c-exercises/binsearch.c

/*
Write a recursive function that searches for a target in a sorted array using
binay search, where the array, its size and the target are given as parameters.
*/

#include <limits.h>
#include <stdio.h>
#include <assert.h>

#define DEBUG 1
#define debug_printf(fmt, ...) \
            do { if (DEBUG) fprintf(stderr, fmt, __VA_ARGS__); } while (0)

void assert_sorted(int array[], int arraylen) {
  int n = INT_MIN;

  for (int i = 0; i < arraylen; i++) {
    assert(n <= array[i]);
    n = array[i];
  }
}

int binsearchr(int array[], int from, int to, int target) {
  int m = from + (to-from)/2;
  debug_printf("from = %d, to = %d, m = %d\n", from, to, m);

  if (to < from)
    return -1;

  if (array[m] == target) {
    return m;
  } else if (array[m] > target) {
    return binsearchr(array, from, m-1, target);
  } else {
    /* array[m] < target */
    return binsearchr(array, m+1,  to, target);
  }
}

int binsearch(int array[], int arraylen, int target) {
  return binsearchr(array, 0, arraylen-1, target);
}

int main(void) {
  int a[43] = {
    912, 1204, 1761, 2143, 3691,
    5150, 5249, 6864, 6871, 6902,
    9567, 9703, 10411, 10686, 13501,
    14378, 16249, 17393, 17911, 18614,
    19284, 19595, 19960, 20052, 20702,
    21090, 22277, 22624, 24634, 24877,
    25367, 25586, 26706, 26720, 26958,
    27478, 28269, 29142, 30630, 30804,
    31984, 32398, 32588 };
  assert_sorted(a, 43);

  assert(binsearch(a, 43, 912) == 0);
  assert(binsearch(a, 43, 3691) == 4);
  assert(binsearch(a, 43, 32588) == 42);
  assert(binsearch(a, 43, 12345) == -1);

  int b[2] = { 23, 42 };
  assert_sorted(b, 2);

  assert(binsearch(b, 2, 23) == 0);
  assert(binsearch(b, 2, 42) == 1);
  assert(binsearch(b, 2, 5) == -1);
  assert(binsearch(b, 2, 99) == -1);

  int c[1] = { 0 };
  assert_sorted(c, 1);

  assert(binsearch(c, 1, 0) == 0);
  assert(binsearch(c, 1, 1) == -1);
}