How to simplify inputs for a decoder in verilog hdl - boolean

Question

I'm making a decoder for an FPGA. The verilog code compiles, but the switches don't do anything. I quadruple-checked the pin assignments, and they are correct, so I guess there are some logic problems with my code.
The truth table is:
enable p a1 a0 y0 y1 y2 y3
0 0 0 0 1 0 0 0
0 0 0 1 0 1 0 0
0 0 1 0 0 0 1 0
0 0 1 1 0 0 0 1
0 1 0 0 0 1 1 1
0 1 0 1 1 0 1 1
0 1 1 0 1 1 0 1
0 1 1 1 1 1 1 0
1 x x x x z z z
This is my code:
module decoder2x4(
input enable,
input p,
input [1:0] a,
output [3:0] y
);
assign y[0] = (( ~p & ~a[1] & ~a[0]) | (~a[1] & a[0]) |(a[1] & ~a[0]) | ( p & a[1] & a[0]) ) & ~enable;
assign y[1] = (( ~p & ~a[1] & a[0]) | ( p & ~a[1] & ~a[0]) |( p & a[1] & ~a[0])| ( p & a[1] & a[0])) & ~enable;
assign y[2] = (( ~p & a[1] & ~a[0]) | ( p & ~a[1] & ~a[0]) |( p & ~a[1] & a[0])| ( p & a[1] & a[0])) & ~enable;
assign y[3] = (( ~p & a[1] & a[0]) | ( p & ~a[1] & ~a[0]) |( p & ~a[1] & a[0])| ( p & a[1] & ~a[0])) & ~enable;
endmodule

Answer 1

That is not the way you use HDL code, unless it is a school assignment.
I have copied this from the work Pierre.Vriens did to you table:
enable p a1 a0 y0 y1 y2 y3
0 0 0 0 1 0 0 0
0 0 0 1 0 1 0 0
0 0 1 0 0 0 1 0
0 0 1 1 0 0 0 1
0 1 0 0 0 1 1 1
0 1 0 1 1 0 1 1
0 1 1 0 1 1 0 1
0 1 1 1 1 1 1 0
1 x x x x z z z <<< Probably a typo here
The bottom line is a bit special so I use an if for that. The reset we can put in a case:
always #( * )
begin
if (enable)
{y0, y1, y2, y3} = 4'hz; // **
else
case ( {p, a1, a0 } )
3'b000 : {y0, y1, y2, y3} = 4'b1000;
3'b001 : {y0, y1, y2, y3} = 4'b0100;
3'b010 : {y0, y1, y2, y3} = 4'b0010;
3'b011 : {y0, y1, y2, y3} = 4'b0001;
3'b100 : {y0, y1, y2, y3} = 4'b0111;
3'b101 : {y0, y1, y2, y3} = 4'b1011;
3'b110 : {y0, y1, y2, y3} = 4'b1101;
3'b111 : {y0, y1, y2, y3} = 4'b1110;
endcase
end // always
Note that this copies the orignal table as much as possible, which reduced the possibilities of errors. An editor with column editing helps a lot!
** I assume you want all outputs to be 'z'. In your table you have xzzz.