zlib: More precise matching (#386)

* demo bug

* check 2 bytes of zlib header

* add .zz test

formats_test.go

@@ -3,6 +3,7 @@ package archiver
 import (
 	"bytes"
 	"context"
+	"errors"
 	"io"
 	"io/fs"
 	"math/rand"
@@ -370,6 +371,13 @@ func TestIdentifyFindFormatByStreamContent(t *testing.T) {
 			compressorName:        "",
 			wantFormatName:        ".rar",
 		},
+		{
+			name:                  "should recognize zz",
+			openCompressionWriter: Zlib{}.OpenWriter,
+			content:               []byte("this is text"),
+			compressorName:        ".zz",
+			wantFormatName:        ".zz",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -410,3 +418,30 @@ func TestIdentifyAndOpenZip(t *testing.T) {
 	})
 	checkErr(t, err, "extracting zip")
 }
+
+func TestIdentifyASCIIFileStartingWithX(t *testing.T) {
+	// Create a temporary file starting with the letter 'x'
+	tmpFile, err := os.CreateTemp("", "TestIdentifyASCIIFileStartingWithX-tmp-*.txt")
+	if err != nil {
+		t.Fatalf("fail to create tmp test file for archive tests: err=%v", err)
+	}
+
+	_, err = tmpFile.Write([]byte("xThis is a test file"))
+	if err != nil {
+		t.Fatalf("Failed to write to temp file: %v", err)
+	}
+	tmpFile.Close()
+
+	// Open the file and use the Identify function
+	file, err := os.Open(tmpFile.Name())
+	if err != nil {
+		t.Fatalf("Failed to open temp file: %v", err)
+	}
+	defer file.Close()
+
+	_, _, err = Identify(tmpFile.Name(), file)
+	if !errors.Is(err, ErrNoMatch) {
+		t.Fatalf("Identify failed: %v", err)
+	}
+
+}

zlib.go

@@ -1,7 +1,6 @@
 package archiver
 
 import (
-	"bytes"
 	"io"
 	"strings"
 
@@ -28,11 +27,13 @@ func (zz Zlib) Match(filename string, stream io.Reader) (MatchResult, error) {
 	}
 
 	// match file header
-	buf, err := readAtMost(stream, len(ZlibHeader))
-	if err != nil {
+	buf, err := readAtMost(stream, 2)
+	// If an error occurred or buf is not 2 bytes we can't check the header
+	if err != nil || len(buf) < 2 {
 		return mr, err
 	}
-	mr.ByStream = bytes.Equal(buf, ZlibHeader)
+
+	mr.ByStream = isValidZlibHeader(buf[0], buf[1])
 
 	return mr, nil
 }
@@ -49,4 +50,23 @@ func (Zlib) OpenReader(r io.Reader) (io.ReadCloser, error) {
 	return zlib.NewReader(r)
 }
 
-var ZlibHeader = []byte{0x78}
+func isValidZlibHeader(first, second byte) bool {
+	// Define all 32 valid zlib headers, see https://stackoverflow.com/questions/9050260/what-does-a-zlib-header-look-like/54915442#54915442
+	validHeaders := map[uint16]struct{}{
+		0x081D: {}, 0x085B: {}, 0x0899: {}, 0x08D7: {},
+		0x1819: {}, 0x1857: {}, 0x1895: {}, 0x18D3: {},
+		0x2815: {}, 0x2853: {}, 0x2891: {}, 0x28CF: {},
+		0x3811: {}, 0x384F: {}, 0x388D: {}, 0x38CB: {},
+		0x480D: {}, 0x484B: {}, 0x4889: {}, 0x48C7: {},
+		0x5809: {}, 0x5847: {}, 0x5885: {}, 0x58C3: {},
+		0x6805: {}, 0x6843: {}, 0x6881: {}, 0x68DE: {},
+		0x7801: {}, 0x785E: {}, 0x789C: {}, 0x78DA: {},
+	}
+
+	// Combine the first and second bytes into a single 16-bit, big-endian value
+	header := uint16(first)<<8 | uint16(second)
+
+	// Check if the header is in the map of valid headers
+	_, isValid := validHeaders[header]
+	return isValid
+}