```
#' Calculate tree per ha factor for both fix and variable area plot
#'
#' @description Calculates tree per ha factor for both fix and variable area plots.
#'
#' @param sampleType character, Specifies how the plot is sampled among fixed area plot or variable area plot,
#' must be either V for variable area plot or F for fixed area plot.
#'
#' @param blowUp numeric, Specifies the blowup factor. For fixed area plot, it is calculated as 1/plotarea.
#' For variable area plot, it is basal area factor (BAF).
#'
#' @param treeWeight numeric, Specifies whether a tree is zero counted (tree is out),
#' one time counted (regular count) or two times counted (double counted)
#' in the walk through sampling protocal.
#'
#'
#' @param plotWeight numeric, Specifies how a plot is measured, i.e., full plot measured (valued as 1),
#' half plot measured (valued as 2) or quarter plot measured (valued as 4).
#'
#'
#' @param treeBasalArea numeric, When plot is measured using variable area plot, this value must be given,
#' otherwise, can be missing
#'
#' @return Tree per ha factor
#'
#' @importFrom data.table ':=' data.table
#' @author Yong Luo
#' @export
#' @docType methods
#' @rdname PHFCalculator
PHFCalculator <- function(sampleType,
blowUp,
treeWeight,
plotWeight,
treeBasalArea){
Data <- data.table(obsindex = 1:length(blowUp),
SAMP_TYP = sampleType, BLOWUP = blowUp,
PLOT_WT = plotWeight, TREE_WT = treeWeight,
BA_TREE = treeBasalArea)
if(nrow(Data[!(SAMP_TYP %in% c("V", "F"))])){
stop("sampleType must be either V or F. Please check your input.")
}
for(i in c("blowUp", "treeWeight", "plotWeight")){
if(length(get(i)[is.na(get(i))]) > 0){
stop("Input ", i, " has NA value. Please check your input")
}
}
if(nrow(Data[SAMP_TYP == "V" & is.na(BA_TREE),]) > 0){
stop("Non-NA TreeBasalArea values must be provided for variable area plot. Please check your input")
}
Data[SAMP_TYP == "V", PHF_TREE := TREE_WT*PLOT_WT*BLOWUP/BA_TREE]
Data[SAMP_TYP == "F", PHF_TREE := TREE_WT*PLOT_WT*BLOWUP]
return(Data[order(obsindex),]$PHF_TREE)
}
```

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