// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/03/a/PC.hdl

/**
 * A 16-bit counter with load and reset control bits.
 * if      (reset[t] == 1) out[t+1] = 0
 * else if (load[t] == 1)  out[t+1] = in[t]
 * else if (inc[t] == 1)   out[t+1] = out[t] + 1  (integer addition)
 * else                    out[t+1] = out[t]
 */

CHIP PC {
    IN in[16],load,inc,reset;
    OUT out[16];

    PARTS:
    // Put your code here:
    
    //全部0にする
    And16(a[0..15]=in[0..15], b[0..15]=false, out=wall0);
    
    //インクリメント(winc)
    Inc16(in[0..15]=ret, out=winc);
    
    //resetが1のときは、wall0を使う。loadが1のときは、inを使う。incが1のときはwincを使う。そうでなければw2が0となり更新されない
    //実際にはクレノー図
    //reset, load, inc
    //1,?,? = 00 (reset)
    //0,1,? = 01 (load)
    //0,0,1 = 10 (inc)
    //0,0,0 = 11 (---)
    //resetかloadが1の場合はsel[1]は必ず0
    Or(a=reset, b=load, out=nsel1);
    //sel1がsel[1]
    Not(in=nsel1, out=sel1);
    //0,1,0
    //0,1,1
    //0,0,0
    //上記3つのパターンの時のみsel[0]は1 nreset and ((load and ninc) or (load and inc) or (nload and ninc))
    Not(in=load, out=nload);
    Not(in=inc, out=ninc);
    Not(in=reset, out=nreset);
    And(a=load, b=ninc, out=wand1);
    And(a=load, b=inc, out=wand2);
    And(a=nload, b=ninc, out=wand3);
    Or(a=wand1, b=wand2, out=wor1);
    Or(a=wand3, b=wor1, out=wor2);
    And(a=nreset, b=wor2, out=sel0);
    
    Mux4Way16(a[0..15]=wall0, b[0..15]=in[0..15], c[0..15]=winc, d[0..15]=ret, sel[0]=sel0, sel[1]=sel1, out=wreg);
    
    //loadやincやresetが1の場合は更新
    Or(a=load, b=inc, out=w1);
    Or(a=reset, b=w1, out=w2);
    Register(in[0..15]=wreg, load=w2, out[0..15]=out[0..15], out=ret);
}