eic-opticks/sysrap/sbitmask.h

0001 #pragma once
0002
0003 /**
0004 https://stackoverflow.com/questions/1392059/algorithm-to-generate-bit-mask
0005 **/
0006
0007 #include <climits>
0008
0009 template <typename T>
0010 static constexpr T sbitmask(unsigned n)
0011 {
0012     //return static_cast<T>(-(n != 0)) & (static_cast<T>(-1) >> ((sizeof(T) * CHAR_BIT) - n));
0013     //return n == 0 ? 0 : (static_cast<T>(-1) >> ((sizeof(T) * CHAR_BIT) - n));
0014     return n == 0 ? 0 : ( (~static_cast<T>(0)) >> ((sizeof(T) * CHAR_BIT) - n));
0015 }
0016
0017 /**
0018 sbitmask_ = lambda i:np.uint64(-1) >> np.uint64(64-i)
0019
0020 In [51]: for i in range(64+1):print(" {0:2d} {1:064b} ".format(i, sbitmask_(i)))
0021   0 0000000000000000000000000000000000000000000000000000000000000000
0022   1 0000000000000000000000000000000000000000000000000000000000000001
0023   2 0000000000000000000000000000000000000000000000000000000000000011
0024   3 0000000000000000000000000000000000000000000000000000000000000111
0025   4 0000000000000000000000000000000000000000000000000000000000001111
0026   5 0000000000000000000000000000000000000000000000000000000000011111
0027   6 0000000000000000000000000000000000000000000000000000000000111111
0028
0029 In [61]: "{0:064b}".format( ~sbitmask_(64-8) )
0030 Out[61]: '1111111100000000000000000000000000000000000000000000000000000000'
0031
0032 In [62]: "{0:064b}".format( ~sbitmask_(64-7) )
0033 Out[62]: '1111111000000000000000000000000000000000000000000000000000000000'
0034
0035 In [63]: "{0:064b}".format( ~sbitmask_(64-3) )
0036 Out[63]: '1110000000000000000000000000000000000000000000000000000000000000'
0037
0038
0039 **/
0040
0041
0042 template <typename T>
0043 static constexpr T sbitmask_0(unsigned n)
0044 {
0045     return static_cast<T>(-(n != 0)) ;
0046     // all bits set, except for n=0 which gives all bits unset
0047 }
0048
0049 template <typename T>
0050 static constexpr T sbitmask_1(unsigned n)
0051 {
0052    return static_cast<T>(-1) >> ((sizeof(T) * CHAR_BIT) - n);
0053 }
0054
0055 template <typename T>
0056 static constexpr T sbitmask_2(unsigned n)
0057 {
0058    return static_cast<T>(-1) ;
0059 }
0060
0061