//----------------------------------------------------
// A four level, round-robin arbiter. This was
// orginally coded by WD Peterson in VHDL.
//----------------------------------------------------
module arbiter (
  clk,    
  rst,    
  req3,   
  req2,   
  req1,   
  req0,   
  gnt3,   
  gnt2,   
  gnt1,   
  gnt0   
);
// --------------Port Declaration----------------------- 
input           clk;    
input           rst;    
input           req3;   
input           req2;   
input           req1;   
input           req0;   
output          gnt3;   
output          gnt2;   
output          gnt1;   
output          gnt0;   

//--------------Internal Registers----------------------
wire    [1:0]   gnt       ;   
wire            comreq    ; 
wire            beg       ;
wire   [1:0]    lgnt      ;
wire            lcomreq   ;
reg             lgnt0     ;
reg             lgnt1     ;
reg             lgnt2     ;
reg             lgnt3     ;
reg             lasmask   ;
reg             lmask0    ;
reg             lmask1    ;
reg             ledge     ;

//--------------Code Starts Here----------------------- 
always @ (posedge clk)
if (rst) begin
  lgnt0 <= 0;
  lgnt1 <= 0;
  lgnt2 <= 0;
  lgnt3 <= 0;
end else begin                                     
  lgnt0 <=(~lcomreq & ~lmask1 & ~lmask0 & ~req3 & ~req2 & ~req1 & req0)
        | (~lcomreq & ~lmask1 &  lmask0 & ~req3 & ~req2 &  req0)
        | (~lcomreq &  lmask1 & ~lmask0 & ~req3 &  req0)
        | (~lcomreq &  lmask1 &  lmask0 & req0  )
        | ( lcomreq & lgnt0 );
  lgnt1 <=(~lcomreq & ~lmask1 & ~lmask0 &  req1)
        | (~lcomreq & ~lmask1 &  lmask0 & ~req3 & ~req2 &  req1 & ~req0)
        | (~lcomreq &  lmask1 & ~lmask0 & ~req3 &  req1 & ~req0)
        | (~lcomreq &  lmask1 &  lmask0 &  req1 & ~req0)
        | ( lcomreq &  lgnt1);
  lgnt2 <=(~lcomreq & ~lmask1 & ~lmask0 &  req2  & ~req1)
        | (~lcomreq & ~lmask1 &  lmask0 &  req2)
        | (~lcomreq &  lmask1 & ~lmask0 & ~req3 &  req2  & ~req1 & ~req0)
        | (~lcomreq &  lmask1 &  lmask0 &  req2 & ~req1 & ~req0)
        | ( lcomreq &  lgnt2);
  lgnt3 <=(~lcomreq & ~lmask1 & ~lmask0 & req3  & ~req2 & ~req1)
        | (~lcomreq & ~lmask1 &  lmask0 & req3  & ~req2)
        | (~lcomreq &  lmask1 & ~lmask0 & req3)
        | (~lcomreq &  lmask1 &  lmask0 & req3  & ~req2 & ~req1 & ~req0)
        | ( lcomreq & lgnt3);
end 

//----------------------------------------------------
// lasmask state machine.
//----------------------------------------------------
assign beg = (req3 | req2 | req1 | req0) & ~lcomreq;
always @ (posedge clk)
begin                                     
  lasmask <= (beg & ~ledge & ~lasmask);
  ledge   <= (beg & ~ledge &  lasmask) 
          |  (beg &  ledge & ~lasmask);
end 

//----------------------------------------------------
// comreq logic.
//----------------------------------------------------
assign lcomreq = ( req3 & lgnt3 )
                | ( req2 & lgnt2 )
                | ( req1 & lgnt1 )
                | ( req0 & lgnt0 );

//----------------------------------------------------
// Encoder logic.
//----------------------------------------------------
assign  lgnt =  {(lgnt3 | lgnt2),(lgnt3 | lgnt1)};

//----------------------------------------------------
// lmask register.
//----------------------------------------------------
always @ (posedge clk )
if( rst ) begin
  lmask1 <= 0;
  lmask0 <= 0;
end else if(lasmask) begin
  lmask1 <= lgnt[1];
  lmask0 <= lgnt[0];
end else begin
  lmask1 <= lmask1;
  lmask0 <= lmask0;
end 

assign comreq = lcomreq;
assign gnt    = lgnt;
//----------------------------------------------------
// Drive the outputs
//----------------------------------------------------
assign gnt3   = lgnt3;
assign gnt2   = lgnt2;
assign gnt1   = lgnt1;
assign gnt0   = lgnt0;

endmodule