// 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:
    Mux16(a=s, b=false, sel=reset, out=s1);

    /* Make sure we only load if we didn't reset. */
    Not(in=reset, out=notreset);
    And(a=notreset, b=load, out=load1);

    Mux16(a=s1, b=in, sel=load1, out=s2);

    /* Make sure we only inc if we didn't reset or load. */
    Not(in=load, out=notload);
    And(a=notreset, b=notload, out=notresetorload);
    And(a=notresetorload, b=inc, out=inc1);

    /* Calculate increment and set depending on inc flag. */
    Inc16(in=s, out=si);
    Mux16(a=s2, b=si, sel=inc1, out=s3);

    /* s3 will have the original value if none of load/inc/reset was set. */
    Register(in=s3, load=true, out=s);

    /* Output nextstate */
    Id(in=s, out=out);
}