add some helper methods for text

src/main/java/io/github/ititus/text/SubCharSequence.java

@@ -0,0 +1,57 @@
+package io.github.ititus.text;
+
+import java.util.Objects;
+
+public class SubCharSequence implements CharSequence {
+
+    private final CharSequence sequence;
+    private final int start;
+    private final int end;
+
+    public SubCharSequence(CharSequence sequence, int start, int end) {
+        Objects.requireNonNull(sequence);
+        if (start < 0 || end < 0 || end < start) {
+            throw new IllegalArgumentException();
+        }
+
+        int length = sequence.length();
+        if (start > length || end > length) {
+            throw new IllegalArgumentException();
+        }
+
+        this.sequence = sequence;
+        this.start = start;
+        this.end = end;
+    }
+
+    @Override
+    public int length() {
+        return end - start;
+    }
+
+    @Override
+    public char charAt(int index) {
+        index += start;
+        if (index < 0 || index >= end) {
+            throw new IndexOutOfBoundsException();
+        }
+
+        return sequence.charAt(index);
+    }
+
+    @Override
+    public CharSequence subSequence(int start, int end) {
+        start += this.start;
+        end += this.start;
+        if (start < 0 || start >= this.end || end < 0 || end >= this.end) {
+            throw new IndexOutOfBoundsException();
+        }
+
+        return new SubCharSequence(sequence, this.start + start, this.start + end);
+    }
+
+    @Override
+    public String toString() {
+        return sequence.subSequence(start, end).toString();
+    }
+}

src/main/java/io/github/ititus/text/TextUtil.java

@@ -5,4 +5,25 @@ public class TextUtil {
     public static String reverse(String s) {
         return new StringBuilder(s).reverse().toString();
     }
+
+    public static boolean startsWith(CharSequence s, CharSequence prefix) {
+        if (s instanceof String && prefix instanceof String) {
+            return ((String) s).startsWith((String) prefix);
+        }
+
+        int prefixLength = prefix.length();
+        if (prefixLength == 0) {
+            return true;
+        } else if (s.length() < prefixLength) {
+            return false;
+        }
+
+        for (int i = 0; i < prefixLength; i++) {
+            if (s.charAt(i) != prefix.charAt(i)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
 }