## Compute the x-y coordinates for a tree
rpartco <- function(tree, parms)
{
    if (missing(parms)) {
        pn <- paste0("device", dev.cur())
        if (!exists(pn, envir = rpart_env, inherits = FALSE))
            stop("no information available on parameters from previous call to plot()")
        parms <- get(pn, envir = rpart_env, inherits = FALSE)
    }

    frame <- tree$frame
    node <- as.numeric(row.names(frame))
    depth <- tree.depth(node)
    is.leaf <- (frame$var == "<leaf>")
    if (length(parms)) {
	uniform <- parms$uniform
	nspace <- parms$nspace
	minbranch <- parms$minbranch
    } else {
	uniform <- FALSE
	nspace <- -1
	minbranch <- 0.3
    }

    if (uniform)
        y <- (1 + max(depth) - depth) / max(depth, 4L)
    else {                    # make y- (parent y) = change in deviance
	y <- dev <- frame$dev
        temp <- split(seq(node), depth)     #d epth 0 nodes, then 1, then ...
        parent <- match(node %/% 2L, node)
        sibling <- match(ifelse(node %% 2L, node - 1L, node + 1L), node)

        ## assign the depths
        for (i in temp[-1L]) {
	    temp2 <- dev[parent[i]] - (dev[i] + dev[sibling[i]])
            y[i] <- y[parent[i]] - temp2
        }
	##
	## For some problems, classification & loss matrices in particular
	##   the gain from a split may be 0.  This is ugly on the plot.
	## Hence the "fudge" factor of  0.3 * the average step
	##
	fudge <- minbranch * diff(range(y)) / max(depth)
        for (i in temp[-1L]) {
	    temp2 <- dev[parent[i]] - (dev[i] + dev[sibling[i]])
	    haskids <- !(is.leaf[i] & is.leaf[sibling[i]])
	    y[i] <- y[parent[i]] - ifelse(temp2 <= fudge & haskids, fudge, temp2)
        }
	y <- y / (max(y))
    }

    # Now compute the x coordinates, by spacing out the leaves and then
    #   filling in
    x <- double(length(node))         # allocate, then fill it in below
    x[is.leaf] <- seq(sum(is.leaf))      # leaves at 1, 2, 3, ....
    left.child <- match(node * 2L, node)
    right.child <- match(node * 2L + 1L, node)

    ## temp is a list of non-is.leaf, by depth
    temp <- split(seq(node)[!is.leaf], depth[!is.leaf])
    for (i in rev(temp))
        x[i] <- 0.5 * (x[left.child[i]] + x[right.child[i]])

    if (nspace < 0) return(list(x = x, y = y))

    ##
    ## Now we get fancy, and try to do overlapping
    ##
    ##  The basic algorithm is, at each node:
    ##      1: get the left & right edges, by depth, for the left and
    ##           right sons, of the x-coordinate spacing.
    ##      2: find the minimal free spacing.  If this is >0, slide the
    ##           right hand son over to the left
    ##      3: report the left & right extents of the new tree up to the
    ##           parent
    ##   A way to visualize steps 1 and 2 is to imagine, for a given node,
    ##      that the left son, with all its descendants, is drawn on a
    ##      slab of wood.  The left & right edges, per level, give the
    ##      width of this board.  (The board is not a rectangle, it has
    ##      'stair step' edges). Do the same for the right son.  Now
    ##      insert some spacers, one per level, and slide right hand
    ##      board over until they touch.  Glue the boards and spacer
    ##      together at that point.
    ##
    ##  If a node has children, its 'space' is considered to extend left
    ##    and right by the amount "nspace", which accounts for space
    ##    used by the arcs from this node to its children.  For
    ##    horseshoe connections nspace usually is 1.
    ##
    compress <- function(x, me, depth)
    {
        lson <- me + 1L
	if (is.leaf[lson]) left <- list(left = x[lson], right = x[lson],
                                        depth = depth + 1L, sons = lson)
        else {
            left <- compress(x, me + 1L, depth + 1L)
            x <- left$x
        }

        rson <- me + 1L + length(left$sons) #index of right son
	if (is.leaf[rson]) right <- list(left = x[rson], right = x[rson],
                                         depth = depth + 1L, sons = rson)
	else {
            right <- compress(x, rson, depth + 1L)
            x <- right$x
        }

	maxd <- max(left$depth, right$depth) - depth
        mind <- min(left$depth, right$depth) - depth

        ## Find the smallest distance between the two subtrees
        ##   But only over depths that they have in common
        ## 1 is a minimum distance allowed
	slide <- min(right$left[1L:mind] - left$right[1L:mind]) - 1L
	if (slide > 0) {        # slide the right hand node to the left
	    x[right$sons] <- x[right$sons] - slide
	    x[me] <- (x[right$sons[1L]] + x[left$sons[1L]])/2
        }
	else slide <- 0

        ## report back
        if (left$depth > right$depth) {
	    templ <- left$left
            tempr <- left$right
            tempr[1L:mind] <- pmax(tempr[1L:mind], right$right - slide)
        } else {
	    templ <- right$left  - slide
	    tempr <- right$right - slide
	    templ[1L:mind] <- pmin(templ[1L:mind], left$left)
        }

	list(x = x,
             left = c(x[me] - nspace * (x[me] - x[lson]), templ),
	     right = c(x[me] - nspace * (x[me] - x[rson]), tempr),
	     depth = maxd + depth, sons = c(me, left$sons, right$sons))
    }
    x <- compress(x, 1L, 1L)$x
    list(x = x, y = y)
}