// Creates the Calculator Object
function Calculator() {
    // Set current and buffer values to 0
    this.current = 0;
    this.buffer = "0";
    // Set previous operator to null
    this.previousOperator = null;
    // When button is clicked, if not a number, buffer an operator
    // if a number, buffer the operand.
    this.buttonClicked = ( value ) => {
        if (isNaN(parseInt(value))) {
            calculator.bufferOperator(value);
        } else {
            calculator.bufferOperand(value);
        }
        // Push to value
        this.renderResult();
    }
    // Function to buffer an operator given a value
    this.bufferOperator = (value) =>
    {
        switch( value ) {
            // If the value is Clear, reset all values to default.
            case "C":
                this.buffer = "0";
                this.current = 0;
                this.previousOperator = null;
                break;
            // If the value is equals, set previous operator to null,
            // flush the operation using the typecasted buffer, and set
            // buffer to the current value in order to continue operations.
            // Finally set current value and previous operator to default.
            case "=":
                if( this.previousOperator === null ) {
                    return;
                }
                this.flushOperation( parseInt( this.buffer ));
                this.buffer = "" + this.current;
                this.previousOperator = null;
                this.current = 0;
                break;
            // If value is backspace, delete the last digit in the buffer.
            case "←":
                if( this.buffer.length === 1 ) {
                    this.buffer = 0;
                }
                else {
                    this.buffer = this.buffer.substring( 0, this.buffer.length - 1 );
                }
                break;
            default:
                this.calculate( value );
        }
    }
    // Adds operand, given a value, to the buffer.
    this.bufferOperand = ( value ) =>
    {
        if( this.buffer === "0" )
        {
            this.buffer = value;
        }
        else
        {
            this.buffer += value;
        }
    }
    // Performs operator functions depending on which operator was clicked.
    // Given a tempBuffer, perform the corresponding operation to the current value.
    this.flushOperation = ( tempBuffer ) => {
        if( this.previousOperator === "+" ) { this.current += tempBuffer}
        else if( this.previousOperator === "-" ) { this.current -= tempBuffer}
        else if( this.previousOperator === "x" ) { this.current *= tempBuffer}
        else { this.current /= tempBuffer}
    }
    // Given a value, create a temporary buffer equal to current buffer,
    // perform the corresponding operation, and reset operator and buffer to
    // default values.
    this.calculate = ( value ) => {
        const tempBuffer = parseInt( this.buffer );
        if ( this.current === 0 ) {
            this.current = tempBuffer;
        }
        else {
            this.flushOperation(tempBuffer)
        }
        this.previousOperator = value;
        this.buffer = "0";
    }
    // Exclusively called when a button is clicked, display a value
    // rounded to 4 decimal places in the result box.
    this.renderResult = () => {
        // Note: this is the proper way to round to decimal places. Math.round( ( x * 100 ) / 100 )
        // will almost always give rounding errors
        result.value = +(Math.round(this.buffer + 'e+4') + 'e-4');
    }
}

// Instantiate Calculator
let calculator = new Calculator();

// Create constant for result display HTML object
const result = document.querySelector("#result");

// Create Event Listener for button
document.querySelector( '.buttons' ).addEventListener( 'click', function( event ) {
    // Pass the value of the HTML button to the buttonClicked() function.
    calculator.buttonClicked( event.target.innerHTML );
});