tester: add very basic mock structs for testing i2c devices, based on work done by @rogpeppe

Signed-off-by: deadprogram <ron@hybridgroup.com>

tester/device.go

@@ -0,0 +1,58 @@
+package tester
+
+import (
+	qt "github.com/frankban/quicktest"
+)
+
+const maxRegisters = 200
+
+// I2CDevice represents a mock I2C device on a mock I2C bus.
+type I2CDevice struct {
+	C    *qt.C
+	Addr uint8
+	// Registers holds the device registers. It can be inspected
+	// or changed as desired for testing.
+	Registers [maxRegisters]uint8
+	// If Err is non-nil, it will be returned as the error from the
+	// I2C methods.
+	Err error
+}
+
+// NewI2CDevice returns a new mock I2C device.
+func NewI2CDevice(c *qt.C, addr uint8) *I2CDevice {
+	return &I2CDevice{
+		C:    c,
+		Addr: addr,
+	}
+}
+
+// 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:])
+	return nil
+}
+
+// WriteRegister implements I2C.WriteRegister.
+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)
+	return nil
+}
+
+// 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) {
+		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) {
+		d.C.Fatalf("register read/write [%#x, %#x] end out of range", r, int(r)+len(buf))
+	}
+}