class Interpolator

class Interpolator {
    
    /**
     * The character sequence that identifies the start of a token.
     *
     * @var string
     */
    protected $startToken;
    
    /**
     * The character sequence that identifies the end of a token.
     *
     * @var string
     */
    protected $endToken;
    
    /**
     * The associative array of replacements.
     *
     * @var array
     */
    protected $data = [];
    
    /**
     * Interpolator constructor.
     *
     * @param string $start_token
     *   The start marker for a token, e.g. '['.
     * @param string $end_token
     *   The end marker for a token, e.g. ']'.
     */
    public function __construct($start_token = '\\[', $end_token = '\\]') {
        $this->startToken = $start_token;
        $this->endToken = $end_token;
    }
    
    /**
     * Sets the data set to use when interpolating.
     *
     * @param array $data
     *   The key:value pairs to use when interpolating.
     *
     * @return $this
     */
    public function setData(array $data) {
        $this->data = $data;
        return $this;
    }
    
    /**
     * Adds to the data set to use when interpolating.
     *
     * @param array $data
     *   The key:value pairs to use when interpolating.
     *
     * @return $this
     */
    public function addData(array $data) {
        $this->data = array_merge($this->data, $data);
        return $this;
    }
    
    /**
     * Replaces tokens in a string with values from an associative array.
     *
     * Tokens are surrounded by delimiters, e.g. square brackets "[key]". The
     * characters that surround the key may be defined when the Interpolator is
     * constructed.
     *
     * Example:
     * If the message is 'Hello, [user.name]', then the value of the user.name
     * item is fetched from the array, and the token [user.name] is replaced with
     * the result.
     *
     * @param string $message
     *   Message containing tokens to be replaced.
     * @param array $extra
     *   Data to use for interpolation in addition to whatever was provided to
     *   self::setData().
     * @param string|bool $default
     *   (optional) The value to substitute for tokens that are not found in the
     *   data. If FALSE, then missing tokens are not replaced. Defaults to an
     *   empty string.
     *
     * @return string
     *   The message after replacements have been made.
     */
    public function interpolate($message, array $extra = [], $default = '') {
        $data = $extra + $this->data;
        $replacements = $this->replacements($message, $data, $default);
        return strtr($message, $replacements);
    }
    
    /**
     * Finds the tokens that exist in a message and builds a replacement array.
     *
     * All of the replacements in the data array are looked up given the token
     * keys from the provided message. Keys that do not exist in the configuration
     * are replaced with the default value.
     *
     * @param string $message
     *   String with tokens.
     * @param array $data
     *   Data to use for interpolation.
     * @param string $default
     *   (optional) The value to substitute for tokens that are not found in the
     *   data. If FALSE, then missing tokens are not replaced. Defaults to an
     *   empty string.
     *
     * @return string[]
     *   An array of replacements to make. Keyed by tokens and the replacements
     *   are the values.
     */
    protected function replacements($message, array $data, $default = '') {
        $tokens = $this->findTokens($message);
        $replacements = [];
        foreach ($tokens as $sourceText => $key) {
            $replacement_text = array_key_exists($key, $data) ? $data[$key] : $default;
            if ($replacement_text !== FALSE) {
                $replacements[$sourceText] = $replacement_text;
            }
        }
        return $replacements;
    }
    
    /**
     * Finds all of the tokens in the provided message.
     *
     * @param string $message
     *   String with tokens.
     *
     * @return string[]
     *   map of token to key, e.g. {{key}} => key
     */
    protected function findTokens($message) {
        $reg_ex = '#' . $this->startToken . '([a-zA-Z0-9._-]+)' . $this->endToken . '#';
        if (!preg_match_all($reg_ex, $message, $matches, PREG_SET_ORDER)) {
            return [];
        }
        $tokens = [];
        foreach ($matches as $matchSet) {
            [
                $sourceText,
                $key,
            ] = $matchSet;
            $tokens[$sourceText] = $key;
        }
        return $tokens;
    }

}