windowMask := uint64(width - 1) pos := uint(0) carry := uint64(0) for pos < 256 { indexU64 := pos / 64 indexBit := pos % 64 var bitBuf uint64 if indexBit < 64-w { // This window's bits are contained in a single u64 bitBuf = digits[indexU64] >> indexBit } else { // Combine the current 64 bits with bits from the next 64 bitBuf = (digits[indexU64] >> indexBit) | (digits[1+indexU64] << (64 - indexBit)) } // Add carry into the current window window := carry + (bitBuf & windowMask) if window&1 == 0 { //...