From ebb124e8120c01de9fc4b5b74b49507d7f1d035e Mon Sep 17 00:00:00 2001
From: Evgenii <evgeniy@nspcc.ru>
Date: Tue, 29 Jan 2019 14:25:17 +0300
Subject: [PATCH] add random GF(2^127)-element generator

---
 gf127/gf127.go      |  7 +++++++
 gf127/gf127_test.go | 25 +++++++++++++++++++------
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/gf127/gf127.go b/gf127/gf127.go
index 1dadb3b..f923cf9 100644
--- a/gf127/gf127.go
+++ b/gf127/gf127.go
@@ -12,6 +12,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"math/bits"
+	"math/rand"
 )
 
 // GF127 represents element of GF(2^127)
@@ -36,6 +37,12 @@ func New(lo, hi uint64) *GF127 {
 	return &GF127{lo, hi}
 }
 
+// Random returns random element from GF(2^127).
+// Is used mostly for testing.
+func Random() *GF127 {
+	return &GF127{rand.Uint64(), rand.Uint64() >> 1}
+}
+
 // String returns hex-encoded representation, starting with MSB.
 func (c *GF127) String() string {
 	return hex.EncodeToString(c.ByteArray())
diff --git a/gf127/gf127_test.go b/gf127/gf127_test.go
index 6d0c7ae..eca46ad 100644
--- a/gf127/gf127_test.go
+++ b/gf127/gf127_test.go
@@ -1,17 +1,18 @@
 package gf127
 
 import (
-	"math/rand"
 	"testing"
 )
 
 const maxUint64 = ^uint64(0)
 
 func TestAdd(t *testing.T) {
-	a := &GF127{rand.Uint64(), rand.Uint64() >> 1}
-	b := &GF127{rand.Uint64(), rand.Uint64() >> 1}
-	e := &GF127{a[0] ^ b[0], a[1] ^ b[1]}
-	c := &GF127{0, 0}
+	var (
+		a = Random()
+		b = Random()
+		e = &GF127{a[0] ^ b[0], a[1] ^ b[1]}
+		c = new(GF127)
+	)
 	c.Add(a, b)
 	if e[0] != c[0] || e[1] != c[1] {
 		t.Errorf("expected (%s), got (%s)", e.String(), c.String())
@@ -74,10 +75,22 @@ var testCasesInv = [][2]*GF127{
 }
 
 func TestInv(t *testing.T) {
-	c := new(GF127)
+	var a, b, c = new(GF127), new(GF127), new(GF127)
 	for _, tc := range testCasesInv {
 		if Inv(tc[0], c); !c.Equals(tc[1]) {
 			t.Errorf("expected (%s), got (%s)", tc[1].String(), c.String())
 		}
 	}
+
+	for i := 0; i < 3; i++ {
+		// 0 has no inverse
+		if a = Random(); a.Equals(&GF127{0, 0}) {
+			continue
+		}
+		Inv(a, b)
+		Mul(a, b, c)
+		if !c.Equals(&GF127{1, 0}) {
+			t.Errorf("expected inverse of (%s), got (%s)", a.String(), b.String())
+		}
+	}
 }