How can I use square brackets in perl like C? - perl

Question

Yesterday Computerphile uploaded a video about code golf and bitshift variations and I was really fascinated of the program generating music.
This is my formatted version of the code in the video.
int g(int sample, int x, int t, int overdrive) {
return (
(
3 & x & (
sample *
(
(
3 & sample >> 16
?
"BY}6YB6%"
:
"Qj}6jQ6%"
)[t % 8]
+
51
) >> overdrive
)
) << 4
);
}
int main(int n, int s) {
for (int sample=0 ;; sample++)
putchar(
g(sample, 1, n=sample >> 14, 12)
+
g(sample, s=sample >> 17, n^sample >> 13, 10)
+
g(sample, s/3, n + ((sample >> 11) % 3), 10)
+
g(sample, s/5, 8 + n-((sample >> 10) % 3), 9)
);
}
I wanted to try convert the program to pure perl and this is my attempt.
sub g {
return (
(
3 & $_[1] & (
$_[0] *
(
(
3 & $_[0] >> 16
?
"BY}6YB6%"
:
"Qj}6jQ6%"
)[$_[2] % 8]
+
51
) >> $_[3]
)
) << 4
);
}
for($i=0;;$i++){
print pack('C',
g($i, 1, $n=$i >> 14, 12)
+
g($i, $s=$i >> 17, $n^$i >> 13, 10)
+
g($i, $s/3, $n + (($i >> 11) % 3), 10)
+
g($i, $s/5, 8 + $n-(($i >> 10) % 3), 9)
);
}
According to the manual for putchar the the value is internally converted to an unsigned char when written. So I used the pack function in perl with C template which is an unsigned char. However when I pipe the result of both programs to aplay they don't produce the same music.
If I use inline C and call the function g from perl it does work correctly.
use Inline C => <<'END_C';
int g(int sample, int x, int t, int overdrive) {
return (
(
3 & x & (
sample *
(
(
3 & sample >> 16
?
"BY}6YB6%"
:
"Qj}6jQ6%"
)[t % 8]
+
51
) >> overdrive
)
) << 4
);
}
END_C
for($i=0;;$i++){
print pack('C',
g($i, 1, $n=$i >> 14, 12)
+
g($i, $s=$i >> 17, $n^$i >> 13, 10)
+
g($i, $s/3, $n + (($i >> 11) % 3), 10)
+
g($i, $s/5, 8 + $n-(($i >> 10) % 3), 9)
);
}
The only explanation I have is that [$_[2] % 8] is not doing what I think it is in perl.
How can I make the programs produce the same music in perl and C? On windows you can use perl theprogram.pl | sox -c 1 -b 8 -e unsigned -t raw -r 8k - -t waveaudio 0 if you have sox installed.

Answer 1

String is not an array in perl, you need to replace array access with call to substr() function:
...
ord(substr((
3 & $_[0] >> 16
?
"BY}6YB6%"
:
"Qj}6jQ6%"
), $_[2] % 8, 1))
...
More efficient:
# Outside the sub.
my $a1 = [ unpack 'C*', "BY}6YB6%" ];
my $a2 = [ unpack 'C*', "Qj}6jQ6%" ];
...
${ 3 & $_[0] >> 16 ? $a1 : $a2 }[ $_[2] % 8 ]
...