// fig12_09.cpp
// binarySearch function with a precondition requiring a sorted vector.
#include <algorithm>
#include <iostream>
#include <vector>

template<class T>
int binarySearch(const std::vector<T>& items, const T& key)
   [[pre: items.size() > 0]]
   [[pre audit: std::is_sorted(items.begin(), items.end())]] {
   size_t low{0}; // low index of elements to search                 
   size_t high{items.size() - 1}; // high index
   size_t middle{(low + high + 1) / 2}; // middle element            
   int loc{-1}; // key's index; -1 if not found              

   do { // loop to search for element                             
      // if the element is found at the middle                
      if (key == items[middle]) {                             
         loc = middle; // loc is the current middle 
      }                                                       
      else if (key < items[middle]) { // middle is too high   
         high = middle - 1; // eliminate the higher half      
      }                                                       
      else { // middle element is too low                     
         low = middle + 1; // eliminate the lower half        
      }                                                       

      middle = (low + high + 1) / 2; // recalculate the middle
   } while ((low <= high) && (loc == -1));               

   return loc; // return location of key
} 

int main() {
   // sorted vector v1 satisfies binarySearch's sorted vector precondition
   std::vector v1{10, 20, 30, 40, 50, 60, 70, 80, 90};
   int result1{binarySearch(v1, 70)};
   std::cout << "70 was " << (result1 != -1 ? "" : "not ") << "found in v1\n";

   // unsorted vector v2 violates binarySearch's sorted vector precondition
   std::vector v2{60, 70, 80, 90, 10, 20, 30, 40, 50};
   int result2{binarySearch(v2, 60)};
   std::cout << "60 was " << (result2 != -1 ? "" : "not ") << "found in v2\n";
}



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