// Copyright 2011 The Snappy-Go Authors. All rights reserved.
// Copyright (c) 2019 Klaus Post. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package s2

import (
	"bytes"
	"encoding/binary"
	"flag"
	"fmt"
	"io"
	"math"
	"math/rand"
	"net/http"
	"os"
	"path/filepath"
	"runtime"
	"strings"
	"testing"

	"github.com/klauspost/compress/internal/snapref"
	"github.com/klauspost/compress/zip"
	"github.com/klauspost/compress/zstd"
)

const maxUint = ^uint(0)
const maxInt = int(maxUint >> 1)

var (
	download     = flag.Bool("download", false, "If true, download any missing files before running benchmarks")
	testdataDir  = flag.String("testdataDir", "testdata", "Directory containing the test data")
	benchdataDir = flag.String("benchdataDir", "testdata/bench", "Directory containing the benchmark data")
)

func TestMaxEncodedLen(t *testing.T) {
	testSet := []struct {
		in, out int64
	}{
		0:  {in: 0, out: 1},
		1:  {in: 1 << 24, out: 1<<24 + int64(binary.PutVarint([]byte{binary.MaxVarintLen32: 0}, int64(1<<24))) + literalExtraSize(1<<24)},
		2:  {in: MaxBlockSize, out: math.MaxUint32},
		3:  {in: math.MaxUint32 - binary.MaxVarintLen32 - literalExtraSize(math.MaxUint32), out: math.MaxUint32},
		4:  {in: math.MaxUint32 - 9, out: -1},
		5:  {in: math.MaxUint32 - 8, out: -1},
		6:  {in: math.MaxUint32 - 7, out: -1},
		7:  {in: math.MaxUint32 - 6, out: -1},
		8:  {in: math.MaxUint32 - 5, out: -1},
		9:  {in: math.MaxUint32 - 4, out: -1},
		10: {in: math.MaxUint32 - 3, out: -1},
		11: {in: math.MaxUint32 - 2, out: -1},
		12: {in: math.MaxUint32 - 1, out: -1},
		13: {in: math.MaxUint32, out: -1},
		14: {in: -1, out: -1},
		15: {in: -2, out: -1},
	}
	// 32 bit platforms have a different threshold.
	if maxInt == math.MaxInt32 {
		testSet[2].out = math.MaxInt32
		testSet[3].out = -1
	}
	t.Log("Maxblock:", MaxBlockSize, "reduction:", intReduction)
	// Test all sizes up to maxBlockSize.
	for i := range int64(maxBlockSize) {
		testSet = append(testSet, struct{ in, out int64 }{in: i, out: i + int64(binary.PutVarint([]byte{binary.MaxVarintLen32: 0}, i)) + literalExtraSize(i)})
	}
	for i := range testSet {
		tt := testSet[i]
		want := tt.out
		got := int64(MaxEncodedLen(int(tt.in)))
		if got != want {
			t.Errorf("test %d: input: %d, want: %d, got: %d", i, tt.in, want, got)
		}
	}
}

func cmp(got, want []byte) error {
	if bytes.Equal(got, want) {
		return nil
	}
	if len(got) != len(want) {
		return fmt.Errorf("got %d bytes, want %d", len(got), len(want))
	}
	for i := range got {
		if got[i] != want[i] {
			return fmt.Errorf("byte #%d: got 0x%02x, want 0x%02x", i, got[i], want[i])
		}
	}
	return nil
}

func roundtrip(b, ebuf, dbuf []byte) error {
	bOrg := make([]byte, len(b))
	copy(bOrg, b)
	asmEnc := Encode(nil, b)
	if err := cmp(bOrg, b); err != nil {
		return fmt.Errorf("src was changed: %v", err)
	}
	goEnc := encodeGo(nil, b)
	if err := cmp(bOrg, b); err != nil {
		return fmt.Errorf("src was changed: %v", err)
	}

	//fmt.Println("asm:", len(asmEnc), "go:", len(goEnc))
	dGo, err := Decode(nil, goEnc)
	if err != nil {
		return fmt.Errorf("decoding error: %v", err)
	}

	if err := cmp(dGo, b); err != nil {
		return fmt.Errorf("roundtrip mismatch: %v", err)
	}

	// fmt.Println("decode asm...")
	d, err := Decode(nil, asmEnc)
	if err != nil {
		return fmt.Errorf("decoding error: %v", err)
	}
	if err := cmp(d, b); err != nil {
		return fmt.Errorf("roundtrip mismatch: %v", err)
	}
	d, err = Decode(dbuf, EncodeBetter(ebuf, b))
	if err != nil {
		return fmt.Errorf("decoding better error: %v", err)
	}
	if err := cmp(d, b); err != nil {
		return fmt.Errorf("roundtrip better mismatch: %v", err)
	}

	// Test concat with some existing data.
	dst := []byte("existing")
	// Add 3 different encodes and a 0 length block.
	concat, err := ConcatBlocks(dst, Encode(nil, b), EncodeBetter(nil, b), []byte{0}, EncodeSnappy(nil, b))
	if err != nil {
		return fmt.Errorf("concat error: %v", err)
	}
	if err := cmp(concat[:len(dst)], dst); err != nil {
		return fmt.Errorf("concat existing mismatch: %v", err)
	}
	concat = concat[len(dst):]

	d, _ = Decode(nil, concat)
	want := append(make([]byte, 0, len(b)*3), b...)
	want = append(want, b...)
	want = append(want, b...)

	if err := cmp(d, want); err != nil {
		return fmt.Errorf("roundtrip concat mismatch: %v", err)
	}

	return nil
}

func TestEmpty(t *testing.T) {
	if err := roundtrip(nil, nil, nil); err != nil {
		t.Fatal(err)
	}
}

func TestSmallCopy(t *testing.T) {
	for _, ebuf := range [][]byte{nil, make([]byte, 20), make([]byte, 64)} {
		for _, dbuf := range [][]byte{nil, make([]byte, 20), make([]byte, 64)} {
			for i := range 32 {
				s := "aaaa" + strings.Repeat("b", i) + "aaaabbbb"
				if err := roundtrip([]byte(s), ebuf, dbuf); err != nil {
					t.Errorf("len(ebuf)=%d, len(dbuf)=%d, i=%d: %v", len(ebuf), len(dbuf), i, err)
				}
			}
		}
	}
}

func TestSmallRand(t *testing.T) {
	rng := rand.New(rand.NewSource(1))
	for n := 1; n < 20000; n += 23 {
		b := make([]byte, n)
		for i := range b {
			b[i] = uint8(rng.Intn(256))
		}
		if err := roundtrip(b, nil, nil); err != nil {
			t.Fatal(err)
		}
	}
}

func TestSmallRegular(t *testing.T) {
	for n := 1; n < 20000; n += 23 {
		b := make([]byte, n)
		for i := range b {
			b[i] = uint8(i%10 + 'a')
		}
		if err := roundtrip(b, nil, nil); err != nil {
			t.Fatal(err)
		}
	}
}

func TestSmallRepeat(t *testing.T) {
	for n := 1; n < 20000; n += 23 {
		b := make([]byte, n)
		for i := range b[:n/2] {
			b[i] = uint8(i * 255 / n)
		}
		for i := range b[n/2:] {
			b[i+n/2] = uint8(i%10 + 'a')
		}
		if err := roundtrip(b, nil, nil); err != nil {
			t.Fatal(err)
		}
	}
}

func TestInvalidVarint(t *testing.T) {
	testCases := []struct {
		desc  string
		input string
	}{{
		"invalid varint, final byte has continuation bit set",
		"\xff",
	}, {
		"invalid varint, value overflows uint64",
		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00",
	}, {
		// https://github.com/google/snappy/blob/master/format_description.txt
		// says that "the stream starts with the uncompressed length [as a
		// varint] (up to a maximum of 2^32 - 1)".
		"valid varint (as uint64), but value overflows uint32",
		"\x80\x80\x80\x80\x10",
	}, {
		// 8-byte overlong varint encoding value 4. The format spec limits
		// the varint preamble to 5 bytes. binary.Uvarint accepts up to 10
		// bytes so this decodes to a valid uint32 value (4), but the
		// overlong encoding must be rejected.
		"overlong varint (8 bytes encoding value 4), exceeds 5-byte max",
		"\x84\x80\x80\x80\x80\x80\x80\x00" +
			"\x00\x00\x00\x00\x00\x00\x00" +
			"\x30",
	}}

	for _, tc := range testCases {
		input := []byte(tc.input)
		if _, err := DecodedLen(input); err != ErrCorrupt {
			t.Errorf("%s: DecodedLen: got %v, want ErrCorrupt", tc.desc, err)
		}
		if _, err := Decode(nil, input); err != ErrCorrupt {
			t.Errorf("%s: Decode: got %v, want ErrCorrupt", tc.desc, err)
		}
	}
}

func TestDecode(t *testing.T) {
	lit40Bytes := make([]byte, 40)
	for i := range lit40Bytes {
		lit40Bytes[i] = byte(i)
	}
	lit40 := string(lit40Bytes)

	testCases := []struct {
		desc    string
		input   string
		want    string
		wantErr error
	}{{
		`decodedLen=0; valid input`,
		"\x00",
		"",
		nil,
	}, {
		`decodedLen=3; tagLiteral, 0-byte length; length=3; valid input`,
		"\x03" + "\x08\xff\xff\xff",
		"\xff\xff\xff",
		nil,
	}, {
		`decodedLen=2; tagLiteral, 0-byte length; length=3; not enough dst bytes`,
		"\x02" + "\x08\xff\xff\xff",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=3; tagLiteral, 0-byte length; length=3; not enough src bytes`,
		"\x03" + "\x08\xff\xff",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=40; tagLiteral, 0-byte length; length=40; valid input`,
		"\x28" + "\x9c" + lit40,
		lit40,
		nil,
	}, {
		`decodedLen=1; tagLiteral, 1-byte length; not enough length bytes`,
		"\x01" + "\xf0",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=3; tagLiteral, 1-byte length; length=3; valid input`,
		"\x03" + "\xf0\x02\xff\xff\xff",
		"\xff\xff\xff",
		nil,
	}, {
		`decodedLen=1; tagLiteral, 2-byte length; not enough length bytes`,
		"\x01" + "\xf4\x00",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=3; tagLiteral, 2-byte length; length=3; valid input`,
		"\x03" + "\xf4\x02\x00\xff\xff\xff",
		"\xff\xff\xff",
		nil,
	}, {
		`decodedLen=1; tagLiteral, 3-byte length; not enough length bytes`,
		"\x01" + "\xf8\x00\x00",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=3; tagLiteral, 3-byte length; length=3; valid input`,
		"\x03" + "\xf8\x02\x00\x00\xff\xff\xff",
		"\xff\xff\xff",
		nil,
	}, {
		`decodedLen=1; tagLiteral, 4-byte length; not enough length bytes`,
		"\x01" + "\xfc\x00\x00\x00",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=1; tagLiteral, 4-byte length; length=3; not enough dst bytes`,
		"\x01" + "\xfc\x02\x00\x00\x00\xff\xff\xff",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=4; tagLiteral, 4-byte length; length=3; not enough src bytes`,
		"\x04" + "\xfc\x02\x00\x00\x00\xff",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=3; tagLiteral, 4-byte length; length=3; valid input`,
		"\x03" + "\xfc\x02\x00\x00\x00\xff\xff\xff",
		"\xff\xff\xff",
		nil,
	}, {
		`decodedLen=4; tagCopy1, 1 extra length|offset byte; not enough extra bytes`,
		"\x04" + "\x01",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=4; tagCopy2, 2 extra length|offset bytes; not enough extra bytes`,
		"\x04" + "\x02\x00",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=4; tagCopy4, 4 extra length|offset bytes; not enough extra bytes`,
		"\x04" + "\x03\x00\x00\x00",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=4; tagLiteral (4 bytes "abcd"); valid input`,
		"\x04" + "\x0cabcd",
		"abcd",
		nil,
	}, {
		`decodedLen=13; tagLiteral (4 bytes "abcd"); tagCopy1; length=9 offset=4; valid input`,
		"\x0d" + "\x0cabcd" + "\x15\x04",
		"abcdabcdabcda",
		nil,
	}, {
		`decodedLen=8; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=4; valid input`,
		"\x08" + "\x0cabcd" + "\x01\x04",
		"abcdabcd",
		nil,
	}, {
		`decodedLen=8; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=2; valid input`,
		"\x08" + "\x0cabcd" + "\x01\x02",
		"abcdcdcd",
		nil,
	}, {
		`decodedLen=8; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=1; valid input`,
		"\x08" + "\x0cabcd" + "\x01\x01",
		"abcddddd",
		nil,
	}, {
		`decodedLen=8; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=0; repeat offset as first match`,
		"\x08" + "\x0cabcd" + "\x01\x00",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=13; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=1; literal: 'z'; tagCopy1; length=4 offset=0; repeat offset as second match`,
		"\x0d" + "\x0cabcd" + "\x01\x01" + "\x00z" + "\x01\x00",
		"abcdddddzzzzz",
		nil,
	}, {
		`decodedLen=9; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=4; inconsistent dLen`,
		"\x09" + "\x0cabcd" + "\x01\x04",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=8; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=5; offset too large`,
		"\x08" + "\x0cabcd" + "\x01\x05",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=7; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=4; length too large`,
		"\x07" + "\x0cabcd" + "\x01\x04",
		"",
		ErrCorrupt,
	}, {
		`decodedLen=6; tagLiteral (4 bytes "abcd"); tagCopy2; length=2 offset=3; valid input`,
		"\x06" + "\x0cabcd" + "\x06\x03\x00",
		"abcdbc",
		nil,
	}, {
		`decodedLen=6; tagLiteral (4 bytes "abcd"); tagCopy4; length=2 offset=3; valid input`,
		"\x06" + "\x0cabcd" + "\x07\x03\x00\x00\x00",
		"abcdbc",
		nil,
	}}

	const (
		// notPresentXxx defines a range of byte values [0xa0, 0xc5) that are
		// not present in either the input or the output. It is written to dBuf
		// to check that Decode does not write bytes past the end of
		// dBuf[:dLen].
		//
		// The magic number 37 was chosen because it is prime. A more 'natural'
		// number like 32 might lead to a false negative if, for example, a
		// byte was incorrectly copied 4*8 bytes later.
		notPresentBase = 0xa0
		notPresentLen  = 37
	)

	var dBuf [100]byte
loop:
	for i, tc := range testCases {
		input := []byte(tc.input)
		for _, x := range input {
			if notPresentBase <= x && x < notPresentBase+notPresentLen {
				t.Errorf("#%d (%s): input shouldn't contain %#02x\ninput: % x", i, tc.desc, x, input)
				continue loop
			}
		}

		dLen, n := binary.Uvarint(input)
		if n <= 0 {
			t.Errorf("#%d (%s): invalid varint-encoded dLen", i, tc.desc)
			continue
		}
		if dLen > uint64(len(dBuf)) {
			t.Errorf("#%d (%s): dLen %d is too large", i, tc.desc, dLen)
			continue
		}

		for j := range dBuf {
			dBuf[j] = byte(notPresentBase + j%notPresentLen)
		}
		g, gotErr := Decode(dBuf[:], input)
		if got := string(g); got != tc.want || gotErr != tc.wantErr {
			t.Errorf("#%d (%s):\ngot  %q, %v\nwant %q, %v",
				i, tc.desc, got, gotErr, tc.want, tc.wantErr)
			continue
		}
		for j, x := range dBuf {
			if uint64(j) < dLen {
				continue
			}
			if w := byte(notPresentBase + j%notPresentLen); x != w {
				t.Errorf("#%d (%s): Decode overrun: dBuf[%d] was modified: got %#02x, want %#02x\ndBuf: % x",
					i, tc.desc, j, x, w, dBuf)
				continue loop
			}
		}
	}
}

func TestDecodeCopy4(t *testing.T) {
	dots := strings.Repeat(".", 65536)

	input := strings.Join([]string{
		"\x89\x80\x04",         // decodedLen = 65545.
		"\x0cpqrs",             // 4-byte literal "pqrs".
		"\xf4\xff\xff" + dots,  // 65536-byte literal dots.
		"\x13\x04\x00\x01\x00", // tagCopy4; length=5 offset=65540.
	}, "")

	gotBytes, err := Decode(nil, []byte(input))
	if err != nil {
		t.Fatal(err)
	}
	got := string(gotBytes)
	want := "pqrs" + dots + "pqrs."
	if len(got) != len(want) {
		t.Fatalf("got %d bytes, want %d", len(got), len(want))
	}
	if got != want {
		for i := 0; i < len(got); i++ {
			if g, w := got[i], want[i]; g != w {
				t.Fatalf("byte #%d: got %#02x, want %#02x", i, g, w)
			}
		}
	}
}

// TestDecodeLengthOffset tests decoding an encoding of the form literal +
// copy-length-offset + literal. For example: "abcdefghijkl" + "efghij" + "AB".
func TestDecodeLengthOffset(t *testing.T) {
	const (
		prefix = "abcdefghijklmnopqr"
		suffix = "ABCDEFGHIJKLMNOPQR"

		// notPresentXxx defines a range of byte values [0xa0, 0xc5) that are
		// not present in either the input or the output. It is written to
		// gotBuf to check that Decode does not write bytes past the end of
		// gotBuf[:totalLen].
		//
		// The magic number 37 was chosen because it is prime. A more 'natural'
		// number like 32 might lead to a false negative if, for example, a
		// byte was incorrectly copied 4*8 bytes later.
		notPresentBase = 0xa0
		notPresentLen  = 37
	)
	var gotBuf, wantBuf, inputBuf [128]byte
	for length := 1; length <= 18; length++ {
		for offset := 1; offset <= 18; offset++ {
		loop:
			for suffixLen := 0; suffixLen <= 18; suffixLen++ {
				totalLen := len(prefix) + length + suffixLen

				inputLen := binary.PutUvarint(inputBuf[:], uint64(totalLen))
				inputBuf[inputLen] = tagLiteral + 4*byte(len(prefix)-1)
				inputLen++
				inputLen += copy(inputBuf[inputLen:], prefix)
				inputBuf[inputLen+0] = tagCopy2 + 4*byte(length-1)
				inputBuf[inputLen+1] = byte(offset)
				inputBuf[inputLen+2] = 0x00
				inputLen += 3
				if suffixLen > 0 {
					inputBuf[inputLen] = tagLiteral + 4*byte(suffixLen-1)
					inputLen++
					inputLen += copy(inputBuf[inputLen:], suffix[:suffixLen])
				}
				input := inputBuf[:inputLen]

				for i := range gotBuf {
					gotBuf[i] = byte(notPresentBase + i%notPresentLen)
				}
				got, err := Decode(gotBuf[:], input)
				if err != nil {
					t.Errorf("length=%d, offset=%d; suffixLen=%d: %v", length, offset, suffixLen, err)
					continue
				}

				wantLen := 0
				wantLen += copy(wantBuf[wantLen:], prefix)
				for i := 0; i < length; i++ {
					wantBuf[wantLen] = wantBuf[wantLen-offset]
					wantLen++
				}
				wantLen += copy(wantBuf[wantLen:], suffix[:suffixLen])
				want := wantBuf[:wantLen]

				for _, x := range input {
					if notPresentBase <= x && x < notPresentBase+notPresentLen {
						t.Errorf("length=%d, offset=%d; suffixLen=%d: input shouldn't contain %#02x\ninput: % x",
							length, offset, suffixLen, x, input)
						continue loop
					}
				}
				for i, x := range gotBuf {
					if i < totalLen {
						continue
					}
					if w := byte(notPresentBase + i%notPresentLen); x != w {
						t.Errorf("length=%d, offset=%d; suffixLen=%d; totalLen=%d: "+
							"Decode overrun: gotBuf[%d] was modified: got %#02x, want %#02x\ngotBuf: % x",
							length, offset, suffixLen, totalLen, i, x, w, gotBuf)
						continue loop
					}
				}
				for _, x := range want {
					if notPresentBase <= x && x < notPresentBase+notPresentLen {
						t.Errorf("length=%d, offset=%d; suffixLen=%d: want shouldn't contain %#02x\nwant: % x",
							length, offset, suffixLen, x, want)
						continue loop
					}
				}

				if !bytes.Equal(got, want) {
					t.Errorf("length=%d, offset=%d; suffixLen=%d:\ninput % x\ngot   % x\nwant  % x",
						length, offset, suffixLen, input, got, want)
					continue
				}
			}
		}
	}
}

const (
	goldenText       = "Mark.Twain-Tom.Sawyer.txt"
	goldenCompressed = goldenText + ".rawsnappy"
)

func TestDecodeGoldenInput(t *testing.T) {
	tDir := filepath.FromSlash(*testdataDir)
	src, err := os.ReadFile(filepath.Join(tDir, goldenCompressed))
	if err != nil {
		t.Fatalf("ReadFile: %v", err)
	}
	got, err := Decode(nil, src)
	if err != nil {
		t.Fatalf("Decode: %v", err)
	}
	want, err := os.ReadFile(filepath.Join(tDir, goldenText))
	if err != nil {
		t.Fatalf("ReadFile: %v", err)
	}
	if err := cmp(got, want); err != nil {
		t.Fatal(err)
	}
}

// TestSlowForwardCopyOverrun tests the "expand the pattern" algorithm
// described in decode_amd64.s and its claim of a 10 byte overrun worst case.
func TestSlowForwardCopyOverrun(t *testing.T) {
	const base = 100

	for length := 1; length < 18; length++ {
		for offset := 1; offset < 18; offset++ {
			highWaterMark := base
			d := base
			l := length
			o := offset

			// makeOffsetAtLeast8
			for o < 8 {
				if end := d + 8; highWaterMark < end {
					highWaterMark = end
				}
				l -= o
				d += o
				o += o
			}

			// fixUpSlowForwardCopy
			a := d
			d += l

			// finishSlowForwardCopy
			for l > 0 {
				if end := a + 8; highWaterMark < end {
					highWaterMark = end
				}
				a += 8
				l -= 8
			}

			dWant := base + length
			overrun := highWaterMark - dWant
			if d != dWant || overrun < 0 || 10 < overrun {
				t.Errorf("length=%d, offset=%d: d and overrun: got (%d, %d), want (%d, something in [0, 10])",
					length, offset, d, overrun, dWant)
			}
		}
	}
}

// TestEncoderSkip will test skipping various sizes and block types.
func TestEncoderSkip(t *testing.T) {
	for ti, origLen := range []int{10 << 10, 256 << 10, 2 << 20, 8 << 20} {
		if testing.Short() && ti > 1 {
			break
		}
		t.Run(fmt.Sprint(origLen), func(t *testing.T) {
			src := make([]byte, origLen)
			rng := rand.New(rand.NewSource(1))
			firstHalf, secondHalf := src[:origLen/2], src[origLen/2:]
			bonus := secondHalf[len(secondHalf)-origLen/10:]
			for i := range firstHalf {
				// Incompressible.
				firstHalf[i] = uint8(rng.Intn(256))
			}
			for i := range secondHalf {
				// Easy to compress.
				secondHalf[i] = uint8(i & 32)
			}
			for i := range bonus {
				// Incompressible.
				bonus[i] = uint8(rng.Intn(256))
			}
			var dst bytes.Buffer
			enc := NewWriter(&dst, WriterBlockSize(64<<10))
			_, err := io.Copy(enc, bytes.NewBuffer(src))
			if err != nil {
				t.Fatal(err)
			}
			err = enc.Close()
			if err != nil {
				t.Fatal(err)
			}
			compressed := dst.Bytes()
			dec := NewReader(nil)
			for i := 0; i < len(src); i += len(src)/20 - 17 {
				t.Run(fmt.Sprint("skip-", i), func(t *testing.T) {
					want := src[i:]
					dec.Reset(bytes.NewBuffer(compressed))
					// Read some of it first
					read, err := io.CopyN(io.Discard, dec, int64(len(want)/10))
					if err != nil {
						t.Fatal(err)
					}
					// skip what we just read.
					want = want[read:]
					err = dec.Skip(int64(i))
					if err != nil {
						t.Fatal(err)
					}
					got, err := io.ReadAll(dec)
					if err != nil {
						t.Errorf("Skipping %d returned error: %v", i, err)
						return
					}
					if !bytes.Equal(want, got) {
						t.Log("got  len:", len(got))
						t.Log("want len:", len(want))
						t.Errorf("Skipping %d did not return correct data (content mismatch)", i)
						return
					}
				})
				if testing.Short() && i > 0 {
					return
				}
			}
		})
	}
}

// TestEncodeNoiseThenRepeats encodes input for which the first half is very
// incompressible and the second half is very compressible. The encoded form's
// length should be closer to 50% of the original length than 100%.
func TestEncodeNoiseThenRepeats(t *testing.T) {
	for _, origLen := range []int{256 * 1024, 2048 * 1024} {
		src := make([]byte, origLen)
		rng := rand.New(rand.NewSource(1))
		firstHalf, secondHalf := src[:origLen/2], src[origLen/2:]
		for i := range firstHalf {
			firstHalf[i] = uint8(rng.Intn(256))
		}
		for i := range secondHalf {
			secondHalf[i] = uint8(i >> 8)
		}
		dst := Encode(nil, src)
		if got, want := len(dst), origLen*3/4; got >= want {
			t.Fatalf("origLen=%d: got %d encoded bytes, want less than %d", origLen, got, want)
		}
		t.Log(len(dst))
	}
}

func TestFramingFormat(t *testing.T) {
	// src is comprised of alternating 1e5-sized sequences of random
	// (incompressible) bytes and repeated (compressible) bytes. 1e5 was chosen
	// because it is larger than maxBlockSize (64k).
	src := make([]byte, 1e6)
	rng := rand.New(rand.NewSource(1))
	for i := range 10 {
		if i%2 == 0 {
			for j := range int(1e5) {
				src[1e5*i+j] = uint8(rng.Intn(256))
			}
		} else {
			for j := range int(1e5) {
				src[1e5*i+j] = uint8(i)
			}
		}
	}

	buf := new(bytes.Buffer)
	bw := NewWriter(buf)
	if _, err := bw.Write(src); err != nil {
		t.Fatalf("Write: encoding: %v", err)
	}
	err := bw.Close()
	if err != nil {
		t.Fatal(err)
	}
	dst, err := io.ReadAll(NewReader(buf))
	if err != nil {
		t.Fatalf("ReadAll: decoding: %v", err)
	}
	if err := cmp(dst, src); err != nil {
		t.Fatal(err)
	}
}

func TestFramingFormatBetter(t *testing.T) {
	// src is comprised of alternating 1e5-sized sequences of random
	// (incompressible) bytes and repeated (compressible) bytes. 1e5 was chosen
	// because it is larger than maxBlockSize (64k).
	src := make([]byte, 1e6)
	rng := rand.New(rand.NewSource(1))
	for i := range 10 {
		if i%2 == 0 {
			for j := range int(1e5) {
				src[1e5*i+j] = uint8(rng.Intn(256))
			}
		} else {
			for j := range int(1e5) {
				src[1e5*i+j] = uint8(i)
			}
		}
	}

	buf := new(bytes.Buffer)
	bw := NewWriter(buf, WriterBetterCompression())
	if _, err := bw.Write(src); err != nil {
		t.Fatalf("Write: encoding: %v", err)
	}
	err := bw.Close()
	if err != nil {
		t.Fatal(err)
	}
	dst, err := io.ReadAll(NewReader(buf))
	if err != nil {
		t.Fatalf("ReadAll: decoding: %v", err)
	}
	if err := cmp(dst, src); err != nil {
		t.Fatal(err)
	}
}

func TestEmitLiteral(t *testing.T) {
	testCases := []struct {
		length int
		want   string
	}{
		{1, "\x00"},
		{2, "\x04"},
		{59, "\xe8"},
		{60, "\xec"},
		{61, "\xf0\x3c"},
		{62, "\xf0\x3d"},
		{254, "\xf0\xfd"},
		{255, "\xf0\xfe"},
		{256, "\xf0\xff"},
		{257, "\xf4\x00\x01"},
		{65534, "\xf4\xfd\xff"},
		{65535, "\xf4\xfe\xff"},
		{65536, "\xf4\xff\xff"},
	}

	dst := make([]byte, 70000)
	nines := bytes.Repeat([]byte{0x99}, 65536)
	for _, tc := range testCases {
		lit := nines[:tc.length]
		n := emitLiteral(dst, lit)
		if !bytes.HasSuffix(dst[:n], lit) {
			t.Errorf("length=%d: did not end with that many literal bytes", tc.length)
			continue
		}
		got := string(dst[:n-tc.length])
		if got != tc.want {
			t.Errorf("length=%d:\ngot  % x\nwant % x", tc.length, got, tc.want)
			continue
		}
	}
}

func TestEmitCopy(t *testing.T) {
	testCases := []struct {
		offset int
		length int
		want   string
	}{
		{8, 04, "\x01\x08"},
		{8, 11, "\x1d\x08"},
		{8, 12, "\x2e\x08\x00"},
		{8, 13, "\x32\x08\x00"},
		{8, 59, "\xea\x08\x00"},
		{8, 60, "\xee\x08\x00"},
		{8, 61, "\xf2\x08\x00"},
		{8, 62, "\xf6\x08\x00"},
		{8, 63, "\xfa\x08\x00"},
		{8, 64, "\xfe\x08\x00"},
		{8, 65, "\x11\b\x15\x001"},
		{8, 66, "\x11\b\x15\x002"},
		{8, 67, "\x11\b\x15\x003"},
		{8, 68, "\x11\b\x15\x004"},
		{8, 69, "\x11\b\x15\x005"},
		{8, 80, "\x11\b\x15\x00@"},
		{8, 800, "\x11\b\x19\x00\x14\x02"},
		{8, 800000, "\x11\b\x1d\x00\xf44\v"},

		{256, 04, "\x21\x00"},
		{256, 11, "\x3d\x00"},
		{256, 12, "\x2e\x00\x01"},
		{256, 13, "\x32\x00\x01"},
		{256, 59, "\xea\x00\x01"},
		{256, 60, "\xee\x00\x01"},
		{256, 61, "\xf2\x00\x01"},
		{256, 62, "\xf6\x00\x01"},
		{256, 63, "\xfa\x00\x01"},
		{256, 64, "\xfe\x00\x01"},
		{256, 65, "1\x00\x15\x001"},
		{256, 66, "1\x00\x15\x002"},
		{256, 67, "1\x00\x15\x003"},
		{256, 68, "1\x00\x15\x004"},
		{256, 69, "1\x00\x15\x005"},
		{256, 80, "1\x00\x15\x00@"},
		{256, 800, "1\x00\x19\x00\x14\x02"},
		{256, 80000, "1\x00\x1d\x00t8\x00"},

		{2048, 04, "\x0e\x00\x08"},
		{2048, 11, "\x2a\x00\x08"},
		{2048, 12, "\x2e\x00\x08"},
		{2048, 13, "\x32\x00\x08"},
		{2048, 59, "\xea\x00\x08"},
		{2048, 60, "\xee\x00\x08"},
		{2048, 61, "\xf2\x00\x08"},
		{2048, 62, "\xf6\x00\x08"},
		{2048, 63, "\xfa\x00\x08"},
		{2048, 64, "\xfe\x00\x08"},
		{2048, 65, "\xee\x00\x08\x05\x00"},
		{2048, 66, "\xee\x00\x08\x09\x00"},
		{2048, 67, "\xee\x00\x08\x0d\x00"},
		{2048, 68, "\xee\x00\x08\x11\x00"},
		{2048, 69, "\xee\x00\x08\x15\x00\x01"},
		{2048, 80, "\xee\x00\x08\x15\x00\x0c"},
		{2048, 800, "\xee\x00\x08\x19\x00\xe0\x01"},
		{2048, 80000, "\xee\x00\x08\x1d\x00\x40\x38\x00"},

		{204800, 04, "\x0f\x00\x20\x03\x00"},
		{204800, 65, "\xff\x00\x20\x03\x00\x03\x00\x20\x03\x00"},
		{204800, 69, "\xff\x00\x20\x03\x00\x05\x00"},
		{204800, 800, "\xff\x00\x20\x03\x00\x19\x00\xdc\x01"},
		{204800, 80000, "\xff\x00\x20\x03\x00\x1d\x00\x3c\x38\x00"},
	}

	dst := make([]byte, 1024)
	for _, tc := range testCases {
		n := emitCopy(dst, tc.offset, tc.length)
		got := string(dst[:n])
		if got != tc.want {
			t.Errorf("offset=%d, length=%d:\ngot  %q\nwant %q", tc.offset, tc.length, got, tc.want)
		}
	}
}

func TestNewWriter(t *testing.T) {
	// Test all 32 possible sub-sequences of these 5 input slices.
	//
	// Their lengths sum to 400,000, which is over 6 times the Writer ibuf
	// capacity: 6 * maxBlockSize is 393,216.
	inputs := [][]byte{
		bytes.Repeat([]byte{'a'}, 40000),
		bytes.Repeat([]byte{'b'}, 150000),
		bytes.Repeat([]byte{'c'}, 60000),
		bytes.Repeat([]byte{'d'}, 120000),
		bytes.Repeat([]byte{'e'}, 30000),
	}
loop:
	for i := 0; i < 1<<uint(len(inputs)); i++ {
		var want []byte
		buf := new(bytes.Buffer)
		w := NewWriter(buf)
		for j, input := range inputs {
			if i&(1<<uint(j)) == 0 {
				continue
			}
			if _, err := w.Write(input); err != nil {
				t.Errorf("i=%#02x: j=%d: Write: %v", i, j, err)
				continue loop
			}
			want = append(want, input...)
		}
		if err := w.Close(); err != nil {
			t.Errorf("i=%#02x: Close: %v", i, err)
			continue
		}
		got, err := io.ReadAll(NewReader(buf))
		if err != nil {
			t.Errorf("i=%#02x: ReadAll: %v", i, err)
			continue
		}
		if err := cmp(got, want); err != nil {
			t.Errorf("i=%#02x: %v", i, err)
			continue
		}
	}
}

func TestFlush(t *testing.T) {
	buf := new(bytes.Buffer)
	w := NewWriter(buf)
	defer w.Close()
	if _, err := w.Write(bytes.Repeat([]byte{'x'}, 20)); err != nil {
		t.Fatalf("Write: %v", err)
	}
	if n := buf.Len(); n != 0 {
		t.Fatalf("before Flush: %d bytes were written to the underlying io.Writer, want 0", n)
	}
	if err := w.Flush(); err != nil {
		t.Fatalf("Flush: %v", err)
	}
	if n := buf.Len(); n == 0 {
		t.Fatalf("after Flush: %d bytes were written to the underlying io.Writer, want non-0", n)
	}
}

func TestReaderUncompressedDataOK(t *testing.T) {
	r := NewReader(strings.NewReader(magicChunk +
		"\x01\x08\x00\x00" + // Uncompressed chunk, 8 bytes long (including 4 byte checksum).
		"\x68\x10\xe6\xb6" + // Checksum.
		"\x61\x62\x63\x64", // Uncompressed payload: "abcd".
	))
	g, err := io.ReadAll(r)
	if err != nil {
		t.Fatal(err)
	}
	if got, want := string(g), "abcd"; got != want {
		t.Fatalf("got %q, want %q", got, want)
	}
}

func TestReaderUncompressedDataNoPayload(t *testing.T) {
	r := NewReader(strings.NewReader(magicChunk +
		"\x01\x04\x00\x00" + // Uncompressed chunk, 4 bytes long.
		"", // No payload; corrupt input.
	))
	if _, err := io.ReadAll(r); err != ErrCorrupt {
		t.Fatalf("got %v, want %v", err, ErrCorrupt)
	}
}

func TestReaderUncompressedDataTooLong(t *testing.T) {
	// The maximum legal chunk length... is 4MB + 4 bytes checksum.
	n := maxBlockSize + checksumSize
	n32 := uint32(n)
	r := NewReader(strings.NewReader(magicChunk +
		// Uncompressed chunk, n bytes long.
		string([]byte{chunkTypeUncompressedData, uint8(n32), uint8(n32 >> 8), uint8(n32 >> 16)}) +
		strings.Repeat("\x00", n),
	))
	// CRC is not set, so we should expect that error.
	if _, err := io.ReadAll(r); err != ErrCRC {
		t.Fatalf("got %v, want %v", err, ErrCRC)
	}

	// test first invalid.
	n++
	n32 = uint32(n)
	r = NewReader(strings.NewReader(magicChunk +
		// Uncompressed chunk, n bytes long.
		string([]byte{chunkTypeUncompressedData, uint8(n32), uint8(n32 >> 8), uint8(n32 >> 16)}) +
		strings.Repeat("\x00", n),
	))
	if _, err := io.ReadAll(r); err != ErrCorrupt {
		t.Fatalf("got %v, want %v", err, ErrCorrupt)
	}
}

func TestReaderReset(t *testing.T) {
	gold := bytes.Repeat([]byte("All that is gold does not glitter,\n"), 10000)
	buf := new(bytes.Buffer)
	w := NewWriter(buf)
	_, err := w.Write(gold)
	if err != nil {
		t.Fatalf("Write: %v", err)
	}
	err = w.Close()
	if err != nil {
		t.Fatalf("Close: %v", err)
	}

	encoded, invalid, partial := buf.String(), "invalid", "partial"
	r := NewReader(nil)
	for i, s := range []string{encoded, invalid, partial, encoded, partial, invalid, encoded, encoded} {
		if s == partial {
			r.Reset(strings.NewReader(encoded))
			if _, err := r.Read(make([]byte, 101)); err != nil {
				t.Errorf("#%d: %v", i, err)
				continue
			}
			continue
		}
		r.Reset(strings.NewReader(s))
		got, err := io.ReadAll(r)
		switch s {
		case encoded:
			if err != nil {
				t.Errorf("#%d: %v", i, err)
				continue
			}
			if err := cmp(got, gold); err != nil {
				t.Errorf("#%d: %v", i, err)
				continue
			}
		case invalid:
			if err == nil {
				t.Errorf("#%d: got nil error, want non-nil", i)
				continue
			}
		}
	}
}

func TestWriterReset(t *testing.T) {
	gold := bytes.Repeat([]byte("Not all those who wander are lost;\n"), 10000)
	const n = 20
	w := NewWriter(nil)
	defer w.Close()

	var gots, wants [][]byte
	failed := false
	for i := 0; i <= n; i++ {
		buf := new(bytes.Buffer)
		w.Reset(buf)
		want := gold[:len(gold)*i/n]
		if _, err := w.Write(want); err != nil {
			t.Errorf("#%d: Write: %v", i, err)
			failed = true
			continue
		}
		if err := w.Flush(); err != nil {
			t.Errorf("#%d: Flush: %v", i, err)
			failed = true
			got, err := io.ReadAll(NewReader(buf))
			if err != nil {
				t.Errorf("#%d: ReadAll: %v", i, err)
				failed = true
				continue
			}
			gots = append(gots, got)
			wants = append(wants, want)
		}
		if failed {
			continue
		}
		for i := range gots {
			if err := cmp(gots[i], wants[i]); err != nil {
				t.Errorf("#%d: %v", i, err)
			}
		}
	}
}

func TestWriterResetWithoutFlush(t *testing.T) {
	buf0 := new(bytes.Buffer)
	buf1 := new(bytes.Buffer)
	w := NewWriter(buf0)
	if _, err := w.Write([]byte("xxx")); err != nil {
		t.Fatalf("Write #0: %v", err)
	}
	// Note that we don't Flush the Writer before calling Reset.
	w.Reset(buf1)
	if _, err := w.Write([]byte("yyy")); err != nil {
		t.Fatalf("Write #1: %v", err)
	}
	if err := w.Flush(); err != nil {
		t.Fatalf("Flush: %v", err)
	}
	got, err := io.ReadAll(NewReader(buf1))
	if err != nil {
		t.Fatalf("ReadAll: %v", err)
	}
	if err := cmp(got, []byte("yyy")); err != nil {
		t.Fatal(err)
	}
}

type writeCounter int

func (c *writeCounter) Write(p []byte) (int, error) {
	*c++
	return len(p), nil
}

// TestNumUnderlyingWrites tests that each Writer flush only makes one or two
// Write calls on its underlying io.Writer, depending on whether or not the
// flushed buffer was compressible.
func TestNumUnderlyingWrites(t *testing.T) {
	testCases := []struct {
		input []byte
		want  int
	}{
		// Magic header + block
		{bytes.Repeat([]byte{'x'}, 100), 2},
		// One block each:
		{bytes.Repeat([]byte{'y'}, 100), 1},
		{[]byte("ABCDEFGHIJKLMNOPQRST"), 1},
	}

	// If we are doing sync writes, we write uncompressed as two writes.
	if runtime.GOMAXPROCS(0) == 1 {
		testCases[2].want++
	}
	var c writeCounter
	w := NewWriter(&c)
	defer w.Close()
	for i, tc := range testCases {
		c = 0
		if _, err := w.Write(tc.input); err != nil {
			t.Errorf("#%d: Write: %v", i, err)
			continue
		}
		if err := w.Flush(); err != nil {
			t.Errorf("#%d: Flush: %v", i, err)
			continue
		}
		if int(c) != tc.want {
			t.Errorf("#%d: got %d underlying writes, want %d", i, c, tc.want)
			continue
		}
	}
}

func testWriterRoundtrip(t *testing.T, src []byte, opts ...WriterOption) {
	var buf bytes.Buffer
	enc := NewWriter(&buf, opts...)
	n, err := enc.Write(src)
	if err != nil {
		t.Error(err)
		return
	}
	if n != len(src) {
		t.Error(io.ErrShortWrite)
		return
	}
	err = enc.Flush()
	if err != nil {
		t.Error(err)
		return
	}
	// Extra flush and close should be noops.
	err = enc.Flush()
	if err != nil {
		t.Error(err)
		return
	}
	err = enc.Close()
	if err != nil {
		t.Error(err)
		return
	}

	t.Logf("encoded to %d -> %d bytes", len(src), buf.Len())
	dec := NewReader(&buf)
	decoded, err := io.ReadAll(dec)
	if err != nil {
		t.Error(err)
		return
	}
	if len(decoded) != len(src) {
		t.Error("decoded len:", len(decoded), "!=", len(src))
		return
	}
	err = cmp(src, decoded)
	if err != nil {
		t.Error(err)
	}
}

func testBlockRoundtrip(t *testing.T, src []byte) {
	dst := Encode(nil, src)
	t.Logf("encoded to %d -> %d bytes", len(src), len(dst))
	decoded, err := Decode(nil, dst)
	if err != nil {
		t.Error(err)
		return
	}
	if len(decoded) != len(src) {
		t.Error("decoded len:", len(decoded), "!=", len(src))
		return
	}
	err = cmp(decoded, src)
	if err != nil {
		t.Error(err)
	}
}

func testBetterBlockRoundtrip(t *testing.T, src []byte) {
	dst := EncodeBetter(nil, src)
	t.Logf("encoded to %d -> %d bytes", len(src), len(dst))
	decoded, err := Decode(nil, dst)
	if err != nil {
		t.Error(err)
		return
	}
	if len(decoded) != len(src) {
		t.Error("decoded len:", len(decoded), "!=", len(src))
		return
	}
	err = cmp(src, decoded)
	if err != nil {
		t.Error(err)
	}
}

func testBestBlockRoundtrip(t *testing.T, src []byte) {
	dst := EncodeBest(nil, src)
	t.Logf("encoded to %d -> %d bytes", len(src), len(dst))
	decoded, err := Decode(nil, dst)
	if err != nil {
		t.Error(err)
		return
	}
	if len(decoded) != len(src) {
		t.Error("decoded len:", len(decoded), "!=", len(src))
		return
	}
	err = cmp(src, decoded)
	if err != nil {
		t.Error(err)
	}
}

func testSnappyBlockRoundtrip(t *testing.T, src []byte) {
	// Write with s2, decode with snapref.
	t.Run("regular", func(t *testing.T) {
		dst := EncodeSnappy(nil, src)
		t.Logf("encoded to %d -> %d bytes", len(src), len(dst))
		decoded, err := snapref.Decode(nil, dst)
		if err != nil {
			t.Error(err)
			return
		}
		if len(decoded) != len(src) {
			t.Error("decoded len:", len(decoded), "!=", len(src))
			return
		}
		err = cmp(src, decoded)
		if err != nil {
			t.Error(err)
		}
	})
	t.Run("better", func(t *testing.T) {
		dst := EncodeSnappyBetter(nil, src)
		t.Logf("encoded to %d -> %d bytes", len(src), len(dst))
		decoded, err := snapref.Decode(nil, dst)
		if err != nil {
			t.Error(err)
			return
		}
		if len(decoded) != len(src) {
			t.Error("decoded len:", len(decoded), "!=", len(src))
			return
		}
		err = cmp(src, decoded)
		if err != nil {
			t.Error(err)
		}
	})
	t.Run("best", func(t *testing.T) {
		dst := EncodeSnappyBest(nil, src)
		t.Logf("encoded to %d -> %d bytes", len(src), len(dst))
		decoded, err := snapref.Decode(nil, dst)
		if err != nil {
			t.Error(err)
			return
		}
		if len(decoded) != len(src) {
			t.Error("decoded len:", len(decoded), "!=", len(src))
			return
		}
		err = cmp(src, decoded)
		if err != nil {
			t.Error(err)
		}
	})
}

func testSnappyDecode(t *testing.T, src []byte) {
	var buf bytes.Buffer
	enc := snapref.NewBufferedWriter(&buf)
	n, err := enc.Write(src)
	if err != nil {
		t.Error(err)
		return
	}
	if n != len(src) {
		t.Error(io.ErrShortWrite)
		return
	}
	enc.Close()
	t.Logf("encoded to %d -> %d bytes", len(src), buf.Len())
	dec := NewReader(&buf)
	decoded, err := io.ReadAll(dec)
	if err != nil {
		t.Error(err)
		return
	}
	if len(decoded) != len(src) {
		t.Error("decoded len:", len(decoded), "!=", len(src))
		return
	}
	err = cmp(src, decoded)
	if err != nil {
		t.Error(err)
	}
}

func benchDecode(b *testing.B, src []byte) {
	b.Run("default", func(b *testing.B) {
		encoded := Encode(nil, src)
		buf := make([]byte, 0, len(src))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		var err error
		for i := 0; i < b.N; i++ {
			buf, err = Decode(buf[:0], encoded)
			if err != nil {
				b.Fatal(err)
			}
		}
		b.ReportMetric(100*float64(len(encoded))/float64(len(src)), "pct")
	})
	b.Run("better", func(b *testing.B) {
		encoded := EncodeBetter(nil, src)
		buf := make([]byte, 0, len(src))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		var err error
		for i := 0; i < b.N; i++ {
			buf, err = Decode(buf[:0], encoded)
			if err != nil {
				b.Fatal(err)
			}
		}
		b.ReportMetric(100*float64(len(encoded))/float64(len(src)), "pct")
	})
	b.Run("best", func(b *testing.B) {
		encoded := EncodeBest(nil, src)
		buf := make([]byte, 0, len(src))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		var err error
		for i := 0; i < b.N; i++ {
			buf, err = Decode(buf[:0], encoded)
			if err != nil {
				b.Fatal(err)
			}
		}
		b.ReportMetric(100*float64(len(encoded))/float64(len(src)), "pct")
	})
	b.Run("snappy-input", func(b *testing.B) {
		encoded := snapref.Encode(nil, src)
		buf := make([]byte, 0, len(src))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		var err error
		for i := 0; i < b.N; i++ {
			buf, err = Decode(buf[:0], encoded)
			if err != nil {
				b.Fatal(err)
			}
		}
		b.ReportMetric(100*float64(len(encoded))/float64(len(src)), "pct")
	})
}

func benchEncode(b *testing.B, src []byte) {
	// Bandwidth is in amount of uncompressed data.
	b.Run("default", func(b *testing.B) {
		dst := make([]byte, MaxEncodedLen(len(src)))
		b.ResetTimer()
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		for i := 0; i < b.N; i++ {
			dst = Encode(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
	b.Run("better", func(b *testing.B) {
		dst := make([]byte, MaxEncodedLen(len(src)))
		b.ResetTimer()
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		for i := 0; i < b.N; i++ {
			dst = EncodeBetter(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
	b.Run("best", func(b *testing.B) {
		dst := make([]byte, MaxEncodedLen(len(src)))
		b.ResetTimer()
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		for i := 0; i < b.N; i++ {
			dst = EncodeBest(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
	b.Run("snappy-default", func(b *testing.B) {
		dst := make([]byte, MaxEncodedLen(len(src)))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			dst = EncodeSnappy(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
	b.Run("snappy-better", func(b *testing.B) {
		dst := make([]byte, MaxEncodedLen(len(src)))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			dst = EncodeSnappyBetter(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
	b.Run("snappy-best", func(b *testing.B) {
		dst := make([]byte, MaxEncodedLen(len(src)))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			dst = EncodeSnappyBest(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
	b.Run("snappy-ref-noasm", func(b *testing.B) {
		dst := make([]byte, snapref.MaxEncodedLen(len(src)))
		b.SetBytes(int64(len(src)))
		b.ReportAllocs()
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			dst = snapref.Encode(dst[:0], src)
		}
		b.ReportMetric(100*float64(len(dst))/float64(len(src)), "pct")
	})
}

func testOrBenchmark(b testing.TB) string {
	if _, ok := b.(*testing.B); ok {
		return "benchmark"
	}
	return "test"
}

func readFile(b testing.TB, filename string) []byte {
	src, err := os.ReadFile(filename)
	if err != nil {
		b.Skipf("skipping %s: %v", testOrBenchmark(b), err)
	}
	if len(src) == 0 {
		b.Fatalf("%s has zero length", filename)
	}
	return src
}

// expand returns a slice of length n containing mutated copies of src.
func expand(src []byte, n int) []byte {
	dst := make([]byte, n)
	cnt := uint8(0)
	for x := dst; len(x) > 0; cnt++ {
		idx := copy(x, src)
		for i := range x {
			if i >= len(src) {
				break
			}
			x[i] = src[i] ^ cnt
		}
		x = x[idx:]
	}
	return dst
}

func benchTwain(b *testing.B, n int, decode bool) {
	data := expand(readFile(b, "../testdata/Mark.Twain-Tom.Sawyer.txt"), n)
	if decode {
		benchDecode(b, data)
	} else {
		benchEncode(b, data)
	}
}

func BenchmarkTwainDecode1e1(b *testing.B) { benchTwain(b, 1e1, true) }
func BenchmarkTwainDecode1e2(b *testing.B) { benchTwain(b, 1e2, true) }
func BenchmarkTwainDecode1e3(b *testing.B) { benchTwain(b, 1e3, true) }
func BenchmarkTwainDecode1e4(b *testing.B) { benchTwain(b, 1e4, true) }
func BenchmarkTwainDecode1e5(b *testing.B) { benchTwain(b, 1e5, true) }
func BenchmarkTwainDecode1e6(b *testing.B) { benchTwain(b, 1e6, true) }
func BenchmarkTwainDecode1e7(b *testing.B) { benchTwain(b, 1e7, true) }
func BenchmarkTwainEncode1e1(b *testing.B) { benchTwain(b, 1e1, false) }
func BenchmarkTwainEncode1e2(b *testing.B) { benchTwain(b, 1e2, false) }
func BenchmarkTwainEncode1e3(b *testing.B) { benchTwain(b, 1e3, false) }
func BenchmarkTwainEncode1e4(b *testing.B) { benchTwain(b, 1e4, false) }
func BenchmarkTwainEncode1e5(b *testing.B) { benchTwain(b, 1e5, false) }
func BenchmarkTwainEncode1e6(b *testing.B) { benchTwain(b, 1e6, false) }
func BenchmarkTwainEncode1e7(b *testing.B) { benchTwain(b, 1e7, false) }

func BenchmarkRandomEncodeBlock1MB(b *testing.B) {
	rng := rand.New(rand.NewSource(1))
	data := make([]byte, 1<<20)
	for i := range data {
		data[i] = uint8(rng.Intn(256))
	}
	benchEncode(b, data)
}

func BenchmarkRandomEncodeBetterBlock16MB(b *testing.B) {
	rng := rand.New(rand.NewSource(1))
	data := make([]byte, 16<<20)
	for i := range data {
		data[i] = uint8(rng.Intn(256))
	}
	benchEncode(b, data)
}

// testFiles' values are copied directly from
// https://raw.githubusercontent.com/google/snappy/master/snappy_unittest.cc
// The label field is unused in snappy-go.
var testFiles = []struct {
	label     string
	filename  string
	sizeLimit int
}{
	{"html", "html", 0},
	{"urls", "urls.10K", 0},
	{"jpg", "fireworks.jpeg", 0},
	{"jpg_200b", "fireworks.jpeg", 200},
	{"pdf", "paper-100k.pdf", 0},
	{"html4", "html_x_4", 0},
	{"txt1", "alice29.txt", 0},
	{"txt2", "asyoulik.txt", 0},
	{"txt3", "lcet10.txt", 0},
	{"txt4", "plrabn12.txt", 0},
	{"pb", "geo.protodata", 0},
	{"gaviota", "kppkn.gtb", 0},
	{"txt1_128b", "alice29.txt", 128},
	{"txt1_1000b", "alice29.txt", 1000},
	{"txt1_10000b", "alice29.txt", 10000},
	{"txt1_20000b", "alice29.txt", 20000},
}

const (
	// The benchmark data files are at this canonical URL.
	benchURL = "https://raw.githubusercontent.com/google/snappy/master/testdata/"
)

func downloadBenchmarkFiles(b testing.TB, basename string) (errRet error) {
	bDir := filepath.FromSlash(*benchdataDir)
	filename := filepath.Join(bDir, basename)
	if stat, err := os.Stat(filename); err == nil && stat.Size() != 0 {
		return nil
	}

	if !*download {
		b.Skipf("test data not found; skipping %s without the -download flag", testOrBenchmark(b))
	}
	// Download the official snappy C++ implementation reference test data
	// files for benchmarking.
	if err := os.MkdirAll(bDir, 0777); err != nil && !os.IsExist(err) {
		return fmt.Errorf("failed to create %s: %s", bDir, err)
	}

	f, err := os.Create(filename)
	if err != nil {
		return fmt.Errorf("failed to create %s: %s", filename, err)
	}
	defer f.Close()
	defer func() {
		if errRet != nil {
			os.Remove(filename)
		}
	}()
	url := benchURL + basename
	resp, err := http.Get(url)
	if err != nil {
		return fmt.Errorf("failed to download %s: %s", url, err)
	}
	defer resp.Body.Close()
	if s := resp.StatusCode; s != http.StatusOK {
		return fmt.Errorf("downloading %s: HTTP status code %d (%s)", url, s, http.StatusText(s))
	}
	_, err = io.Copy(f, resp.Body)
	if err != nil {
		return fmt.Errorf("failed to download %s to %s: %s", url, filename, err)
	}
	return nil
}

func TestEstimateBlockSize(t *testing.T) {
	var input []byte
	for range 100 {
		EstimateBlockSize(input)
		input = append(input, 0)
	}
}

func benchFile(b *testing.B, i int, decode bool) {
	if err := downloadBenchmarkFiles(b, testFiles[i].filename); err != nil {
		b.Fatalf("failed to download testdata: %s", err)
	}
	bDir := filepath.FromSlash(*benchdataDir)
	data := readFile(b, filepath.Join(bDir, testFiles[i].filename))
	if !decode {
		b.Run("est-size", func(b *testing.B) {
			if n := testFiles[i].sizeLimit; 0 < n && n < len(data) {
				data = data[:n]
			}
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				for pb.Next() {
					_ = EstimateBlockSize(data)
				}
			})
			sz := float64(EstimateBlockSize(data))
			if sz > 0 {
				b.ReportMetric(100*sz/float64(len(data)), "pct")
				b.ReportMetric(sz, "B")
			}
		})
	}
	b.Run("block", func(b *testing.B) {
		if n := testFiles[i].sizeLimit; 0 < n && n < len(data) {
			data = data[:n]
		}
		if decode {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				encoded := Encode(nil, data)
				tmp := make([]byte, len(data))
				for pb.Next() {
					var err error
					tmp, err = Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, MaxEncodedLen(len(data)))
				tmp := make([]byte, len(data))
				for pb.Next() {
					res := Encode(dst, data)
					if len(res) == 0 {
						panic(0)
					}
					if false {
						tmp, _ = Decode(tmp, res)
						if !bytes.Equal(tmp, data) {
							panic("wrong")
						}
					}
				}
			})
		}
		b.ReportMetric(100*float64(len(Encode(nil, data)))/float64(len(data)), "pct")
		b.ReportMetric(float64(len(Encode(nil, data))), "B")
	})
	b.Run("block-better", func(b *testing.B) {
		if decode {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				encoded := EncodeBetter(nil, data)
				tmp := make([]byte, len(data))
				for pb.Next() {
					var err error
					tmp, err = Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, MaxEncodedLen(len(data)))
				tmp := make([]byte, len(data))
				for pb.Next() {
					res := EncodeBetter(dst, data)
					if len(res) == 0 {
						panic(0)
					}
					if false {
						tmp, _ = Decode(tmp, res)
						if !bytes.Equal(tmp, data) {
							panic("wrong")
						}
					}
				}
			})
		}
		b.ReportMetric(100*float64(len(EncodeBetter(nil, data)))/float64(len(data)), "pct")
		b.ReportMetric(float64(len(EncodeBetter(nil, data))), "B")
	})

	b.Run("block-best", func(b *testing.B) {
		if decode {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				encoded := EncodeBest(nil, data)
				tmp := make([]byte, len(data))
				for pb.Next() {
					var err error
					tmp, err = Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeBest(nil, data)))/float64(len(data)), "pct")
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, MaxEncodedLen(len(data)))
				tmp := make([]byte, len(data))
				for pb.Next() {
					res := EncodeBest(dst, data)
					if len(res) == 0 {
						panic(0)
					}
					if false {
						tmp, _ = Decode(tmp, res)
						if !bytes.Equal(tmp, data) {
							panic("wrong")
						}
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeBest(nil, data)))/float64(len(data)), "pct")
		}
		b.ReportMetric(float64(len(EncodeBest(nil, data))), "B")
	})
}

func benchFileSnappy(b *testing.B, i int, decode bool) {
	if err := downloadBenchmarkFiles(b, testFiles[i].filename); err != nil {
		b.Fatalf("failed to download testdata: %s", err)
	}
	bDir := filepath.FromSlash(*benchdataDir)
	data := readFile(b, filepath.Join(bDir, testFiles[i].filename))
	if n := testFiles[i].sizeLimit; 0 < n && n < len(data) {
		data = data[:n]
	}

	b.Run("s2-snappy", func(b *testing.B) {
		if decode {
			b.SetBytes(int64(len(data)))
			b.ResetTimer()
			b.ReportAllocs()
			b.RunParallel(func(pb *testing.PB) {
				encoded := EncodeSnappy(nil, data)
				tmp := make([]byte, len(data))
				for pb.Next() {
					var err error
					tmp, err = Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeSnappy(nil, data)))/float64(len(data)), "pct")
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()

			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, MaxEncodedLen(len(data)))
				for pb.Next() {
					res := EncodeSnappy(dst, data)
					if len(res) == 0 {
						panic(0)
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeSnappy(nil, data)))/float64(len(data)), "pct")
		}
		b.ReportMetric(float64(len(EncodeSnappy(nil, data))), "B")
	})

	b.Run("s2-snappy-better", func(b *testing.B) {
		if decode {
			b.SetBytes(int64(len(data)))
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				encoded := EncodeSnappyBetter(nil, data)
				tmp := make([]byte, len(data))
				b.ReportAllocs()
				b.ResetTimer()

				for pb.Next() {
					var err error
					tmp, err = Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeSnappyBetter(nil, data)))/float64(len(data)), "pct")
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, MaxEncodedLen(len(data)))
				tmp := make([]byte, len(data))
				for pb.Next() {
					res := EncodeSnappyBetter(dst, data)
					if len(res) == 0 {
						panic(0)
					}
					if false {
						tmp, _ = Decode(tmp, res)
						if !bytes.Equal(tmp, data) {
							panic("wrong")
						}
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeSnappyBetter(nil, data)))/float64(len(data)), "pct")
		}
		b.ReportMetric(float64(len(EncodeSnappyBetter(nil, data))), "B")
	})

	b.Run("s2-snappy-best", func(b *testing.B) {
		if decode {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				encoded := EncodeSnappyBest(nil, data)
				tmp := make([]byte, len(data))
				for pb.Next() {
					var err error
					tmp, err = Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeSnappyBest(nil, data)))/float64(len(data)), "pct")
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, MaxEncodedLen(len(data)))
				tmp := make([]byte, len(data))
				for pb.Next() {
					res := EncodeSnappyBest(dst, data)
					if len(res) == 0 {
						panic(0)
					}
					if false {
						tmp, _ = snapref.Decode(tmp, res)
						if !bytes.Equal(tmp, data) {
							panic("wrong")
						}
					}
				}
			})
			b.ReportMetric(100*float64(len(EncodeSnappyBest(nil, data)))/float64(len(data)), "pct")
		}
		b.ReportMetric(float64(len(EncodeSnappyBest(nil, data))), "B")
	})
	b.Run("snappy-noasm", func(b *testing.B) {
		if decode {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				encoded := snapref.Encode(nil, data)
				tmp := make([]byte, len(data))
				for pb.Next() {
					var err error
					tmp, err = snapref.Decode(tmp, encoded)
					if err != nil {
						b.Fatal(err)
					}
				}
			})
			b.ReportMetric(100*float64(len(snapref.Encode(nil, data)))/float64(len(data)), "pct")
		} else {
			b.SetBytes(int64(len(data)))
			b.ReportAllocs()
			b.ResetTimer()
			b.RunParallel(func(pb *testing.PB) {
				dst := make([]byte, snapref.MaxEncodedLen(len(data)))
				tmp := make([]byte, len(data))
				for pb.Next() {
					res := snapref.Encode(dst, data)
					if len(res) == 0 {
						panic(0)
					}
					if false {
						tmp, _ = snapref.Decode(tmp, res)
						if !bytes.Equal(tmp, data) {
							panic("wrong")
						}
					}
				}
			})
			b.ReportMetric(100*float64(len(snapref.Encode(nil, data)))/float64(len(data)), "pct")
		}
	})
}

func TestRoundtrips(t *testing.T) {
	testFile(t, 0, 10)
	testFile(t, 1, 10)
	testFile(t, 2, 10)
	testFile(t, 3, 10)
	testFile(t, 4, 10)
	testFile(t, 5, 10)
	testFile(t, 6, 10)
	testFile(t, 7, 10)
	testFile(t, 8, 10)
	testFile(t, 9, 10)
	testFile(t, 10, 10)
	testFile(t, 11, 10)
	testFile(t, 12, 0)
	testFile(t, 13, 0)
	testFile(t, 14, 0)
	testFile(t, 15, 0)
}

func testFile(t *testing.T, i, repeat int) {
	if err := downloadBenchmarkFiles(t, testFiles[i].filename); err != nil {
		t.Skipf("failed to download testdata: %s", err)
	}

	if testing.Short() {
		repeat = 0
	}
	t.Run(fmt.Sprint(i, "-", testFiles[i].label), func(t *testing.T) {
		bDir := filepath.FromSlash(*benchdataDir)
		data := readFile(t, filepath.Join(bDir, testFiles[i].filename))
		if testing.Short() && len(data) > 10000 {
			t.SkipNow()
		}
		oSize := len(data)
		for i := 0; i < repeat; i++ {
			data = append(data, data[:oSize]...)
		}
		t.Run("s2", func(t *testing.T) {
			testWriterRoundtrip(t, data)
		})
		t.Run("s2-better", func(t *testing.T) {
			testWriterRoundtrip(t, data, WriterBetterCompression())
		})
		t.Run("s2-best", func(t *testing.T) {
			testWriterRoundtrip(t, data, WriterBestCompression())
		})
		t.Run("s2-uncompressed", func(t *testing.T) {
			testWriterRoundtrip(t, data, WriterUncompressed())
		})
		t.Run("block", func(t *testing.T) {
			d := data
			testBlockRoundtrip(t, d)
		})
		t.Run("block-better", func(t *testing.T) {
			d := data
			testBetterBlockRoundtrip(t, d)
		})
		t.Run("block-best", func(t *testing.T) {
			d := data
			testBestBlockRoundtrip(t, d)
		})
		t.Run("s2-snappy", func(t *testing.T) {
			d := data
			testSnappyBlockRoundtrip(t, d)
		})
		t.Run("snappy", func(t *testing.T) {
			testSnappyDecode(t, data)
		})
	})
}

func TestDataRoundtrips(t *testing.T) {
	test := func(t *testing.T, data []byte) {
		t.Run("s2", func(t *testing.T) {
			testWriterRoundtrip(t, data)
		})
		t.Run("s2-better", func(t *testing.T) {
			testWriterRoundtrip(t, data, WriterBetterCompression())
		})
		t.Run("s2-best", func(t *testing.T) {
			testWriterRoundtrip(t, data, WriterBestCompression())
		})
		t.Run("block", func(t *testing.T) {
			d := data
			testBlockRoundtrip(t, d)
		})
		t.Run("block-better", func(t *testing.T) {
			d := data
			testBetterBlockRoundtrip(t, d)
		})
		t.Run("block-best", func(t *testing.T) {
			d := data
			testBestBlockRoundtrip(t, d)
		})
		t.Run("s2-snappy", func(t *testing.T) {
			d := data
			testSnappyBlockRoundtrip(t, d)
		})
		t.Run("snappy", func(t *testing.T) {
			testSnappyDecode(t, data)
		})
	}
	t.Run("longblock", func(t *testing.T) {
		data := make([]byte, 1<<25)
		if testing.Short() {
			data = data[:1<<20]
		}
		test(t, data)
	})
	t.Run("4f9e1a0", func(t *testing.T) {
		comp, _ := os.ReadFile("testdata/4f9e1a0da7915a3d69632f5613ed78bc998a8a23.zst")
		dec, _ := zstd.NewReader(bytes.NewBuffer(comp))
		data, _ := io.ReadAll(dec)
		test(t, data)
	})
	data, err := os.ReadFile("testdata/enc_regressions.zip")
	if err != nil {
		t.Fatal(err)
	}
	zr, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
	if err != nil {
		t.Fatal(err)
	}
	for _, tt := range zr.File {
		if !strings.HasSuffix(t.Name(), "") {
			continue
		}
		t.Run(tt.Name, func(t *testing.T) {
			r, err := tt.Open()
			if err != nil {
				t.Error(err)
				return
			}
			b, err := io.ReadAll(r)
			if err != nil {
				t.Error(err)
				return
			}
			test(t, b[:len(b):len(b)])
		})
	}

}

func BenchmarkDecodeS2BlockParallel(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			benchFile(b, i, true)
		})
	}
}

func BenchmarkEncodeS2BlockParallel(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			benchFile(b, i, false)
		})
	}
}

func BenchmarkDecodeSnappyBlockParallel(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			benchFileSnappy(b, i, true)
		})
	}
}

func BenchmarkEncodeSnappyBlockParallel(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			benchFileSnappy(b, i, false)
		})
	}
}

func TestMatchLen(t *testing.T) {
	// ref is a simple, reference implementation of matchLen.
	ref := func(a, b []byte) int {
		n := 0
		for i := range a {
			if a[i] != b[i] {
				break
			}
			n++
		}
		return n
	}

	// We allow slightly shorter matches at the end of slices
	const maxBelow = 0
	nums := []int{0, 1, 2, 7, 8, 9, 16, 20, 29, 30, 31, 32, 33, 34, 38, 39, 40}
	for yIndex := 40; yIndex > 30; yIndex-- {
		xxx := bytes.Repeat([]byte("x"), 40)
		if yIndex < len(xxx) {
			xxx[yIndex] = 'y'
		}
		for _, i := range nums {
			for _, j := range nums {
				if i >= j {
					continue
				}
				got := matchLen(xxx[j:], xxx[i:])
				want := ref(xxx[j:], xxx[i:])
				if got > want {
					t.Errorf("yIndex=%d, i=%d, j=%d: got %d, want %d", yIndex, i, j, got, want)
					continue
				}
				if got < want-maxBelow {
					t.Errorf("yIndex=%d, i=%d, j=%d: got %d, want %d", yIndex, i, j, got, want)
				}
			}
		}
	}
}

func BenchmarkDecodeBlockSingle(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			if err := downloadBenchmarkFiles(b, testFiles[i].filename); err != nil {
				b.Fatalf("failed to download testdata: %s", err)
			}
			bDir := filepath.FromSlash(*benchdataDir)
			data := readFile(b, filepath.Join(bDir, testFiles[i].filename))
			if testFiles[i].sizeLimit > 0 && len(data) > testFiles[i].sizeLimit {
				data = data[:testFiles[i].sizeLimit]
			}
			benchDecode(b, data)
		})
	}
}

func BenchmarkDecodeBlockParallel(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			benchFile(b, i, true)
		})
	}
}

func BenchmarkEncodeBlockSingle(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			if err := downloadBenchmarkFiles(b, testFiles[i].filename); err != nil {
				b.Fatalf("failed to download testdata: %s", err)
			}
			bDir := filepath.FromSlash(*benchdataDir)
			data := readFile(b, filepath.Join(bDir, testFiles[i].filename))
			if testFiles[i].sizeLimit > 0 && len(data) > testFiles[i].sizeLimit {
				data = data[:testFiles[i].sizeLimit]
			}
			benchEncode(b, data)
		})
	}
}

func BenchmarkEncodeBlockParallel(b *testing.B) {
	for i := range testFiles {
		b.Run(fmt.Sprint(i, "-", testFiles[i].label), func(b *testing.B) {
			benchFile(b, i, false)
		})
	}
}
