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🔨 refactored get_from_vector function

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Snippet from http://stackoverflow.com/a/41031865/266378
This commit is contained in:
Niels Lohmann 2016-12-08 09:37:23 +01:00
parent 81a4272444
commit 6b84c4155c
2 changed files with 63 additions and 59 deletions
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63
src/json.hpp
View file

@ -6262,36 +6262,35 @@ class basic_json
}
}
template<typename T, typename std::enable_if<sizeof(T) == 1, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
/*
Precondition:
vec: | | | a | b | c | d | | | T: | | | | |
^ ^ ^ ^
current_index idx ptr sizeof(T)
Postcondition:
vec: | | | a | b | c | d | | | T: | d | c | b | a |
^ ^ ^
| idx ptr
current_index
Code from <http://stackoverflow.com/a/41031865/266378>
*/
template<typename T>
static T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_index)
{
return static_cast<T>(vec[current_idx + 1]);
T result;
uint8_t* ptr = reinterpret_cast<uint8_t*>(&result);
size_t idx = current_index + 1 + sizeof(T);
while (idx > current_index)
{
*ptr++ = vec[--idx];
}
template<typename T, typename std::enable_if<sizeof(T) == 2, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
{
return static_cast<T>((static_cast<T>(vec[current_idx + 1]) << 010) +
static_cast<T>(vec[current_idx + 2]));
}
template<typename T, typename std::enable_if<sizeof(T) == 4, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
{
return static_cast<T>((static_cast<T>(vec[current_idx + 1]) << 030) +
(static_cast<T>(vec[current_idx + 2]) << 020) +
(static_cast<T>(vec[current_idx + 3]) << 010) +
static_cast<T>(vec[current_idx + 4]));
}
template<typename T, typename std::enable_if<sizeof(T) == 8, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
{
return static_cast<T>((static_cast<T>(vec[current_idx + 1]) << 070) +
(static_cast<T>(vec[current_idx + 2]) << 060) +
(static_cast<T>(vec[current_idx + 3]) << 050) +
(static_cast<T>(vec[current_idx + 4]) << 040) +
(static_cast<T>(vec[current_idx + 5]) << 030) +
(static_cast<T>(vec[current_idx + 6]) << 020) +
(static_cast<T>(vec[current_idx + 7]) << 010) +
static_cast<T>(vec[current_idx + 8]));
return result;
}
static void to_msgpack_internal(const basic_json& j, std::vector<uint8_t>& v)
@ -7173,7 +7172,13 @@ class basic_json
{
idx += 2; // skip two content bytes
// code from RFC 7049, Appendix D
// code from RFC 7049, Appendix D, Figure 3:
// As half-precision floating-point numbers were only added to IEEE
// 754 in 2008, today's programming platforms often still only have
// limited support for them. It is very easy to include at least
// decoding support for them even without such support. An example
// of a small decoder for half-precision floating-point numbers in
// the C language is shown in Figure 3.
const int half = (v[current_idx + 1] << 8) + v[current_idx + 2];
const int exp = (half >> 10) & 0x1f;
const int mant = half & 0x3ff;

55
src/json.hpp.re2c
View file

@ -6262,36 +6262,35 @@ class basic_json
}
}
template<typename T, typename std::enable_if<sizeof(T) == 1, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
/*
Precondition:
vec: | | | a | b | c | d | | | T: | | | | |
^ ^ ^ ^
current_index idx ptr sizeof(T)
Postcondition:
vec: | | | a | b | c | d | | | T: | d | c | b | a |
^ ^ ^
| idx ptr
current_index
Code from <http://stackoverflow.com/a/41031865/266378>
*/
template<typename T>
static T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_index)
{
return static_cast<T>(vec[current_idx + 1]);
T result;
uint8_t* ptr = reinterpret_cast<uint8_t*>(&result);
size_t idx = current_index + 1 + sizeof(T);
while (idx > current_index)
{
*ptr++ = vec[--idx];
}
template<typename T, typename std::enable_if<sizeof(T) == 2, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
{
return static_cast<T>((static_cast<T>(vec[current_idx + 1]) << 010) +
static_cast<T>(vec[current_idx + 2]));
}
template<typename T, typename std::enable_if<sizeof(T) == 4, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
{
return static_cast<T>((static_cast<T>(vec[current_idx + 1]) << 030) +
(static_cast<T>(vec[current_idx + 2]) << 020) +
(static_cast<T>(vec[current_idx + 3]) << 010) +
static_cast<T>(vec[current_idx + 4]));
}
template<typename T, typename std::enable_if<sizeof(T) == 8, int>::type = 0>
static constexpr T get_from_vector(const std::vector<uint8_t>& vec, const size_t current_idx)
{
return static_cast<T>((static_cast<T>(vec[current_idx + 1]) << 070) +
(static_cast<T>(vec[current_idx + 2]) << 060) +
(static_cast<T>(vec[current_idx + 3]) << 050) +
(static_cast<T>(vec[current_idx + 4]) << 040) +
(static_cast<T>(vec[current_idx + 5]) << 030) +
(static_cast<T>(vec[current_idx + 6]) << 020) +
(static_cast<T>(vec[current_idx + 7]) << 010) +
static_cast<T>(vec[current_idx + 8]));
return result;
}
static void to_msgpack_internal(const basic_json& j, std::vector<uint8_t>& v)