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package bitset
func select64(w uint64, j uint) uint {
seen := 0
// Divide 64bit
part := w & 0xFFFFFFFF
n := uint(popcount(part))
if n <= j {
part = w >> 32
seen += 32
j -= n
}
ww := part
// Divide 32bit
part = ww & 0xFFFF
n = uint(popcount(part))
if n <= j {
part = ww >> 16
seen += 16
j -= n
}
ww = part
// Divide 16bit
part = ww & 0xFF
n = uint(popcount(part))
if n <= j {
part = ww >> 8
seen += 8
j -= n
}
ww = part
// Lookup in final byte
counter := 0
for ; counter < 8; counter++ {
j -= uint((ww >> counter) & 1)
if j+1 == 0 {
break
}
}
return uint(seen + counter)
}
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