resolve #34

assira.core/src/main/java/net/ranides/assira/text/StringUtils.java

@@ -1027,6 +1027,44 @@ public final class StringUtils {
         throw new NoSuchElementException();
     }
 
+    /**
+     * Extracts 'length' characters from 'begin'. Both parameters accept negative values.
+     * Negative value 'x' means "x less than length".
+     * @param text original string
+     * @param begin allowed to be negative, then index is counted from the end
+     * @param length allowed to negative, then length is counted as 'less than original length'
+     * @return substring
+     */
+    public static String substr(String text, int begin, int length) {
+        if(text == null || length==0) {
+            return "";
+        }
+        if(begin < 0) {
+            begin += text.length();
+        }
+        if(length < 0) {
+            length += text.length();
+        }
+        int b = MathUtils.clip(begin, 0, text.length());
+        int e = MathUtils.clip(begin + length, 0, text.length());
+        return text.substring(b, e);
+    }
+
+    /**
+     * Extracts 'length' characters from string. If length is positive, extracts characters from begin.
+     * If length is negative, extracts characters from the end.
+     * @param text original text
+     * @param length number of characters
+     * @return substring
+     */
+    public static String substr(String text, int length) {
+        if (length < 0) {
+            return substr(text, length, -length);
+        } else {
+            return substr(text, 0, length);
+        }
+    }
+
     private static String replaceEndl(String text) {
         return text.replaceAll("\r\n", "\037").replace('\n', '\037').replace('\r', '\037');
     }