos:], l.leftDelim) {
			if l.pos > l.start {
				l.emit(itemText)
			}
			return lexLeftDelim
		}
		if l.next() == eof {
			break
		}
	}
	// Correctly reached EOF.
	if l.pos > l.start {
		l.emit(itemText)
	}
	l.emit(itemEOF)
	return nil
}

// lexLeftDelim scans the left delimiter, which is known to be present.
func lexLeftDelim(l *lexer) stateFn {
	if strings.HasPrefix(l.input[l.pos:], l.leftDelim+leftComment) {
		return lexComment
	}
	l.pos += len(l.leftDelim)
	l.emit(itemLeftDelim)
	return lexInsideAction
}

// lexComment scans a comment. The left comment marker is known to be present.
func lexComment(l *lexer) stateFn {
	i := strings.Index(l.input[l.pos:], rightComment+l.rightDelim)
	if i < 0 {
		return l.errorf("unclosed comment")
	}
	l.pos += i + len(rightComment) + len(l.rightDelim)
	l.ignore()
	return lexText
}

// lexRightDelim scans the right delimiter, which is known to be present.
func lexRightDelim(l *lexer) stateFn {
	l.pos += len(l.rightDelim)
	l.emit(itemRightDelim)
	return lexText
}

// lexInsideAction scans the elements inside action delimiters.
func lexInsideAction(l *lexer) stateFn {
	// Either number, quoted string, or identifier.
	// Spaces separate and are ignored.
	// Pipe symbols separate and are emitted.
	if strings.HasPrefix(l.input[l.pos:], l.rightDelim) {
		return lexRightDelim
	}
	switch r := l.next(); {
	case r == eof || r == '\n':
		return l.errorf("unclosed action")
	case isSpace(r):
		l.ignore()
	case r == ':':
		if l.next() != '=' {
			return l.errorf("expected :=")
		}
		l.emit(itemColonEquals)
	case r == '|':
		l.emit(itemPipe)
	case r == '"':
		return lexQuote
	case r == '`':
		return lexRawQuote
	case r == '$':
		return lexIdentifier
	case r == '\'':
		return lexChar
	case r == '.':
		// special look-ahead for ".field" so we don't break l.backup().
		if l.pos < len(l.input) {
			r := l.input[l.pos]
			if r < '0' || '9' < r {
				return lexIdentifier // itemDot comes from the keyword table.
			}
		}
		fallthrough // '.' can start a number.
	case r == '+' || r == '-' || ('0' <= r && r <= '9'):
		l.backup()
		return lexNumber
	case isAlphaNumeric(r):
		l.backup()
		return lexIdentifier
	case r <= unicode.MaxASCII && unicode.IsPrint(r):
		l.emit(itemChar)
		return lexInsideAction
	default:
		return l.errorf("unrecognized character in action: %#U", r)
	}
	return lexInsideAction
}

// lexIdentifier scans an alphanumeric or field.
func lexIdentifier(l *lexer) stateFn {
Loop:
	for {
		switch r := l.next(); {
		case isAlphaNumeric(r):
			// absorb.
		case r == '.' && (l.input[l.start] == '.' || l.input[l.start] == '$'):
			// field chaining; absorb into one token.
		default:
			l.backup()
			word := l.input[l.start:l.pos]
			switch {
			case key[word] > itemKeyword:
				l.emit(key[word])
			case word[0] == '.':
				l.emit(itemField)
			case word[0] == '$':
				l.emit(itemVariable)
			case word == "true", word == "false":
				l.emit(itemBool)
			default:
				l.emit(itemIdentifier)
			}
			break Loop
		}
	}
	return lexInsideAction
}

// lexChar scans a character constant. The initial quote is already
// scanned.  Syntax checking is done by the parse.
func lexChar(l *lexer) stateFn {
Loop:
	for {
		switch l.next() {
		case '\\':
			if r := l.next(); r != eof && r != '\n' {
				break
			}
			fallthrough
		case eof, '\n':
			return l.errorf("unterminated character constant")
		case '\'':
			break Loop
		}
	}
	l.emit(itemCharConstant)
	return lexInsideAction
}

// lexNumber scans a number: decimal, octal, hex, float, or imaginary.  This
// isn't a perfect number scanner - for instance it accepts "." and "0x0.2"
// and "089" - but when it's wrong the input is invalid and the parser (via
// strconv) will notice.
func lexNumber(l *lexer) stateFn {
	if !l.scanNumber() {
		return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
	}
	if sign := l.peek(); sign == '+' || sign == '-' {
		// Complex: 1+2i.  No spaces, must end in 'i'.
		if !l.scanNumber() || l.input[l.pos-1] != 'i' {
			return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
		}
		l.emit(itemComplex)
	} else {
		l.emit(itemNumber)
	}
	return lexInsideAction
}

func (l *lexer) scanNumber() bool {
	// Optional leading sign.
	l.accept("+-")
	// Is it hex?
	digits := "0123456789"
	if l.accept("0") && l.accept("xX") {
		digits = "0123456789abcdefABCDEF"
	}
	l.acceptRun(digits)
	if l.accept(".") {
		l.acceptRun(digits)
	}
	if l.accept("eE") {
		l.accept("+-")
		l.acceptRun("0123456789")
	}
	// Is it imaginary?
	l.accept("i")
	// Next thing mustn't be alphanumeric.
	if isAlphaNumeric(l.peek()) {
		l.next()
		return false
	}
	return true
}

// lexQuote scans a quoted string.
func lexQuote(l *lexer) stateFn {
Loop:
	for {
		switch l.next() {
		case '\\':
			if r := l.next(); r != eof && r != '\n' {
				break
			}
			fallthrough
		case eof, '\n':
			return l.errorf("unterminated quoted string")
		case '"':
			break Loop
		}
	}
	l.emit(itemString)
	return lexInsideAction
}

// lexRawQuote scans a raw quoted string.
func lexRawQuote(l *lexer) stateFn {
Loop:
	for {
		switch l.next() {
		case eof, '\n':
			return l.errorf("unterminated raw quoted string")
		case '`':
			break Loop
		}
	}
	l.emit(itemRawString)
	return lexInsideAction
}

// isSpace reports whether r is a space character.
func isSpace(r rune) bool {
	switch r {
	case ' ', '\t', '\n', '\r':
		return true
	}
	return false
}

// isAlphaNumeric reports whether r is an alphabetic, digit, or underscore.
func isAlphaNumeric(r rune) bool {
	return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
}