compress/flate: test if results are deterministic

This will test if deflate output is deterministic between two runs
of the deflater, when write sizes differ.

The deflater makes no official promises that results are
deterministic between runs, but this is a good test to determine
unintentional randomness.

Note that this does not guarantee that results are deterministic
across platforms nor that results will be deterministic between
Go versions. This is also not guarantees we should imply.

Change-Id: Id7dd89fe276060fd83a43d0b34ac35d50fcd32d9
Reviewed-on: https://go-review.googlesource.com/20573Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/flate/writer_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"runtime"
 	"testing"
 )
@@ -127,3 +128,68 @@ func TestWriteError(t *testing.T) {
 		}
 	}
 }
+
+// Test if two runs produce identical results
+// even when writing different sizes to the Writer.
+func TestDeterministicL0(t *testing.T) { testDeterministic(0, t) }
+func TestDeterministicL1(t *testing.T) { testDeterministic(1, t) }
+func TestDeterministicL2(t *testing.T) { testDeterministic(2, t) }
+func TestDeterministicL3(t *testing.T) { testDeterministic(3, t) }
+func TestDeterministicL4(t *testing.T) { testDeterministic(4, t) }
+func TestDeterministicL5(t *testing.T) { testDeterministic(5, t) }
+func TestDeterministicL6(t *testing.T) { testDeterministic(6, t) }
+func TestDeterministicL7(t *testing.T) { testDeterministic(7, t) }
+func TestDeterministicL8(t *testing.T) { testDeterministic(8, t) }
+func TestDeterministicL9(t *testing.T) { testDeterministic(9, t) }
+
+func testDeterministic(i int, t *testing.T) {
+	// Test so much we cross a good number of block boundaries.
+	var length = maxStoreBlockSize*30 + 500
+	if testing.Short() {
+		length /= 10
+	}
+
+	// Create a random, but compressible stream.
+	rng := rand.New(rand.NewSource(1))
+	t1 := make([]byte, length)
+	for i := range t1 {
+		t1[i] = byte(rng.Int63() & 7)
+	}
+
+	// Do our first encode.
+	var b1 bytes.Buffer
+	br := bytes.NewBuffer(t1)
+	w, err := NewWriter(&b1, i)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Use a very small prime sized buffer.
+	cbuf := make([]byte, 787)
+	_, err = io.CopyBuffer(w, br, cbuf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+
+	// We choose a different buffer size,
+	// bigger than a maximum block, and also a prime.
+	var b2 bytes.Buffer
+	cbuf = make([]byte, 81761)
+	br2 := bytes.NewBuffer(t1)
+	w2, err := NewWriter(&b2, i)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = io.CopyBuffer(w2, br2, cbuf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w2.Close()
+
+	b1b := b1.Bytes()
+	b2b := b2.Bytes()
+
+	if bytes.Compare(b1b, b2b) != 0 {
+		t.Errorf("level %d did not produce deterministic result, result mismatch, len(a) = %d, len(b) = %d", i, len(b1b), len(b2b))
+	}
+}