Counting the position of ones in a binary vector

no bugs, no hassle of spaghetti

additionally you can add clock or change indexing

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity test is
port(
din : in  std_logic_vector(0 to 15);
dout: out std_logic_vector(0 to 63)
);
end entity;
architecture a of test is
begin
process(din)
begin
for i in 0 to 15 loop
    if din(i) = '0' then
          dout(i*4 to i*4+3) <= "0000";
     else
          dout(i*4 to i*4+3) <= std_logic_vector(to_unsigned(i,4));
     end if;
end loop;
end process;
end a;

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no bugs, no hassle of spaghetti

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This solution is of course consuming much less logic resources, but not corresponding to the original specification - whatever may be it's purpose.