feat: Add RLE and Shannon-Fano compression algorithms (#6779)

* feat: Add RLE and Shannon-Fano compression algorithms

* Fix: Resolve CI failures for compression algorithms

* chore: trigger CI rebuild

src/test/java/com/thealgorithms/compression/RunLengthEncodingTest.java

@@ -0,0 +1,90 @@
+package com.thealgorithms.compression;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class RunLengthEncodingTest {
+
+    @Test
+    void testNullInputs() {
+        // Test that a null input to compress returns an empty string
+        assertEquals("", RunLengthEncoding.compress(null));
+
+        // Test that a null input to decompress returns an empty string
+        assertEquals("", RunLengthEncoding.decompress(null));
+    }
+
+    @Test
+    void testCompressionSimple() {
+        // Test a typical string with multiple runs
+        String input = "AAAABBBCCDAA";
+        String expected = "4A3B2C1D2A";
+        assertEquals(expected, RunLengthEncoding.compress(input));
+    }
+
+    @Test
+    void testCompressionWithNoRuns() {
+        // Test a string with no consecutive characters
+        String input = "ABCDE";
+        String expected = "1A1B1C1D1E";
+        assertEquals(expected, RunLengthEncoding.compress(input));
+    }
+
+    @Test
+    void testCompressionEdgeCases() {
+        // Test with an empty string
+        assertEquals("", RunLengthEncoding.compress(""));
+
+        // Test with a single character
+        assertEquals("1A", RunLengthEncoding.compress("A"));
+
+        // Test with a long run of a single character
+        assertEquals("10Z", RunLengthEncoding.compress("ZZZZZZZZZZ"));
+    }
+
+    @Test
+    void testDecompressionSimple() {
+        // Test decompression of a typical RLE string
+        String input = "4A3B2C1D2A";
+        String expected = "AAAABBBCCDAA";
+        assertEquals(expected, RunLengthEncoding.decompress(input));
+    }
+
+    @Test
+    void testDecompressionWithNoRuns() {
+        // Test decompression of a string with single characters
+        String input = "1A1B1C1D1E";
+        String expected = "ABCDE";
+        assertEquals(expected, RunLengthEncoding.decompress(input));
+    }
+
+    @Test
+    void testDecompressionWithMultiDigitCount() {
+        // Test decompression where a run count is greater than 9
+        String input = "12A1B3C";
+        String expected = "AAAAAAAAAAAABCCC";
+        assertEquals(expected, RunLengthEncoding.decompress(input));
+    }
+
+    @Test
+    void testDecompressionEdgeCases() {
+        // Test with an empty string
+        assertEquals("", RunLengthEncoding.decompress(""));
+
+        // Test with a single character run
+        assertEquals("A", RunLengthEncoding.decompress("1A"));
+    }
+
+    @Test
+    void testSymmetry() {
+        // Test that compressing and then decompressing returns the original string
+        String original1 = "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB";
+        String compressed = RunLengthEncoding.compress(original1);
+        String decompressed = RunLengthEncoding.decompress(compressed);
+        assertEquals(original1, decompressed);
+
+        String original2 = "A";
+        assertEquals(original2, RunLengthEncoding.decompress(RunLengthEncoding.compress(original2)));
+    }
+}

src/test/java/com/thealgorithms/compression/ShannonFanoTest.java

@@ -0,0 +1,71 @@
+package com.thealgorithms.compression;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+
+class ShannonFanoTest {
+
+    @Test
+    void testNullInput() {
+        // Test with a null string, should return an empty map
+        assertTrue(ShannonFano.generateCodes(null).isEmpty());
+    }
+
+    @Test
+    void testSimpleString() {
+        // A simple string to test basic code generation
+        String text = "AAABBC";
+        Map<Character, String> codes = ShannonFano.generateCodes(text);
+
+        assertEquals(3, codes.size());
+        assertEquals("0", codes.get('A'));
+        assertEquals("10", codes.get('B'));
+        assertEquals("11", codes.get('C'));
+    }
+
+    @Test
+    void testExampleFromStringIssue() {
+        // Example from the original issue proposal: A:15, B:7, C:6, D:6, E:5
+        // The code finds a more optimal split: {A,B} | {C,D,E} -> |22-17|=5
+        // instead of {A} | {B,C,D,E} -> |15-24|=9.
+        String text = "AAAAAAAAAAAAAAABBBBBBBCCCCCCDDDDDDEEEEE";
+        Map<Character, String> codes = ShannonFano.generateCodes(text);
+
+        assertEquals(5, codes.size());
+        assertEquals("00", codes.get('A'));
+        assertEquals("01", codes.get('B'));
+        assertEquals("10", codes.get('C'));
+        assertEquals("110", codes.get('D'));
+        assertEquals("111", codes.get('E'));
+    }
+
+    @Test
+    void testEdgeCases() {
+        // Test with an empty string
+        assertTrue(ShannonFano.generateCodes("").isEmpty());
+
+        // Test with a single character
+        Map<Character, String> singleCharCodes = ShannonFano.generateCodes("AAAAA");
+        assertEquals(1, singleCharCodes.size());
+        assertEquals("0", singleCharCodes.get('A')); // A single symbol gets code "0"
+
+        // Test with all unique characters
+        String uniqueCharsText = "ABCDEF";
+        Map<Character, String> uniqueCharCodes = ShannonFano.generateCodes(uniqueCharsText);
+        assertEquals(6, uniqueCharCodes.size());
+        // Check that codes are unique and have varying lengths as expected
+        assertEquals(6, uniqueCharCodes.values().stream().distinct().count());
+    }
+
+    @Test
+    void testStringWithTwoChars() {
+        String text = "ABABAB";
+        Map<Character, String> codes = ShannonFano.generateCodes(text);
+
+        assertEquals(2, codes.size());
+        assertTrue(codes.get('A').equals("0") && codes.get('B').equals("1") || codes.get('A').equals("1") && codes.get('B').equals("0"));
+    }
+}