#' @title Export Top PC Regions
#'
#' @description This function identifies a user-specified number of regions that contribute the most to an individual principal component.
#' If there are any brain regions that contribute negatively to that a PC, it places those in a separate column. It then exports a
#' .csv file that can be fed into the Matlab function CustomBrainRegionStack so that the anatomical organization of these brain regions
#' can be visualized in 3D. Regions with positive scores will be colored green while regions with negative scores will be purple.
#' @param PCAobject This is the output of your prcomp() function. It is a list that includes "rotation"
#' @param PCnum This is which principal component you want to examine (e.g. PC1, PC2). Enter a number.
#' @param num2show This is the number of regions in EACH color you want displayed on the graph.
#' @export
#' @examples


export_top_PC_regions <- function(PCAobject, PCnum, num2show) {

  PCnum = as.numeric(PCnum)
  num2show = as.numeric(num2show)

  loading_scores_PC <- PCAobject$rotation[,PCnum]
  loading_scores_PC_ranked <- sort(loading_scores_PC, decreasing=TRUE)

  loading_scores_PC_ranked <- as_tibble(loading_scores_PC_ranked, rownames = "ROIname")

  top_regions_scores <- slice_head(loading_scores_PC_ranked, n=num2show)

  bottom_regions_scores <- slice_tail(loading_scores_PC_ranked, n=num2show)
  bottom_regions_filtered <- bottom_regions_scores %>%      # only include bottom regions if their scores are actually <0 (subtract from the PC)
    filter(value < 0) %>%
    mutate(value = abs(value)) #...but Matlab needs positive numbers to graph

  brain_reg_columnar <- read_csv("293_brain_regions_formatted.csv") # THIS ONLY IS COMPATIBLE WITH THE MATLAB PROGRAM IF YOU USE THE FULL 293 BRAIN REGIONS
  colnames(brain_reg_columnar) <- "ROIname"

  tryJoin <- left_join(brain_reg_columnar, top_regions_scores)  # fills in scores at corresponding brain regions
  tryJoin[is.na(tryJoin)] = 0 # replaces NAs with 0s

  tryJoin2 <- left_join(brain_reg_columnar, bottom_regions_filtered, by = "ROIname") # fills in scores at corresponding brain regions
  tryJoin2[is.na(tryJoin2)] = 0 # replaces NAs with 0s

  tryJoin$valueNeg <- tryJoin2$value  #adds negative value column to main object

  tryJoin <- tryJoin %>%
    select(value, valueNeg)  # gets rid of ROI names because they make Matlab unhappy

  filename <- paste("PC", as.character(PCnum), "_", as.character(num2show), "Regions_Custom_Map.csv", sep = "") #descriptive filename

  write_csv(tryJoin, filename) #write csv

}