The R Project SVN R

/* Original (by permission) from
 * MASS/MASS.c by W. N. Venables and B. D. Ripley  Copyright (C) 1994-9

 * Find maximum column: designed for probabilities.
 * Uses reservoir sampling to break ties at random.
 */

#include "R_ext/Arith.h" /* NA handling */
#include <Rmath.h> /* fmax2 */
#include "R_ext/Random.h" /* ..RNGstate */

#include "R_ext/Applic.h" /* NA handling */

#define RELTOL 1e-5

void R_max_col(double *matrix, int *nr, int *nc, int *maxes)
{
    int   r, c, m, ntie, n_r = *nr;
    double a, b, tol;

#define NA_check                \
    if (ISNAN(a)) { /* NA or NaN */     \
        maxes[r] = NA_INTEGER;      \
        continue;               \
    }

    GetRNGstate();
    for (r = 0; r < n_r; r++) {
    m = 0;
    ntie = 1;
    a = matrix[r];
    NA_check;
    for (c = 1; c < *nc; c++) {
        b = matrix[r + c * n_r];
        NA_check;
        tol = RELTOL * fmax2(fabs(a), fabs(b));
        if (b >= a + tol) {
        ntie = 1;
        a = b;
        m = c;
        } else if (b >= a - tol) {
        ntie++;
        if (ntie * unif_rand() < 1.) m = c;
        }
    }
    maxes[r] = m + 1;
    }
    PutRNGstate();
}