Implement correct width computation

include/fmt/format.h

@@ -923,9 +923,29 @@ inline size_t compute_width(string_view s) {
 #if FMT_USE_TEXT
   basic_memory_buffer<uint32_t> code_points;
   for (auto cp: boost::text::make_to_utf32_range(s)) code_points.push_back(cp);
-  size_t num_graphemes = 0;
-  for (auto g: boost::text::graphemes(code_points)) ++num_graphemes;
-  return num_graphemes;
+  size_t width = 0;
+  for (auto g: boost::text::graphemes(code_points)) {
+    auto cp = *g.begin();
+    // Based on http://www.cl.cam.ac.uk/~mgk25/ucs/wcwidth.c by Markus Kuhn.
+    width += 1 +
+      (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)));
+  }
+  return width;
 #else
   return s.size();
 #endif  // FMT_USE_TEXT