[hash] PKU 2002 Squares

# include <stdio.h>
# include <string.h>

# define odd(x) (((x)+100000)&0x1)

# define MAXN 1005
# define MOD 10007

int n;
int x[MAXN], y[MAXN];
int head[MOD], next[MAXN];

int hash(int xx, int yy)
{
int hkey = xx*13131 + yy;
return (hkey%MOD + MOD)%MOD;
}

void init_hash_table(void)
{
memset(head, 0, sizeof(head));
}

void insert_hash_table(int i)
{
int h = hash(x[i], y[i]);
next[i] = head[h];
head[h] = i;
}

int lookup_hash_table(int hkey, int xx, int yy)
{
int u = head[hkey];
while (u)
{
if (x[u]==xx && y[u]==yy) return u;
u = next[u];
}
return 0;
}

void init(void)
{
int i;
init_hash_table();
for (i = 1; i <= n; ++i)
{
scanf("%d%d", &x[i], &y[i]);
insert_hash_table(i);
}
}

void solve(void)
{
int i, j, k, ans = 0, xx, yy, t;
for (i = 1; i < n; ++i)
for (j = i+1; j <= n; ++j)
{
xx = x[i]+x[j]+y[j]-y[i];
yy = y[i]+y[j]-x[j]+x[i];
if (odd(xx) || odd(yy)) ;
else if (k = lookup_hash_table(hash(xx>>1, yy>>1), xx>>1, yy>>1))
{
xx = x[i]+x[j]-y[j]+y[i];
yy = y[i]+y[j]+x[j]-x[i];
if (odd(xx) || odd(yy)) ;
else if (lookup_hash_table(hash(xx>>1, yy>>1), xx>>1, yy>>1))
++ans;
}
}
printf("%d\n", ans>>1);
}

int main()
{
while (scanf("%d", &n), n)
{
init();
solve();
}

return 0;
}