RunLengthEncoder.java

public class RunLengthEncoder {

    /**
     * Encodes a string using run-length encoding.
     *
     * Consecutive repeated characters are replaced by the count followed by
     * the character. A count of 1 is still written explicitly.
     *
     * Examples:
     *   "aaabbc"  -> "3a2b1c"
     *   "abcd"    -> "1a1b1c1d"
     *   "aabbcc"  -> "2a2b2c"
     *   "aaaaaaaaaaaab" -> "12a1b"   (counts can be more than one digit)
     *   ""        -> ""
     *
     * @param input the string to encode (non-null)
     * @return the run-length encoded string
     */
    public static String encode(String input) {
        String result = "";

        for(int i=0;i<input.length();i++) {
            char c = input.charAt(i);
            if (String.valueOf(c).matches("[A-Za-z]")) {
                int count = 1;
                while(i<input.length()-1) {
                    if (input.charAt(i + 1) == c) {
                        count++;
                        i++;
                    } else {
                        break;
                    }
                }
                result += String.valueOf(count) + String.valueOf(c);
            }
        }

        return result;
    }

    /**
     * Decodes a run-length encoded string back to its original form.
     *
     * Each token is a number followed by a character. The number may be
     * more than one digit.
     *
     * Examples:
     *   "3a2b1c"  -> "aaabbc"
     *   "1a1b1c1d" -> "abcd"
     *   "12a1b"   -> "aaaaaaaaaaaab"
     *   ""        -> ""
     *
     * @param input the encoded string to decode (non-null)
     * @return the decoded original string
     */
    public static String decode(String input) {
        // Define result
        // iterate over input
        // if it's a number, look at subsequent chars to see if they are also numbers until you hit a letter
        // put the digits together and parse them, then add that many of the letter to the result string
        String result = "";

        for(int i=0;i<input.length();i++) {
            char c = input.charAt(i);
            String num = String.valueOf(c);
            if (num.matches("[0-9]")) {
                while(i<input.length()-1) {
                    String nextC = String.valueOf(input.charAt(i+1));
                    if (nextC.matches("[0-9]")) {
                        num += nextC;
                        i++;
                    } else if (nextC.matches("[a-zA-Z]")) {
                        result += nextC.repeat(Integer.parseInt(num));
                        i++;
                        break;
                    }
                }
            }
        }

        return result;
    }

    // -------------------------------------------------------------------------
    // Test runner
    // -------------------------------------------------------------------------

    public static void main(String[] args) {
        int passed = 0, failed = 0;

        Object[][] encodeTests = {
            // { input, expected }
            { "aaabbc",          "3a2b1c"       },
            { "abcd",            "1a1b1c1d"     },
            { "aabbcc",          "2a2b2c"       },
            { "aaaaaaaaaaaab",   "12a1b"        },  // count > 9
            { "a",               "1a"           },  // single char
            { "aaa",             "3a"           },  // all same
            { "",                ""             },  // empty
        };

        System.out.println("=== encode ===");
        for (Object[] test : encodeTests) {
            String input    = (String) test[0];
            String expected = (String) test[1];
            String actual   = encode(input);
            boolean ok      = expected.equals(actual);
            System.out.printf("  %-20s -> %-15s %s%n",
                "\"" + input + "\"",
                actual == null ? "null" : "\"" + actual + "\"",
                ok ? "PASS" : "FAIL (expected \"" + expected + "\")");
            if (ok) passed++; else failed++;
        }

        Object[][] decodeTests = {
            // { input, expected }
            { "3a2b1c",          "aaabbc"             },
            { "1a1b1c1d",        "abcd"               },
            { "2a2b2c",          "aabbcc"             },
            { "12a1b",           "aaaaaaaaaaaab"      },  // count > 9
            { "1a",              "a"                  },  // single char
            { "3a",              "aaa"                },  // all same
            { "",                ""                   },  // empty
        };

        System.out.println("=== decode ===");
        for (Object[] test : decodeTests) {
            String input    = (String) test[0];
            String expected = (String) test[1];
            String actual   = decode(input);
            boolean ok      = expected.equals(actual);
            System.out.printf("  %-20s -> %-25s %s%n",
                "\"" + input + "\"",
                actual == null ? "null" : "\"" + actual + "\"",
                ok ? "PASS" : "FAIL (expected \"" + expected + "\")");
            if (ok) passed++; else failed++;
        }

        System.out.printf("%nResults: %d passed, %d failed%n", passed, failed);
    }
}