RealGCD.scala

import chisel3._
import chisel3.util.{Valid, DeqIO}

// Problem:
// Implement a GCD circuit (the greatest common divisor of two numbers).
// Input numbers are bundled as 'RealGCDInput' and communicated using the Decoupled handshake protocol
//
class RealGCDInput extends Bundle {
  val a = UInt(16.W)
  val b = UInt(16.W)
}

class RealGCD extends Module {
  val io  = IO(new Bundle {
    val in  = DeqIO(new RealGCDInput())
    val out = Output(Valid(UInt(16.W)))
  })

  val x = Reg(UInt())
  val y = Reg(UInt())
  val p = RegInit(false.B)

  io.in.ready := !p

  when (io.in.valid && !p) {
    x := io.in.bits.a
    y := io.in.bits.b
    p := true.B
  } 

  when (p) {
    when (x > y)  { x := y; y := x } 
    .otherwise    { y := y - x }
  }

  io.out.bits  := x
  io.out.valid := y === 0.U && p
  when (io.out.valid) {
    p := false.B
  }
}