** CHANGES ARE EXPERIMENTAL (FOR TESTING ONLY)

Bockchain:
1. Optim: Multi-thread long-hash computation when encountering groups of blocks.
2. Optim: Cache verified txs and return result from cache instead of re-checking whenever possible.
3. Optim: Preload output-keys when encoutering groups of blocks. Sort by amount and global-index before bulk querying database and multi-thread when possible.
4. Optim: Disable double spend check on block verification, double spend is already detected when trying to add blocks.
5. Optim: Multi-thread signature computation whenever possible.
6. Patch: Disable locking (recursive mutex) on called functions from check_tx_inputs which causes slowdowns (only seems to happen on ubuntu/VMs??? Reason: TBD)
7. Optim: Removed looped full-tx hash computation when retrieving transactions from pool (???).
8. Optim: Cache difficulty/timestamps (735 blocks) for next-difficulty calculations so that only 2 db reads per new block is needed when a new block arrives (instead of 1470 reads).

Berkeley-DB:
1. Fix: 32-bit data errors causing wrong output global indices and failure to send blocks to peers (etc).
2. Fix: Unable to pop blocks on reorganize due to transaction errors.
3. Patch: Large number of transaction aborts when running multi-threaded bulk queries.
4. Patch: Insufficient locks error when running full sync.
5. Patch: Incorrect db stats when returning from an immediate exit from "pop block" operation.
6. Optim: Add bulk queries to get output global indices.
7. Optim: Modified output_keys table to store public_key+unlock_time+height for single transaction lookup (vs 3)
8. Optim: Used output_keys table retrieve public_keys instead of going through output_amounts->output_txs+output_indices->txs->output:public_key
9. Optim: Added thread-safe buffers used when multi-threading bulk queries.
10. Optim: Added support for nosync/write_nosync options for improved performance (*see --db-sync-mode option for details)
11. Mod: Added checkpoint thread and auto-remove-logs option.
12. *Now usable on 32-bit systems like RPI2.

LMDB:
1. Optim: Added custom comparison for 256-bit key tables (minor speed-up, TBD: get actual effect)
2. Optim: Modified output_keys table to store public_key+unlock_time+height for single transaction lookup (vs 3)
3. Optim: Used output_keys table retrieve public_keys instead of going through output_amounts->output_txs+output_indices->txs->output:public_key
4. Optim: Added support for sync/writemap options for improved performance (*see --db-sync-mode option for details)
5. Mod: Auto resize to +1GB instead of multiplier x1.5

ETC:
1. Minor optimizations for slow-hash for ARM (RPI2). Incomplete.
2. Fix: 32-bit saturation bug when computing next difficulty on large blocks.

[PENDING ISSUES]
1. Berkely db has a very slow "pop-block" operation. This is very noticeable on the RPI2 as it sometimes takes > 10 MINUTES to pop a block during reorganization.
   This does not happen very often however, most reorgs seem to take a few seconds but it possibly depends on the number of outputs present. TBD.
2. Berkeley db, possible bug "unable to allocate memory". TBD.

[NEW OPTIONS] (*Currently all enabled for testing purposes)
1. --fast-block-sync arg=[0:1] (default: 1)
	a. 0 = Compute long hash per block (may take a while depending on CPU)
	b. 1 = Skip long-hash and verify blocks based on embedded known good block hashes (faster, minimal CPU dependence)
2. --db-sync-mode arg=[[safe|fast|fastest]:[sync|async]:[nblocks_per_sync]] (default: fastest:async:1000)
	a. safe = fdatasync/fsync (or equivalent) per stored block. Very slow, but safest option to protect against power-out/crash conditions.
	b. fast/fastest = Enables asynchronous fdatasync/fsync (or equivalent). Useful for battery operated devices or STABLE systems with UPS and/or systems with battery backed write cache/solid state cache.
	Fast    - Write meta-data but defer data flush.
	Fastest - Defer meta-data and data flush.
	Sync    - Flush data after nblocks_per_sync and wait.
	Async   - Flush data after nblocks_per_sync but do not wait for the operation to finish.
3. --prep-blocks-threads arg=[n] (default: 4 or system max threads, whichever is lower)
        Max number of threads to use when computing long-hash in groups.
4. --show-time-stats arg=[0:1] (default: 1)
	Show benchmark related time stats.
5. --db-auto-remove-logs arg=[0:1] (default: 1)
	For berkeley-db only. Auto remove logs if enabled.

**Note: lmdb and berkeley-db have changes to the tables and are not compatible with official git head version.
	At the moment, you need a full resync to use this optimized version.

[PERFORMANCE COMPARISON]
**Some figures are approximations only.
Using a baseline machine of an i7-2600K+SSD+(with full pow computation):
1. The optimized lmdb/blockhain core can process blocks up to 585K for ~1.25 hours + download time, so it usually takes 2.5 hours to sync the full chain.
2. The current head with memory can process blocks up to 585K for ~4.2 hours + download time, so it usually takes 5.5 hours to sync the full chain.
3. The current head with lmdb can process blocks up to 585K for ~32 hours + download time and usually takes 36 hours to sync the full chain.

Averate procesing times (with full pow computation):
lmdb-optimized:
1. tx_ave = 2.5 ms / tx
2. block_ave = 5.87 ms / block
memory-official-repo:
1. tx_ave = 8.85 ms / tx
2. block_ave = 19.68 ms / block
lmdb-official-repo (0f4a036437)
1. tx_ave = 47.8 ms / tx
2. block_ave = 64.2 ms / block

**Note: The following data denotes processing times only (does not include p2p download time)
lmdb-optimized processing times (with full pow computation):
1. Desktop,  Quad-core / 8-threads 2600k  (8Mb) - 1.25 hours processing time (--db-sync-mode=fastest:async:1000).
2. Laptop,   Dual-core / 4-threads U4200  (3Mb) - 4.90 hours processing time (--db-sync-mode=fastest:async:1000).
3. Embedded, Quad-core / 4-threads Z3735F (2x1Mb) - 12.0 hours processing time (--db-sync-mode=fastest:async:1000).

lmdb-optimized processing times (with per-block-checkpoint)
1. Desktop,  Quad-core / 8-threads 2600k  (8Mb) - 10 minutes processing time (--db-sync-mode=fastest:async:1000).

berkeley-db optimized processing times (with full pow computation)
1. Desktop, Quad-core / 8-threads 2600k  (8Mb) - 1.8 hours processing time (--db-sync-mode=fastest:async:1000).
2. RPI2. Improved from estimated 3 months(???) into 2.5 days (*Need 2AMP supply + Clock:1Ghz + [usb+ssd] to achieve this speed) (--db-sync-mode=fastest:async:1000).

berkeley-db optimized processing times (with per-block-checkpoint)
1. RPI2. 12-15 hours (*Need 2AMP supply + Clock:1Ghz + [usb+ssd] to achieve this speed) (--db-sync-mode=fastest:async:1000).

src/blockchain_db/blockchain_db.h

@@ -138,6 +138,15 @@ namespace cryptonote
 // typedef for convenience
 typedef std::pair<crypto::hash, uint64_t> tx_out_index;
 
+#pragma pack(push, 1)
+struct output_data_t
+{
+	crypto::public_key pubkey;
+	uint64_t unlock_time;
+	uint64_t height;
+};
+#pragma pack(pop)
+	
 /***********************************
  * Exception Definitions
  ***********************************/
@@ -279,7 +288,7 @@ private:
   virtual void remove_transaction_data(const crypto::hash& tx_hash, const transaction& tx) = 0;
 
   // tells the subclass to store an output
-  virtual void add_output(const crypto::hash& tx_hash, const tx_out& tx_output, const uint64_t& local_index) = 0;
+  virtual void add_output(const crypto::hash& tx_hash, const tx_out& tx_output, const uint64_t& local_index, const uint64_t unlock_time) = 0;
 
   // tells the subclass to remove an output
   virtual void remove_output(const tx_out& tx_output) = 0;
@@ -313,7 +322,7 @@ protected:
 
   mutable uint64_t time_tx_exists = 0;
   uint64_t time_commit1 = 0;
-
+  bool m_auto_remove_logs = true;
 
 public:
 
@@ -461,7 +470,8 @@ public:
   virtual uint64_t get_num_outputs(const uint64_t& amount) const = 0;
 
   // return public key for output with global output amount <amount> and index <index>
-  virtual crypto::public_key get_output_key(const uint64_t& amount, const uint64_t& index) const = 0;
+  virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index) = 0;
+  virtual output_data_t get_output_key(const uint64_t& global_index) const = 0;
 
   // returns the output indexed by <index> in the transaction with hash <h>
   virtual tx_out get_output(const crypto::hash& h, const uint64_t& index) const = 0;
@@ -471,9 +481,11 @@ public:
 
   // returns the transaction-local reference for the output with <amount> at <index>
   // return type is pair of tx hash and index
-  virtual tx_out_index get_output_tx_and_index(const uint64_t& amount, const uint64_t& index) const = 0;
-  virtual void get_output_tx_and_index(const uint64_t& amount, std::vector<uint64_t> &offsets, std::vector<tx_out_index> &indices) const = 0;
-  virtual bool  has_bulk_indices() const = 0;
+  virtual tx_out_index get_output_tx_and_index(const uint64_t& amount, const uint64_t& index) = 0;
+  virtual void get_output_tx_and_index(const uint64_t& amount, const std::vector<uint64_t> &offsets, std::vector<tx_out_index> &indices) = 0;
+  virtual void get_output_key(const uint64_t &amount, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs) = 0;
+  
+  virtual bool can_thread_bulk_indices() const = 0;
 
   // return a vector of indices corresponding to the global output index for
   // each output in the transaction with hash <h>
@@ -485,7 +497,10 @@ public:
   // returns true if key image <img> is present in spent key images storage
   virtual bool has_key_image(const crypto::key_image& img) const = 0;
 
+  void set_auto_remove_logs(bool auto_remove) { m_auto_remove_logs = auto_remove; }
+
   bool m_open;
+  mutable epee::critical_section m_synchronization_lock;
 };  // class BlockchainDB