From 57434b509141a6ee9681116b8d552069126e615f Mon Sep 17 00:00:00 2001
From: Andy Balholm <andy@balholm.com>
Date: Mon, 29 Jul 2024 09:56:04 -0700
Subject: [PATCH] Encoder: check for empty block

Fixes #51
---
 brotli_test.go | 44 ++++++++++++++++++++++++++++++++++++++++++++
 encoder.go     |  9 +++++++++
 go.mod         |  2 ++
 go.sum         |  2 ++
 4 files changed, 57 insertions(+)

diff --git a/brotli_test.go b/brotli_test.go
index 5928a57..15b996e 100644
--- a/brotli_test.go
+++ b/brotli_test.go
@@ -14,10 +14,12 @@ import (
 	"math"
 	"math/rand"
 	"os"
+	"strconv"
 	"testing"
 	"time"
 
 	"github.com/andybalholm/brotli/matchfinder"
+	"github.com/xyproto/randomstring"
 )
 
 func checkCompressedData(compressedData, wantOriginalData []byte) error {
@@ -740,3 +742,45 @@ func TestEncodeM0Lazy(t *testing.T) {
 func BenchmarkEncodeM0Lazy(b *testing.B) {
 	benchmark(b, "testdata/Isaac.Newton-Opticks.txt", matchfinder.M0{Lazy: true}, 1<<16)
 }
+
+func TestIssue51(t *testing.T) {
+	for i := 65536; i <= 65536*4; i += 65536 {
+		t.Run("compress data length: "+strconv.Itoa(i)+"bytes", func(t *testing.T) {
+			dataStr := randomstring.HumanFriendlyString(i)
+			dataBytes := []byte(dataStr)
+			buf := bytes.Buffer{}
+			w := NewWriterV2(&buf, 4)
+
+			n, err := w.Write(dataBytes)
+			if err != nil {
+				t.Fatalf("Error while compressing data: %v", err)
+			}
+			if n != len(dataBytes) {
+				t.Fatalf("Bytes written (%d) != len(databytes) (%d)", n, len(dataBytes))
+			}
+			err = w.Close()
+			if err != nil {
+				t.Fatalf("Error closing writer: %v", err)
+			}
+
+			r := NewReader(&buf)
+			dst := make([]byte, len(dataBytes)+100)
+			p := dst
+			total := 0
+			for {
+				n1, err1 := r.Read(p)
+				if err1 != nil {
+					if err1 != io.EOF {
+						t.Fatal(err1)
+					}
+					break
+				}
+				total += n1
+				p = p[n1:]
+			}
+			if !bytes.Equal(dst[:total], dataBytes) {
+				t.Fatal("Decompressed bytes don't match")
+			}
+		})
+	}
+}
diff --git a/encoder.go b/encoder.go
index 650d1e4..1928382 100644
--- a/encoder.go
+++ b/encoder.go
@@ -21,6 +21,15 @@ func (e *Encoder) Encode(dst []byte, src []byte, matches []matchfinder.Match, la
 		e.wroteHeader = true
 	}
 
+	if len(src) == 0 {
+		if lastBlock {
+			e.bw.writeBits(2, 3) // islast + isempty
+			e.bw.jumpToByteBoundary()
+			return e.bw.dst
+		}
+		return dst
+	}
+
 	var literalHisto [256]uint32
 	var commandHisto [704]uint32
 	var distanceHisto [64]uint32
diff --git a/go.mod b/go.mod
index 50324ea..982814a 100644
--- a/go.mod
+++ b/go.mod
@@ -3,3 +3,5 @@ module github.com/andybalholm/brotli
 go 1.13
 
 retract v1.0.1 // occasional panics and data corruption
+
+require github.com/xyproto/randomstring v1.0.5 // indirect
diff --git a/go.sum b/go.sum
index e69de29..505973d 100644
--- a/go.sum
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/xyproto/randomstring v1.0.5 h1:YtlWPoRdgMu3NZtP45drfy1GKoojuR7hmRcnhZqKjWU=
+github.com/xyproto/randomstring v1.0.5/go.mod h1:rgmS5DeNXLivK7YprL0pY+lTuhNQW3iGxZ18UQApw/E=