Refactor random string stuff and seeding

Make sure we have a good random seed on the default RNG, that the
predictable RNG is clearly marked as such, that random strings are
actually the length requested, and that they contain a restricted set of
characters only.

cmd/syncthing/random_test.go

@@ -0,0 +1,80 @@
+// Copyright (C) 2014 The Syncthing Authors.
+//
+// This program is free software: you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+// more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program. If not, see <http://www.gnu.org/licenses/>.
+
+package main
+
+import "testing"
+
+func TestPredictableRandom(t *testing.T) {
+	// predictable random sequence is predictable
+	e := 3440579354231278675
+	if v := predictableRandom.Int(); v != e {
+		t.Errorf("Unexpected random value %d != %d", v, e)
+	}
+}
+
+func TestSeedFromBytes(t *testing.T) {
+	// should always return the same seed for the same bytes
+	tcs := []struct {
+		bs []byte
+		v  int64
+	}{
+		{[]byte("hello world"), -3639725434188061933},
+		{[]byte("hello worlx"), -2539100776074091088},
+	}
+
+	for _, tc := range tcs {
+		if v := seedFromBytes(tc.bs); v != tc.v {
+			t.Errorf("Unexpected seed value %d != %d", v, tc.v)
+		}
+	}
+}
+
+func TestRandomString(t *testing.T) {
+	for _, l := range []int{0, 1, 2, 3, 4, 8, 42} {
+		s := randomString(l)
+		if len(s) != l {
+			t.Errorf("Incorrect length %d != %s", len(s), l)
+		}
+	}
+
+	strings := make([]string, 1000)
+	for i := range strings {
+		strings[i] = randomString(8)
+		for j := range strings {
+			if i == j {
+				continue
+			}
+			if strings[i] == strings[j] {
+				t.Errorf("Repeated random string %q", strings[i])
+			}
+		}
+	}
+}
+
+func TestRandomInt64(t *testing.T) {
+	ints := make([]int64, 1000)
+	for i := range ints {
+		ints[i] = randomInt64()
+		for j := range ints {
+			if i == j {
+				continue
+			}
+			if ints[i] == ints[j] {
+				t.Errorf("Repeated random int64 %d", ints[i])
+			}
+		}
+	}
+}