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+/*
+ *
+ * Copyright 2018 gRPC authors.
+ *
+ * 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.
+ *
+ */
+
+#ifndef GRPC_CORE_TSI_ALTS_FRAME_PROTECTOR_FRAME_HANDLER_H
+#define GRPC_CORE_TSI_ALTS_FRAME_PROTECTOR_FRAME_HANDLER_H
+
+#include <grpc/support/port_platform.h>
+
+#include <stdbool.h>
+#include <stdlib.h>
+
+const size_t kFrameMessageType = 0x06;
+const size_t kFrameLengthFieldSize = 4;
+const size_t kFrameMessageTypeFieldSize = 4;
+const size_t kFrameMaxSize = 1024 * 1024;
+const size_t kFrameHeaderSize =
+    kFrameLengthFieldSize + kFrameMessageTypeFieldSize;
+
+/**
+ * Implementation of frame reader and frame writer. All APIs in the
+ * header are thread-compatible.
+ */
+
+/**
+ * Main struct for a frame writer. It reads frames from an input buffer, and
+ * writes the contents as raw bytes. It does not own the input buffer.
+ */
+typedef struct alts_frame_writer {
+  const unsigned char* input_buffer;
+  unsigned char header_buffer[kFrameHeaderSize];
+  size_t input_bytes_written;
+  size_t header_bytes_written;
+  size_t input_size;
+} alts_frame_writer;
+
+/**
+ * Main struct for a frame reader. It reads raw bytes and puts the framed
+ * result into an output buffer. It does not own the output buffer.
+ */
+typedef struct alts_frame_reader {
+  unsigned char* output_buffer;
+  unsigned char header_buffer[kFrameHeaderSize];
+  size_t header_bytes_read;
+  size_t output_bytes_read;
+  size_t bytes_remaining;
+} alts_frame_reader;
+
+/**
+ * This method creates a frame writer instance and initializes its internal
+ * states.
+ */
+alts_frame_writer* alts_create_frame_writer();
+
+/**
+ * This method resets internal states of a frame writer and prepares to write
+ * a single frame. It does not take ownership of payload_buffer.
+ * The payload_buffer must outlive the writer.
+ *
+ * - writer: a frame writer instance.
+ * - buffer: a buffer storing full payload data to be framed.
+ * - length: size of payload data.
+ *
+ * The method returns true on success and false otherwise.
+ */
+bool alts_reset_frame_writer(alts_frame_writer* writer,
+                             const unsigned char* buffer, size_t length);
+
+/**
+ * This method writes up to bytes_size bytes of a frame to output.
+ *
+ * - writer: a frame writer instance.
+ * - output: an output buffer used to store the frame.
+ * - bytes_size: an in/out parameter that stores the size of output buffer
+ *   before the call, and gets written the number of frame bytes written to the
+ *   buffer.
+ *
+ * The method returns true on success and false otherwise.
+ */
+bool alts_write_frame_bytes(alts_frame_writer* writer, unsigned char* output,
+                            size_t* bytes_size);
+
+/**
+ * This method checks if a reset can be called to write a new frame. It returns
+ * true if it's the first time to frame a payload, or the current frame has
+ * been finished processing. It returns false if it's not ready yet to start a
+ * new frame (e.g., more payload data needs to be accumulated to process the
+ * current frame).
+ *
+ * if (alts_is_frame_writer_done(writer)) {
+ *   // a new frame can be written, call reset.
+ *   alts_reset_frame_writer(writer, payload_buffer, payload_size);
+ * } else {
+ *   // accumulate more payload data until a full frame can be written.
+ * }
+ *
+ * - writer: a frame writer instance.
+ */
+bool alts_is_frame_writer_done(alts_frame_writer* writer);
+
+/**
+ * This method returns the number of bytes left to write before a complete frame
+ * is formed.
+ *
+ * - writer: a frame writer instance.
+ */
+size_t alts_get_num_writer_bytes_remaining(alts_frame_writer* writer);
+
+/**
+ * This method destroys a frame writer instance.
+ *
+ * - writer: a frame writer instance.
+ */
+void alts_destroy_frame_writer(alts_frame_writer* writer);
+
+/**
+ * This method creates a frame reader instance and initializes its internal
+ * states.
+ */
+alts_frame_reader* alts_create_frame_reader();
+
+/**
+ * This method resets internal states of a frame reader (including setting its
+ * output_buffer with buffer), and prepares to write processed bytes to
+ * an output_buffer. It does not take ownership of buffer. The buffer must
+ * outlive reader.
+ *
+ * - reader: a frame reader instance.
+ * - buffer: an output buffer used to store deframed results.
+ *
+ * The method returns true on success and false otherwise.
+ */
+bool alts_reset_frame_reader(alts_frame_reader* reader, unsigned char* buffer);
+
+/**
+ * This method processes up to the number of bytes given in bytes_size. It may
+ * choose not to process all the bytes, if, for instance, more bytes are
+ * given to the method than required to complete the current frame.
+ *
+ * - reader: a frame reader instance.
+ * - bytes: a buffer that stores data to be processed.
+ * - bytes_size: an in/out parameter that stores the size of bytes before the
+ *   call and gets written the number of bytes processed.
+ *
+ * The method returns true on success and false otherwise.
+ */
+bool alts_read_frame_bytes(alts_frame_reader* reader,
+                           const unsigned char* bytes, size_t* bytes_size);
+
+/**
+ * This method checks if a frame length has been read.
+ *
+ * - reader: a frame reader instance.
+ *
+ * The method returns true if a frame length has been read and false otherwise.
+ */
+bool alts_has_read_frame_length(alts_frame_reader* reader);
+
+/**
+ * This method returns the number of bytes the frame reader intends to write.
+ * It may only be called if alts_has_read_frame_length() returns true.
+ *
+ * - reader: a frame reader instance.
+ */
+size_t alts_get_reader_bytes_remaining(alts_frame_reader* reader);
+
+/**
+ * This method resets output_buffer but does not otherwise modify other internal
+ * states of a frame reader instance. After being set, the new output_buffer
+ * will hold the deframed payload held by the original output_buffer. It does
+ * not take ownership of buffer. The buffer must outlive the reader.
+ * To distinguish between two reset methods on a frame reader,
+ *
+ * if (alts_fh_is_frame_reader_done(reader)) {
+ *   // if buffer contains a full payload to be deframed, call reset.
+ *   alts_reset_frame_reader(reader, buffer);
+ * }
+ *
+ * // if remaining buffer space is not enough to hold a full payload
+ * if (buffer_space_remaining < alts_get_reader_bytes_remaining(reader)) {
+ *   // allocate enough space for a new buffer, copy back data processed so far,
+ *   // and call reset.
+ *   alts_reset_reader_output_buffer(reader, new_buffer).
+ * }
+ *
+ * - reader: a frame reader instance.
+ * - buffer: a buffer used to set reader's output_buffer.
+ */
+void alts_reset_reader_output_buffer(alts_frame_reader* reader,
+                                     unsigned char* buffer);
+
+/**
+ * This method checks if reset can be called to start processing a new frame.
+ * If true and reset was previously called, a full frame has been processed and
+ * the content of the frame is available in output_buffer.
+
+ * - reader: a frame reader instance.
+ */
+bool alts_is_frame_reader_done(alts_frame_reader* reader);
+
+/**
+ * This method returns output_bytes_read of a frame reader instance.
+ *
+ * - reader: a frame reader instance.
+ */
+size_t alts_get_output_bytes_read(alts_frame_reader* reader);
+
+/**
+ * This method returns output_buffer of a frame reader instance.
+ *
+ * - reader: a frame reader instance.
+ */
+unsigned char* alts_get_output_buffer(alts_frame_reader* reader);
+
+/**
+ * This method destroys a frame reader instance.
+ *
+ * - reader: a frame reader instance.
+ */
+void alts_destroy_frame_reader(alts_frame_reader* reader);
+
+#endif /* GRPC_CORE_TSI_ALTS_FRAME_PROTECTOR_FRAME_HANDLER_H */