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Fix checksum code, some of the time it did not match Go/Python #18

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Fix checksum code, some of the time it did not match Go/Python #18

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17 changes: 12 additions & 5 deletions lib/checksum.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2,10 +2,17 @@ var crc32c = require('fast-crc32c').calculate
var bufferAlloc = require('buffer-alloc')

module.exports = function (value) {
var x = crc32c(value)
var result = bufferAlloc(4)
var x = crc32c(value)
var result = bufferAlloc(4)

result.writeUInt32LE(((((x >> 15) | (x << 17)) + 0xa282ead8)) >>> 0, 0, true)
// As defined in section 3 of https://github.com/google/snappy/blob/master/framing_format.txt
// And other implementations for reference:
// Go: https://github.com/golang/snappy/blob/2e65f85255dbc3072edf28d6b5b8efc472979f5a/snappy.go#L97
// Python: https://github.com/andrix/python-snappy/blob/602e9c10d743f71bef0bac5e4c4dffa17340d7b3/snappy/snappy.py#L70
// Mask the right hand to (32 - 17) = 15 bits -> 0x7fff, to keep correct 32 bit values.
// Shift the left hand with >>> for correct 32 bit intermediate result.
// Then final >>> 0 for 32 bits output
result.writeUInt32LE(((((x >>> 15) | ((x & 0x7fff) << 17)) + 0xa282ead8)) >>> 0, 0, true)

return result
}
return result
}
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