diff --git a/BUILD.gn b/BUILD.gn
index ce598c6..d9e8d71 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1951,6 +1951,7 @@
       "core/fxcrt/xml/cfx_saxreader_unittest.cpp",
       "core/fxcrt/xml/cfx_xmlsyntaxparser_unittest.cpp",
       "fxbarcode/oned/BC_OnedCode128Writer_unittest.cpp",
+      "fxbarcode/oned/BC_OnedEAN13Writer_unittest.cpp",
       "fxbarcode/pdf417/BC_PDF417HighLevelEncoder_unittest.cpp",
       "xfa/fde/cfde_texteditengine_unittest.cpp",
       "xfa/fgas/crt/cfgas_formatstring_unittest.cpp",
diff --git a/fxbarcode/oned/BC_OnedEAN13Writer.cpp b/fxbarcode/oned/BC_OnedEAN13Writer.cpp
index dddb9cc..e7642de 100644
--- a/fxbarcode/oned/BC_OnedEAN13Writer.cpp
+++ b/fxbarcode/oned/BC_OnedEAN13Writer.cpp
@@ -79,17 +79,16 @@
 int32_t CBC_OnedEAN13Writer::CalcChecksum(const CFX_ByteString& contents) {
   int32_t odd = 0;
   int32_t even = 0;
-  FX_STRSIZE j = 1;
-  for (FX_STRSIZE i = 0; i < contents.GetLength(); i++) {
-    if (j % 2) {
-      odd += FXSYS_DecimalCharToInt(contents[i]);
+  FX_STRSIZE parity = 1;
+  for (FX_STRSIZE i = contents.GetLength(); i > 0; i--) {
+    if (parity % 2) {
+      odd += FXSYS_DecimalCharToInt(contents[i - 1]);
     } else {
-      even += FXSYS_DecimalCharToInt(contents[i]);
+      even += FXSYS_DecimalCharToInt(contents[i - 1]);
     }
-    j++;
+    parity++;
   }
-  int32_t checksum = 10 - (odd * 3 + even) % 10;
-  return checksum;
+  return (10 - (odd * 3 + even) % 10) % 10;
 }
 
 uint8_t* CBC_OnedEAN13Writer::EncodeWithHint(const CFX_ByteString& contents,
diff --git a/fxbarcode/oned/BC_OnedEAN13Writer_unittest.cpp b/fxbarcode/oned/BC_OnedEAN13Writer_unittest.cpp
new file mode 100644
index 0000000..932e1f4
--- /dev/null
+++ b/fxbarcode/oned/BC_OnedEAN13Writer_unittest.cpp
@@ -0,0 +1,98 @@
+// Copyright 2017 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "fxbarcode/oned/BC_OnedEAN13Writer.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+TEST(OnedEAN13WriterTest, Encode) {
+  CBC_OnedEAN13Writer writer;
+  int32_t width;
+  int32_t height;
+  uint8_t* encoded;
+  const char* expected;
+
+  // EAN-13 barcodes encode 13-digit numbers into 95 modules in a unidimensional
+  // disposition.
+  encoded = writer.Encode("", BCFORMAT_EAN_13, width, height);
+  EXPECT_EQ(nullptr, encoded);
+  FX_Free(encoded);
+
+  encoded = writer.Encode("123", BCFORMAT_EAN_13, width, height);
+  EXPECT_EQ(nullptr, encoded);
+  FX_Free(encoded);
+
+  encoded = writer.Encode("123456789012", BCFORMAT_EAN_13, width, height);
+  EXPECT_EQ(nullptr, encoded);
+  FX_Free(encoded);
+
+  encoded = writer.Encode("12345678901234", BCFORMAT_EAN_13, width, height);
+  EXPECT_EQ(nullptr, encoded);
+  FX_Free(encoded);
+
+  encoded = writer.Encode("1234567890128", BCFORMAT_EAN_13, width, height);
+  EXPECT_NE(nullptr, encoded);
+  EXPECT_EQ(1, height);
+  EXPECT_EQ(95, width);
+
+  expected =
+      "# #"  // Start
+      // 1 implicit by LLGLGG in next 6 digits
+      "  #  ##"  // 2 L
+      " #### #"  // 3 L
+      "  ### #"  // 4 G
+      " ##   #"  // 5 L
+      "    # #"  // 6 G
+      "  #   #"  // 7 G
+      " # # "    // Middle
+      "#  #   "  // 8 R
+      "### #  "  // 9 R
+      "###  # "  // 0 R
+      "##  ## "  // 1 R
+      "## ##  "  // 2 R
+      "#  #   "  // 8 R
+      "# #";     // End
+  for (int i = 0; i < 95; i++) {
+    EXPECT_EQ(expected[i] != ' ', !!encoded[i]) << i;
+  }
+  FX_Free(encoded);
+
+  encoded = writer.Encode("7776665554440", BCFORMAT_EAN_13, width, height);
+  EXPECT_NE(nullptr, encoded);
+  EXPECT_EQ(1, height);
+  EXPECT_EQ(95, width);
+
+  expected =
+      "# #"  // Start
+      // 7 implicit by LGLGLG in next 6 digits
+      " ### ##"  // 7 L
+      "  #   #"  // 7 G
+      " # ####"  // 6 L
+      "    # #"  // 6 G
+      " # ####"  // 6 L
+      " ###  #"  // 5 G
+      " # # "    // Middle
+      "#  ### "  // 5 R
+      "#  ### "  // 5 R
+      "# ###  "  // 4 R
+      "# ###  "  // 4 R
+      "# ###  "  // 4 R
+      "###  # "  // 0 R
+      "# #";     // End
+  for (int i = 0; i < 95; i++) {
+    EXPECT_EQ(expected[i] != ' ', !!encoded[i]) << i;
+  }
+  FX_Free(encoded);
+}
+
+TEST(OnedEAN13WriterTest, Checksum) {
+  CBC_OnedEAN13Writer writer;
+  EXPECT_EQ(0, writer.CalcChecksum(""));
+  EXPECT_EQ(6, writer.CalcChecksum("123"));
+  EXPECT_EQ(8, writer.CalcChecksum("123456789012"));
+  EXPECT_EQ(0, writer.CalcChecksum("777666555444"));
+}
+
+}  // namespace