logB(x) # C's logb(x), numeric integer-valued "log2".
         # R's logb() is defined as "log wrt base"
ilogb(x) # == logB(), but of *type* integer

fpclassify(x)
isnormal(x)
nearbyint(x)
signbit(x)

nextafter(x, y)
nexttoward(x, y)

x <- (1:20)*pi
stopifnot(ilogb (x) == logB (x),  is.integer(ilogb(x)),
          ilogb(-x) == logB(-x),  is.double ( logB(x)))
cbind(x, "2^il(x)"= 2^logB(x), ilogb = ilogb(x), signbit = signbit(x),
         fpclassify = fpclassify(x), isnormal = isnormal(x))

x <- c(2^-(10:22), rexp(1000));
summary(x / 2^ilogb(x)) # in   [1, 2)  interval
stopifnot(nearbyint(x) == round(x))

nextafter(-0, +0)
nextafter(+0,  1)
nextafter(+0, -1)
nextafter(Inf, -1)
nextafter(-Inf, 0)