Improve width estimation (#2033)

include/fmt/format.h

@@ -695,7 +695,28 @@ FMT_CONSTEXPR inline size_t compute_width(string_view s) {
   // It is not a lambda for compatibility with C++14.
   struct count_code_points {
     size_t* count;
-    FMT_CONSTEXPR void operator()(uint32_t, int) const { ++*count; }
+    FMT_CONSTEXPR void operator()(uint32_t cp, int error) const {
+      *count +=
+          1 +
+          (error == 0 && cp >= 0x1100 &&
+           (cp <= 0x115f ||  // Hangul Jamo init. consonants
+            cp == 0x2329 ||  // LEFT-POINTING ANGLE BRACKET〈
+            cp == 0x232a ||  // RIGHT-POINTING ANGLE BRACKET 〉
+            // CJK ... Yi except Unicode Character “〿”:
+            (cp >= 0x2e80 && cp <= 0xa4cf && cp != 0x303f) ||
+            (cp >= 0xac00 && cp <= 0xd7a3) ||    // Hangul Syllables
+            (cp >= 0xf900 && cp <= 0xfaff) ||    // CJK Compatibility Ideographs
+            (cp >= 0xfe10 && cp <= 0xfe19) ||    // Vertical Forms
+            (cp >= 0xfe30 && cp <= 0xfe6f) ||    // CJK Compatibility Forms
+            (cp >= 0xff00 && cp <= 0xff60) ||    // Fullwidth Forms
+            (cp >= 0xffe0 && cp <= 0xffe6) ||    // Fullwidth Forms
+            (cp >= 0x20000 && cp <= 0x2fffd) ||  // CJK
+            (cp >= 0x30000 && cp <= 0x3fffd) ||
+            // Miscellaneous Symbols and Pictographs + Emoticons:
+            (cp >= 0x1f300 && cp <= 0x1f64f) ||
+            // Supplemental Symbols and Pictographs:
+            (cp >= 0x1f900 && cp <= 0x1f9ff)));
+    }
   };
   for_each_codepoint(s, count_code_points{&num_code_points});
   return num_code_points;

test/compile-test.cc

@@ -253,7 +253,7 @@ TEST(CompileTimeFormattingTest, String) {
             test_format<17>(FMT_COMPILE("{} is {}"), "The answer", "42"));
 
   EXPECT_EQ("abc**", test_format<6>(FMT_COMPILE("{:*<5}"), "abc"));
-  EXPECT_EQ("**🤡**", test_format<9>(FMT_COMPILE("{:*^5}"), "🤡"));
+  EXPECT_EQ("**🤡**", test_format<9>(FMT_COMPILE("{:*^6}"), "🤡"));
 }
 
 TEST(CompileTimeFormattingTest, Combination) {

test/format-test.cc

@@ -868,7 +868,8 @@ TEST(FormatterTest, Width) {
   EXPECT_EQ("    0xcafe", format("{0:10}", reinterpret_cast<void*>(0xcafe)));
   EXPECT_EQ("x          ", format("{0:11}", 'x'));
   EXPECT_EQ("str         ", format("{0:12}", "str"));
-  EXPECT_EQ(format("{:*^5}", "🤡"), "**🤡**");
+  EXPECT_EQ(format("{:*^6}", "🤡"), "**🤡**");
+  EXPECT_EQ(format("{:*^8}", "你好"), "**你好**");
   EXPECT_EQ(format("{:#6}", 42.0), "  42.0");
   EXPECT_EQ(format("{:6c}", static_cast<int>('x')), "x     ");
   EXPECT_EQ(format("{:>06.0f}", 0.00884311), "000000");