Add revision implementation (#139)

1 files changed, 117 insertions, 0 deletions

diff --git a/internal/revision/scanner.go b/internal/revision/scanner.go
new file mode 100644
index 0000000..fb5f333
--- /dev/null
+++ b/internal/revision/scanner.go
@@ -0,0 +1,117 @@
+package revision
+
+import (
+	"bufio"
+	"io"
+	"unicode"
+)
+
+// runeCategoryValidator takes a rune as input and
+// validates it belongs to a rune category
+type runeCategoryValidator func(r rune) bool
+
+// tokenizeExpression aggegates a series of runes matching check predicate into a single
+// string and provides given tokenType as token type
+func tokenizeExpression(ch rune, tokenType token, check runeCategoryValidator, r *bufio.Reader) (token, string, error) {
+	var data []rune
+	data = append(data, ch)
+
+	for {
+		c, _, err := r.ReadRune()
+
+		if c == zeroRune {
+			break
+		}
+
+		if err != nil {
+			return tokenError, "", err
+		}
+
+		if check(c) {
+			data = append(data, c)
+		} else {
+			err := r.UnreadRune()
+
+			if err != nil {
+				return tokenError, "", err
+			}
+
+			return tokenType, string(data), nil
+		}
+	}
+
+	return tokenType, string(data), nil
+}
+
+var zeroRune = 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)}
+}
+
+// Scan extracts tokens and their strings counterpart
+// from the reader
+func (s *scanner) scan() (token, string, error) {
+	ch, _, err := s.r.ReadRune()
+
+	if err != nil && err != io.EOF {
+		return tokenError, "", err
+	}
+
+	switch ch {
+	case zeroRune:
+		return eof, "", nil
+	case ':':
+		return colon, string(ch), nil
+	case '~':
+		return tilde, string(ch), nil
+	case '^':
+		return caret, string(ch), nil
+	case '.':
+		return dot, string(ch), nil
+	case '/':
+		return slash, string(ch), nil
+	case '{':
+		return obrace, string(ch), nil
+	case '}':
+		return cbrace, string(ch), nil
+	case '-':
+		return minus, string(ch), nil
+	case '@':
+		return at, string(ch), nil
+	case '\\':
+		return aslash, string(ch), nil
+	case '?':
+		return qmark, string(ch), nil
+	case '*':
+		return asterisk, string(ch), nil
+	case '[':
+		return obracket, string(ch), nil
+	case '!':
+		return emark, string(ch), nil
+	}
+
+	if unicode.IsSpace(ch) {
+		return space, string(ch), nil
+	}
+
+	if unicode.IsControl(ch) {
+		return control, string(ch), nil
+	}
+
+	if unicode.IsLetter(ch) {
+		return tokenizeExpression(ch, word, unicode.IsLetter, s.r)
+	}
+
+	if unicode.IsNumber(ch) {
+		return tokenizeExpression(ch, number, unicode.IsNumber, s.r)
+	}
+
+	return tokenError, string(ch), nil
+}