I have a project where a function receives four 8-bit characters and needs to convert the resulting 32-bit IEEE-754 float to a regular Perl number. It seems like there should be a faster way than the working code below, but I have not been able to figure out a simpler pack function that works.
It does not work, but it seems like it is close:
$float = unpack("f", pack("C4", @array[0..3]); # Fails for small numbers
Works:
@bits0 = split('', unpack("B8", pack("C", shift)));
@bits1 = split('', unpack("B8", pack("C", shift)));
@bits2 = split('', unpack("B8", pack("C", shift)));
@bits3 = split('', unpack("B8", pack("C", shift)));
push @bits, @bits3, @bits2, @bits1, @bits0;
$mantbit = shift(@bits);
$mantsign = $mantbit ? -1 : 1;
$exp = ord(pack("B8", join("",@bits[0..7])));
splice(@bits, 0, 8);
# Convert fractional float to decimal
for (my $i = 0; $i < 23; $i++) {
$f = $bits[$i] * 2 ** (-1 * ($i + 1));
$mant += $f;
}
$float = $mantsign * (1 + $mant) * (2 ** ($exp - 127));
Anyone have a better way?
I'd take the opposite approach: forget unpacking, stick to bit twiddling.
First, assemble your 32 bit word. Depending on endianness, this might have to be the other way around:
my $word = ($byte0 << 24) + ($byte1 << 16) + ($byte2 << 8) + $byte3;
Now extract the parts of the word: the sign bit, exponent and mantissa:
my $sign = ($word & 0x80000000) ? -1 : 1;
my $expo = (($word & 0x7F800000) >> 23) - 127;
my $mant = ($word & 0x007FFFFF | 0x00800000);
Assemble your float:
my $num = $sign * (2 ** $expo) * ( $mant / (1 << 23));
There's some examples on Wikipedia.