[OT] fastest fibbonacci

[Era Scarecrow via Digitalmars-d]

On Monday, 24 October 2016 at 08:54:38 UTC, Andrea Fontana wrote:
> You can simply write it as:
> round(phi^n/sqrt(5));
>
> Check my example above :)

Ran your example and it's perfect for 32bit code. But 64bit, not 
so much. It's only good through 71 iterations (longs) then it 
starts having errors. Also for some odd reason the input is one 
off, so i had to add a -1 to the input for it to align. This 
makes it accurate to 41/64 bit results.

   for(int i = 1; i < 100; ++i) {
     auto cf = computeFib(i);
     auto cfm = computeFibMagic(i-1); //with magic numbers exampled
     writeln(i, ": ", cf, "\t", cfm, "\t", cf == cfm);
   }

64: 10610209857723      10610209857723  true
65: 17167680177565      17167680177565  true
66: 27777890035288      27777890035288  true
67: 44945570212853      44945570212853  true
68: 72723460248141      72723460248141  true
69: 117669030460994     117669030460994 true
70: 190392490709135     190392490709135 true
71: 308061521170129     308061521170129 true
72: 498454011879264     498454011879265 false
73: 806515533049393     806515533049395 false
74: 1304969544928657    1304969544928660        false
75: 2111485077978050    2111485077978055        false
76: 3416454622906707    3416454622906715        false
77: 5527939700884757    5527939700884771        false
78: 8944394323791464    8944394323791487        false
79: 14472334024676221   14472334024676258       false
80: 23416728348467685   23416728348467746       false