/*
 * PBACS REV3 — Policy-Based Adaptive Constraint System
 * Low-Level Signal Transport Layer (Formal Extractions)
 * 
 * Target: Tang Nano 9K (Gowin GW1NR-9)
 * Specification: linear/RES-2311
 * 
 * Formal Anchor: Semantics.PBACSSignal
 */

module pbacs_rev3 (
    input wire clk,
    input wire reset_n,
    input wire signed [15:0] sample_in,
    output reg b_t,
    output reg [1:0] policy_state,
    output reg [31:0] pressure_out
);

    // L2 Phi-accumulator (Golden Ratio constant: 106070)
    reg [31:0] phi;
    
    // L1 Error accumulator
    reg signed [31:0] error;
    
    // L4 Pressure accumulator (SLUQ)
    reg [31:0] pressure;
    
    // Threshold constant from specification
    localparam signed [31:0] THRESH_POS = 32768;
    localparam signed [31:0] THRESH_NEG = -32768;

    always @(posedge clk or negedge reset_n) begin
        if (!reset_n) begin
            phi <= 0;
            error <= 0;
            pressure <= 0;
            b_t <= 0;
            policy_state <= 2'b00; // CYAN
        end else begin
            // Step 1: Phi increment
            phi <= phi + 32'd106070;
            
            // Step 2: Threshold lookup (MSB-based simplification)
            // L2: theta_t = LUT[phi]
            wire signed [31:0] theta_t = (phi[31]) ? THRESH_POS : THRESH_NEG;
            
            // Step 3 & 4: Error Accumulation and Symbol Decision
            // b_t = (v_t + e_t-1) > theta_t
            if ((sample_in + error) > theta_t) begin
                b_t <= 1'b1;
                error <= sample_in + error - theta_t;
            end else begin
                b_t <= 1'b0;
                error <= sample_in + error;
            end
            
            // Step 5-8: Pressure Management (SLUQ)
            // stress = |e_next|
            wire [31:0] stress = (error[31]) ? -error : error;
            
            // exponential pressure tracking (0.9 decay + 0.1 gain)
            // pressure <= (pressure * 0.9) + (stress * 0.1)
            // Using 10-bit shifts for approximation: (P*921 + S*103) >> 10
            pressure <= (pressure * 32'd921 + stress * 32'd103) >> 10;
            
            // Policy Transition (CMYK)
            if (pressure > 32'd50000)      policy_state <= 2'b11; // BLACK (Halt)
            else if (pressure > 32'd30000) policy_state <= 2'b10; // YELLOW (Warning)
            else if (pressure > 32'd10000) policy_state <= 2'b01; // MAGENTA (Active)
            else                           policy_state <= 2'b00; // CYAN (Idle)
            
            pressure_out <= pressure;
        end
    end

endmodule