Enable hash index for block-based table

Summary: Based on previous patches, this diff eventually provides the end-to-end mechanism for users to specify the hash-index.

Test Plan: Wrote several new unit tests.

Reviewers: sdong, haobo, dhruba

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16539

table/block.h

@@ -10,6 +10,7 @@
 #pragma once
 #include <stddef.h>
 #include <stdint.h>
+
 #include "rocksdb/iterator.h"
 #include "rocksdb/options.h"
 
@@ -17,6 +18,7 @@ namespace rocksdb {
 
 struct BlockContents;
 class Comparator;
+class BlockHashIndex;
 
 class Block {
  public:
@@ -26,20 +28,28 @@ class Block {
   ~Block();
 
   size_t size() const { return size_; }
-  bool   cachable() const { return cachable_; }
+  const char* data() const { return data_; }
+  bool cachable() const { return cachable_; }
+  uint32_t NumRestarts() const;
   CompressionType compression_type() const { return compression_type_; }
+
+  // If hash index lookup is enabled and `use_hash_index` is true. This block
+  // will do hash lookup for the key prefix.
+  //
+  // NOTE: for the hash based lookup, if a key prefix doesn't match any key,
+  // the iterator will simply be set as "invalid", rather than returning
+  // the key that is just pass the target key.
   Iterator* NewIterator(const Comparator* comparator);
-  const char* data() { return data_; }
+  void SetBlockHashIndex(BlockHashIndex* hash_index);
 
  private:
-  uint32_t NumRestarts() const;
-
   const char* data_;
   size_t size_;
   uint32_t restart_offset_;     // Offset in data_ of restart array
   bool owned_;                  // Block owns data_[]
   bool cachable_;
   CompressionType compression_type_;
+  std::unique_ptr<BlockHashIndex> hash_index_;
 
   // No copying allowed
   Block(const Block&);