slip.c

#include <stdio.h>
#include "slip.h"

enum slip_result slip_encode(
  unsigned char *frame,
  unsigned long frameLength,
  unsigned char *output,
  unsigned long maxOutputSize,
  unsigned long *outputSize
) {
  unsigned long outputIndex = 0;
  for (unsigned long inIndex = 0; inIndex < frameLength; inIndex++) {
    // Check if we ran out of space on the output
    if (maxOutputSize <= outputIndex) {
      return SLIP_BUFFER_OVERFLOW;
    }

    // Grab one byte from the input and check if we need to escape it
    unsigned char c = frame[inIndex];
    switch (c) {
      case SLIP_END:
        output[outputIndex] = SLIP_ESC;
        output[outputIndex + 1] = SLIP_ESC_END;
        outputIndex += 2;
        break;
      case SLIP_ESC:
        output[outputIndex] = SLIP_ESC;
        output[outputIndex + 1] = SLIP_ESC_ESC;
        outputIndex += 2;
        break;
      default:
        // No need to escape, copy as it is
        output[outputIndex] = c;
        outputIndex += 1;
        break;
    }
  }

  // Mark the frame end
  output[outputIndex] = SLIP_END;

  // Return the output size
  *outputSize = outputIndex + 1;
  return SLIP_OK;
}

enum slip_result slip_decode(
  unsigned char *encodedFrame,
  unsigned long frameLength,
  unsigned char *output,
  unsigned long maxOutputSize,
  unsigned long *outputSize
) {
  int invalidEscape = 0;

  unsigned long outputIndex = 0;
  for (unsigned long inIndex = 0; inIndex < frameLength; inIndex++) {
    // Check if we ran out of space on the output buffer
    if (maxOutputSize <= outputIndex) {
      return SLIP_BUFFER_OVERFLOW;
    }

    unsigned char inByte = encodedFrame[inIndex];
    if (inByte == SLIP_ESC) {
      switch (encodedFrame[inIndex + 1]) {
        case SLIP_ESC_END:
          output[outputIndex] = SLIP_END;
          break;
        case SLIP_ESC_ESC:
          output[outputIndex] = SLIP_ESC;
          break;
        default:
          // Escape sequence invalid, complain on stderr
          output[outputIndex] = SLIP_ESC;
          invalidEscape = 1;
          fprintf(stderr, "SLIP escape error! (Input bytes 0x%02x, 0x%02x)\n", inByte, encodedFrame[inIndex + 1]);
          break;
      }
      inIndex += 1;
    } else if (inByte == SLIP_END) {
      // End of packet, stop the loop
      outputIndex++;
      break;
    } else {
      output[outputIndex] = inByte;
    }

    outputIndex++;
  }

  *outputSize = outputIndex;
  if (invalidEscape) return SLIP_INVALID_ESCAPE;
  return SLIP_OK;
}