ICU4J: Undo change to backup() and make uses of backup() and readCode…

…Point() consistent

Fix three locations where `cp = readCodePoint()` was followed by `backup(1)`, in a context where
`cp` might be a wide char:

`getIdentifier()`: Use `peekChar()` rather than `readCodePoint()` followed by `backup()`
   (an identifier could be followed by a wide char that is not a name-char)
`skipWhitespaces()`: The same change (a whitespace could be followed by a wide char)
`getName()`: names can include wide chars

The tests are in 4429088

icu4j/main/core/src/main/java/com/ibm/icu/message2/InputSource.java

@@ -29,6 +29,24 @@ int peekChar() {
         return buffer.charAt(cursor);
     }
 
+    int peekCodePoint() {
+        if (atEnd()) {
+            return -1;
+        }
+        char c = buffer.charAt(cursor);
+        if (Character.isHighSurrogate(c)) {
+            if (cursor + 1 < buffer.length()) {
+                char c2 = buffer.charAt(cursor + 1);
+                if (Character.isLowSurrogate(c2)) {
+                    return Character.toCodePoint(c, c2);
+                } else { // invalid, high surrogate followed by non-surrogate
+                    return c;
+                }
+            }
+        }
+        return c;
+    }
+
     int readCodePoint() {
         // TODO: remove this?
         // START Detect possible infinite loop
@@ -62,22 +80,10 @@ int readCodePoint() {
         return c;
     }
 
-    // Backup a number of code points.
+    // Backup a number of characters.
     void backup(int amount) {
-        int amountBackedUp = 0;
         // TODO: validate
-        while (amountBackedUp < amount) {
-            backupOneCodePoint();
-            amountBackedUp++;
-        }
-    }
-
-    void backupOneCodePoint() {
-        if (Character.isLowSurrogate(buffer.charAt(cursor - 1))) {
-            cursor -= 2;
-        } else {
-            cursor--;
-        }
+        cursor -= amount;
     }
 
     int getPosition() {

icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java

@@ -53,12 +53,12 @@ private MFDataModel.Message parseImpl() throws MFParseException {
             cp = input.readCodePoint();
             cp = input.peekChar();
             if (cp == '{') { // `{{`, complex body without declarations
-                input.backupOneCodePoint(); // let complexBody deal with the wrapping {{ and }}
+                input.backup(1); // let complexBody deal with the wrapping {{ and }}
                 MFDataModel.Pattern pattern = getQuotedPattern();
                 skipOptionalWhitespaces();
                 result = new MFDataModel.PatternMessage(new ArrayList<>(), pattern);
             } else { // placeholder
-                input.backupOneCodePoint(); // We want the '{' present, to detect the part as placeholder.
+                input.backup(1); // We want the '{' present, to detect the part as placeholder.
                 MFDataModel.Pattern pattern = getPattern();
                 result = new MFDataModel.PatternMessage(new ArrayList<>(), pattern);
             }
@@ -129,7 +129,7 @@ private String getText() {
                     if (StringUtils.isContentChar(cp) || StringUtils.isWhitespace(cp)) {
                         result.appendCodePoint(cp);
                     } else {
-                        input.backupOneCodePoint();
+                        input.backup(1);
                         return result.toString();
                     }
             }
@@ -413,13 +413,12 @@ private String getIdentifier() throws MFParseException {
         if (namespace == null) {
             return null;
         }
-        int cp = input.readCodePoint();
+        int cp = input.peekChar();
         if (cp == ':') { // the previous name was namespace
+            input.readCodePoint(); // Consume the ':'
             String name = getName();
             checkCondition(name != null, "Expected name after namespace '" + namespace + "'");
             return namespace + ":" + name;
-        } else {
-            input.backupOneCodePoint();
         }
         return namespace;
     }
@@ -490,7 +489,7 @@ private MFDataModel.Literal getLiteral() throws MFParseException {
                 MFDataModel.Literal ql = getQuotedLiteral();
                 return ql;
             default: // unquoted
-                input.backupOneCodePoint();
+                input.backup(1);
                 MFDataModel.Literal unql = getUnQuotedLiteral();
                 return unql;
         }
@@ -567,14 +566,14 @@ private int skipOptionalWhitespaces() {
     private int skipWhitespaces() {
         int skipCount = 0;
         while (true) {
-            int cp = input.readCodePoint();
+            int cp = input.peekChar();
             if (cp == EOF) {
                 return skipCount;
             }
             if (!StringUtils.isWhitespace(cp)) {
-                input.backupOneCodePoint();
                 return skipCount;
             }
+            input.readCodePoint();
             skipCount++;
         }
     }
@@ -798,13 +797,14 @@ private String getName() throws MFParseException {
         }
         result.appendCodePoint(cp);
         while (true) {
-            cp = input.readCodePoint();
+            cp = input.peekCodePoint();
             if (StringUtils.isNameChar(cp)) {
+                cp = input.readCodePoint();
                 result.appendCodePoint(cp);
             } else if (cp == EOF) {
+                input.readCodePoint();
                 break;
             } else {
-                input.backup(1);
                 break;
             }
         }