Import of the watch repository from Pebble

platform/snowy/boot/src/util/cobs.c

@@ -0,0 +1,51 @@
+/*
+ * 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.
+ */
+
+#include "cobs.h"
+
+size_t cobs_encode(void *dst_ptr, const void *src_ptr, size_t length) {
+  const char *src = src_ptr;
+  char *dst = dst_ptr;
+  uint8_t code = 0x01;
+  size_t code_idx = 0;
+  size_t dst_idx = 1;
+
+  for (size_t src_idx = 0; src_idx < length; ++src_idx) {
+    if (src[src_idx] == '\0') {
+      dst[code_idx] = code;
+      code_idx = dst_idx++;
+      code = 0x01;
+    } else {
+      dst[dst_idx++] = src[src_idx];
+      code++;
+      if (code == 0xff) {
+        if (src_idx == length - 1) {
+          // Special case: the final encoded block is 254 bytes long with no
+          // zero after it. While it's technically a valid encoding if a
+          // trailing zero is appended, it causes the output to be one byte
+          // longer than it needs to be. This violates consistent overhead
+          // contract and could overflow a carefully sized buffer.
+          break;
+        }
+        dst[code_idx] = code;
+        code_idx = dst_idx++;
+        code = 0x01;
+      }
+    }
+  }
+  dst[code_idx] = code;
+  return dst_idx;
+}