Go: refactor (#16602)

### Summary

1. update doc
2. refactor route code

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/binding/cpp/re2/bitmap256.cc

@@ -0,0 +1,44 @@
+// Copyright 2023 The RE2 Authors.  All Rights Reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "re2/bitmap256.h"
+
+#include <stdint.h>
+
+#include "util/logging.h"
+#include "util/util.h"
+
+namespace re2 {
+
+int Bitmap256::FindNextSetBit(int c) const {
+    DCHECK_GE(c, 0);
+    DCHECK_LE(c, 255);
+
+    // Check the word that contains the bit. Mask out any lower bits.
+    int i = c / 64;
+    uint64_t word = words_[i] & (~uint64_t{0} << (c % 64));
+    if (word != 0)
+        return (i * 64) + FindLSBSet(word);
+
+    // Check any following words.
+    i++;
+    switch (i) {
+        case 1:
+            if (words_[1] != 0)
+                return (1 * 64) + FindLSBSet(words_[1]);
+            FALLTHROUGH_INTENDED;
+        case 2:
+            if (words_[2] != 0)
+                return (2 * 64) + FindLSBSet(words_[2]);
+            FALLTHROUGH_INTENDED;
+        case 3:
+            if (words_[3] != 0)
+                return (3 * 64) + FindLSBSet(words_[3]);
+            FALLTHROUGH_INTENDED;
+        default:
+            return -1;
+    }
+}
+
+} // namespace re2