Disk: ReadDriver: Add unified cache

Previously, we had to copy an entire block out of the
old cache every time we wanted to read even a single
byte from it.

This ended up being a fairly significant performance
issue, in addition to the fact that the caching code
was duplicated.

include/icsneo/disk/diskreaddriver.h

@@ -22,6 +22,9 @@ public:
 	virtual optional<uint64_t> readLogicalDisk(Communication& com, device_eventhandler_t report,
 		uint64_t pos, uint8_t* into, uint64_t amount, std::chrono::milliseconds timeout = DefaultTimeout);
 
+	void invalidateCache(uint64_t pos = 0,
+		uint64_t amount = std::numeric_limits<uint32_t>::max() /* large value, but avoid overflow */);
+
 protected:
 	/**
 	 * Perform a read which the driver can do in one shot.
@@ -31,6 +34,15 @@ protected:
 	 */
 	virtual optional<uint64_t> readLogicalDiskAligned(Communication& com, device_eventhandler_t report,
 		uint64_t pos, uint8_t* into, uint64_t amount, std::chrono::milliseconds timeout) = 0;
+
+private:
+	std::vector<uint8_t> cache;
+	uint64_t cachePos = 0;
+	std::chrono::time_point<std::chrono::steady_clock> cachedAt;
+
+	static constexpr const std::chrono::milliseconds CacheTime = std::chrono::milliseconds(1000);
+
+	optional<uint64_t> readFromCache(uint64_t pos, uint8_t* into, uint64_t amount, std::chrono::milliseconds staleAfter = CacheTime);
 };
 
 } // namespace Disk