R/kern.R

In SPEI: Calculation of the Standardised Precipitation-Evapotranspiration Index

# A function for generating kernels needed for computing the SPI and SPEI at different time scales.
#
kern <- function(scale, type='rectangular', shift=0) {
  if(type!='rectangular' & type!='triangular' & type!='circular' & type!='gaussian') {
    stop('type must be one of: rectangular, triangular, circular, gaussian')
  }
  #
  s <- scale
  h <- shift
  if(h>=s) {
    stop('Parameter "shift" must be lower than "scale"')
  }
  if(h<0) {
    stop('Parameter "shift" cannot have a negative value')
  }
  if(s<=0) {
    stop('Parameter "scale" must be higher than zero')
  }
  #
  if(type=='rectangular' | s==1) k <- rep(1,s)
  if(type=='triangular') k <- s:1
  if(type=='circular') k <- (s^2+(1-(1:s)^2))
  if(type=='gaussian') k <- (1/0.4)*1/sqrt(2*pi*1^2)*
    exp(-(seq(0,-3,-3/(s-1))-0)^2/2*1^2)
  if(h) k <- c(k[(h+1):2],k[1:(s-h)])
  return(k/sum(k))
}