`timescale 1ns/1ps

// =============================================================================
// Multiply a value by a constant
// Use a shift-add algorithm instead of a multiplier
// parameters:
//      -- W : data width
//      -- C : constant
// inout:
//      -- x : input of width W
//      -- y : output of widht 2W
// =============================================================================
module mul_const_shiftadd#(
    parameter integer W   = 16,
    parameter integer C   = 16'sh7fff
)(
    input  wire signed [W-1:0] x,
    output wire signed [2*W-1:0] y
);
    // Sign and magnitude of constant
    localparam integer C_NEG = (C < 0) ? 1 : 0;
    localparam integer C_ABS = (C < 0) ? -C : C;

    // MSB index of |C| (0-based). Keeps network minimal.
    function integer msb_index;
        input integer v;
        integer i;
        begin
            msb_index = -1;
            for (i = 0; i < 32; i = i + 1)
                if (v >> i) msb_index = i;
        end
    endfunction

    localparam integer I_MAX = (C_ABS == 0) ? 0 : msb_index(C_ABS);

    // Width big enough for the largest partial product: W bits shifted by I_MAX
    localparam integer PPW = W + I_MAX + 1;

    // Pre-extend x to PPW so shifts don’t truncate high/sign bits
    wire signed [PPW-1:0] x_ext = {{(PPW-W){x[W-1]}}, x};

    // Partial products (only where C’s bit is 1)
    wire signed [PPW-1:0] part [0:I_MAX];
    genvar i;
    generate
        for (i = 0; i <= I_MAX; i = i + 1) begin : GEN_PARTS
            assign part[i] = C_ABS[i] ? (x_ext <<< i) : {PPW{1'b0}};
        end
    endgenerate

    // Adder chain (you can replace with a balanced tree for speed)
    wire signed [PPW-1:0] sum [0:I_MAX];
    generate
        if (I_MAX == 0) begin
            assign sum[0] = part[0];
        end else begin
            assign sum[0] = part[0];
            for (i = 1; i <= I_MAX; i = i + 1) begin : GEN_SUM
                assign sum[i] = sum[i-1] + part[i];
            end
        end
    endgenerate

    // Apply sign of C
    wire signed [PPW-1:0] mag  = (I_MAX == 0) ? part[0] : sum[I_MAX];
    wire signed [PPW-1:0] prod = C_NEG ? -mag : mag;

    // Stretch/extend to 2W result width
    assign y = {{((2*W)-PPW){prod[PPW-1]}}, prod};
endmodule