feat: add StripSource to Scanner (#68)

- Add StripSource to Scanner

- Use pure string instead of string address since origin string is changed during parsing time

ast/node.go

@@ -70,17 +70,17 @@ func (l Leaf) collapse(level int) Node {
 
 type Branch map[string]Children
 
-func (n Branch) One(name string) Node {
-	if c, has := n[name]; has {
+func (b Branch) One(name string) Node {
+	if c, has := b[name]; has {
 		if one, ok := c.(One); ok {
 			return one.Node
 		}
 	}
 	return nil
 }
 
-func (n Branch) Many(name string) []Node {
-	if c, has := n[name]; has {
+func (b Branch) Many(name string) []Node {
+	if c, has := b[name]; has {
 		if many, ok := c.(Many); ok {
 			return many
 		}
@@ -138,9 +138,9 @@ func (l Leaf) clone() Node {
 	return l
 }
 
-func (n Branch) clone() Node {
+func (b Branch) clone() Node {
 	result := Branch{}
-	for name, node := range n {
+	for name, node := range b {
 		result[name] = node.clone()
 	}
 	return result
@@ -162,12 +162,12 @@ func (l Leaf) narrow() bool {
 	return true
 }
 
-func (n Branch) narrow() bool {
-	switch len(n) {
+func (b Branch) narrow() bool {
+	switch len(b) {
 	case 0:
 		return true
 	case 1:
-		for _, group := range n {
+		for _, group := range b {
 			return group.narrow()
 		}
 	}
@@ -193,18 +193,17 @@ func (b Branch) ContentEquals(other Node) bool {
 			return false
 		}
 		for k, v := range b {
-			switch v.(type) {
+			switch v := v.(type) {
 			case One:
-				if !v.(One).Node.ContentEquals(other.One(k)) {
+				if !v.Node.ContentEquals(other.One(k)) {
 					return false
 				}
 			case Many:
-				nodes := v.(Many)
 				otherNodes := other.Many(k)
-				if len(nodes) != len(otherNodes) {
+				if len(v) != len(otherNodes) {
 					return false
 				}
-				for i, n := range nodes {
+				for i, n := range v {
 					if !n.ContentEquals(otherNodes[i]) {
 						return false
 					}

parser/scanner.go

@@ -8,38 +8,41 @@ import (
 
 type Scanner struct {
 	src         source // the source the scanner is drawing from
-	sliceStart  int    // the start of the slice visible to the scanner
-	sliceLength int    // the length of the slice visible to the scanner
+	sliceStart  int    // the start of the slice visible to the scanner, based on the original src
+	sliceLength int    // the length of the slice visible to the scanner, based on the original src
 }
 
 type source interface {
 	length() int                // the length of the entire source string
 	slice(i, length int) string // the string of the given slice
 	filename() string           // the name of the file from which the source is derived (or empty if none)
+	stripSource(i, length int) source
 }
 
 type stringSource struct {
-	origin *string // the entire source string
-	f      string  // the source filename
+	origin string // the entire source string
+	f      string // the source filename
 }
 
 func NewScanner(str string) *Scanner {
-	return &Scanner{stringSource{origin: &str}, 0, len(str)}
+	return &Scanner{stringSource{origin: str}, 0, len(str)}
 }
 
 func NewScannerWithFilename(str, filename string) *Scanner {
-	return &Scanner{stringSource{&str, filename}, 0, len(str)}
+	return &Scanner{stringSource{str, filename}, 0, len(str)}
 }
 
 func NewScannerAt(str string, offset, size int) *Scanner {
-	if offset+size > len(str) {
-		panic(fmt.Errorf("%d is out of range [:%d]", offset+size, len(str)))
-	}
-	return &Scanner{stringSource{origin: &str}, offset, size}
+	return &Scanner{stringSource{origin: str}, offset, size}
 }
 
 // - Scanner
 
+func (r Scanner) StripSource() Scanner {
+	r.src = r.src.stripSource(r.sliceStart, r.sliceLength)
+	return r
+}
+
 // The name of the file from which the source is derived (or empty if none).
 func (s Scanner) Filename() string {
 	return s.src.filename()
@@ -162,12 +165,21 @@ func (s *Scanner) EatRegexp(re *regexp.Regexp, match *Scanner, captures []Scanne
 
 // - stringSource
 
+func (s stringSource) stripSource(offset, size int) source {
+	s.origin = s.slice(offset, size)
+	return s
+}
+
 func (s stringSource) length() int {
-	return len(*s.origin)
+	return len(s.origin)
 }
 
 func (s stringSource) slice(i, length int) string {
-	return (*s.origin)[i : i+length]
+	// Since offset and length based on the original origin string, so they might be out of range
+	if i < 0 || i+length < 0 || i > len(s.origin) || i+length > len(s.origin) {
+		return s.origin
+	}
+	return (s.origin)[i : i+length]
 }
 
 func (s stringSource) filename() string {