Optimize string finder.

Core/Text/Utils/StringFinder.cs

@@ -50,6 +50,15 @@ public StringFinder(string s)
         Value = Previous = string.Empty;
     }
 
+    /// <summary>
+    /// Initializes a new instance of the StringFinder class.
+    /// </summary>
+    /// <param name="s">The source string.</param>
+    public StringFinder(StringBuilder s)
+        : this(s?.ToString())
+    {
+    }
+
     /// <summary>
     /// Gets the source string.
     /// </summary>
@@ -85,6 +94,34 @@ public StringFinder(string s)
     /// </summary>
     public int RestLength => Rest.Length;
 
+    /// <summary>
+    /// Gets the sub-string before a search string.
+    /// </summary>
+    /// <param name="q">The search string.</param>
+    /// <param name="offset">The zero-based offset.</param>
+    /// <returns>The sub-string.</returns>
+    public string PreviewBefore(string q, out int offset)
+    {
+        if (string.IsNullOrEmpty(q))
+        {
+            offset = Offset;
+            return string.Empty;
+        }
+
+        var s = Rest;
+        var index = s.IndexOf(q);
+        if (index < 0)
+        {
+            Value = string.Empty;
+            offset = Source.Length;
+            return string.Empty;
+        }
+
+        var value = Rest.Substring(0, index);
+        offset = Offset + index;
+        return value;
+    }
+
     /// <summary>
     /// Gets the sub-string before a search string. The search string is in the rest string.
     /// </summary>

UnitTest/Text/StringUnitTest.cs

@@ -47,6 +47,7 @@ public void TestStringFinder()
             "."
         });
         Assert.IsFalse(finder.IsEnd);
+        Assert.AreEqual(TestString.Length - 1, finder.Offset);
         Assert.AreEqual("\"", finder.Before(1));
         Assert.AreEqual("\"", finder.Value);
         Assert.IsTrue(finder.IsEnd);