tower_of_hanoi.R

#' Demonstrate the Tower of Hanoi puzzle in R
#'
#' This function uses the recursive algorithm to solve the Tower of Hanoi
#' puzzle, and demonstrates the game in animation.
#'
#' This function was written by Linlin Yan <linlin.yan@@cos.name> in a Chinese
#' forum (See 'References') to show the usage of recursive algorithm.
#' @param n an integer indicating the number of disks on the rot.
#' @seealso \code{\link[graphics]{barplot}}
#' @references Original code: \url{http://cos.name/cn/topic/101199}
#'
#'   About the Tower of Hanoi: \url{http://en.wikipedia.org/wiki/Tower_of_Hanoi}
#' @author Linlin Yan <\email{linlin.yan@@cos.name}>
#' @export
#' @examples
#' \dontrun{
#' tower_of_hanoi(7)
#' }
tower_of_hanoi <- function(n = 7) {
    if (!interactive()) return()
    tower <- list(1:n, NULL, NULL)
    color <- rainbow(n)
    par(mfrow = c(1, 3), mar = rep(0, 4), ann = FALSE)
    bgcolor <- par("bg")
    if (bgcolor == "transparent") bgcolor <- "white"
    
    draw.hanoi <- function() {
        for (i in 1:3) {
            plot(c(-n, n), c(0, n + 2), type = "n", xlab = "",
                 ylab = "", axes = FALSE)
            rect(-n, 0, n, n + 2, border = bgcolor, col = bgcolor)
            if (length(tower[[i]]) > 0) {
                barplot(rev(tower[[i]]), add = TRUE, horiz = TRUE,
                        col = color[rev(tower[[i]])])
                barplot(-rev(tower[[i]]), add = TRUE, horiz = TRUE,
                        col = color[rev(tower[[i]])])
            }
        }
    }
    
    move.hanoi <- function(k, from, to, via) {
        if (k > 1) {
            move.hanoi(k - 1, from, via, to)
            move.hanoi(1, from, to, via)
            move.hanoi(k - 1, via, to, from)
        }
        else {
            cat("Move ", tower[[from]][1], " from ", LETTERS[from],
                " to ", LETTERS[to], "\n")
            #print(tower)
            tower[[to]] <<- c(tower[[from]][1], tower[[to]])
            tower[[from]] <<- tower[[from]][-1]
            draw.hanoi()
            Sys.sleep(0.5)
        }
    }
    
    draw.hanoi()
    move.hanoi(n, 1, 2, 3)
    par(mfrow = c(1, 1))
}

tower_of_hanoi(3)