Paul Terwilliger - 6 months ago 28

R Question

I'm working in R. I need to find a fast function that will mask the highest set bit of an integer. For example:

`# 6 binary is 110, this should turn into 010 which is 2`

function_mask(6) = 2

# 8 in binary is 1000, this should turn into 0000

function_mask(8) = 0

This is equivalent to subtracting the closest lower power of two. I will be happy if I can find a fast function that will simply find the closest lower power of two. For example:

`# 6 in binary is 110, the MSB is 100`

function_power_two(6) = 4

function_mask(6) = 6 - function_power_two(6) = 2

# 8 in binary is 1000, the MSB is 1000 which is 8 in base 10

function_power_two(8) = 8

function_mask(8) = 8 - function_power_two(8) = 0

I have found bitwise operations in R: for example, bitwShiftL and bitwShiftR. However, I do not know how to implement a solution in R.

I have seen solutions in other languages: Java, C, and C++. However, I do not know how to implement these solutions in R.

There are solutions in C++ using Rcpp, however Rcpp does not support integers larger than 32-bit. I need larger integers than that.

Answer

For an R solution:

```
function_mask <- function(x) {
xb <-intToBits(x)
highestbit <- length(xb) - match("01", rev(xb))
x-2L^highestbit
}
```

Comparing speeds to the other answers, we see this one is fastest, so far.

```
function_mask1 <- function(x) {
bits = intToBits(x) # Convert integer to binary vector
ii = tail(which(bits > 0), n=1) # Identify most significant bit
bits[ii] = as.raw(0) # Mask the most significant bit
out = packBits(bits,type='integer') # Convert binary back to integer
return(out)
}
maskHighBit <- function(x){
strtoi(gsub("^1", "", R.utils::intToBin(as.integer(x))), base=2)}
library(microbenchmark)
microbenchmark(function_mask(112L), function_mask1(112L), maskHighBit(112L), times=1000)
#Unit: microseconds
#expr min lq mean median uq max neval cld
#function_mask(112L) 17.961 20.014 23.65080 23.092 24.632 57.475 1000 a
#function_mask1(112L) 39.514 44.132 49.79744 47.724 49.777 107.765 1000 b
#maskHighBit(112L) 108.791 114.435 127.53792 118.540 133.422 2054.189 1000 c
```