add to_utf8 and to_utf32

Author: jube

include/gf2/core/StringUtils.h

@@ -138,6 +138,9 @@ namespace gf {
     return { string };
   }
 
+  GF_CORE_API std::u32string to_utf32(std::string_view utf8);
+  GF_CORE_API std::string to_utf8(std::u32string_view utf32);
+
 }
 
 #endif // GF_STRING_UTILS_H

library/core/StringUtils.cc

@@ -127,4 +127,37 @@ namespace gf {
     return result;
   }
 
+  std::u32string to_utf32(std::string_view utf8)
+  {
+    CodepointRange range = codepoints(utf8);
+    return { range.begin(), range.end() };
+  }
+
+  std::string to_utf8(std::u32string_view utf32)
+  {
+    std::string utf8;
+
+    for (char32_t c : utf32) {
+      if (c < 0x80) {
+        utf8.push_back(static_cast<char>(c & 0x7F));
+      } else if (c < 0x800) {
+        utf8.push_back(static_cast<char>(((c >> 6) & 0x1F) | 0xC0));
+        utf8.push_back(static_cast<char>(((c >> 0) & 0x3F) | 0x80));
+      } else if (c < 0x10000) {
+        utf8.push_back(static_cast<char>(((c >> 12) & 0x0F) | 0xE0));
+        utf8.push_back(static_cast<char>(((c >>  6) & 0x3F) | 0x80));
+        utf8.push_back(static_cast<char>(((c >>  0) & 0x3F) | 0x80));
+      } else if (c < 0x110000) {
+        utf8.push_back(static_cast<char>(((c >> 18) & 0x07) | 0xF0));
+        utf8.push_back(static_cast<char>(((c >> 12) & 0x3F) | 0x80));
+        utf8.push_back(static_cast<char>(((c >>  6) & 0x3F) | 0x80));
+        utf8.push_back(static_cast<char>(((c >>  0) & 0x3F) | 0x80));
+      } else {
+        assert(false);
+      }
+    }
+
+    return utf8;
+  }
+
 }

tests/tests_core_StringUtils.cc

@@ -123,3 +123,43 @@ TEST(StringUtilsTest, Codepoints) {
     EXPECT_EQ(*cp.begin(), U'\U0001F3AE');
   }
 }
+
+TEST(StringUtilsTest, ToUtf32) {
+  auto str1 = gf::to_utf32("");
+  EXPECT_EQ(str1.size(), 0u);
+  EXPECT_EQ(str1, U"");
+
+  auto str2 = gf::to_utf32("abc");
+  EXPECT_EQ(str2.size(), 3u);
+  EXPECT_EQ(str2, U"abc");
+
+  auto str3 = gf::to_utf32("éçù€");
+  EXPECT_EQ(str3.size(), 4u);
+  EXPECT_EQ(str3, U"éçù€");
+
+  auto str4 = gf::to_utf32("東京");
+  EXPECT_EQ(str4.size(), 2u);
+  EXPECT_EQ(str4, U"東京");
+
+  auto str5 = gf::to_utf32("🎮");
+  EXPECT_EQ(str5.size(), 1u);
+  EXPECT_EQ(str5, U"🎮");
+}
+
+
+TEST(StringUtilsTest, ToUtf8) {
+  auto str1 = gf::to_utf8(U"");
+  EXPECT_EQ(str1, "");
+
+  auto str2 = gf::to_utf8(U"abc");
+  EXPECT_EQ(str2, "abc");
+
+  auto str3 = gf::to_utf8(U"éçù€");
+  EXPECT_EQ(str3, "éçù€");
+
+  auto str4 = gf::to_utf8(U"東京");
+  EXPECT_EQ(str4, "東京");
+
+  auto str5 = gf::to_utf8(U"🎮");
+  EXPECT_EQ(str5, "🎮");
+}