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reproducing javascripts >> operator

by mje (Curate)
on Mar 20, 2015 at 11:06 UTC ( #1120729=perlquestion: print w/replies, xml ) Need Help??
mje has asked for the wisdom of the Perl Monks concerning the following question:

I've got some javascript I need to rewrite in Perl and I'm stuck on the javascript >> operator which I believe is sign-propagating right bitwise shift. I found this node and it looked promising but none of the suggestions work for me.

I was thinking it may be because my ivsize is 8. Anyway, here are some examples from javascript of numbers shifted right 8 bits:

-1 >> 8 = -1 2140837940 >> 8 = 8362648 -1934825877 >> 8 = -7557914 -2004116368 >> 8 = -7828580 1978483312 >> 8 = 7728450 -293893010 >> 8 = -1148020

I cannot seem to reproduce this in Perl. Any ideas?

javascript >> operator

EDIT: added link to Javascript docs

Replies are listed 'Best First'.
Re: reproducing javascripts >> operator
by shmem (Chancellor) on Mar 20, 2015 at 11:46 UTC

    Looking at the results, the JavaScript bitwise shift isn't. It's a division with rounding.

    This is ugly, but does what you want:

    sub rshift {int($_[0]/(1<<$_[1])+($_[0]<0?-0.5:0.5))} for (2140837940,-1934825877,-2004116368,1978483312,-293893010) { print "$_ >> 8 = ",rshift($_,8),"\n" } __END__ 2140837940 >> 8 = 8362648 -1934825877 >> 8 = -7557914 -2004116368 >> 8 = -7828580 1978483312 >> 8 = 7728450 -293893010 >> 8 = -1148020
    perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

      shmem:

      Yeah, if the numbers are large enough, JavaScript silently converts them to float before doing some operations like this--it can be surprising.

      ...roboticus

      When your only tool is a hammer, all problems look like your thumb.

      Thanks. That does seem to work for everything except -1.

      It appears on a 32 bit perl use integer and the >> operator work.

      I'm using a 64 bit perl with 8 bytes integers. The actual problem I'm having is the script below works on a 32 bit perl and does not on a 64 bit perl. It is a hack of some javascript hence the way it is written right now.

      use 5.016; use strict; use warnings; use integer; challenge('932209653','YRM1k2jP', 'a', 'e', 'xxx:'); sub challenge { my ($c, $slt, $s1, $s2, $value) = @_; my $table = "00000000 77073096 EE0E612C 990951BA 076DC419 706AF48F + E963A535 9E6495A3 0EDB8832 79DCB8A4 E0D5E91E 97D2D988 09B64C2B 7EB17 +CBD E7B82D07 90BF1D91 1DB71064 6AB020F2 F3B97148 84BE41DE 1ADAD47D 6D +DDE4EB F4D4B551 83D385C7 136C9856 646BA8C0 FD62F97A 8A65C9EC 14015C4F + 63066CD9 FA0F3D63 8D080DF5 3B6E20C8 4C69105E D56041E4 A2677172 3C03E +4D1 4B04D447 D20D85FD A50AB56B 35B5A8FA 42B2986C DBBBC9D6 ACBCF940 32 +D86CE3 45DF5C75 DCD60DCF ABD13D59 26D930AC 51DE003A C8D75180 BFD06116 + 21B4F4B5 56B3C423 CFBA9599 B8BDA50F 2802B89E 5F058808 C60CD9B2 B10BE +924 2F6F7C87 58684C11 C1611DAB B6662D3D 76DC4190 01DB7106 98D220BC EF +D5102A 71B18589 06B6B51F 9FBFE4A5 E8B8D433 7807C9A2 0F00F934 9609A88E + E10E9818 7F6A0DBB 086D3D2D 91646C97 E6635C01 6B6B51F4 1C6C6162 85653 +0D8 F262004E 6C0695ED 1B01A57B 8208F4C1 F50FC457 65B0D9C6 12B7E950 8B +BEB8EA FCB9887C 62DD1DDF 15DA2D49 8CD37CF3 FBD44C65 4DB26158 3AB551CE + A3BC0074 D4BB30E2 4ADFA541 3DD895D7 A4D1C46D D3D6F4FB 4369E96A 346ED +9FC AD678846 DA60B8D0 44042D73 33031DE5 AA0A4C5F DD0D7CC9 5005713C 27 +0241AA BE0B1010 C90C2086 5768B525 206F85B3 B966D409 CE61E49F 5EDEF90E + 29D9C998 B0D09822 C7D7A8B4 59B33D17 2EB40D81 B7BD5C3B C0BA6CAD EDB88 +320 9ABFB3B6 03B6E20C 74B1D29A EAD54739 9DD277AF 04DB2615 73DC1683 E3 +630B12 94643B84 0D6D6A3E 7A6A5AA8 E40ECF0B 9309FF9D 0A00AE27 7D079EB1 + F00F9344 8708A3D2 1E01F268 6906C2FE F762575D 806567CB 196C3671 6E6B0 +6E7 FED41B76 89D32BE0 10DA7A5A 67DD4ACC F9B9DF6F 8EBEEFF9 17B7BE43 60 +B08ED5 D6D6A3E8 A1D1937E 38D8C2C4 4FDFF252 D1BB67F1 A6BC5767 3FB506DD + 48B2364B D80D2BDA AF0A1B4C 36034AF6 41047A60 DF60EFC3 A867DF55 316E8 +EEF 4669BE79 CB61B38C BC66831A 256FD2A0 5268E236 CC0C7795 BB0B4703 22 +0216B9 5505262F C5BA3BBE B2BD0B28 2BB45A92 5CB36A04 C2D7FFA7 B5D0CF31 + 2CD99E8B 5BDEAE1D 9B64C2B0 EC63F226 756AA39C 026D930A 9C0906A9 EB0E3 +63F 72076785 05005713 95BF4A82 E2B87A14 7BB12BAE 0CB61B38 92D28E9B E5 +D5BE0D 7CDCEFB7 0BDBDF21 86D3D2D4 F1D4E242 68DDB3F8 1FDA836E 81BE16CD + F6B9265B 6FB077E1 18B74777 88085AE6 FF0F6A70 66063BCA 11010B5C 8F659 +EFF F862AE69 616BFFD3 166CCF45 A00AE278 D70DD2EE 4E048354 3903B3C2 A7 +672661 D06016F7 4969474D 3E6E77DB AED16A4A D9D65ADC 40DF0B66 37D83BF0 + A9BCAE53 DEBB9EC5 47B2CF7F 30B5FFE9 BDBDF21C CABAC28A 53B39330 24B4A +3A6 BAD03605 CDD70693 54DE5729 23D967BF B3667A2E C4614AB8 5D681B02 2A +6F2B94 B40BBE37 C30C8EA1 5A05DF1B 2D02EF8D"; my $n = 4; my $start = ord($s1); my $end = ord($s2); my @arr; my $chlg; my $crc; my $m = (($end - $start) + 1) ** $n; for (my $i=0; $i < $n; $i++) { $arr[$i] = $s1; } XXX: for (my $i=0; $i < ($m-1); $i++) { YYY: for (my $j = $n-1; $j >= 0;--$j) { my $t = ord($arr[$j]); $t++; say $t if $i == 0; $arr[$j] = chr($t); if (ord($arr[$j]) <= $end) { last YYY; } else { $arr[$j] = $s1; } } $chlg = join("", @arr); my $str = $chlg . $slt; $crc = -1; for (my $k = 0, my $iTop = length($str); $k < $iTop; $k++ ) { my $code = ord(substr($str, $k, 1)); $crc = ($crc >> 8) ^ (hex(substr($table, ((($crc ^ $code) & 0x000000FF) * 9 +), 8))); } $crc = $crc ^ (-1); $crc = abs($crc); if ($crc == $c) { last XXX; } } my $result = $value . $chlg . ":" . $slt . ":" . $crc; say $result; return $result; } # outputs xxx:dede:YRM1k2jP:932209653 on 32 bit perl # and xxx:eeee:YRM1k2jP:2651920522 on 64 bit perl
        The actual problem I'm having is the script below works on a 32 bit perl and does not on a 64 bit perl

        At least rshift() fixes the different-output issue; it works

        # 32bit: perl -e 'printf"%vd\n",$^V; do pop' 1120752.pl 5.8.8 98 xxx:eeee:YRM1k2jP:1749307403 # 64bit: perl -e 'printf"%vd\n",$^V; do pop' 1120752.pl 5.14.2 98 xxx:eeee:YRM1k2jP:1749307403

        ...for some value of "works". The 98 ostensibly comes from the line

        say $t if $i == 0;

        ...and something else besides that may need to be fixed. Your turn.

        perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'
Re: reproducing javascripts >> operator
by ww (Archbishop) on Mar 20, 2015 at 11:35 UTC
Re: reproducing javascripts >> operator
by oiskuu (Hermit) on Mar 20, 2015 at 19:26 UTC

    The (canonical) way to perform sign extension is via xor and subtract: sub sxtb { (shift ^ 0x80) - 0x80 }

    For CRC digest, I'd consider using a specific module, but in a pinch, the following should work:

    sub sxt { ((0xffffffff & shift) ^ 0x80000000) - 0x80000000 } ... $crc = ($crc >> 8) ^ ... $crc = sxt $crc; ...

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