factor out and simplify get_syncoffset

include/libtorrent/bt_peer_connection.hpp

@@ -321,12 +321,6 @@ namespace libtorrent {
 		void write_pe_vc_cryptofield(span<char> write_buf
 			, int crypto_field, std::size_t pad_size);
 
-		// Returns offset at which bytestream (src, src + src_size)
-		// matches bytestream(target, target + target_size).
-		// If no sync found, return -1
-		int get_syncoffset(char const* src, int src_size
-			, char const* target, int target_size) const;
-
 		// helper to cut down on boilerplate
 		void rc4_decrypt(span<char> buf);
 #endif

include/libtorrent/string_util.hpp

@@ -35,6 +35,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #include "libtorrent/config.hpp"
 #include "libtorrent/string_view.hpp"
+#include "libtorrent/span.hpp"
 
 #include <vector>
 #include <string>
@@ -71,6 +72,10 @@ namespace libtorrent {
 
 	TORRENT_EXTRA_EXPORT bool string_ends_with(string_view s1, string_view s2);
 
+	// Returns offset at which src matches target.
+	// If no sync found, return -1
+	TORRENT_EXTRA_EXPORT int search(span<char const> src, span<char const> target);
+
 	struct listen_interface_t
 	{
 		std::string device;

src/bt_peer_connection.cpp

@@ -68,6 +68,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/socket_type.hpp"
 #include "libtorrent/performance_counters.hpp" // for counters
 #include "libtorrent/alert_manager.hpp" // for alert_manager
+#include "libtorrent/string_util.hpp" // for search
 
 #if !defined(TORRENT_DISABLE_ENCRYPTION) && !defined(TORRENT_DISABLE_EXTENSIONS)
 #include "libtorrent/pe_crypto.hpp"
@@ -668,29 +669,6 @@ namespace {
 			aux::write_uint16(handshake_len, write_buf); // len(IA)
 	}
 
-	// TODO: 3 use span instead of (pointer,len) pairs
-	int bt_peer_connection::get_syncoffset(char const* src, int const src_size
-		, char const* target, int const target_size) const
-	{
-		TORRENT_ASSERT(target_size >= src_size);
-		TORRENT_ASSERT(src_size > 0);
-		TORRENT_ASSERT(src);
-		TORRENT_ASSERT(target);
-
-		int const traverse_limit = target_size - src_size;
-
-		// TODO: this could be optimized using knuth morris pratt
-		for (int i = 0; i < traverse_limit; ++i)
-		{
-			char const* target_ptr = target + i;
-			if (std::equal(src, src+src_size, target_ptr))
-				return i;
-		}
-
-		// no complete sync
-		return -1;
-	}
-
 	void bt_peer_connection::rc4_decrypt(span<char> buf)
 	{
 		m_rc4->decrypt(buf);
@@ -2579,8 +2557,7 @@ namespace {
 #endif
 			}
 
-			int const syncoffset = get_syncoffset(m_sync_hash->data(), 20
+			int const syncoffset = search(*m_sync_hash, recv_buffer);
-				, recv_buffer.begin(), int(recv_buffer.size()));
 
 			// No sync
 			if (syncoffset == -1)
@@ -2721,8 +2698,7 @@ namespace {
 			}
 
 			TORRENT_ASSERT(m_sync_vc.get());
-			int const syncoffset = get_syncoffset(m_sync_vc.get(), 8
+			int const syncoffset = search({m_sync_vc.get(), 8}, recv_buffer);
-				, recv_buffer.begin(), int(recv_buffer.size()));
 
 			// No sync
 			if (syncoffset == -1)

src/string_util.cpp

@@ -40,6 +40,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #include <cstdlib> // for malloc
 #include <cstring> // for memmov/strcpy/strlen
+#include <algorithm> // for search
 
 namespace libtorrent {
 
@@ -149,6 +150,20 @@ namespace libtorrent {
 		return s1.size() >= s2.size() && std::equal(s2.rbegin(), s2.rend(), s1.rbegin());
 	}
 
+	int search(span<char const> src, span<char const> target)
+	{
+		TORRENT_ASSERT(!src.empty());
+		TORRENT_ASSERT(!target.empty());
+		TORRENT_ASSERT(target.size() >= src.size());
+		TORRENT_ASSERT(target.size() < std::size_t(std::numeric_limits<int>::max()));
+
+		auto it = std::search(target.begin(), target.end(), src.begin(), src.end());
+
+		// no complete sync
+		if (it == target.end()) return -1;
+		return static_cast<int>(it - target.begin());
+	}
+
 	char* allocate_string_copy(char const* str)
 	{
 		if (str == nullptr) return nullptr;