rawConversion Convert to or from (Bit/Packed) Raw Vectors

Description

Conversion to and from and manipulation of objects of type "raw", both used as bits or “packed” 8 bits.

Usage

charToRaw(x)
rawToChar(x, multiple = FALSE)

rawShift(x, n)

rawToBits(x)
intToBits(x)
packBits(x, type = c("raw", "integer", "double"))

numToInts(x)
numToBits(x)

Arguments

x	object to be converted or shifted.
multiple	logical: should the conversion be to a single character string or multiple individual characters?
n	the number of bits to shift. Positive numbers shift right and negative numbers shift left: allowed values are -8 ... 8.
type	the result type, partially matched.

Details

packBits accepts raw, integer or logical inputs, the last two without any NAs.

numToBits(.) and packBits(., type="double") are inverse functions of each other, see also the examples.

Note that ‘bytes’ are not necessarily the same as characters, e.g. in UTF-8 locales.

Value

charToRaw converts a length-one character string to raw bytes. It does so without taking into account any declared encoding (see Encoding).

rawToChar converts raw bytes either to a single character string or a character vector of single bytes (with "" for 0). (Note that a single character string could contain embedded nuls; only trailing nulls are allowed and will be removed.) In either case it is possible to create a result which is invalid in a multibyte locale, e.g. one using UTF-8. Long vectors are allowed if multiple is true.

rawShift(x, n) shift the bits in x by n positions to the right, see the argument n, above.

rawToBits returns a raw vector of 8 times the length of a raw vector with entries 0 or 1. intToBits returns a raw vector of 32 times the length of an integer vector with entries 0 or 1. (Non-integral numeric values are truncated to integers.) In both cases the unpacking is least-significant bit first.

packBits packs its input (using only the lowest bit for raw or integer vectors) least-significant bit first to a raw, integer or double (“numeric”) vector.

numToInts() and numToBits() split double precision numeric vectors either into to two integers each or into 64 bits each, stored as raw. In both cases the unpacking is least-significant element first.

Examples

x <- "A test string"
(y <- charToRaw(x))
is.vector(y) # TRUE

rawToChar(y)
rawToChar(y, multiple = TRUE)
(xx <- c(y,  charToRaw("&"), charToRaw(" more")))
rawToChar(xx)

rawShift(y, 1)
rawShift(y,-2)

rawToBits(y)

showBits <- function(r) stats::symnum(as.logical(rawToBits(r)))

z <- as.raw(5)
z ; showBits(z)
showBits(rawShift(z, 1)) # shift to right
showBits(rawShift(z, 2))
showBits(z)
showBits(rawShift(z, -1)) # shift to left
showBits(rawShift(z, -2)) # ..
showBits(rawShift(z, -3)) # shifted off entirely

packBits(as.raw(0:31))
i <- -2:3
stopifnot(exprs = {
  identical(i, packBits(intToBits(i), "integer"))
  identical(packBits(       0:31) ,
            packBits(as.raw(0:31)))
})
str(pBi <- packBits(intToBits(i)))
data.frame(B = matrix(pBi, nrow=6, byrow=TRUE),
           hex = format(as.hexmode(i)), i)


## Look at internal bit representation of ...

## ... of integers :
bitI <- function(x) vapply(as.integer(x), function(x) {
            b <- substr(as.character(rev(intToBits(x))), 2L, 2L)
            paste0(c(b[1L], " ", b[2:32]), collapse = "")
          }, "")
print(bitI(-8:8), width = 35, quote = FALSE)

## ... of double precision numbers in format  'sign exp | mantissa'
## where  1 bit sign  1 <==> "-";
##       11 bit exp   is the base-2 exponent biased by 2^10 - 1 (1023)
##       52 bit mantissa is without the implicit leading '1'
#
## Bit representation  [ sign | exponent | mantissa ] of double prec numbers :

bitC <- function(x) noquote(vapply(as.double(x), function(x) { # split one double
    b <- substr(as.character(rev(numToBits(x))), 2L, 2L)
    paste0(c(b[1L], " ", b[2:12], " | ", b[13:64]), collapse = "")
  }, ""))
bitC(17)
bitC(c(-1,0,1))
bitC(2^(-2:5))
bitC(1+2^-(1:53))# from 0.5 converge to 1

###  numToBits(.)  <==>   intToBits(numToInts(.)) :
d2bI <- function(x) vapply(as.double(x), function(x) intToBits(numToInts(x)), raw(64L))
d2b  <- function(x) vapply(as.double(x), function(x)           numToBits(x) , raw(64L))
set.seed(1)
x <- c(sort(rt(2048, df=1.5)),  2^(-10:10), 1+2^-(1:53))
str(bx <- d2b(x)) # a  64 x 2122  raw matrix
stopifnot( identical(bx, d2bI(x)) )

## Show that  packBits(*, "double")  is the inverse of numToBits() :
packBits(numToBits(pi), type="double")
bitC(2050)
b <- numToBits(2050) 
identical(b, numToBits(packBits(b, type="double")))
pbx <- apply(bx, 2, packBits, type="double")
stopifnot( identical(pbx, x))