file_sys: Move IPS patching code into separate source file

In anticipation of a new BPS patcher.

src/core/file_sys/patch.cpp

@@ -0,0 +1,55 @@
+// Copyright 2019 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <cstring>
+#include <string>
+#include "common/logging/log.h"
+#include "core/file_sys/patch.h"
+
+namespace FileSys::Patch {
+
+bool ApplyIpsPatch(const std::vector<u8>& ips, std::vector<u8>& buffer) {
+    u32 cursor = 5;
+    u32 patch_length = ips.size() - 3;
+    std::string ips_header(ips.begin(), ips.begin() + 5);
+
+    if (ips_header != "PATCH") {
+        LOG_INFO(Service_FS, "Attempted to load invalid IPS");
+        return false;
+    }
+
+    while (cursor < patch_length) {
+        std::string eof_check(ips.begin() + cursor, ips.begin() + cursor + 3);
+
+        if (eof_check == "EOF")
+            return false;
+
+        u32 offset = ips[cursor] << 16 | ips[cursor + 1] << 8 | ips[cursor + 2];
+        std::size_t length = ips[cursor + 3] << 8 | ips[cursor + 4];
+
+        // check for an rle record
+        if (length == 0) {
+            length = ips[cursor + 5] << 8 | ips[cursor + 6];
+
+            if (buffer.size() < offset + length)
+                return false;
+
+            for (u32 i = 0; i < length; ++i)
+                buffer[offset + i] = ips[cursor + 7];
+
+            cursor += 8;
+
+            continue;
+        }
+
+        if (buffer.size() < offset + length)
+            return false;
+
+        std::memcpy(&buffer[offset], &ips[cursor + 5], length);
+        cursor += length + 5;
+    }
+    return true;
+}
+
+} // namespace FileSys::Patch