mcp23017: use new tester package

Also change the `tester` package slightly to use an exposed `Registers`
array rather adding yet another accessor method to retrieve a register
value. This seems to me more transparent and "obvious" - we aren't trying
to hide the fact that there's just a simple memory store there.
Also unexport the `assertRegisterRange` method which was never intended
to be part of the public API.

tester/device.go

@@ -10,7 +10,7 @@ type I2CDevice struct {
 	addr uint8
 	// Registers holds the device registers. It can be inspected
 	// or changed as desired for testing.
-	registers [MaxRegisters]uint8
+	Registers [MaxRegisters]uint8
 	// If Err is non-nil, it will be returned as the error from the
 	// I2C methods.
 	Err error
@@ -29,35 +29,13 @@ func (d *I2CDevice) Addr() uint8 {
 	return d.addr
 }
 
-// SetupRegisters sets all of the Device registers.
-// It is intended to be used when setting up a fake device
-// for testing expected vs. actual values.
-func (d *I2CDevice) SetupRegisters(regs []uint8) {
-	if len(regs) > MaxRegisters {
-		panic("exceeded maximum number of registers for fake device")
-	}
-	for k, v := range regs {
-		d.registers[k] = v
-	}
-}
-
-// SetupRegister sets one of the Device registers.
-// It is intended to be used when setting up a fake device
-// for testing expected vs. actual values.
-func (d *I2CDevice) SetupRegister(r, v uint8) {
-	if r > MaxRegisters {
-		panic("exceeded maximum number of registers for fake device")
-	}
-	d.registers[r] = v
-}
-
 // ReadRegister implements I2C.ReadRegister.
 func (d *I2CDevice) ReadRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
-	d.AssertRegisterRange(r, buf)
-	copy(buf, d.registers[r:])
+	d.assertRegisterRange(r, buf)
+	copy(buf, d.Registers[r:])
 	return nil
 }
 
@@ -66,18 +44,18 @@ func (d *I2CDevice) WriteRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
-	d.AssertRegisterRange(r, buf)
-	copy(d.registers[r:], buf)
+	d.assertRegisterRange(r, buf)
+	copy(d.Registers[r:], buf)
 	return nil
 }
 
-// AssertRegisterRange asserts that reading or writing the given
+// assertRegisterRange asserts that reading or writing the given
 // register and subsequent registers is in range of the available registers.
-func (d *I2CDevice) AssertRegisterRange(r uint8, buf []byte) {
-	if int(r) >= len(d.registers) {
+func (d *I2CDevice) assertRegisterRange(r uint8, buf []byte) {
+	if int(r) >= len(d.Registers) {
 		d.c.Fatalf("register read/write [%#x, %#x] start out of range", r, int(r)+len(buf))
 	}
-	if int(r)+len(buf) > len(d.registers) {
+	if int(r)+len(buf) > len(d.Registers) {
 		d.c.Fatalf("register read/write [%#x, %#x] end out of range", r, int(r)+len(buf))
 	}
 }

tester/i2c.go

@@ -1,5 +1,7 @@
 package tester
 
+import "fmt"
+
 // I2CBus implements the I2C interface in memory for testing.
 type I2CBus struct {
 	c       Failer
@@ -16,10 +18,24 @@ func NewI2CBus(c Failer) *I2CBus {
 }
 
 // AddDevice adds a new mock device to the mock I2C bus.
+// It panics if a device with the same address is added more than once.
 func (bus *I2CBus) AddDevice(d *I2CDevice) {
+	for _, dev := range bus.devices {
+		if dev.Addr() == d.addr {
+			panic(fmt.Errorf("device already added at address %#x", d))
+		}
+	}
 	bus.devices = append(bus.devices, d)
 }
 
+// NewDevice creates a new device with the given address
+// and adds it to the mock I2C bus.
+func (bus *I2CBus) NewDevice(addr uint8) *I2CDevice {
+	dev := NewI2CDevice(bus.c, addr)
+	bus.AddDevice(dev)
+	return dev
+}
+
 // ReadRegister implements I2C.ReadRegister.
 func (bus *I2CBus) ReadRegister(addr uint8, r uint8, buf []byte) error {
 	return bus.FindDevice(addr).ReadRegister(r, buf)