The problem is essentially to count the number of bits in an integer. There's a good solution for this, which I'll show now.

This subroutine from MMIXware is supposed to do exactly this. This is for a 32-bit word though, not just 7 bits.

@ Here's a fun way to count the number of bits in a tetrabyte.
[This classical trick is called the ``Gillies--Miller method
for sideways addition'' in {\sl The Preparation of Programs
for an Electronic Digital Computer\/} by Wilkes, Wheeler, and
Gill, second edition (Reading, Mass.:\ Addison--Wesley, 1957),
191--193. Some of the tricks used here were suggested by
Balbir Singh, Peter Rossmanith, and Stefan Schwoon.]
@^Gillies, Donald Bruce@>
@^Miller, Jeffrey Charles Percy@>
@^Wilkes, Maurice Vincent@>
@^Wheeler, David John@>
@^Gill, Stanley@>
@^Singh, Balbir@>
@^Rossmanith, Peter@>
@^Schwoon, Stefan@>

@<Subr...@>=
int count_bits @,@,@[ARGS((tetra))@];@+@t}\6{@>
int count_bits(x)
  tetra x;
{
  register int xx=x;
  xx=xx-((xx>>1)&0x55555555);
  xx=(xx&0x33333333)+((xx>>2)&0x33333333);
  xx=(xx+(xx>>4))&0x0f0f0f0f;
  xx=xx+(xx>>8);
  return (xx+(xx>>16)) & 0xff;
}
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#!perl
use warnings;
use strict;

# the following subroutines couns the number of bits in an 8-bit integ
+er

sub sadd8_a { # my first solution -- without looking closely to your c
+ode
        my($x) = @_;
        my $n = 0;
        for (my $k = 1; $k < 0x100; $k <<= 1) {
                0 != ($x & $k) and $n++;
        }
        $n;
}

sub sadd8_b { # one of your solutions, modified a bit

  my ( $iDays ) = @_;

  my $iRes = 0;
  for ( 1, 2, 4, 8, 16, 32, 64, 128 ){
    if ( $iDays & $_ ){
      $iRes++;
    }
  }

  return $iRes;
}

sub sadd8_c {
        my($x) = @_;
        $x = ($x & 0b01010101) + ($x >> 1 & 0b01010101);
        $x = ($x & 0b00110011) + ($x >> 2 & 0b00110011);
        ($x & 0b00001111) + ($x >> 4);
}

# let's print an example
for my $n (0 .. 31) {
        print sadd8_a($n), " ";
}
print "\n";

# test
for my $n (0 .. 255) {
        my $a = sadd8_a($n);
        my $b = sadd8_b($n);
        my $c = sadd8_c($n);
        $a == $b && $b == $c or
                die "wrong results: $n $a $b $c";
}

__END__
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