1 files changed, 253 insertions, 0 deletions
diff --git a/src/compress/lzw/reader_test.go b/src/compress/lzw/reader_test.go
new file mode 100644
index 0000000..d1eb76d
--- /dev/null
+++ b/src/compress/lzw/reader_test.go
@@ -0,0 +1,253 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package lzw
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"math"
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+type lzwTest struct {
+	desc       string
+	raw        string
+	compressed string
+	err        error
+}
+
+var lzwTests = []lzwTest{
+	{
+		"empty;LSB;8",
+		"",
+		"\x01\x01",
+		nil,
+	},
+	{
+		"empty;MSB;8",
+		"",
+		"\x80\x80",
+		nil,
+	},
+	{
+		"tobe;LSB;7",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x4f\x42\x45\x4f\x52\x4e\x4f\x54\x82\x84\x86\x8b\x85\x87\x89\x81",
+		nil,
+	},
+	{
+		"tobe;LSB;8",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x9e\x08\x29\xf2\x44\x8a\x93\x27\x54\x04\x12\x34\xb8\xb0\xe0\xc1\x84\x01\x01",
+		nil,
+	},
+	{
+		"tobe;MSB;7",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x4f\x42\x45\x4f\x52\x4e\x4f\x54\x82\x84\x86\x8b\x85\x87\x89\x81",
+		nil,
+	},
+	{
+		"tobe;MSB;8",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x2a\x13\xc8\x44\x52\x79\x48\x9c\x4f\x2a\x40\xa0\x90\x68\x5c\x16\x0f\x09\x80\x80",
+		nil,
+	},
+	{
+		"tobe-truncated;LSB;8",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x9e\x08\x29\xf2\x44\x8a\x93\x27\x54\x04",
+		io.ErrUnexpectedEOF,
+	},
+	// This example comes from https://en.wikipedia.org/wiki/Graphics_Interchange_Format.
+	{
+		"gif;LSB;8",
+		"\x28\xff\xff\xff\x28\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff",
+		"\x00\x51\xfc\x1b\x28\x70\xa0\xc1\x83\x01\x01",
+		nil,
+	},
+	// This example comes from http://compgroups.net/comp.lang.ruby/Decompressing-LZW-compression-from-PDF-file
+	{
+		"pdf;MSB;8",
+		"-----A---B",
+		"\x80\x0b\x60\x50\x22\x0c\x0c\x85\x01",
+		nil,
+	},
+}
+
+func TestReader(t *testing.T) {
+	var b bytes.Buffer
+	for _, tt := range lzwTests {
+		d := strings.Split(tt.desc, ";")
+		var order Order
+		switch d[1] {
+		case "LSB":
+			order = LSB
+		case "MSB":
+			order = MSB
+		default:
+			t.Errorf("%s: bad order %q", tt.desc, d[1])
+		}
+		litWidth, _ := strconv.Atoi(d[2])
+		rc := NewReader(strings.NewReader(tt.compressed), order, litWidth)
+		defer rc.Close()
+		b.Reset()
+		n, err := io.Copy(&b, rc)
+		s := b.String()
+		if err != nil {
+			if err != tt.err {
+				t.Errorf("%s: io.Copy: %v want %v", tt.desc, err, tt.err)
+			}
+			if err == io.ErrUnexpectedEOF {
+				// Even if the input is truncated, we should still return the
+				// partial decoded result.
+				if n == 0 || !strings.HasPrefix(tt.raw, s) {
+					t.Errorf("got %d bytes (%q), want a non-empty prefix of %q", n, s, tt.raw)
+				}
+			}
+			continue
+		}
+		if s != tt.raw {
+			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.desc, n, s, len(tt.raw), tt.raw)
+		}
+	}
+}
+
+type devZero struct{}
+
+func (devZero) Read(p []byte) (int, error) {
+	for i := range p {
+		p[i] = 0
+	}
+	return len(p), nil
+}
+
+func TestHiCodeDoesNotOverflow(t *testing.T) {
+	r := NewReader(devZero{}, LSB, 8)
+	d := r.(*decoder)
+	buf := make([]byte, 1024)
+	oldHi := uint16(0)
+	for i := 0; i < 100; i++ {
+		if _, err := io.ReadFull(r, buf); err != nil {
+			t.Fatalf("i=%d: %v", i, err)
+		}
+		// The hi code should never decrease.
+		if d.hi < oldHi {
+			t.Fatalf("i=%d: hi=%d decreased from previous value %d", i, d.hi, oldHi)
+		}
+		oldHi = d.hi
+	}
+}
+
+// TestNoLongerSavingPriorExpansions tests the decoder state when codes other
+// than clear codes continue to be seen after decoder.hi and decoder.width
+// reach their maximum values (4095 and 12), i.e. after we no longer save prior
+// expansions. In particular, it tests seeing the highest possible code, 4095.
+func TestNoLongerSavingPriorExpansions(t *testing.T) {
+	// Iterations is used to calculate how many input bits are needed to get
+	// the decoder.hi and decoder.width values up to their maximum.
+	iterations := []struct {
+		width, n int
+	}{
+		// The final term is 257, not 256, as NewReader initializes d.hi to
+		// d.clear+1 and the clear code is 256.
+		{9, 512 - 257},
+		{10, 1024 - 512},
+		{11, 2048 - 1024},
+		{12, 4096 - 2048},
+	}
+	nCodes, nBits := 0, 0
+	for _, e := range iterations {
+		nCodes += e.n
+		nBits += e.n * e.width
+	}
+	if nCodes != 3839 {
+		t.Fatalf("nCodes: got %v, want %v", nCodes, 3839)
+	}
+	if nBits != 43255 {
+		t.Fatalf("nBits: got %v, want %v", nBits, 43255)
+	}
+
+	// Construct our input of 43255 zero bits (which gets d.hi and d.width up
+	// to 4095 and 12), followed by 0xfff (4095) as 12 bits, followed by 0x101
+	// (EOF) as 12 bits.
+	//
+	// 43255 = 5406*8 + 7, and codes are read in LSB order. The final bytes are
+	// therefore:
+	//
+	// xwwwwwww xxxxxxxx yyyyyxxx zyyyyyyy
+	// 10000000 11111111 00001111 00001000
+	//
+	// or split out:
+	//
+	// .0000000 ........ ........ ........   w = 0x000
+	// 1....... 11111111 .....111 ........   x = 0xfff
+	// ........ ........ 00001... .0001000   y = 0x101
+	//
+	// The 12 'w' bits (not all are shown) form the 3839'th code, with value
+	// 0x000. Just after decoder.read returns that code, d.hi == 4095 and
+	// d.last == 0.
+	//
+	// The 12 'x' bits form the 3840'th code, with value 0xfff or 4095. Just
+	// after decoder.read returns that code, d.hi == 4095 and d.last ==
+	// decoderInvalidCode.
+	//
+	// The 12 'y' bits form the 3841'st code, with value 0x101, the EOF code.
+	//
+	// The 'z' bit is unused.
+	in := make([]byte, 5406)
+	in = append(in, 0x80, 0xff, 0x0f, 0x08)
+
+	r := NewReader(bytes.NewReader(in), LSB, 8)
+	nDecoded, err := io.Copy(io.Discard, r)
+	if err != nil {
+		t.Fatalf("Copy: %v", err)
+	}
+	// nDecoded should be 3841: 3839 literal codes and then 2 decoded bytes
+	// from 1 non-literal code. The EOF code contributes 0 decoded bytes.
+	if nDecoded != int64(nCodes+2) {
+		t.Fatalf("nDecoded: got %v, want %v", nDecoded, nCodes+2)
+	}
+}
+
+func BenchmarkDecoder(b *testing.B) {
+	buf, err := os.ReadFile("../testdata/e.txt")
+	if err != nil {
+		b.Fatal(err)
+	}
+	if len(buf) == 0 {
+		b.Fatalf("test file has no data")
+	}
+
+	for e := 4; e <= 6; e++ {
+		n := int(math.Pow10(e))
+		b.Run(fmt.Sprint("1e", e), func(b *testing.B) {
+			b.StopTimer()
+			b.SetBytes(int64(n))
+			buf0 := buf
+			compressed := new(bytes.Buffer)
+			w := NewWriter(compressed, LSB, 8)
+			for i := 0; i < n; i += len(buf0) {
+				if len(buf0) > n-i {
+					buf0 = buf0[:n-i]
+				}
+				w.Write(buf0)
+			}
+			w.Close()
+			buf1 := compressed.Bytes()
+			buf0, compressed, w = nil, nil, nil
+			runtime.GC()
+			b.StartTimer()
+			for i := 0; i < b.N; i++ {
+				io.Copy(io.Discard, NewReader(bytes.NewReader(buf1), LSB, 8))
+			}
+		})
+	}
+}