Check in the pristine copy of ICU 4.6...

icu46/source/i18n/csrutf8.cpp

+/*
+ **********************************************************************
+ *   Copyright (C) 2005-2008, International Business Machines
+ *   Corporation and others.  All Rights Reserved.
+ **********************************************************************
+ */
+
+#include "unicode/utypes.h"
+
+#if !UCONFIG_NO_CONVERSION
+
+#include "csrutf8.h"
+
+U_NAMESPACE_BEGIN
+
+CharsetRecog_UTF8::~CharsetRecog_UTF8()
+{
+    // nothing to do
+}
+
+const char *CharsetRecog_UTF8::getName() const
+{
+    return "UTF-8";
+}
+
+int32_t CharsetRecog_UTF8::match(InputText* det) {
+    bool hasBOM = FALSE;
+    int32_t numValid = 0;
+    int32_t numInvalid = 0;
+    const uint8_t *input = det->fRawInput;
+    int32_t i;
+    int32_t trailBytes = 0;
+    int32_t confidence;
+
+    if (det->fRawLength >= 3 && 
+        input[0] == 0xEF && input[1] == 0xBB && input[2] == 0xBF) {
+            hasBOM = TRUE;
+    }
+
+    // Scan for multi-byte sequences
+    for (i=0; i < det->fRawLength; i += 1) {
+        int32_t b = input[i];
+
+        if ((b & 0x80) == 0) {
+            continue;   // ASCII
+        }
+
+        // Hi bit on char found.  Figure out how long the sequence should be
+        if ((b & 0x0E0) == 0x0C0) {
+            trailBytes = 1;
+        } else if ((b & 0x0F0) == 0x0E0) {
+            trailBytes = 2;
+        } else if ((b & 0x0F8) == 0xF0) {
+            trailBytes = 3;
+        } else {
+            numInvalid += 1;
+
+            if (numInvalid > 5) {
+                break;
+            }
+
+            trailBytes = 0;
+        }
+
+        // Verify that we've got the right number of trail bytes in the sequence
+        for (;;) {
+            i += 1;
+
+            if (i >= det->fRawLength) {
+                break;
+            }
+
+            b = input[i];
+
+            if ((b & 0xC0) != 0x080) {
+                numInvalid += 1;
+                break;
+            }
+
+            if (--trailBytes == 0) {
+                numValid += 1;
+                break;
+            }
+        }
+
+    }
+
+    // Cook up some sort of confidence score, based on presense of a BOM
+    //    and the existence of valid and/or invalid multi-byte sequences.
+    confidence = 0;
+    if (hasBOM && numInvalid == 0) {
+        confidence = 100;
+    } else if (hasBOM && numValid > numInvalid*10) {
+        confidence = 80;
+    } else if (numValid > 3 && numInvalid == 0) {
+        confidence = 100;
+    } else if (numValid > 0 && numInvalid == 0) {
+        confidence = 80;
+    } else if (numValid == 0 && numInvalid == 0) {
+        // Plain ASCII.
+        confidence = 10;
+    } else if (numValid > numInvalid*10) {
+        // Probably corruput utf-8 data.  Valid sequences aren't likely by chance.
+        confidence = 25;
+    }
+
+    return confidence;
+}
+
+U_NAMESPACE_END
+#endif