Add false positives example

example_test.go

@@ -3,10 +3,12 @@ package bloom_test
 import (
 	"fmt"
 	"github.com/yourbasic/bloom"
+	"math/rand"
+	"strconv"
 )
 
 // Create and use a Bloom filter.
-func Example() {
+func Example_basics() {
 	// Create a Bloom filter with room for 10000 elements
 	// at a false-positives rate less than 0.5 percent.
 	blacklist := bloom.New(10000, 200)
@@ -23,3 +25,28 @@ func Example() {
 	}
 	// Output: https://rascal.com seems to be shady.
 }
+
+// Count the number of false positives.
+func Example_falsePositives() {
+	// Create a Bloom filter with room for n elements
+	// at a false-positives rate less than 1/p.
+	n := 1000
+	p := 100
+	filter := bloom.New(n, p)
+
+	// Add n random strings.
+	for i := 0; i < n; i++ {
+		filter.Add(strconv.FormatUint(rand.Uint64(), 10))
+	}
+
+	// Do n random lookups and count the (mostly accidental) hits.
+	// It shouldn't be much more than n/p, and hopefully less.
+	count := 0
+	for i := 0; i < n; i++ {
+		if filter.Test(strconv.FormatUint(rand.Uint64(), 10)) {
+			count++
+		}
+	}
+	fmt.Println(count, "mistakes were made.")
+	// Output: 1 mistakes were made.
+}