Method decode

src/main/java/com/thealgorithms/conversions/Base64.java:106–163

Decodes the given Base64 encoded string to a byte array. @param input the Base64 encoded string to decode @return the decoded byte array @throws IllegalArgumentException if input is null or contains invalid Base64 characters

(String input)

     * @throws IllegalArgumentException if input is null or contains invalid Base64 characters
     */
    public static byte[] decode(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Input cannot be null");
        }

        if (input.isEmpty()) {
            return new byte[0];
        }

        // Strict RFC 4648 compliance: length must be a multiple of 4
        if (input.length() % 4 != 0) {
            throw new IllegalArgumentException("Invalid Base64 input length; must be multiple of 4");
        }

        // Validate padding: '=' can only appear at the end (last 1 or 2 chars)
        int firstPadding = input.indexOf('=');
        if (firstPadding != -1) {
            if (firstPadding < input.length() - 2) {
                throw new IllegalArgumentException("Padding '=' can only appear at the end (last 1 or 2 characters)");
            }
            for (int i = firstPadding; i < input.length(); i++) {
                if (input.charAt(i) != '=') {
                    throw new IllegalArgumentException("A padding '=' must not be followed by a non-padding character");
                }
            }
        }

        List<Byte> result = new ArrayList<>();

        // Process input in groups of 4 characters
        for (int i = 0; i < input.length(); i += 4) {
            // Get up to 4 characters
            int char1 = getBase64Value(input.charAt(i));
            int char2 = getBase64Value(input.charAt(i + 1));
            int char3 = input.charAt(i + 2) == '=' ? 0 : getBase64Value(input.charAt(i + 2));
            int char4 = input.charAt(i + 3) == '=' ? 0 : getBase64Value(input.charAt(i + 3));

            // Combine four 6-bit groups into a 24-bit number
            int combined = (char1 << 18) | (char2 << 12) | (char3 << 6) | char4;

            // Extract three 8-bit bytes
            result.add((byte) ((combined >> 16) & 0xFF));
            if (input.charAt(i + 2) != '=') {
                result.add((byte) ((combined >> 8) & 0xFF));
            }
            if (input.charAt(i + 3) != '=') {
                result.add((byte) (combined & 0xFF));
            }
        }

        // Convert List<Byte> to byte[]
        byte[] resultArray = new byte[result.size()];
        for (int i = 0; i < result.size(); i++) {
            resultArray[i] = result.get(i);
        }

        return resultArray;
    }