spiral.cpp

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericMatrix rcpp_spiral(std::size_t nrow, std::size_t ncol) {
    Rcpp::NumericMatrix res(nrow, ncol); // result matrix
    int dx[4] = {0, 1, 0, -1}; // shift by rows
    int dy[4] = {1, 0, -1, 0}; // shift by columns
    int x = 0, y = -1; // init coordinates
    std::size_t num = 1; // numbers to fill: 1...nrow * ncol
    int side, len;
    for (std::size_t i = 0; i < nrow + ncol - 1; i++) {
        side = i % 4; // matrix side: 0, 1, 2, 3
        if (side % 2 == 1) // if side is not even
            len = (nrow + nrow - i) / 2; // row limit
        else
            len = (ncol + ncol - i) / 2; // column limit
        for (std::size_t j = 0; j < len && num <= nrow * ncol; j++) {
            x += dx[side]; // shift row
            y += dy[side]; // shift column
            res(x, y) = num++;
        }
    }
    return res;
}

spiral.R

spiral <- function(n) {
    stopifnot(is.numeric(n))
    stopifnot(n > 0)
    offset <- c(1, n, -1, -n)
    reps <- n - seq_len(n * 2 - 1L) %/% 2
    indicies <- rep(rep_len(offset, length(reps)), reps)
    indicies <- cumsum(indicies)
    values <- integer(length(indicies))
    values[indicies] <- seq_along(indicies)
    matrix(values, n, n, byrow = TRUE)
}