I had some problems with earlier examples. So I cobbled up my own version which should work for any N-bit (16, 32, 64, etc) PC.


use strict;  
use Math::BigInt;  

############################
# BEGIN N-BIT SYSTEM CHECK #
############################

# Calculate the bit-width of cells on this system.

sub count_bits {
    my $i = 0;
    my $q = ~0x0;      # Set all bits
    while ( $q ) {     # All shifted out?
        $q = $q << 1;  # Shift left
        $i++;          # Accum shifts
    }
    return $i;         # Return bit-width
}

my $bits    = count_bits;       # System bit-width.
my $cells   = int( $bits / 8 ); # max bytes in array 
my $mask    = ~0x0;             # Full mask = 0xF*
my $top_bit = ~( $mask >> 1 );  # High mask = 0x80*

##########################
# END N-BIT SYSTEM CHECK #
##########################

# Convert a BigInt to an array.
sub bint_to_array {
  my $quo = shift;
  my $msk = Math::BigInt->new($mask);
  my @cls;
  until ( $quo->is_zero() ) {
    my $rem = $quo->copy();      # Lest modify ruin.
    $rem->band($msk);            # REM = LSC
    $quo->brsft($bits);          # QUO = Cells above
    unshift @cls, $rem->bstr();  # Keep remainder
  }
  return @cls;
}

# Convert an array to a BigInt. 
sub array_to_bint {
    my @cls = @_;
    my $out = Math::BigInt->new(0);
    my $i = 0;
    while ( @_ ) {
        my $lsc = Math::BigInt->new( pop @_ );  # LSC
        $lsc->blsft($bits * $i);                # Shift up
        $out->bior($lsc);                       # OR to LSC
        ++$i;
    }
    return $out;
}

sub demo_array_bint {
    print "\n\nDemo of bint/array conversion... \n";
    my $bint_1 = new Math::BigInt($_[0]);
    print "\tINPUT DECIMAL  = ", $bint_1->bstr(), "\n";  
    my @array  = bint_to_array($bint_1);
    print "\tTHRUPUT ARRAY  = ", join ', ', @array, "\n";
    my $bint_2 = array_to_bint(@array);
    print "\tOUTPUT DECIMAL = ", $bint_2->bstr(), "\n";
}

demo_array_bint('1234567890' x 6);

[download]

Output looks like this on NetBSD 2.0 using Perl 5.8.6.

 
baal: {39} perl foo.pl

Demo of bint/array conversion... 
        INPUT DECIMAL  = 123456789012345678901234567890123456789012345
+678901234567890
        THRUPUT ARRAY  = 19, 2868184292, 3156107799, 1065854007, 23524
+60956, 2362438038, 3460238034, 
        OUTPUT DECIMAL = 123456789012345678901234567890123456789012345
+678901234567890
baal: {40}

[download]

In reply to Re: Converting an array of bytes into a BigInt - speed considerations by aplonis
in thread Converting an array of bytes into a BigInt - speed considerations by dempa

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