/* binsearch.c */

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

int bsearch(int  [], int, int);

int main() {

  int a[100], i; 
  int target = 45; 

  /* initialize array */
  for (i=0; i<100; i++) {
    a[i]=i;   
  }

  printf("target %d found at position %d\n",target, bsearch(a, 100, target));
}


/* Assume a sorted in increasing order. Returns pos at which target
   occurs in a and -1 if target not in a */
int bsearch(int a[], int n, int target) {
  int start, end, middle, found, position; 
  
  start = 0; end = n-1; found = 0; middle;
  position = -1; 
  
  while ((!found) && (start < end -1)) {
    middle = (start + end)/2; /* this truncates to an int */
    if (a[middle] == target) {
      found = 1; 
      position = middle; 
    } else {
      if (a[middle] < target) start = middle; 
      else end = middle;
    }
  }
  if (!found) {
    assert(start == end || start == end-1); 
    if (a[start] == target) position = start;
    if (a[end] == target) position = end;
  }
  
  return position;
}