[runLength-en-decoding] run length encoding (#15735)

core-java-modules/core-java-string-algorithms-4/src/test/java/com/baeldung/string/runlength/RunLengthEncodingUnitTest.java

@@ -0,0 +1,93 @@
+package com.baeldung.string.runlength;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class RunLengthEncodingUnitTest {
+    private static final String INPUT = "WWWWWWWWWWWWBAAACCDEEEEE";
+    private static final String RLE = "12W1B3A2C1D5E";
+
+    String runLengthEncode(String input) {
+        StringBuilder result = new StringBuilder();
+        int count = 1;
+        char[] chars = input.toCharArray();
+        for (int i = 0; i < chars.length; i++) {
+            char c = chars[i];
+            if (i + 1 < chars.length && c == chars[i + 1]) {
+                count++;
+            } else {
+                result.append(count).append(c);
+                count = 1;
+            }
+        }
+        return result.toString();
+    }
+
+
+    String runLengthDecode(String rle) {
+        StringBuilder result = new StringBuilder();
+        char[] chars = rle.toCharArray();
+
+        int count = 0;
+        for (char c : chars) {
+            if (Character.isDigit(c)) {
+                count = 10 * count + Character.getNumericValue(c);
+            } else {
+                result.append(String.join("", Collections.nCopies(count, String.valueOf(c))));
+                count = 0;
+            }
+        }
+        return result.toString();
+    }
+
+    String runLengthEncodeByRegEx(String input) {
+        String[] arr = input.split("(?<=(\\D))(?!\\1)");
+        StringBuilder result = new StringBuilder();
+        for (String run : arr) {
+            result.append(run.length()).append(run.charAt(0));
+        }
+        return result.toString();
+    }
+
+    String runLengthDecodeByRegEx(String rle) {
+        if (rle.isEmpty()) {
+            return "";
+        }
+        String[] arr = rle.split("(?<=\\D)|(?=\\D+)");
+        if (arr.length % 2 != 0) {
+            throw new IllegalArgumentException("Not a RLE string");
+        }
+        StringBuilder result = new StringBuilder();
+
+        for (int i = 1; i <= arr.length; i += 2) {
+            int count = Integer.parseInt(arr[i - 1]);
+            String c = arr[i];
+
+            result.append(String.join("", Collections.nCopies(count, c)));
+        }
+        return result.toString();
+    }
+
+    @Test
+    void whenInvokingRunLengthEncode_thenGetExpectedResult() {
+        assertEquals(RLE, runLengthEncode(INPUT));
+    }
+
+    @Test
+    void whenInvokingRunLengthDecode_thenGetExpectedResult() {
+        assertEquals(INPUT, runLengthDecode(RLE));
+    }
+
+    @Test
+    void whenInvokingRunLengthEncodeByRegEx_thenGetExpectedResult() {
+        assertEquals(RLE, runLengthEncodeByRegEx(INPUT));
+    }
+
+    @Test
+    void whenInvokingRunLengthDecodeByRegEx_thenGetExpectedResult() {
+        assertEquals(INPUT, runLengthDecodeByRegEx(RLE));
+    }
+}