Optimize git object parsing with StringScanner

lib/src/commit.dart

@@ -43,6 +43,9 @@ class Commit {
     final commits = <String, Commit>{};
 
     while (!scanner.isDone) {
+      if (scanner.scan(RegExp(r'\r?\n'))) {
+        continue;
+      }
       final tuple = _parse(scanner, true);
       commits[tuple.sha!] = tuple.commit;
     }
@@ -57,37 +60,34 @@ class Commit {
     final headers = <String, List<String>>{};
 
     final startSpot = scanner.position;
-    var lastLine = scanner.readNextLine();
-
-    while (lastLine != null && lastLine.isNotEmpty) {
-      final allHeaderMatches = headerRegExp.allMatches(lastLine);
-      if (allHeaderMatches.isNotEmpty) {
-        final match = allHeaderMatches.single;
-        assert(match.groupCount == 2);
-        final header = match.group(1)!;
-        final value = match.group(2)!;
 
-        headers.putIfAbsent(header, () => <String>[]).add(value);
-      }
+    while (scanner.scan(headerRegExp)) {
+      final match = scanner.lastMatch!;
+      final header = match.group(1)!;
+      final value = match.group(2)!;
 
-      lastLine = scanner.readNextLine()!;
+      headers.putIfAbsent(header, () => <String>[]).add(value);
     }
 
-    assert(lastLine!.isEmpty);
+    // consume the blank line but it might not exist if the commit has no body
+    // at all, or might be empty.
+    scanner.scan(RegExp(r'\r?\n'));
 
-    String message;
+    var message = '';
 
     if (isRevParse) {
       final msgLines = <String>[];
-      lastLine = scanner.readNextLine();
 
-      const revParseMessagePrefix = '    ';
-      while (lastLine != null && lastLine.startsWith(revParseMessagePrefix)) {
-        msgLines.add(lastLine.substring(revParseMessagePrefix.length));
-        lastLine = scanner.readNextLine();
+      while (scanner.scan(RegExp(r'    ([^\r\n]*)(?:\r?\n|$)'))) {
+        msgLines.add(scanner.lastMatch!.group(1)!);
+        if (!scanner.lastMatch!.group(0)!.endsWith('\n')) {
+          break;
+        }
       }
 
-      message = msgLines.join('\n');
+      if (msgLines.isNotEmpty) {
+        message = msgLines.join('\n');
+      }
     } else {
       message = scanner.rest;
       scanner.position = scanner.string.length;

lib/src/tag.dart

@@ -22,19 +22,17 @@ class Tag {
 
     final scanner = StringScanner(content);
 
-    var lastLine = scanner.readNextLine()!;
-
-    while (lastLine.isNotEmpty) {
-      final match = headerRegExp.allMatches(lastLine).single;
-      assert(match.groupCount == 2);
+    while (scanner.scan(headerRegExp)) {
+      final match = scanner.lastMatch!;
       final header = match.group(1)!;
       final value = match.group(2)!;
 
       headers.putIfAbsent(header, () => <String>[]).add(value);
-
-      lastLine = scanner.readNextLine()!;
     }
 
+    // consume the blank line that separates headers from message
+    scanner.scan(RegExp(r'\r?\n'));
+
     String objectSha;
     String type;
     String tag;

lib/src/util.dart

@@ -1,11 +1,12 @@
-import 'package:string_scanner/string_scanner.dart';
-
 import 'bot.dart';
 import 'top_level.dart';
 
 const shaRegexPattern = '[a-f0-9]{40}';
 
-final headerRegExp = RegExp(r'^([a-z]+) (.+)$');
+final headerRegExp = RegExp(
+  r'^([a-z]+) ((?:[^\r\n]|\r?\n[ \t])+)\r?\n',
+  multiLine: true,
+);
 
 void requireArgumentValidSha1(String value, String argName) {
   metaRequireArgumentNotNullOrEmpty(argName);
@@ -16,17 +17,3 @@ void requireArgumentValidSha1(String value, String argName) {
     throw ArgumentError.value(value, argName, message);
   }
 }
-
-extension StringScannerX on StringScanner {
-  String? readNextLine() {
-    if (isDone) return null;
-    if (scan(_lineRegexp)) {
-      return lastMatch![1];
-    }
-    final restStr = rest;
-    position = string.length;
-    return restStr;
-  }
-}
-
-final _lineRegexp = RegExp(r'([^\r\n]*)\r?\n');