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package mtree
import (
"bufio"
"io"
)
const posixPolynomial uint32 = 0x04C11DB7
// cksum is an implementation of the POSIX CRC algorithm
func cksum(r io.Reader) (uint32, int, error) {
in := bufio.NewReader(r)
count := 0
var sum uint32
f := func(b byte) {
for i := 7; i >= 0; i-- {
msb := sum & (1 << 31)
sum = sum << 1
if msb != 0 {
sum = sum ^ posixPolynomial
}
}
sum ^= uint32(b)
}
for done := false; !done; {
switch b, err := in.ReadByte(); err {
case io.EOF:
done = true
case nil:
f(b)
count++
default:
return ^sum, count, err
}
}
for m := count; ; {
f(byte(m) & 0xff)
m = m >> 8
if m == 0 {
break
}
}
f(0)
f(0)
f(0)
f(0)
return ^sum, count, nil
}
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