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Implement base85 in a 32-bit safe way, without bcmath
Summary: See T1635 and the giant inline comment. Test Plan: Ran unit tests on 32-bit and 64-bit machines. Reviewers: btrahan, vrana Reviewed By: btrahan CC: aran Maniphest Tasks: T1635 Differential Revision: https://secure.phabricator.com/D3250
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1 changed files with 29 additions and 5 deletions
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@ -682,6 +682,30 @@ final class ArcanistBundle {
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// This is implemented awkwardly in order to closely mirror git's
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// implementation in base85.c
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// It is also implemeted awkwardly to work correctly on 32-bit machines.
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// Broadly, this algorithm converts the binary input to printable output
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// by transforming each 4 binary bytes of input to 5 printable bytes of
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// output, one piece at a time.
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//
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// To do this, we convert the 4 bytes into a 32-bit integer, then use
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// modulus and division by 85 to pick out printable bytes (85^5 is slightly
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// larger than 2^32). In C, this algorithm is fairly easy to implement
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// because the accumulator can be made unsigned.
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//
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// In PHP, there are no unsigned integers, so values larger than 2^31 break
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// on 32-bit systems under modulus:
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//
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// $ php -r 'print (1 << 31) % 13;' # On a 32-bit machine.
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// -11
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//
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// However, PHP's float type is an IEEE 754 64-bit double precision float,
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// so we can safely store integers up to around 2^53 without loss of
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// precision. To work around the lack of an unsigned type, we just use a
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// double and perform the modulus with fmod().
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//
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// (Since PHP overflows integer operations into floats, we don't need much
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// additional casting.)
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static $map = array(
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'0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
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'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
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@ -700,19 +724,19 @@ final class ArcanistBundle {
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$pos = 0;
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$bytes = strlen($data);
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while ($bytes) {
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$accum = '0';
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$accum = 0;
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for ($count = 24; $count >= 0; $count -= 8) {
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$val = ord($data[$pos++]);
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$val = bcmul($val, (string)(1 << $count));
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$accum = bcadd($accum, $val);
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$val = $val * (1 << $count);
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$accum = $accum + $val;
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if (--$bytes == 0) {
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break;
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}
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}
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$slice = '';
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for ($count = 4; $count >= 0; $count--) {
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$val = bcmod($accum, 85);
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$accum = bcdiv($accum, 85);
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$val = (int)fmod($accum, 85.0);
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$accum = floor($accum / 85.0);
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$slice .= $map[$val];
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}
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$buf .= strrev($slice);
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