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>

diff --git a/src/math/big/example_test.go b/src/math/big/example_test.go
index 078be47f956940d5fed438817a3e0bdc65ab504d..384b50e51ca25176bc926889b3ff54f3ae3936d3 100644 (file)
--- a/src/math/big/example_test.go
+++ b/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
+}