run length algorithm

src/main/java/problems/impl/RunLengthConverter.java

@@ -0,0 +1,56 @@
+package problems.impl;
+
+/**
+ * Run-length encoding is a fast and simple method of encoding strings.
+ * The basic idea is to represent repeated successive characters as a single count and character.
+ * For example, the string "AAAABBBCCDAA" would be encoded as "4A3B2C1D2A".
+ * Implement run-length encoding and decoding.
+ * You can assume the string to be encoded have no digits and consists solely of alphabetic characters.
+ * You can assume the string to be decoded is valid.
+ */
+
+public class RunLengthConverter {
+    public static String encode(String value) {
+        if (value.isEmpty()) {
+            return "";
+        }
+
+        char[] chars = value.toCharArray();
+
+        StringBuilder encodedValue = new StringBuilder();
+
+        char currentChar = chars[0];
+        int charCount = 1;
+
+        for (int i = 0; i < chars.length - 1; i++) {
+            char nextChar = chars[i + 1];
+
+            if (nextChar == currentChar) {
+                charCount++;
+            } else {
+                encodedValue.append(charCount).append(currentChar);
+                charCount = 1;
+            }
+
+            currentChar = nextChar;
+        }
+
+        encodedValue.append(charCount).append(currentChar);
+
+        return encodedValue.toString();
+    }
+
+    public static String decode(String value) {
+        char[] chars = value.toCharArray();
+        StringBuilder decodedValue = new StringBuilder();
+
+        for (int i = 0; i < chars.length - 1; i += 2) {
+            int count = Character.getNumericValue(chars[i]);
+            char c = chars[i + 1];
+            for (int j = 0; j < count; j++) {
+                decodedValue.append(c);
+            }
+        }
+        return decodedValue.toString();
+    }
+}

src/test/java/problems/impl/RunLengthConverterTest.java

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+package problems.impl;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class RunLengthConverterTest {
+
+    @Test
+    public void itShouldReturnEmptyStringWhenEncodingEmptyString() {
+        assertEquals(RunLengthConverter.encode(""), "");
+    }
+
+    @Test
+    public void itShouldEncodeForSingleCharacter() {
+        assertEquals(RunLengthConverter.encode("a"), "1a");
+    }
+
+    @Test
+    public void itShouldEncodeForTwoSingleCharacters() {
+        assertEquals(RunLengthConverter.encode("ab"), "1a1b");
+    }
+
+    @Test
+    public void itShouldEncodeRepeatingCharacter() {
+        assertEquals(RunLengthConverter.encode("aaaaaa"), "6a");
+    }
+
+    @Test
+    public void itShouldEncodeRepeatingCharactersThatEndsWithADifferentCharacter() {
+        assertEquals(RunLengthConverter.encode("aaaaab"), "5a1b");
+    }
+
+    @Test
+    public void itShouldEncodeRepeatingCharactersThatEndsWithMultipleDifferentCharacters() {
+        assertEquals(RunLengthConverter.encode("aaaaabb"), "5a2b");
+    }
+
+    @Test
+    public void itShouldDecodeEmptyString() {
+        assertEquals(RunLengthConverter.decode(""), "");
+    }
+
+    @Test
+    public void itShouldDecodeSingleCharacter() {
+        assertEquals(RunLengthConverter.decode("1a"), "a");
+    }
+
+    @Test
+    public void itShouldDecodeTwoSingleCharacters() {
+        assertEquals(RunLengthConverter.decode("1a1b"), "ab");
+    }
+
+    @Test
+    public void itShouldDecodeRepeatingCharacters() {
+        assertEquals(RunLengthConverter.decode("5a"), "aaaaa");
+    }
+
+    @Test
+    public void itShouldDecodeRepeatingCharactersThatEndsWithADifferentCharacter() {
+        assertEquals(RunLengthConverter.decode("5a1b"), "aaaaab");
+    }
+
+    @Test
+    public void itShouldDecodeRepeatingCharactersThatEndWithMultipleDifferentCharacters() {
+        assertEquals(RunLengthConverter.decode("5a2b"), "aaaaabb");
+    }
+
+}