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Timing string comparisons
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <time.h>
using namespace std;
// this is just strcmp() wrapped in timer calls
bool CheckEqual(char* a, char* b, bool show)
{
int i, r, trv;
long time_pre, time_post;
struct timespec ts;
if((trv = clock_gettime(CLOCK_MONOTONIC,&ts)) != 0) {
printf("Timing error, pre. Code = %d\n", trv);
}
time_pre = ts.tv_nsec + (ts.tv_sec * 1000000000L);
r = strcmp(a, b);
if((trv = clock_gettime(CLOCK_MONOTONIC,&ts)) != 0) {
printf("Timing error, post. Code = %d\n", trv);
}
time_post = ts.tv_nsec + (ts.tv_sec * 1000000000L);
if (show)
printf("Time: %ld\n", time_post - time_pre);
else
printf("First compare done.\n");
return r == 0;
}
// this function helps us avoid the problem of the compiler being too smart.
// if we were to use string literals that were equal, it'd optimise compile-time
// constant strings into a single instance, so strcmp() would just early-out on
// the fact that the two pointers were equal, which is not what we want to
// demonstrate here - in reality we'd be comparing buffers that aren't static.
char* MakeStr(char c, int n)
{
int i;
char* s = (char*)calloc(n+1, sizeof(c));
for(i=0; i<n; i++)
s[i] = c;
return s;
}
int main()
{
// equal apart from last
char* aa = MakeStr('0', 64);
char* ab = "0000000000000000000000000000000000000000000000000000000000000001";
// equal apart from first
char* ba = MakeStr('0', 64);
char* bb = "1000000000000000000000000000000000000000000000000000000000000000";
// inequality in the middle
char* ca = "0000000000000000000000000000010000000000000000000000000000000000";
char* cb = "0000000000000000000000000000001000000000000000000000000000000000";
// equal
char* da = MakeStr('0', 64);
char* db = MakeStr('0', 64);
// first call to strcmp() and time functions will result in cache misses
// so we call it once and don't display output to account for that.
CheckEqual(aa, ab, false);
printf("Equal but last, ");
CheckEqual(aa, ab, true);
printf("Equal but last, ");
CheckEqual(aa, ab, true);
printf("Equal but first, ");
CheckEqual(ba, bb, true);
printf("Equal but mid, ");
CheckEqual(ca, cb, true);
printf("Equal but last, ");
CheckEqual(aa, ab, true);
printf("Completely equal, ");
CheckEqual(da, db, true);
printf("Completely equal, ");
CheckEqual(da, db, true);
free(aa); free(ba); free(da); free(db);
return 0;
}
# ./test
First compare done.
Equal but last, Time: 1303
Equal but last, Time: 660
Equal but first, Time: 633
Equal but mid, Time: 616
Equal but last, Time: 632
Completely equal, Time: 630
Completely equal, Time: 981
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