From cb241b7289abfc10111d3180def55ca1fbb2edb4 Mon Sep 17 00:00:00 2001
From: crupest <crupest@outlook.com>
Date: Sun, 5 Jul 2020 23:13:51 +0800
Subject: ...

---
 src/common/StringUtil.cpp      | 4 ++--
 test/common/StringUtilTest.cpp | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/common/StringUtil.cpp b/src/common/StringUtil.cpp
index 0b6332ce..0ebfd85e 100644
--- a/src/common/StringUtil.cpp
+++ b/src/common/StringUtil.cpp
@@ -16,7 +16,7 @@ CodePoint Utf16Iterator::Previous() {
 
   if (cu0 < 0xd800u || cu0 >= 0xe000u) {  // 1-length code point
     return static_cast<CodePoint>(cu0);
-  } else if (cu0 < 0xdc00u || cu0 > 0xdfffu) {  // 2-length code point
+  } else if (cu0 >= 0xdc00u || cu0 <= 0xdfffu) {  // 2-length code point
     if (position_ <= 0) {
       throw TextEncodeException(
           "Unexpected end when reading first code unit of surrogate pair "
@@ -25,7 +25,7 @@ CodePoint Utf16Iterator::Previous() {
     const auto cu1 = static_cast<std::uint16_t>(string_[--position_]);
 
 #ifdef CRU_DEBUG
-    if (cu1 <= 0xdbffu) {
+    if (cu1 < 0xd800u || cu1 > 0xdbffu) {
       throw TextEncodeException(
           "Unexpected bad-range first code unit of surrogate pair during "
           "backward.");
diff --git a/test/common/StringUtilTest.cpp b/test/common/StringUtilTest.cpp
index e454505b..21a9ea9c 100644
--- a/test/common/StringUtilTest.cpp
+++ b/test/common/StringUtilTest.cpp
@@ -26,6 +26,12 @@ TEST(WinString, Utf16Iterator) {
   ASSERT_EQ(i.Next(), 0x1F923);
   ASSERT_EQ(i.Next(), 0x0021);
   ASSERT_EQ(i.Next(), k_invalid_code_point);
+  ASSERT_EQ(i.Previous(), 0x0021);
+  ASSERT_EQ(i.Previous(), 0x1F923);
+  ASSERT_EQ(i.Previous(), 0x4F60);
+  ASSERT_EQ(i.Previous(), 0x03C0);
+  ASSERT_EQ(i.Previous(), 0x0061);
+  ASSERT_EQ(i.Previous(), k_invalid_code_point);
 }
 
 // TEST(WinString, IndexUtf8ToUtf16) {
-- 
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