I was reading this Wikipedia article, and attempting to implement a 'map' based solution in C, where a 'map' is just an int array, initialized to 0.
For some reason it works up to fib(93)
, then starts outputting strange numbers. If it matter's I'm specifying -std=c99
:
#include <stdio.h>
#include <stdlib.h>
// represents a fib num
typedef unsigned long long fib_t;
// the default value of our 'map'
const int FIB_NULL = 0;
// could get from input, set here for now
const int FIB_MAX = 100;
// our 'map' for fib nums
static fib_t *fibMap;
// calculate the fib num n
fib_t fib( unsigned int n )
{
// if this num, n, is not 0 or 1, and is FIB_NULL, then calculate it
if( n > 1 && FIB_NULL == fibMap[n] )
{
fibMap[n] = fib( n-1 ) + fib( n-2 );
}
// get it from the map
return fibMap[n];
}
// used to setup the 'map' for the fibs
static void initFibMap()
{
// emulate a map
fibMap = malloc( sizeof(fib_t) * FIB_MAX);
// initialize it to 'null'
memset(fibMap, FIB_NULL, sizeof(fib_t) * FIB_MAX);
// by definition
fibMap[0] = 0;
fibMap[1] = 1;
}
int main(int argc, char *argv[])
{
// setup our 'map'
initFibMap();
for( unsigned int i=0; i<FIB_MAX; i++ )
{
// breaks on 94
printf("Fib #%d: %llu\n",i, fib(i));
}
}
Strange output:
// . . .
// . . .
// Fib #90: 2880067194370816120 // good
// Fib #91: 4660046610375530309 // good
// Fib #92: 7540113804746346429 // good
// Fib #93: 12200160415121876738 // good
// Fib #94: 1293530146158671551 // WHAT?
// Fib #95: 13493690561280548289
// Fib #96: 14787220707439219840
// Fib #97: 9834167195010216513
// Fib #98: 6174643828739884737
// Fib #99: 16008811023750101250