A new string scanner thats faster than bufio.Scanner and has zero-allocation

tools/utils/strings.go

@@ -3,7 +3,6 @@
 package utils
 
 import (
-	"bufio"
 	"fmt"
 	"strings"
 	"unicode/utf8"
@@ -23,7 +22,7 @@ func Capitalize(x string) string {
 type ScanLines struct {
 	entries []string
 
-	scanner *bufio.Scanner
+	scanner *StringScanner
 }
 
 func NewScanLines(entries ...string) *ScanLines {
@@ -35,7 +34,7 @@ func (self *ScanLines) Scan() bool {
 		if len(self.entries) == 0 {
 			return false
 		}
-		self.scanner = bufio.NewScanner(strings.NewReader(self.entries[0]))
+		self.scanner = NewLineScanner(self.entries[0])
 		self.entries = self.entries[1:]
 		return self.Scan()
 	} else {
@@ -54,15 +53,86 @@ func (self *ScanLines) Text() string {
 	return self.scanner.Text()
 }
 
-func Splitlines(x string, expected_number_of_lines ...int) (ans []string) {
-	if len(expected_number_of_lines) > 0 {
-		ans = make([]string, 0, expected_number_of_lines[0])
-	} else {
-		ans = make([]string, 0, 8)
+type StringScannerScanFunc = func(data string) (remaining_data, token string)
+type StringScannerPostprocessFunc = func(token string) string
+
+func ScanFuncForSeparator(sep string) StringScannerScanFunc {
+	if len(sep) == 1 {
+		sb := sep[0]
+		return func(data string) (remaining_data, token string) {
+			idx := strings.IndexByte(data, sb)
+			if idx < 0 {
+				return "", data
+			}
+			return data[idx+len(sep):], data[:idx]
+		}
+
 	}
-	scanner := bufio.NewScanner(strings.NewReader(x))
-	for scanner.Scan() {
-		ans = append(ans, scanner.Text())
+	return func(data string) (remaining_data, token string) {
+		idx := strings.Index(data, sep)
+		if idx < 0 {
+			return "", data
+		}
+		return data[idx+len(sep):], data[:idx]
 	}
-	return ans
+}
+
+// Faster, better designed, zero-allocation version of bufio.Scanner for strings
+type StringScanner struct {
+	ScanFunc             StringScannerScanFunc
+	PostProcessTokenFunc StringScannerPostprocessFunc
+
+	data  string
+	token string
+}
+
+func (self *StringScanner) Scan() bool {
+	if self.data == "" {
+		self.token = ""
+		return false
+	}
+	self.data, self.token = self.ScanFunc(self.data)
+	if self.PostProcessTokenFunc != nil {
+		self.token = self.PostProcessTokenFunc(self.token)
+	}
+	return true
+}
+
+func (self *StringScanner) Text() string {
+	return self.token
+}
+
+func (self *StringScanner) Split(data string, expected_number ...int) (ans []string) {
+	if len(expected_number) != 0 {
+		ans = make([]string, 0, expected_number[0])
+	} else {
+		ans = []string{}
+	}
+	self.data = data
+	for self.Scan() {
+		ans = append(ans, self.Text())
+	}
+	return
+}
+
+func NewLineScanner(text string) *StringScanner {
+	return &StringScanner{
+		data: text, ScanFunc: ScanFuncForSeparator("\n"),
+		PostProcessTokenFunc: func(s string) string {
+			if len(s) > 0 && s[len(s)-1] == '\r' {
+				s = s[:len(s)-1]
+			}
+			return s
+		},
+	}
+}
+
+func NewSeparatorScanner(text, separator string) *StringScanner {
+	return &StringScanner{
+		data: text, ScanFunc: ScanFuncForSeparator(separator),
+	}
+}
+
+func Splitlines(x string, expected_number_of_lines ...int) (ans []string) {
+	return NewLineScanner("").Split(x, expected_number_of_lines...)
 }

tools/utils/strings_test.go

@@ -0,0 +1,35 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"bufio"
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+var _ = fmt.Print
+
+func TestStringScanner(t *testing.T) {
+	for _, text := range []string{
+		"a\nb\nc",
+		"a\nb\nc\r",
+		"a\n\n\nb\nc",
+		"a\r\r\nb\r\nc\n",
+		"\n1",
+		"",
+	} {
+		actual := Splitlines(text)
+		expected := make([]string, 0, len(actual))
+		s := bufio.NewScanner(strings.NewReader(text))
+		for s.Scan() {
+			expected = append(expected, s.Text())
+		}
+		if diff := cmp.Diff(expected, actual); diff != "" {
+			t.Fatalf("Failed for: %#v\n%s", text, diff)
+		}
+	}
+}