Own code for UTF-32 <-> UTF-8 conversion

Avoids codecvt dependency on Unix where it might not be present, eg
with GCC 4.8; on Windows we use <codecvt> since it’s present in VS2015
to avoid writing a seperate UTF-16 <-> UTF-8 conversion.

simgear/misc/strutils.cxx

@@ -29,9 +29,8 @@
 #include <cstring>              // strerror_r() and strerror_s()
 #include <cctype>
 #include <cerrno>
-#include <codecvt>
-#include <locale>
- 
+#include <cassert>
+
 #include "strutils.hxx"
 
 #include <simgear/debug/logstream.hxx>
@@ -42,6 +41,8 @@
 
 #if defined(SG_WINDOWS)
 	#include <windows.h>
+    #include <codecvt>
+    #include <locale>    
 #endif
 
 using std::string;
@@ -654,14 +655,86 @@ static std::string convertWStringToMultiByte(DWORD encoding, const std::wstring&
 
 std::wstring convertUtf8ToWString(const std::string& a)
 {
+#if defined(SG_WINDOWS)
     std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> ucs2conv;
     return ucs2conv.from_bytes(a);
+#else
+    assert(sizeof(wchar_t) == 4);
+    std::wstring result;
+    int expectedContinuationCount = 0;
+    wchar_t wc = 0;
+    
+    for (uint8_t utf8CodePoint : a) {
+        // ASCII 7-bit range
+        if (utf8CodePoint <= 0x7f) {
+            if (expectedContinuationCount != 0) {
+                throw sg_format_exception();
+            }
+            
+            result.push_back(static_cast<wchar_t>(utf8CodePoint));
+        } else if (expectedContinuationCount > 0) {
+            if ((utf8CodePoint & 0xC0) != 0x80) {
+                throw sg_format_exception();
+            }
+            
+            wc = (wc << 6) | (utf8CodePoint & 0x3F);
+            if (--expectedContinuationCount == 0) {
+                result.push_back(wc);
+            }
+        } else {
+            if ((utf8CodePoint & 0xE0) == 0xC0) {
+                expectedContinuationCount = 1;
+                wc = utf8CodePoint & 0x1f;
+            } else if ((utf8CodePoint & 0xF0) == 0xE0) {
+                expectedContinuationCount = 2;
+                wc = utf8CodePoint & 0x0f;
+            } else if ((utf8CodePoint & 0xF8) == 0xF0) {
+                expectedContinuationCount = 3;
+                wc =utf8CodePoint & 0x07;
+            } else {
+                // illegal UTF-8 encoding
+                throw sg_format_exception();
+            }
+        }
+    } // of UTF-8 code point iteration
+    
+    return result;
+    
+#endif
+
 }
 
 std::string convertWStringToUtf8(const std::wstring& w)
 {
+#if defined(SG_WINDOWS)
     std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> ucs2conv;
     return ucs2conv.to_bytes(w);
+#else
+    assert(sizeof(wchar_t) == 4);
+    std::string result;
+    
+    for (wchar_t cp : w) {
+        if (cp <= 0x7f) {
+            result.push_back(static_cast<uint8_t>(cp));
+        } else if (cp <= 0x07ff) {
+            result.push_back(0xC0 | ((cp >> 6) & 0x1f));
+            result.push_back(0x80 | (cp & 0x3f));
+        } else if (cp <= 0xffff) {
+            result.push_back(0xE0 | ((cp >> 12) & 0x0f));
+            result.push_back(0x80 | ((cp >> 6) & 0x3f));
+            result.push_back(0x80 | (cp & 0x3f));
+        } else if (cp < 0x10ffff) {
+            result.push_back(0xF0 | ((cp >> 18) & 0x07));
+            result.push_back(0x80 | ((cp >> 12) & 0x3f));
+            result.push_back(0x80 | ((cp >> 6) & 0x3f));
+            result.push_back(0x80 | (cp & 0x3f));
+        } else {
+            throw sg_format_exception();
+        }
+    }
+    
+    return result;
+#endif
 }
 
 std::string convertWindowsLocal8BitToUtf8(const std::string& a)

simgear/misc/strutils_test.cxx

@@ -605,6 +605,20 @@ void test_readTime()
     SG_CHECK_EQUAL_EP(strutils::readTime("-0:0:28"), -28 * seconds);
 }
 
+void test_utf8Convert()
+{
+    // F, smiley emoticon, Maths summation symbol, section sign
+    std::wstring a(L"\u0046\U0001F600\u2211\u00A7");
+    
+    
+    std::string utf8A = strutils::convertWStringToUtf8(a);
+    SG_VERIFY(utf8A == std::string("F\xF0\x9F\x98\x80\xE2\x88\x91\xC2\xA7"));
+    
+    
+    std::wstring aRoundTrip = strutils::convertUtf8ToWString(utf8A);
+    SG_VERIFY(a == aRoundTrip);
+}
+
 int main(int argc, char* argv[])
 {
     test_strip();
@@ -624,6 +638,7 @@ int main(int argc, char* argv[])
     test_error_string();
     test_propPathMatch();
     test_readTime();
+    test_utf8Convert();
     
     return EXIT_SUCCESS;
 }