scanner.go

package forge

import (
	"bufio"
	"io"
	"strings"

	"github.com/go-aah/forge/token"
)

var eof = rune(0)

func isLetter(ch rune) bool {
	return ('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z')
}

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

func isNonNewlineWhitespace(ch rune) bool {
	return (ch == ' ' || ch == '\t' || ch == '\r')
}

func isBoolean(str string) bool {
	lower := strings.ToLower(str)
	return lower == "true" || lower == "false"
}

func isNull(str string) bool {
	return strings.ToLower(str) == "null"
}

func isInclude(str string) bool {
	return strings.ToLower(str) == "include"
}

// Scanner struct used to hold data necessary for parsing tokens
// from the input reader
type Scanner struct {
	curLine int
	curCol  int
	curTok  token.Token
	curCh   rune
	newline bool
	reader  *bufio.Reader
	freader io.Reader
}

// NewScanner creates and initializes a new *Scanner from an io.Readerx
func NewScanner(reader io.Reader) *Scanner {
	scanner := &Scanner{
		reader:  bufio.NewReader(reader),
		freader: reader,
		curLine: 1,
		curCol:  1,
		newline: false,
	}
	scanner.readRune()
	return scanner
}

func (scanner *Scanner) readRune() {
	if scanner.newline {
		scanner.curLine++
		scanner.curCol = 0
		scanner.newline = false
	} else {
		scanner.curCol++
	}

	nextCh, _, err := scanner.reader.ReadRune()
	if err == io.EOF || err != nil {
		scanner.curCh = eof
		return
	}

	scanner.curCh = nextCh

	if scanner.curCh == '\n' {
		scanner.newline = true
	}
}

func (scanner *Scanner) parseIdentifier() {
	scanner.curTok.ID = token.IDENTIFIER
	scanner.curTok.Literal = string(scanner.curCh)
	for {
		scanner.readRune()
		if !isLetter(scanner.curCh) && !isDigit(scanner.curCh) && scanner.curCh != '_' {
			break
		}
		scanner.curTok.Literal += string(scanner.curCh)
	}

	if isBoolean(scanner.curTok.Literal) {
		scanner.curTok.ID = token.BOOLEAN
	} else if isNull(scanner.curTok.Literal) {
		scanner.curTok.ID = token.NULL
	} else if isInclude(scanner.curTok.Literal) {
		scanner.curTok.ID = token.INCLUDE
	}
}

func (scanner *Scanner) parseEnvironment() {
	scanner.curTok.ID = token.ENVIRONMENT
	scanner.curTok.Literal = ""
	for {
		scanner.readRune()
		if !isLetter(scanner.curCh) && scanner.curCh != '_' {
			break
		}
		scanner.curTok.Literal += string(scanner.curCh)
	}
}

func (scanner *Scanner) parseNumber(negative bool) {
	scanner.curTok.ID = token.INTEGER
	scanner.curTok.Literal = string(scanner.curCh)
	if negative {
		scanner.curTok.Literal = "-" + scanner.curTok.Literal
	}

	digit := false
	for {
		scanner.readRune()
		if scanner.curCh == '.' && !digit {
			scanner.curTok.ID = token.FLOAT
			digit = true
		} else if !isDigit(scanner.curCh) {
			break
		}
		scanner.curTok.Literal += string(scanner.curCh)
	}
}

func (scanner *Scanner) parseString(delimiter rune) {
	scanner.curTok.ID = token.STRING
	scanner.curTok.Literal = ""
	if scanner.curCh == delimiter {
		scanner.readRune()
		return
	}
	// Whether or not we are trying to escape a character
	escape := false

	// If the first character is an escape
	if scanner.curCh == '\\' {
		escape = true
	} else {
		scanner.curTok.Literal = string(scanner.curCh)
	}
	for {
		scanner.readRune()
		if !escape {
			// We haven't seen a backslash yet
			if scanner.curCh == '\\' {
				// A new backslash
				escape = true
				continue
			} else if scanner.curCh == delimiter {
				// An unescaped quote, time to bail out
				break
			}
			// Continue as normal, no escaping necessary
		} else if scanner.curCh == '\\' || scanner.curCh == delimiter {
			// A valid escape character, continue as normal
			escape = false
		} else {
			// We had a backslash, but an invalid escape character
			// TODO: "Unsupported escape character found"
			break
		}
		scanner.curTok.Literal += string(scanner.curCh)
	}
	scanner.readRune()
}

func (scanner *Scanner) parseComment() {
	scanner.curTok.ID = token.COMMENT
	scanner.curTok.Literal = ""
	for {
		scanner.readRune()
		if scanner.curCh == '\n' || scanner.curCh == eof {
			break
		}
		scanner.curTok.Literal += string(scanner.curCh)
	}
	scanner.readRune()
}

func (scanner *Scanner) skipNonNewlineWhitespace() {
	for {
		scanner.readRune()
		if !isNonNewlineWhitespace(scanner.curCh) {
			break
		}
	}
}

// NextToken will read in the next valid token from the Scanner
func (scanner *Scanner) NextToken() token.Token {
	if isNonNewlineWhitespace(scanner.curCh) {
		scanner.skipNonNewlineWhitespace()
	}

	scanner.curTok = token.Token{
		ID:      token.ILLEGAL,
		Literal: string(scanner.curCh),
		Line:    scanner.curLine,
		Column:  scanner.curCol,
	}

	switch ch := scanner.curCh; {
	case isLetter(ch) || ch == '_':
		scanner.parseIdentifier()
	case isDigit(ch):
		scanner.parseNumber(false)
	case ch == '#':
		scanner.parseComment()
	case ch == '$':
		scanner.parseEnvironment()
	case ch == eof:
		scanner.curTok.ID = token.EOF
		scanner.curTok.Literal = "EOF"
		_ = close(scanner.freader)
	default:
		scanner.readRune()
		scanner.curTok.Literal = string(ch)
		switch ch {
		case ',':
			scanner.curTok.ID = token.COMMA
		case '=':
			scanner.curTok.ID = token.EQUAL
		case '"', '\'':
			scanner.parseString(ch)
		case '[':
			scanner.curTok.ID = token.LBRACKET
		case ']':
			scanner.curTok.ID = token.RBRACKET
		case '{':
			scanner.curTok.ID = token.LBRACE
		case '}':
			scanner.curTok.ID = token.RBRACE
		case ';':
			scanner.curTok.ID = token.SEMICOLON
		case '\n':
			scanner.curTok.ID = token.NEWLINE
		case '.':
			scanner.curTok.ID = token.PERIOD
		case '-':
			if isDigit(scanner.curCh) {
				scanner.parseNumber(true)
			}
		}
	}

	return scanner.curTok
}