Detect icu4c error codes more precisely

Negative values are warnings, not errors; failing on them breals encoding when
it would otherwise work.

Unfortunately, these negative numbers are being converted into large positive
ones through the cgo bridge, so this becomes a slightly more complicated
check than necessary.

c_bridge.c

@@ -15,7 +15,7 @@ const int detectCharset(void        *detector,
 
     // Put input bytes in the detector.
     ucsdet_setText((UCharsetDetector*)detector, (char*)input, input_len, status);
-    if (*status != U_ZERO_ERROR) {
+    if U_FAILURE(*status) {
         return 0;
     }
 
@@ -25,7 +25,7 @@ const int detectCharset(void        *detector,
 
     // Perform analysis and return all guesses and their count.
     bestGuesses = ucsdet_detectAll((UCharsetDetector*)detector, &matchCount, status);
-    if (*status != U_ZERO_ERROR) {
+    if U_FAILURE(*status) {
         return 0;
     }
 
@@ -42,19 +42,19 @@ const int detectCharset(void        *detector,
 
         // Fill guessed encoding
         bestGuessedCharset = ucsdet_getName(bestGuess, status);
-        if (*status != U_ZERO_ERROR) {
+        if U_FAILURE(*status) {
             return 0;
         }
 
         // Fill guessed language
         bestGuessedLanguage = ucsdet_getLanguage(bestGuess, status);
-        if (*status != U_ZERO_ERROR) {
+        if U_FAILURE(*status) {
             return 0;
         }
 
         // Fill its confidence rating
         int32_t conf = ucsdet_getConfidence(bestGuess, status);
-        if (*status != U_ZERO_ERROR) {
+        if U_FAILURE(*status) {
             return 0;
         }
 
@@ -77,13 +77,13 @@ int convertToUtf16(const char   *srcEncoding,
     UConverter *conv;
 
     conv = ucnv_open(srcEncoding, status);
-    if (*status != U_ZERO_ERROR) {
+    if U_FAILURE(*status) {
         return 0;
     }
 
     /* Convert from original encoding to UTF-16 */
     int len = ucnv_toUChars(conv, dest, destCapacity, src, srcLength, status);
-    if (*status != U_ZERO_ERROR) {
+    if U_FAILURE(*status) {
         return 0;
     }
 
@@ -102,13 +102,13 @@ int convertFromUtf16(const char   *destEncoding,
     UConverter *conv;
 
     conv = ucnv_open(destEncoding, status);
-    if (*status != U_ZERO_ERROR) {
+    if U_FAILURE(*status) {
         return 0;
     }
 
     /* Convert from UTF-16 to destination encoding */
     int len = ucnv_fromUChars(conv, dest, destCapacity, src, srcLength, status);
-    if (*status != U_ZERO_ERROR) {
+    if U_FAILURE(*status) {
         return 0;
     }
 

convert.go

@@ -79,7 +79,7 @@ func (conv *CharsetConverter) ConvertToUtf8(input []byte, srcEncoding string) ([
             C.int32_t(len(input)),
             (*C.int)(unsafe.Pointer(&status)))
 
-    if status == U_ZERO_ERROR {
+    if isSuccess(status) {
         nConvLen := C.convertFromUtf16(
             Utf8CString,
             (*C.char)(unsafe.Pointer(&conv.utf8Buffer[0])),
@@ -88,7 +88,7 @@ func (conv *CharsetConverter) ConvertToUtf8(input []byte, srcEncoding string) ([
             C.int32_t(convLen),
             (*C.int)(unsafe.Pointer(&status)))
 
-        if status == U_ZERO_ERROR {
+        if isSuccess(status) {
             resStr := conv.utf8Buffer[:nConvLen]
             return ([]byte)(resStr), nil
         }

detect.go

@@ -11,10 +11,22 @@ import (
 )
 
 const (
-    U_ZERO_ERROR        = 0     // ICU common constant error code which means that no error occured
-    MatchDataBufferSize = 25    // Size of the buffer for detection results (Max count of returned guesses per detect call)
+    U_ZERO_ERROR        = 0          // ICU common constant error code which means that no error occured
+    U_ERROR_LIMIT       = 0x7FFFFFFF // Dirty hack, negative error codes are are being turned into large positive ints
+    MatchDataBufferSize = 25         // Size of the buffer for detection results (Max count of returned guesses per detect call)
 )
 
+// Go implementation of the icu U_SUCCESS macro. Negative status codes are
+// warnings, 0 is a success without warnings, > 0 is an error
+func isSuccess(status int) bool {
+    return status <= U_ZERO_ERROR || status >= U_ERROR_LIMIT
+}
+
+// Go implementation of the icu U_FAILURE macro.
+func isFailure(status int) bool {
+    return status > U_ZERO_ERROR && status < U_ERROR_LIMIT
+}
+
 // CharsetDetector provides ICU charset detection functionality.
 type CharsetDetector struct {
     ptr         *C.UCharsetDetector // ICU struct needed for detection
@@ -39,7 +51,7 @@ func NewCharsetDetector() (*CharsetDetector, error) {
 
     det.ptr = C.ucsdet_open((*C.UErrorCode)(statusPtr))
 
-    if status != U_ZERO_ERROR {
+    if isFailure(status) {
         return nil, fmt.Errorf("ICU Error code returned: %d", status)
     }
 
@@ -70,7 +82,7 @@ func (det *CharsetDetector) GuessCharset(input []byte) (matches []Match, err err
         (*C.MatchData)(unsafe.Pointer(&det.resBuffer[0])),
         C.int(MatchDataBufferSize))
 
-    if status == U_ZERO_ERROR {
+    if isSuccess(status) {
         // Convert the returned number of entries from result buffer to a slice
         // that will be returned
         count := int(guessCount)