Quickselect with some optimizations

qsel.c

#include <stdio.h>
#include <stdlib.h>

static int med3(int a, int b, int c) {
  return a < b
    ? c < a ? a : (c < b ? c : b)
    : c > a ? a : (c > b ? c : b);
}

static void swap(int *a, int i, int j) {
  int t = a[i]; a[i] = a[j]; a[j] = t;
}

// Quick select with Dutch national flag and median-3 pivot.
int sel(int *a, int n, int r) {
  while (n != 1) {
    int mid = med3(a[n / 4], a[n / 2], a[3 * n / 4]);
    int i = 0, j = 0, k = n - 1;
    while (j <= k)
      if (a[j] < mid) swap(a, i++, j++);
      else if (a[j] > mid) swap(a, j, k--);
      else j++;
    if (r < i) {
      n = i;
    } else if (r >= j) {
      a += j;
      n -= j;
      r -= j;
    } else {
      return a[i];
    }
  }
  return a[0];
}