package calculator.model;

import calculator.model.mode.*;
import java.util.Observable;

/**
 * The Display class keeps in sync with the textBox for display in
 * CalculatorForm. This is managed by making Display a Observable class and
 * CalculatorForm a Observer class. Anytime the buffer is changed by
 * the caller, after done with it, the caller should call doNotify().
 */
public class Display extends Observable
{
    /**
     * Clear the display
     */
    public void clear()
    {
        canChange = true;
        buffer.setLength(0);
        buffer.append(Base.ZERO_DIGIT);
        doNotify();
    }

    /**
     * Set the string as display
     */
    public void setDisplay(String s)
    {
        canChange = false;
        buffer.setLength(0);
        buffer.append(s);
        doNotify();
    }

    /**
     * Append a character.
     */
    public boolean appendChar(char ch)
    {
        if (!canChange)
        {
            return false;
        }

        if (ch != Base.DECIMAL_SEPARATOR && isZero())
        {
            // Clear off the 0 to avoid buffer being 00 or 01.
            // For decimal, it should still be "0.".
            buffer.setLength(0);
        }

        buffer.append(ch);
        doNotify();
        return true;
    }

    /**
     * Remove last char from buffer
     */
    public boolean removeChar()
    {
        if (!canChange)
        {
            return false;
        }

        buffer.setLength(buffer.length() - 1);

        if (buffer.length() == 0)
        {
            // If this was the only char, put back 0
            buffer.append(Base.ZERO_DIGIT);
        }
        else if (buffer.length() == 1 && buffer.charAt(0) == Base.MINUS_SIGN)
        {
            // If only -, make buffer as 0.
            buffer.setCharAt(0, Base.ZERO_DIGIT);
        }

        doNotify();
        return true;
    }

    /**
     * Toggle unary minus sign
     */
    public boolean toggleMinus()
    {
        if (buffer.charAt(0) == Base.MINUS_SIGN)
        {
            // - was there. Remove it
            buffer.deleteCharAt(0);
        }
        else if (!isZero())
        {
            // If number is not 0, add - at start.
            buffer.insert(0, Base.MINUS_SIGN);
        }

        doNotify();
        return true;
    }

    /**
     * Returns the display string.
     */
    public String toString()
    {
        return buffer.toString();
    }

    /**
     * Whether the display can change or not?
     */
    public boolean canChange()
    {
        return canChange;
    }

    /**
     * Fires notification to all observers about the change.
     */
    private void doNotify()
    {
        setChanged();
        notifyObservers(buffer.toString());
    }

    /**
     * Is display 0?
     */
    private boolean isZero()
    {
        // Display is 0 if there is one char and that is 0
        return (buffer.length() == 1 && buffer.charAt(0) == Base.ZERO_DIGIT);
    }

    /**
     * Stores the current display in the calculator
     */
    private StringBuffer buffer = new StringBuffer();

    /**
     * Whether the current display can change
     */
    private boolean canChange;
}