Import of the watch repository from Pebble

platform/robert/boot/src/util/sle.c

@@ -0,0 +1,88 @@
+/*
+ * 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 "sle.h"
+
+#include "system/passert.h"
+
+// See waftools/sparse_length_encoding.py for more info on SLE encoding/decoding
+
+typedef struct {
+  const uint8_t *data;
+  uint32_t index;
+  uint32_t length;
+} ReadByteStream;
+
+typedef struct {
+  uint8_t *data;
+  uint32_t index;
+  uint32_t length;
+} WriteByteStream;
+
+static uint8_t prv_byte_stream_read(ReadByteStream *stream) {
+  PBL_ASSERTN(stream->index < stream->length);
+  return stream->data[stream->index++];
+}
+
+static void prv_byte_stream_write(WriteByteStream *stream, uint8_t data) {
+  PBL_ASSERTN(stream->index < stream->length);
+  stream->data[stream->index++] = data;
+}
+
+uint32_t sle_decode(const uint8_t *in, uint32_t in_len, uint8_t *out, uint32_t out_len) {
+  ReadByteStream in_stream = {
+    .data = in,
+    .length = in_len
+  };
+  WriteByteStream out_stream = {
+    .data = out,
+    .length = out_len
+  };
+
+  const uint8_t escape = prv_byte_stream_read(&in_stream);
+  while (true) {
+    const uint8_t byte = prv_byte_stream_read(&in_stream);
+    if (byte != escape) {
+      // simply write the byte into the output stream
+      prv_byte_stream_write(&out_stream, byte);
+      continue;
+    }
+
+    // read the escape code
+    const uint8_t code = prv_byte_stream_read(&in_stream);
+    if (code == 0) {
+      // end of stream
+      break;
+    } else if (code == 1) {
+      // literal escape byte
+      prv_byte_stream_write(&out_stream, escape);
+    } else {
+      // a sequence of zeros
+      uint16_t count;
+      if ((code & 0x80) == 0) {
+        // the count is only 1 byte (1-127)
+        count = code;
+      } else {
+        // the count is 2 bytes
+        count = (((uint16_t)(code & 0x7f) << 8) | prv_byte_stream_read(&in_stream)) + 0x80;
+      }
+      for (int i = 0; i < count; ++i) {
+        prv_byte_stream_write(&out_stream, 0);
+      }
+    }
+  }
+  return out_stream.index;
+}