Calculating the convolution of a piecewise function 
Let
$$f(x) = \begin{cases} \frac{1}{2},  & \text{if $\rvert x\lvert \le 1$
 } \\ 0, & \text{otherwise} \end{cases}$$
I want to calculate the convolution of $f$ with itself.

I am given the following formula:
$$f*g=\int_{-\infty}^{+\infty} f(y)g(x-y) dy$$
so, $ \space f*f=\int_{-\infty}^{+\infty} f(y)f(x-y) dy$. How do I evaluate this integral? 
While doing some research online I found that one can calculate the convolution by using the fourier-transform. $$\mathcal F(f(x)f(x))=\frac{1}{\sqrt{2 \pi}} \hat{f}(k) *\hat{f}(k)$$
The problem with using this method is that I don't know how to multiply a piecewise function with itself. Would it just be:
$$f(x) =
\begin{cases}
\color{red}{\frac{1}{4}},  & \text{if $\rvert x\lvert \le 1$ } \\
0, & \text{otherwise}
\end{cases}$$
or am I doing something wrong here?
 A: Use the original definition:
$$\space f*f=\int_{-\infty}^{+\infty} f(y)f(x-y) dy$$ 
When $|y|>-1$, $f(y)=0$, otherwise it is $\frac{1}{2}$. So the integral is reduced to
$$\frac{1}{2}\int_{-1}^{1} f(x-y) dy$$
Now substitute $t=x-y$,
$$\frac{1}{2}\int_{x-1}^{x+1} f(t) dt$$
It is easy to see when $x\leq -2$ or $x\geq 2$, $f(t)$ is entirely $0$, so this function is equal to $0$ in those intervals. Between $-2$ and $2$, $f(t)$ is $\frac{1}{2}$ in the green region:

In this picture, the upper boundary of the green region is $y=x+1$, the lower boundary is $x-1$. The height of the green region is the distance we want. So you can see that the integral is equal to 
$$(x+1)-(-1)=x+2, \quad\quad -2\leq x\leq 0\\
1-(x-1)=2-x, \quad\quad 0\leq x\leq 2$$
And this is the required function after multiplying by $\frac{1}{4}$.
A: We can use the unit step function $u(x)$ defined as
$$
u(x)=\begin{cases}
1&,x>0\\\\
0&,x<0
\end{cases}$$
to write $f(x)$ as $f(x)=\frac12 (u(x+1)-u(x-1))$.  
Then, the convolution $(f*f)$ is given by
$$\begin{align}
(f*f)(x)&=\int_{-\infty}^\infty f(y)\,f(x-y)\,dy\\\\
&=\frac14\int_{-\infty}^\infty (u(y+1)-u(y-1))(u(x-y+1)-u(x-y-1))\,dy\\\\
&=\frac14 \int_{-1}^{1}(u(x-y+1)-u(x-y-1))\,dy\\\\
&=\frac14\left(\int_{-1}^{x+1} 1\,dy\right)(u(x+2)-u(x))+\frac12u(x)\\\\
&-\frac14\left(\int_{-1}^{x-1} 1\,dy\right)(u(x)-u(x-2))-\frac12u(x-2)\\\\
&=\frac14(x+2)(u(x+2)-u(x))+\frac12 u(x)\\\\
&-\frac14(x)(u(x)-u(x-2))-\frac12u(x-2)\\\\
&=\frac14(x+2)(u(x+2)-u(x))+\frac14(2-x)(u(x)-u(x-2))\\\\
\end{align}$$
Therefore, the convolution $f*f$ is 
$$
(f*f)(x)=
\begin{cases}
x+2&,-2\le x\le 0\\\\
2-x&,0\le x\le 2\\\\
0&,\text{elsewhere}
\end{cases}
$$
