sysrap/tests/SignedIntPackTest.cc

0001 //https://stackoverflow.com/questions/2437283/c-c-packing-signed-char-into-int
0002
0003 #include <stdint.h>
0004 #include <iostream>
0005 #include <iomanip>
0006
0007 // bits of signed integers can be carried within unsigned ones without problem
0008 uint32_t pack_helper(uint32_t c0, uint32_t c1, uint32_t c2, uint32_t c3) {
0009     return c0 | (c1 << 8) | (c2 << 16) | (c3 << 24);
0010 }
0011
0012 uint32_t pack(uint8_t c0, uint8_t c1, uint8_t c2, uint8_t c3) {
0013     return pack_helper(c0, c1, c2, c3);
0014 }
0015
0016 template <int N>
0017 uint8_t unpack_u(uint32_t packed) {   // cast to avoid potential warnings for implicit narrowing conversion
0018     return static_cast<uint8_t>(packed >> (N*8));
0019 }
0020
0021 template <int N>
0022 int8_t unpack_s(uint32_t packed) {
0023     uint8_t r = unpack_u<N>(packed);
0024     return (r <= 0x7f ? r : r - 0x100 );   // b0111 = x7,   0x7f = 127, 0x100 = 256
0025 }
0026
0027 /**
0028 https://en.wikipedia.org/wiki/Two%27s_complement
0029
0030 The two's complement of an N-bit number is defined as its complement with respect to 2^N = 0x1 << N ; (one bit past the end)
0031 the sum of a number and its two's complement is 2^N = 0x1 << N
0032
0033 **/
0034
0035
0036 int main() {
0037
0038     int x0 = 254 ;
0039     int y0 = 127 ;
0040     int z0 = -128 ;
0041     int w0 = -7 ;
0042
0043     uint32_t pk = pack(x0,y0,z0,w0);
0044
0045     int x1u = (int)unpack_u<0>(pk) ;
0046     int y1u = (int)unpack_u<1>(pk) ;
0047     int z1u = (int)unpack_u<2>(pk) ;
0048     int w1u = (int)unpack_u<3>(pk) ;
0049
0050     int x1s = (int)unpack_s<0>(pk) ;
0051     int y1s = (int)unpack_s<1>(pk) ;
0052     int z1s = (int)unpack_s<2>(pk) ;
0053     int w1s = (int)unpack_s<3>(pk) ;
0054
0055
0056     std::cout
0057         << std::setw(10) << x0
0058         << std::setw(10) << x1u
0059         << std::setw(10) << x1s
0060         << std::endl
0061         << std::setw(10) << y0
0062         << std::setw(10) << y1u
0063         << std::setw(10) << y1s
0064         << std::endl
0065         << std::setw(10) << z0
0066         << std::setw(10) << z1u
0067         << std::setw(10) << z1s
0068         << std::endl
0069         << std::setw(10) << w0
0070         << std::setw(10) << w1u
0071         << std::setw(10) << w1s
0072         << std::endl
0073         ;
0074
0075     return 0 ;
0076 }
0077
0078 // clang SignedIntPackTest.cc -lc++ -o /tmp/SignedIntPackTest &&  /tmp/SignedIntPackTest