src/cbor/encoding.c - master - githubmirror/libcbor

src/cbor/encoding.c
224 строки9 KB
Pavel Kalvoda
Preserve NaN sign+payload and tiny-float sign in cbor_encode_half (#412)
23 мар 2026, 00:18
23 мар 2026, 00:18bdf710e
/*
 * Copyright (c) 2014-2020 Pavel Kalvoda <me@pavelkalvoda.com>
 *
 * libcbor is free software; you can redistribute it and/or modify
 * it under the terms of the MIT license. See LICENSE for details.
 */

#include "encoding.h"

#include <math.h>

#include "internal/encoders.h"

size_t cbor_encode_uint8(uint8_t value, unsigned char* buffer,
                         size_t buffer_size) {
  return _cbor_encode_uint8(value, buffer, buffer_size, 0x00);
}

size_t cbor_encode_uint16(uint16_t value, unsigned char* buffer,
                          size_t buffer_size) {
  return _cbor_encode_uint16(value, buffer, buffer_size, 0x00);
}

size_t cbor_encode_uint32(uint32_t value, unsigned char* buffer,
                          size_t buffer_size) {
  return _cbor_encode_uint32(value, buffer, buffer_size, 0x00);
}

size_t cbor_encode_uint64(uint64_t value, unsigned char* buffer,
                          size_t buffer_size) {
  return _cbor_encode_uint64(value, buffer, buffer_size, 0x00);
}

size_t cbor_encode_uint(uint64_t value, unsigned char* buffer,
                        size_t buffer_size) {
  return _cbor_encode_uint(value, buffer, buffer_size, 0x00);
}

size_t cbor_encode_negint8(uint8_t value, unsigned char* buffer,
                           size_t buffer_size) {
  return _cbor_encode_uint8(value, buffer, buffer_size, 0x20);
}

size_t cbor_encode_negint16(uint16_t value, unsigned char* buffer,
                            size_t buffer_size) {
  return _cbor_encode_uint16(value, buffer, buffer_size, 0x20);
}

size_t cbor_encode_negint32(uint32_t value, unsigned char* buffer,
                            size_t buffer_size) {
  return _cbor_encode_uint32(value, buffer, buffer_size, 0x20);
}

size_t cbor_encode_negint64(uint64_t value, unsigned char* buffer,
                            size_t buffer_size) {
  return _cbor_encode_uint64(value, buffer, buffer_size, 0x20);
}

size_t cbor_encode_negint(uint64_t value, unsigned char* buffer,
                          size_t buffer_size) {
  return _cbor_encode_uint(value, buffer, buffer_size, 0x20);
}

size_t cbor_encode_bytestring_start(size_t length, unsigned char* buffer,
                                    size_t buffer_size) {
  return _cbor_encode_uint((size_t)length, buffer, buffer_size, 0x40);
}

size_t _cbor_encode_byte(uint8_t value, unsigned char* buffer,
                         size_t buffer_size) {
  if (buffer_size >= 1) {
    buffer[0] = value;
    return 1;
  } else
    return 0;
}

size_t cbor_encode_indef_bytestring_start(unsigned char* buffer,
                                          size_t buffer_size) {
  return _cbor_encode_byte(0x5F, buffer, buffer_size);
}

size_t cbor_encode_string_start(size_t length, unsigned char* buffer,
                                size_t buffer_size) {
  return _cbor_encode_uint((size_t)length, buffer, buffer_size, 0x60);
}

size_t cbor_encode_indef_string_start(unsigned char* buffer,
                                      size_t buffer_size) {
  return _cbor_encode_byte(0x7F, buffer, buffer_size);
}

size_t cbor_encode_array_start(size_t length, unsigned char* buffer,
                               size_t buffer_size) {
  return _cbor_encode_uint((size_t)length, buffer, buffer_size, 0x80);
}

size_t cbor_encode_indef_array_start(unsigned char* buffer,
                                     size_t buffer_size) {
  return _cbor_encode_byte(0x9F, buffer, buffer_size);
}

size_t cbor_encode_map_start(size_t length, unsigned char* buffer,
                             size_t buffer_size) {
  return _cbor_encode_uint((size_t)length, buffer, buffer_size, 0xA0);
}

size_t cbor_encode_indef_map_start(unsigned char* buffer, size_t buffer_size) {
  return _cbor_encode_byte(0xBF, buffer, buffer_size);
}

size_t cbor_encode_tag(uint64_t value, unsigned char* buffer,
                       size_t buffer_size) {
  return _cbor_encode_uint(value, buffer, buffer_size, 0xC0);
}

size_t cbor_encode_bool(bool value, unsigned char* buffer, size_t buffer_size) {
  return value ? _cbor_encode_byte(0xF5, buffer, buffer_size)
               : _cbor_encode_byte(0xF4, buffer, buffer_size);
}

size_t cbor_encode_null(unsigned char* buffer, size_t buffer_size) {
  return _cbor_encode_byte(0xF6, buffer, buffer_size);
}

size_t cbor_encode_undef(unsigned char* buffer, size_t buffer_size) {
  return _cbor_encode_byte(0xF7, buffer, buffer_size);
}

size_t cbor_encode_half(float value, unsigned char* buffer,
                        size_t buffer_size) {
  // TODO: Broken on systems that do not use IEEE 754
  /* Assuming value is normalized */
  uint32_t val = ((union _cbor_float_helper){.as_float = value}).as_uint;
  uint16_t res;
  uint8_t exp = (uint8_t)((val & 0x7F800000u) >>
                          23u); /* 0b0111_1111_1000_0000_0000_0000_0000_0000 */
  uint32_t mant =
      val & 0x7FFFFFu; /* 0b0000_0000_0111_1111_1111_1111_1111_1111 */
  if (exp == 0xFF) {   /* Infinity or NaNs */
    if (isnan(value)) {
      /* Preserve the sign bit and NaN payload. The top 10 bits of the 23-bit
       * single-precision mantissa map directly onto the 10-bit half-precision
       * mantissa. If the payload fits entirely in the bottom 13 bits (which
       * cannot be represented in half precision), fall back to a quiet NaN
       * to avoid accidentally producing an infinity encoding (mantissa == 0).
       * Note: signaling NaN payloads are preserved on a best-effort basis;
       * some CPUs canonicalize them to quiet NaNs when loaded into registers.
       * See https://github.com/PJK/libcbor/issues/215 */
      uint16_t half_mant = (uint16_t)(mant >> 13u);
      if (half_mant == 0) half_mant = 0x0200u; /* quiet NaN fallback */
      res = (uint16_t)((val & 0x80000000u) >> 16u | 0x7C00u | half_mant);
    } else {
      // If the mantissa is non-zero, we have a NaN, but those are handled
      // above. See
      // https://en.wikipedia.org/wiki/Half-precision_floating-point_format
      CBOR_ASSERT(mant == 0u);
      res = (uint16_t)((val & 0x80000000u) >> 16u | 0x7C00u);
    }
  } else if (exp == 0x00) { /* Zeroes or subnorms */
    res = (uint16_t)((val & 0x80000000u) >> 16u | mant >> 13u);
  } else { /* Normal numbers */
    int8_t logical_exp = (int8_t)(exp - 127);
    CBOR_ASSERT(logical_exp == exp - 127);

    // Now we know that 2^exp <= 0 logically
    if (logical_exp < -24) {
      /* Too small to represent even as a half-precision subnormal; round to
       * zero. The sign bit is preserved so that small negative values round
       * to negative zero rather than positive zero. */
      res = (uint16_t)((val & 0x80000000u) >> 16u);
    } else if (logical_exp < -14) {
      /* Offset the remaining decimal places by shifting the significand, the
         value is lost. This is an implementation decision that works around the
         absence of standard half-float in the language. */
      res = (uint16_t)((val & 0x80000000u) >> 16u) |  // Extract sign bit
            ((uint16_t)(1u << (24u + logical_exp)) +
             (uint16_t)(((mant >> (-logical_exp - 2)) + 1) >>
                        1));  // Round half away from zero for simplicity
    } else {
      res = (uint16_t)((val & 0x80000000u) >> 16u |
                       ((((uint8_t)logical_exp) + 15u) << 10u) |
                       (uint16_t)(mant >> 13u));
    }
  }
  return _cbor_encode_uint16(res, buffer, buffer_size, 0xE0);
}

size_t cbor_encode_single(float value, unsigned char* buffer,
                          size_t buffer_size) {
  // Note: Values of signaling NaNs are discarded. There is no standard
  // way to extract it without assumptions about the internal float
  // representation.
  if (isnan(value)) {
    return _cbor_encode_uint32((uint32_t)0x7FC0 << 16, buffer, buffer_size,
                               0xE0);
  }
  // TODO: Broken on systems that do not use IEEE 754
  return _cbor_encode_uint32(
      ((union _cbor_float_helper){.as_float = value}).as_uint, buffer,
      buffer_size, 0xE0);
}

size_t cbor_encode_double(double value, unsigned char* buffer,
                          size_t buffer_size) {
  // Note: Values of signaling NaNs are discarded. See `cbor_encode_single`.
  if (isnan(value)) {
    return _cbor_encode_uint64((uint64_t)0x7FF8 << 48, buffer, buffer_size,
                               0xE0);
  }
  // TODO: Broken on systems that do not use IEEE 754
  return _cbor_encode_uint64(
      ((union _cbor_double_helper){.as_double = value}).as_uint, buffer,
      buffer_size, 0xE0);
}

size_t cbor_encode_break(unsigned char* buffer, size_t buffer_size) {
  return _cbor_encode_byte(0xFF, buffer, buffer_size);
}

size_t cbor_encode_ctrl(uint8_t value, unsigned char* buffer,
                        size_t buffer_size) {
  return _cbor_encode_uint8(value, buffer, buffer_size, 0xE0);
}