1 files changed, 94 insertions, 0 deletions
diff --git a/misc/ips.c b/misc/ips.c
new file mode 100644
index 00000000..ec7e673e
--- /dev/null
+++ b/misc/ips.c
@@ -0,0 +1,94 @@
+#include <stdio.h>
+#include <math.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+int h[65536];
+
+/*
+ *  Probability analysis of hashing function:
+ *
+ *  Let n be number of items and k number of boxes. For uniform distribution
+ *  we get:
+ *
+ *  Expected value of "item i is in given box": Xi = 1/k
+ *  Expected number of items in given box: a = EX = E(sum Xi) = sum E(Xi) = n/k
+ *  Expected square value: E(X^2) = E((sum Xi)^2) = E((sum_i Xi^2) + (sum_i,j i<>j XiXj)) =
+ *	= sum_i E(Xi^2) + sum_i,j i<>j E(XiXj) =
+ *	= sum_i E(Xi) [Xi is binary] + sum_i,j i<>j E(XiXj) [those are independent] =
+ *	= n/k + n*(n-1)/k^2
+ *  Variance: var X = E(X^2) - (EX)^2 = n/k + n*(n-1)/k^2 - n^2/k^2 =
+ *	= n/k - n/k^2 = a * (1-1/k)
+ *  Probability of fixed box being zero: Pz = ((k-1)/k)^n = (1-1/k)^n = (1-1/k)^(ak) =
+ *	= ((1-1/k)^k)^a which we can approximate by e^-a.
+ */
+
+unsigned int hf(unsigned int n)
+{
+#if 0
+	n = (n ^ (n >> 16)) & 0xffff;
+	n = (n ^ (n << 8)) & 0xffff;
+#elif 1
+	n = (n >> 16) ^ n;
+	n = (n ^ (n << 10)) & 0xffff;
+#elif 0
+	n = (n >> 16) ^ n;
+	n *= 259309;
+#elif 0
+	n ^= (n >> 20);
+	n ^= (n >> 10);
+	n ^= (n >> 5);
+#elif 0
+	n = (n * 259309) + ((n >> 16) * 123479);
+#else
+	return random();
+#endif
+	return n;
+}
+
+int
+main(int argc, char **argv)
+{
+	int cnt=0;
+	int i;
+
+	int bits = atol(argv[1]);
+	int z = 1 << bits;
+	int max = atol(argv[2]);
+
+	while (max--)
+	  {
+	    unsigned int i, e;
+	    if (scanf("%x/%d", &i, &e) != 2)
+	      if (feof(stdin))
+		break;
+	    else
+	      fprintf(stderr, "BUGGG\n");
+//	    i >>= (32-e);
+//	    i |= (i >> e);
+            cnt++;
+	    h[(hf(i) >> 1*(16 - bits)) & (z-1)]++;
+	  }
+//	printf(">>> %d addresses\n", cnt);
+#if 0
+	for(i=0; i<z; i++)
+		printf("%d\t%d\n", i, h[i]);
+#else
+{
+	int min=cnt, max=0, zer=0;
+	double delta=0;
+	double avg = (double) cnt / z;
+	double exdelta = avg*(1-1/z);
+	double exzer = exp(-avg);
+	for(i=0; i<z; i++) {
+		if (h[i] < min) min=h[i];
+		if (h[i] > max) max=h[i];
+		delta += (h[i] - avg) * (h[i] - avg);
+		if (!h[i]) zer++;
+	}
+	printf("size=%5d, min=%d, max=%2d, delta=%-7.6g (%-7.6g), avg=%-5.3g, zero=%g%% (%g%%)\n", z, min, max, delta/z, exdelta, avg, zer/(double)z*100, exzer*100);
+}
+#endif
+
+	return 0;
+}