little math puzzle. Map 1 0 -1 to 1 0.5 0

[pier4r]

Similar to "which kind of programmer are you": http://holyjoe.net/hp/flash07.txt
(there the e^x solution was quite neat)

One get in input a value that can be 1 or 0 or -1 .
The objective is to map those input to:
1 -> 1
0 -> 0.5
-1 -> 0

Of course, the first approach is a "switch case" block. Other ideas? (note that the first range is scaled to the second, but one ending point stays the same)