1 files changed, 239 insertions, 0 deletions
diff --git a/2021/05/line/line.go b/2021/05/line/line.go
new file mode 100644
index 0000000..aedf455
--- /dev/null
+++ b/2021/05/line/line.go
@@ -0,0 +1,239 @@
+package line
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"strconv"
+)
+
+type Line struct {
+	X1, Y1, X2, Y2 int
+}
+
+// Token represents a lexical token.
+type Token int
+
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	WHITESPACE
+
+	// Literals
+	INT // integers
+
+	// Misc characters
+	COMMA // ,
+	ARROW // ->
+)
+
+func isWhitespace(ch rune) bool {
+	return ch == ' ' || ch == '\t' || ch == '\n'
+}
+
+func isDigit(ch rune) bool {
+	return (ch >= '0' && ch <= '9')
+}
+
+var eof = rune(0)
+
+// Scanner represents a lexical scanner.
+type Scanner struct {
+	r *bufio.Reader
+}
+
+// NewScanner returns a new instance of Scanner.
+func NewScanner(r io.Reader) *Scanner {
+	return &Scanner{r: bufio.NewReader(r)}
+}
+
+// read reads the next rune from the bufferred reader.
+// Returns the rune(0) if an error occurs (or io.EOF is returned).
+func (s *Scanner) read() rune {
+	ch, _, err := s.r.ReadRune()
+	if err != nil {
+		return eof
+	}
+	return ch
+}
+
+// unread places the previously read rune back on the reader.
+func (s *Scanner) unread() { _ = s.r.UnreadRune() }
+
+// Scan returns the next token and literal value.
+func (s *Scanner) Scan() (tok Token, lit string) {
+	// Read the next rune.
+	ch := s.read()
+
+	// If we see whitespace then consume all contiguous whitespace.
+	// If we see a digit then consume as an int.
+	if isWhitespace(ch) {
+		return s.scanWhitespace(ch)
+	} else if isDigit(ch) {
+		return s.scanInt(ch)
+	}
+
+	// Otherwise read the individual character.
+	switch ch {
+	case eof:
+		return EOF, ""
+	case ',':
+		return COMMA, string(ch)
+	case '-':
+		return s.scanArrow(ch)
+	}
+
+	return ILLEGAL, string(ch)
+}
+
+// scanWhitespace consumes the current rune and all contiguous whitespace.
+func (s *Scanner) scanWhitespace(read rune) (tok Token, lit string) {
+	// Create a buffer and read the current character into it.
+	var buf bytes.Buffer
+	buf.WriteRune(read)
+
+	// Read every subsequent whitespace character into the buffer.
+	// Non-whitespace characters and EOF will cause the loop to exit.
+	for {
+		if ch := s.read(); ch == eof {
+			break
+		} else if !isWhitespace(ch) {
+			s.unread()
+			break
+		} else {
+			buf.WriteRune(ch)
+		}
+	}
+
+	return WHITESPACE, buf.String()
+}
+
+// scanInt consumes the current rune and all contiguous digit runes.
+func (s *Scanner) scanInt(read rune) (tok Token, lit string) {
+	// Create a buffer and read the current character into it.
+	var buf bytes.Buffer
+	buf.WriteRune(read)
+
+	// Read every subsequent ident character into the buffer.
+	// Non-ident characters and EOF will cause the loop to exit.
+	for {
+		if ch := s.read(); ch == eof {
+			break
+		} else if !isDigit(ch) {
+			s.unread()
+			break
+		} else {
+			_, _ = buf.WriteRune(ch)
+		}
+	}
+
+	// Otherwise return as a regular identifier.
+	return INT, buf.String()
+}
+
+func (s *Scanner) scanArrow(read rune) (tok Token, lit string) {
+	// Create a buffer and read the current character into it.
+	var buf bytes.Buffer
+	buf.WriteRune(read)
+
+	// Read every subsequent ident character into the buffer.
+	// Non-ident characters and EOF will cause the loop to exit.
+	for {
+		if ch := s.read(); ch == eof {
+			break
+		} else if ch == '>' {
+			_, _ = buf.WriteRune(ch)
+			return ARROW, buf.String()
+		} else {
+			_, _ = buf.WriteRune(ch)
+			break
+		}
+	}
+
+	// Otherwise return as a regular identifier.
+	return ILLEGAL, buf.String()
+}
+
+// Parser represents a parser.
+type Parser struct {
+	s   *Scanner
+	buf struct {
+		tok Token  // last read token
+		lit string // last read literal
+		n   int    // buffer size (max=1)
+	}
+}
+
+// NewParser returns a new instance of Parser.
+func NewParser(r io.Reader) *Parser {
+	return &Parser{s: NewScanner(r)}
+}
+
+// scan returns the next token from the underlying scanner.
+// If a token has been unscanned then read that instead.
+func (p *Parser) scan() (tok Token, lit string) {
+	// If we have a token on the buffer, then return it.
+	if p.buf.n != 0 {
+		p.buf.n = 0
+		return p.buf.tok, p.buf.lit
+	}
+
+	// Otherwise read the next token from the scanner.
+	tok, lit = p.s.Scan()
+
+	// Save it to the buffer in case we unscan later.
+	p.buf.tok, p.buf.lit = tok, lit
+
+	return
+}
+
+// unscan pushes the previously read token back onto the buffer.
+func (p *Parser) unscan() { p.buf.n = 1 }
+
+// scanIgnoreWhitespace scans the next non-whitespace token.
+func (p *Parser) scanIgnoreWhitespace() (tok Token, lit string) {
+	tok, lit = p.scan()
+	if tok == WHITESPACE {
+		tok, lit = p.scan()
+	}
+	return
+}
+
+func (p *Parser) Parse() (*Line, error) {
+	l := &Line{}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != INT {
+		return nil, fmt.Errorf("found %q, expected INT", lit)
+	} else {
+		l.X1, _ = strconv.Atoi(lit)
+	}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != COMMA {
+		return nil, fmt.Errorf("found %q, expected COMMA", lit)
+	}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != INT {
+		return nil, fmt.Errorf("found %q, expected INT", lit)
+	} else {
+		l.Y1, _ = strconv.Atoi(lit)
+	}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != ARROW {
+		return nil, fmt.Errorf("found %q, expected ARROW", lit)
+	}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != INT {
+		return nil, fmt.Errorf("found %q, expected INT", lit)
+	} else {
+		l.X2, _ = strconv.Atoi(lit)
+	}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != COMMA {
+		return nil, fmt.Errorf("found %q, expected COMMA", lit)
+	}
+	if tok, lit := p.scanIgnoreWhitespace(); tok != INT {
+		return nil, fmt.Errorf("found %q, expected INT", lit)
+	} else {
+		l.Y2, _ = strconv.Atoi(lit)
+	}
+	if tok, lit := p.scan(); tok != WHITESPACE {
+		return nil, fmt.Errorf("found %q, expected WHITESPACE", lit)
+	}
+	return l, nil
+}