Adding Slice::difference_offset() function

Summary:
There are some use cases in MyRocks to compare two slices
and to return the first byte where they differ. It may be
useful to add it as a RocksDB Slice function.

Test Plan: db_test

Reviewers: sdong, rven, igor

Reviewed By: igor

Subscribers: jkedgar, dhruba

Differential Revision: https://reviews.facebook.net/D46935

db/db_test.cc

@@ -5937,6 +5937,8 @@ TEST_F(DBTest, PrefixScan) {
     snprintf(buf, sizeof(buf), "03______:");
     prefix = Slice(buf, 8);
     key = Slice(buf, 9);
+    ASSERT_EQ(key.difference_offset(prefix), 8);
+    ASSERT_EQ(prefix.difference_offset(key), 8);
     // db configs
     env_->count_random_reads_ = true;
     Options options = CurrentOptions();

include/rocksdb/slice.h

@@ -88,6 +88,9 @@ class Slice {
             (memcmp(data_, x.data_, x.size_) == 0));
   }
 
+  // Compare two slices and returns the first byte where they differ
+  size_t difference_offset(const Slice& b) const;
+
  // private: make these public for rocksdbjni access
   const char* data_;
   size_t size_;
@@ -125,6 +128,15 @@ inline int Slice::compare(const Slice& b) const {
   return r;
 }
 
+inline size_t Slice::difference_offset(const Slice& b) const {
+  size_t off = 0;
+  const size_t len = (size_ < b.size_) ? size_ : b.size_;
+  for (; off < len; off++) {
+    if (data_[off] != b.data_[off]) break;
+  }
+  return off;
+}
+
 }  // namespace rocksdb
 
 #endif  // STORAGE_ROCKSDB_INCLUDE_SLICE_H_