Calculate FWHM.

fwhm.R

fwhm <- function(spectrum, peaks) {

  ## work horse
  .fwhm <- function(spectrum, i) {
    n <- length(spectrum)
    left <- ifelse(i <= 1, 1, i)
    right <- ifelse(i >= n, n, i)
    
    hm <- spectrum@intensity[i]/2
    
    while (left > 1 && spectrum@intensity[left] > hm) {
      left <- left-1
    }
    while (right < n && spectrum@intensity[right] > hm) {
      right <- right+1
    }
    
    ## interpolate x values
    xleft <- approx(x=spectrum@intensity[left:(left+1)],
                    y=spectrum@mass[left:(left+1)], xout=hm)$y
    xright <- approx(x=spectrum@intensity[(right-1):right],
                    y=spectrum@mass[(right-1):right], xout=hm)$y
                    
    return(abs(xleft-xright))
  }
  
  return(unlist(lapply(which(spectrum@mass %in% peaks@mass), .fwhm, spectrum=spectrum)))
}