refactor: use extensions (#62)

lib/src/extensions.dart

@@ -0,0 +1,36 @@
+// coverage:ignore-file
+
+import 'dart:ffi';
+
+import 'package:ffi/ffi.dart';
+import 'package:libgit2dart/src/bindings/libgit2_bindings.dart';
+
+extension IsValidSHA on String {
+  bool isValidSHA() {
+    final hexRegExp = RegExp(r'^[0-9a-fA-F]+$');
+    return hexRegExp.hasMatch(this) &&
+        (GIT_OID_MINPREFIXLEN <= length && GIT_OID_HEXSZ >= length);
+  }
+}
+
+extension ToChar on String {
+  /// Creates a zero-terminated [Utf8] code-unit array from this String,
+  /// casts it to the C `char` type and returns allocated pointer to result.
+  Pointer<Char> toChar() => toNativeUtf8().cast<Char>();
+}
+
+extension ToDartString on Pointer<Char> {
+  /// Converts this UTF-8 encoded string to a Dart string.
+  ///
+  /// Decodes the UTF-8 code units of this zero-terminated byte array as
+  /// Unicode code points and creates a Dart string containing those code
+  /// points.
+  ///
+  /// If [length] is provided, zero-termination is ignored and the result can
+  /// contain NUL characters.
+  ///
+  /// If [length] is not provided, the returned string is the string up til but
+  /// not including the first NUL character.
+  String toDartString({int? length}) =>
+      cast<Utf8>().toDartString(length: length);
+}