math/big: Added small complete example of big.Int usage

Updates #11241

Change-Id: I9639c4f66cf805a57b087c9f648d3918df105d86
Reviewed-on: https://go-review.googlesource.com/11034
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/example_test.go

@@ -49,3 +49,31 @@ func ExampleInt_Scan() {
 	}
 	// Output: 18446744073709551617
 }
+
+// Example_fibonacci demonstrates how to use big.Int to compute the smallest
+// Fibonacci number with 100 decimal digits, and find out whether it is prime.
+func Example_fibonacci() {
+	// create and initialize big.Ints from int64s
+	fib1 := big.NewInt(0)
+	fib2 := big.NewInt(1)
+
+	// initialize limit as 10^99 (the smallest integer with 100 digits)
+	var limit big.Int
+	limit.Exp(big.NewInt(10), big.NewInt(99), nil)
+
+	// loop while fib1 is smaller than 1e100
+	for fib1.Cmp(&limit) < 0 {
+		fib1, fib2 = fib2, fib1.Add(fib1, fib2)
+	}
+
+	fmt.Println(fib1) // 100-digits fibonacci number
+
+	// Test fib1 for primality. The ProbablyPrimes parameter sets the number
+	// of Miller-Rabin rounds to be performed. 20 is a good value.
+	isPrime := fib1.ProbablyPrime(20)
+	fmt.Println(isPrime) // false
+
+	// Output:
+	// 1344719667586153181419716641724567886890850696275767987106294472017884974410332069524504824747437757
+	// false
+}