#' Heatmap Plot (Upper Triangle)
#'
#' Minimal ggplot2 relationship structure plot showing the upper triangle
#' in the top-right corner with no axis labels.
#' @param R A square numeric correlation matrix
#' @param title Optional plot title
#' @param labels Logical to plot column names or not (default FALSE)
#' @param order Optional character vector of the order you want the plot sorted in
#'
#' @return A ggplot object
#'
#' @examples
#' X <- matrix(data = rnorm(100), nrow = 10)
#' R <- cor(X)
#' heatmap(R, title = 'Example Correlation Plot')
#'
#' @importFrom reshape2 melt
#' @import ggplot2
#' @export
heatmap <- function(R, title = NULL, labels = FALSE, order = NULL){
	R_list <- tryCatch({
		if(is.matrix(R)){
			Rm <- R
		}else{
			Rm <- as.matrix(R)
		}

		if(nrow(Rm) != ncol(Rm)){
			stop('Input must be coercible to a square matrix')
		}

		cnames <- colnames(Rm)
		rnames <- rownames(Rm)
		if(is.null(cnames)){
			cnames <- as.character(seq_len(ncol(Rm)))
		}
		if(is.null(rnames)){
			rnames <- as.character(seq_len(nrow(Rm)))
		}

			
		if(!is.null(order)){
			if(length(order) != nrow(Rm)){
				stop('Provided order is not the same length as the width of R')
			}else{
				rownames(Rm) <- rnames
				colnames(Rm) <- cnames
				Rm <- Rm[order, order, drop = FALSE]
			}
		}
		list(
			cnames = colnames(Rm),
			rnames = rownames(Rm),
			R = Rm[upper.tri(Rm)],
			p = ncol(Rm)
		)

	}, error = function(e){
		stop(paste('Invalid input for heatmap:', e$message))
	})

	# Now get a df from the upper triangle
	n <- R_list$p
	M <- matrix(NA, n, n)
	M[upper.tri(M)] <- R_list$R
	colnames(M) <- R_list$cnames
	rownames(M) <- R_list$rnames
	df <- reshape2::melt(M, na.rm = TRUE)
	names(df) <- c('i', 'j', 'value')

	df$i <- match(df$i, rownames(M))
	df$j <- match(df$j, colnames(M))

	# And plot it
	p <- ggplot2::ggplot(
		data = df,
		ggplot2::aes(x = .data$j, y = .data$i, fill = .data$value)
	) +
	ggplot2::geom_tile() +
	ggplot2::scale_y_reverse() +
	ggplot2::coord_fixed(clip = 'off') +
	ggplot2::scale_fill_gradient2(
		low = 'red',
		mid = 'white',
		high = 'blue',
		midpoint = 0,
		limits = c(-1, 1),
		name = NULL,
		guide = 'none'
	) +
	ggplot2::theme_void()

	if(!is.null(title)){
		p <- p +
		ggplot2::ggtitle(title) +
		ggplot2::theme(
			plot.title = ggplot2::element_text(hjust = 0.5)
		)
	}

	if(labels){
		col_label_df <- data.frame(
			j = seq_len(n)[-1],
			i = 0,
			label = R_list$cnames[-1]
		)
		row_label_df <- data.frame(
			j = seq_len(n)[-n],
			i = seq_len(n)[-n],
			label = R_list$rnames[-n]
		)

		max_nchar <- max(nchar(R_list$cnames))
		top_margin <- 5 + max_nchar * 2

		p <- p +
		ggplot2::geom_text(
			data = col_label_df,
			ggplot2::aes(x = .data$j, y = .data$i, label = .data$label),
			inherit.aes = FALSE,
			angle = 90,
			hjust = 0,
			vjust = 0.5
		) +
		ggplot2::geom_text(
			data = row_label_df,
			ggplot2::aes(x = .data$j, y = .data$i, label = .data$label),
			inherit.aes = FALSE,
			hjust = 1,
			vjust = 0.5
		) +
		ggplot2::theme(
			plot.margin = ggplot2::margin(
				t = top_margin,
				r = 5,
				b = 5,
				l = 5
			)
		)
	}

	return(p)
}