Import of the watch repository from Pebble

src/fw/util/legacy_checksum.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <string.h>
+
+//! \file
+//! Calculate the legacy checksum of data.
+//!
+//! The calculation is somewhat like a CRC with the CRC-32 polynomial, but with
+//! the data bytes reordered oddly. The checksum is calculated 32 bits at a
+//! time, little-endian, MSB-first. The legacy checksum of bytes A B C D E F G H
+//! is equal to the CRC-32 of bytes D C B A H G F E (xor 0xFFFFFFFF). When the
+//! data being checksummmed is not a multiple of four bytes in length, the
+//! remainder bytes are zero-padded and byte-swapped(!) before being checksummed
+//! like the previous full words. For example, the legacy checksum of bytes
+//! 1 2 3 4 5 6 is equal to the checksum of bytes 1 2 3 4 6 5 0 0, which is
+//! equivalent to the CRC-32 of bytes 4 3 2 1 0 0 5 6 (xor 0xFFFFFFFF).
+//!
+//! The legacy checksum should not be used except when required for
+//! backwards-compatibility purposes.
+
+typedef struct LegacyChecksum {
+  uint32_t reg;
+  uint8_t accumulator[3];
+  uint8_t accumulated_length;
+} LegacyChecksum;
+
+void legacy_defective_checksum_init(LegacyChecksum *checksum);
+
+void legacy_defective_checksum_update(
+    LegacyChecksum * restrict checksum,
+    const void * restrict data, size_t length);
+
+uint32_t legacy_defective_checksum_finish(LegacyChecksum *checksum);
+
+//! Convenience wrapper to checksum memory in one shot.
+uint32_t legacy_defective_checksum_memory(const void * restrict data,
+                                          size_t length);