From d8158772bf9d1fafd3cdb360f10516d2788e5d47 Mon Sep 17 00:00:00 2001
From: Elias Kosunen <elias.kosunen@gmail.com>
Date: Wed, 6 Nov 2024 22:27:49 +0200
Subject: [PATCH] Fix bug with double-wide characters triggering invalid field
 width calculation

---
 src/scn/impl.h                          | 19 +++++++++++++++----
 tests/unittests/align_and_fill_test.cpp |  8 ++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/src/scn/impl.h b/src/scn/impl.h
index 8a9acd94..94f9ca88 100644
--- a/src/scn/impl.h
+++ b/src/scn/impl.h
@@ -2521,6 +2521,13 @@ class counted_width_iterator {
         return m_multibyte_left;
     }
 
+    bool is_current_double_wide() const
+    {
+        assert(count() != 0 || multibyte_left() != 0);
+        return _get_width_at_current_cp_start(
+                   _get_cp_length_at_current()) == 2;
+    }
+
     constexpr decltype(auto) operator*()
     {
         return *m_current;
@@ -2634,12 +2641,14 @@ class counted_width_iterator {
     friend constexpr bool operator==(const counted_width_iterator& x,
                                      ranges::default_sentinel_t)
     {
-        return x.count() == 0 && x.multibyte_left() == 0;
+        return (x.count() == 0 && x.multibyte_left() == 0) ||
+               (x.count() == 1 && x.multibyte_left() == 0 &&
+                x.is_current_double_wide());
     }
-    friend constexpr bool operator==(ranges::default_sentinel_t,
+    friend constexpr bool operator==(ranges::default_sentinel_t s,
                                      const counted_width_iterator& x)
     {
-        return x.count() == 0 && x.multibyte_left() == 0;
+        return x == s;
     }
 
     friend constexpr bool operator!=(const counted_width_iterator& a,
@@ -2868,7 +2877,9 @@ class take_width_view : public ranges::view_interface<take_width_view<View>> {
         friend constexpr bool operator==(const CWI& y, const sentinel& x)
         {
             return (y.count() == 0 && y.multibyte_left() == 0) ||
-                   y.base() == x.m_end;
+                   y.base() == x.m_end ||
+                   (y.count() == 1 && y.multibyte_left() == 0 &&
+                    y.is_current_double_wide());
         }
 
         friend constexpr bool operator==(const sentinel& x, const CWI& y)
diff --git a/tests/unittests/align_and_fill_test.cpp b/tests/unittests/align_and_fill_test.cpp
index e9e47bfe..dcacba4c 100644
--- a/tests/unittests/align_and_fill_test.cpp
+++ b/tests/unittests/align_and_fill_test.cpp
@@ -629,3 +629,11 @@ TEST(CustomPrecisionTest, DoubleWidthEmoji)
     EXPECT_EQ(r->value(), "😂");
     EXPECT_STREQ(r->begin(), "a");
 }
+
+TEST(CustomPrecisionTest, Fuzz1)
+{
+    auto r = scn::scan<std::string>("a😂", "{:^.2}");
+    ASSERT_TRUE(r);
+    EXPECT_EQ(r->value(), "a");
+    EXPECT_STREQ(r->begin(), "😂");
+}