#!/usr/bin/perl


# see 
# http://www.newwaveinstruments.com/resources/articles/m_sequence_linear_feedback_shift_register_lfsr.htm
# Google search terms LSFR (linear feedback shift register)
# 

# in all cases, missing seed is $FF if $seed=0, and $newbit = 1.

#@plist = (1,2,3,4);
#@plist = (1,3,4,7);


### 8 bit values
#@plist = (1,2,3,7);  # works with newbit=1, seed =0, also with seed=1, newbit =0

#@plist = (3,4,5,7);  # with seed=1, newbit =0, or with seed=0, newbit=1

#@plist = (2,4,6,7);  # with seed=1, newbit =0, or with seed=0, newbit=1
#@plist = (1,2,5,7);  # with seed=1, newbit =0, or with seed=0, newbit=1

# other maximum length sequences, subtract one from these bit numbers
# [8, 7, 6, 1]
# [8, 7, 5, 3]
# [8, 7, 3, 2]
# [8, 6, 5, 4]
# [8, 6, 5, 3]
# [8, 6, 5, 2]

@plist = (1,4,5,7);

$num_bits = 8;

# 16 bit values

#@plist = (1,2,4,15);  # works with newbit=1, seed =0, also with seed=1, newbit =0
#@plist = (5,8,9,15);  # works with newbit=1, seed =0, also with seed=1, newbit =0
# other maxium length sequences, subtract one from these bit numbers
# [16, 15, 13, 4]
# [16, 15, 12, 10]
# [16, 15, 12, 1]
# [16, 15, 10, 4]
# [16, 15, 9, 6]
# [16, 15, 9, 4]
# [16, 15, 7, 2]
# [16, 15, 4, 2]
# [16, 14, 13, 11]
# [16, 14, 13, 5]
# [16, 14, 12, 7]
# [16, 14, 11, 7]
# [16, 14, 9, 7]
# [16, 14, 9, 4]
# [16, 14, 8, 3]
# [16, 13, 12, 11]
# [16, 13, 12, 7]
# [16, 13, 11, 6]
# [16, 13, 9, 6]
# [16, 13, 6, 4]
# [16, 12, 9, 7]
# [16, 12, 9, 6]
# [16, 11, 10, 5]
# [16, 11, 9, 8]
# [16, 11, 9, 7]
# [16, 10, 9, 6]


#$num_bits = 16;


$max_value = 1 << $num_bits;


$seed = 0;


for ($i=0;$i<$max_value;$i++){
 $bitarray[$i] = 0;
}

for ($i=0;$i<$max_value;$i++) {
   $newbit = 1;
#    $newbit = 0;
    foreach $bit (@plist) {
	$newbit = $newbit ^ &get_bit($seed,$bit)
	}

    $newseed = ($seed << 1) | $newbit;
    $newseed = $newseed & ($max_value-1);
    if ($bitarray[$newseed] == 1) {
	printf("Duplicate seed: %x\n",$newseed);
    }
    $bitarray[$newseed] = 1;
    if ($i > ($max_value-5)) {
      printf ("$i: %x\n",$newseed);
  }
    $seed = $newseed ;
}

for ($i=0;$i<$max_value;$i++){
    if ($bitarray[$i] == 0) {
	printf("Missing seed: %x\n",$i);
    }
}

print "Finished\n";

exit;



sub get_bit {
    my($val,$bitnum) = @_;
    my($mask) = 1 << $bitnum;
#    printf ("val: $val,  maske:$mask\n");
    $val = $val & $mask;
    if ($val > 0) {
	return(1);
    } else {
	return(0);
    }
}
    

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