Countones method simplify some constraints.. Here is another example.. The program is to constrain such a way that adjacent elements in an array should change only by 4 bits.
class b;
rand bit [7:0] b_v[32];
constraint b_c {
foreach(b_v[i]) {
if(i!=0) {
$countones(b_v[i] ^ b_v[i-1]) == 4;
}
}
}
function new();
endfunction
function void post_randomize();
foreach(b_v[i]) begin
$display("post_randomize b_v[%0d]=%b",i, b_v[i]);
end
endfunction
endclass : b
module flip_only_4bits;
reg a;
b B = new();
initial
begin
$display("a=%0d",a);
for(int i=0;i<1;i++)
begin
B.randomize();
end
end
endmodule
Output:-
post_randomize b_v[0]=00011000
post_randomize b_v[1]=01110100
post_randomize b_v[2]=01101111
post_randomize b_v[3]=00111001
post_randomize b_v[4]=01010000
post_randomize b_v[5]=11110110
post_randomize b_v[6]=00110101
post_randomize b_v[7]=10010110
post_randomize b_v[8]=10101100
post_randomize b_v[9]=10110001
post_randomize b_v[10]=10101111
post_randomize b_v[11]=01001110
post_randomize b_v[12]=11000000
post_randomize b_v[13]=01011001
post_randomize b_v[14]=01101111
post_randomize b_v[15]=00011101
post_randomize b_v[16]=01110001
post_randomize b_v[17]=00100111
post_randomize b_v[18]=01000001
post_randomize b_v[19]=11001010
post_randomize b_v[20]=00101011
post_randomize b_v[21]=10011001
post_randomize b_v[22]=11011110
post_randomize b_v[23]=10100110
post_randomize b_v[24]=01100011
post_randomize b_v[25]=10100101
post_randomize b_v[26]=01111101
post_randomize b_v[27]=00100001
post_randomize b_v[28]=01011001
post_randomize b_v[29]=00000011
post_randomize b_v[30]=01001110
post_randomize b_v[31]=10111110
