math/big: add (*Int).Sqrt
This is needed for some of the more complex primality tests
(to filter out exact squares), and while the code is simple the
boundary conditions are not obvious, so it seems worth having
in the library.
Change-Id: Ica994a6b6c1e412a6f6d9c3cf823f9b653c6bcbd
Reviewed-on: https://go-review.googlesource.com/30706
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go
index 18f5be7..b8e0778 100644
--- a/src/math/big/int_test.go
+++ b/src/math/big/int_test.go
@@ -9,6 +9,7 @@
"encoding/hex"
"fmt"
"math/rand"
+ "strings"
"testing"
"testing/quick"
)
@@ -1453,3 +1454,44 @@
n := NewInt(10)
n.Rand(rand.New(rand.NewSource(9)), n)
}
+
+func TestSqrt(t *testing.T) {
+ root := 0
+ r := new(Int)
+ for i := 0; i < 10000; i++ {
+ if (root+1)*(root+1) <= i {
+ root++
+ }
+ n := NewInt(int64(i))
+ r.SetInt64(-2)
+ r.Sqrt(n)
+ if r.Cmp(NewInt(int64(root))) != 0 {
+ t.Errorf("Sqrt(%v) = %v, want %v", n, r, root)
+ }
+ }
+
+ for i := 0; i < 1000; i += 10 {
+ n, _ := new(Int).SetString("1"+strings.Repeat("0", i), 10)
+ r := new(Int).Sqrt(n)
+ root, _ := new(Int).SetString("1"+strings.Repeat("0", i/2), 10)
+ if r.Cmp(root) != 0 {
+ t.Errorf("Sqrt(1e%d) = %v, want 1e%d", i, r, i/2)
+ }
+ }
+
+ // Test aliasing.
+ r.SetInt64(100)
+ r.Sqrt(r)
+ if r.Int64() != 10 {
+ t.Errorf("Sqrt(100) = %v, want 10 (aliased output)", r.Int64())
+ }
+}
+
+func BenchmarkSqrt(b *testing.B) {
+ n, _ := new(Int).SetString("1"+strings.Repeat("0", 1001), 10)
+ b.ResetTimer()
+ t := new(Int)
+ for i := 0; i < b.N; i++ {
+ t.Sqrt(n)
+ }
+}
