/*******************************************************************************
 * Module: scrambler
 * Date: 2015-07-11  
 * Author: Alexey     
 * Description: a scrambler for the link layer
 *
 * Copyright (c) 2015 Elphel, Inc.
 * scrambler.v is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * scrambler.v file is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/> .
 *
 * Additional permission under GNU GPL version 3 section 7:
 * If you modify this Program, or any covered work, by linking or combining it
 * with independent modules provided by the FPGA vendor only (this permission
 * does not extend to any 3-rd party modules, "soft cores" or macros) under
 * different license terms solely for the purpose of generating binary "bitstream"
 * files and/or simulating the code, the copyright holders of this Program give
 * you the right to distribute the covered work without those independent modules
 * as long as the source code for them is available from the FPGA vendor free of
 * charge, and there is no dependence on any encrypted modules for simulating of
 * the combined code. This permission applies to you if the distributed code
 * contains all the components and scripts required to completely simulate it
 * with at least one of the Free Software programs.
 *******************************************************************************/
/*
 * Algorithm is taken from the doc, p.565. TODO make it parallel
 */
// TODO another widths support
module scrambler #(
    parameter DATA_BYTE_WIDTH = 4
)
(
    input   wire                                clk,
    input   wire                                rst,

    input   wire                                val_in,
    input   wire    [DATA_BYTE_WIDTH*8 - 1:0]   data_in,
    output  wire    [DATA_BYTE_WIDTH*8 - 1:0]   data_out
);

reg [15:0]  now;
reg [31:0]  next;

always @ (posedge clk)
    now <= rst ? 16'hf0f6 : val_in ? next[31:16] : now;

assign  data_out = val_in ? data_in ^ next : data_in;

always @ (*)
/*    if (rst)
        next    = 32'h0;
    else*/
    begin
        next[31] = now[12] ^ now[10] ^ now[7] ^ now[3] ^ now[1] ^ now[0];
        next[30] = now[15] ^ now[14] ^ now[12] ^ now[11] ^ now[9] ^ now[6] ^ now[3] ^ now[2] ^ now[0];
        next[29] = now[15] ^ now[13] ^ now[12] ^ now[11] ^ now[10] ^ now[8] ^ now[5] ^ now[3] ^ now[2] ^ now[1];
        next[28] = now[14] ^ now[12] ^ now[11] ^ now[10] ^ now[9] ^ now[7] ^ now[4] ^ now[2] ^ now[1] ^ now[0];
        next[27] = now[15] ^ now[14] ^ now[13] ^ now[12] ^ now[11] ^ now[10] ^ now[9] ^ now[8] ^ now[6] ^ now[1] ^ now[0];
        next[26] = now[15] ^ now[13] ^ now[11] ^ now[10] ^ now[9] ^ now[8] ^ now[7] ^ now[5] ^ now[3] ^ now[0];
        next[25] = now[15] ^ now[10] ^ now[9] ^ now[8] ^ now[7] ^ now[6] ^ now[4] ^ now[3] ^ now[2];
        next[24] = now[14] ^ now[9] ^ now[8] ^ now[7] ^ now[6] ^ now[5] ^ now[3] ^ now[2] ^ now[1];
        next[23] = now[13] ^ now[8] ^ now[7] ^ now[6] ^ now[5] ^ now[4] ^ now[2] ^ now[1] ^ now[0];
        next[22] = now[15] ^ now[14] ^ now[7] ^ now[6] ^ now[5] ^ now[4] ^ now[1] ^ now[0];
        next[21] = now[15] ^ now[13] ^ now[12] ^ now[6] ^ now[5] ^ now[4] ^ now[0];
        next[20] = now[15] ^ now[11] ^ now[5] ^ now[4];
        next[19] = now[14] ^ now[10] ^ now[4] ^ now[3];
        next[18] = now[13] ^ now[9] ^ now[3] ^ now[2];
        next[17] = now[12] ^ now[8] ^ now[2] ^ now[1];
        next[16] = now[11] ^ now[7] ^ now[1] ^ now[0];

        next[15] = now[15] ^ now[14] ^ now[12] ^ now[10] ^ now[6] ^ now[3] ^ now[0];
        next[14] = now[15] ^ now[13] ^ now[12] ^ now[11] ^ now[9] ^ now[5] ^ now[3] ^ now[2];
        next[13] = now[14] ^ now[12] ^ now[11] ^ now[10] ^ now[8] ^ now[4] ^ now[2] ^ now[1];
        next[12] = now[13] ^ now[11] ^ now[10] ^ now[9] ^ now[7] ^ now[3] ^ now[1] ^ now[0];
        next[11] = now[15] ^ now[14] ^ now[10] ^ now[9] ^ now[8] ^ now[6] ^ now[3] ^ now[2] ^ now[0];
        next[10] = now[15] ^ now[13] ^ now[12] ^ now[9] ^ now[8] ^ now[7] ^ now[5] ^ now[3] ^ now[2] ^ now[1];
        next[9] = now[14] ^ now[12] ^ now[11] ^ now[8] ^ now[7] ^ now[6] ^ now[4] ^ now[2] ^ now[1] ^ now[0];
        next[8] = now[15] ^ now[14] ^ now[13] ^ now[12] ^ now[11] ^ now[10] ^ now[7] ^ now[6] ^ now[5] ^ now[1] ^ now[0];
        next[7] = now[15] ^ now[13] ^ now[11] ^ now[10] ^ now[9] ^ now[6] ^ now[5] ^ now[4] ^ now[3] ^ now[0];
        next[6] = now[15] ^ now[10] ^ now[9] ^ now[8] ^ now[5] ^ now[4] ^ now[2];
        next[5] = now[14] ^ now[9] ^ now[8] ^ now[7] ^ now[4] ^ now[3] ^ now[1];
        next[4] = now[13] ^ now[8] ^ now[7] ^ now[6] ^ now[3] ^ now[2] ^ now[0];
        next[3] = now[15] ^ now[14] ^ now[7] ^ now[6] ^ now[5] ^ now[3] ^ now[2] ^ now[1];
        next[2] = now[14] ^ now[13] ^ now[6] ^ now[5] ^ now[4] ^ now[2] ^ now[1] ^ now[0];
        next[1] = now[15] ^ now[14] ^ now[13] ^ now[5] ^ now[4] ^ now[1] ^ now[0];
        next[0] = now[15] ^ now[13] ^ now[4] ^ now[0];
    end


endmodule