// 請看看K&R課本附錄 B.10 並仔細研究這程式, 請用很多 n 值測試, 並到bb寫報告!
// 注意課本上寫說 difftime( )是以秒為單位是錯的! 它仍是以 tick 為單位!
//clock.c -- by tsaiwn@csie.nctu.edu.tw
//test using clock( ) function through calling Fibonacci
//====== 電腦很快, 但是若要做的事情很多很多, 還是會感覺很慢 !
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
//第0個月有一對小兔子, 每月會長大, 大兔每月會生出一對, 請問第n月有幾對?
long long fib(int n) {  // recursive version, very SLOW !
   if(n<0) return 0;
   if(n<2) return 1;
   return fib(n-1) + fib(n-2);  //當 n > 50 你就會感覺很慢
} // fib
// 如上的 Recursive version 很好寫, 但因會叫用很多次, 會很慢 !
// === 以下改用 Iteration 方式則會很快很快!
long long fib22(int n) {  // Iteration version using for LOOP
   int i; long long f[999]={1, 1, 0}; // 第0個月與第一個月是 1
   for(i=2; i<=n; ++i) f[i] = f[i-1] + f[i-2];
   return f[n];
} // fib22
int main( ) {
   static char buf[999];
   long long ans;
   long long before, after, start, end, total;
   double elapsed;  // 用來算跑了幾秒?
   int n, w;  // 聽說 w 在籃球火 :-) 若 w 是 3388 則用 fib22( )
   printf("CLK_TCK=%f\n", (double)CLK_TCK );
   printf("CLOCKS_PER_SEC=%f\n", (double)CLOCKS_PER_SEC );
   printf("About to compute Fibonacci number..\n"
      " You can TRY n=19,35,36,37,38,45,46,49, 90,91,92\n");
   printf("fib is a recursive version, fib22 uses LOOP.\n");
   printf("Enter 3838 for calling fib22, or I will call fib: ");
   fgets(buf, sizeof(buf), stdin);   // 讀入字串到 buf
   w = atoi(buf);
   printf("\n n=? ");  fgets(buf, sizeof(buf), stdin); 
   n = atoi(buf);  if(n==0) n = 19;  // 按 ENTER 算 19 :-)
    printf("  ..calling fib%s(%d)...", w==3388?"22":"", n);
   before = time(0);   start = clock();
   if(w==3388) ans =fib22(n);
   else ans = fib(n); //try n=19,30,36,37,38,45,46,49,90,91,92
   end = clock();   after = time(0);
    printf("...return from fib(%d)\n\n", n);
   total = end - start; 
   elapsed = (double)total/CLOCKS_PER_SEC;  // in seconds
// DEV-C++ 底層是使用 GNU 的 MINGW32 gcc/g++
#ifdef __MINGW32__
  printf("Before call, time(0)=%I64d\n", before);
  printf(" After call, time(0)=%I64d\n", after);
#else
  printf("Before call, time(0)=%lld\n", before);
  printf(" After call, time(0)=%lld\n", after);
#endif
  printf("\nfib%s(%d)= ", w==3388?"22":"", n); //w是3388則是fib22
#ifdef __MINGW32__
  printf("%I64d\n", ans);
  printf("\nTime used: %I64d ticks =", total);
#else
  printf("%lld\n",ans);
  printf("\nTime used: %lld ticks =", total);
#endif
   printf("== %f seconds.\n", elapsed);
   printf("Lib function difftime(t2, t1)=%f\n",
             difftime(end, start) );    // 其實它只是做減法 !
   printf(" What is the unit of difftime()?\n");  // tick 滴答
   printf(" Hit ENTER key .."); getchar( );  return 0;
} // main