mfrc522 - Fix 4-byte tag UIDs (#22)

4-byte UIDs incorrectly included a 5th csum byte. 7-byte UIDs were unaffected.

mfrc522/mfrc522.go

@@ -26,6 +26,7 @@ type Dev struct {
 	operationTimeout time.Duration
 	beforeCall       func()
 	afterCall        func()
+	bogusUID         bool
 }
 
 // Key is the access key that consists of 6 bytes. There could be two types of keys - keyA and keyB.
@@ -40,10 +41,22 @@ type config struct {
 	defaultTimeout time.Duration
 	beforeCall     func()
 	afterCall      func()
+	bogusUID       bool
 }
 
 type configF func(*config) *config
 
+// WithBogusUID sets the card reader to return incorrect 4-byte UIDs. In
+// version 3.6.12 and earlier this package ruturned 5-bytes for tags with a
+// 4-byte UID with bytes 0 to 3 being the correct UID and byte 4 being an XOR
+// of bytes 0 to 3. 7-byte UIDs are correct regardless of this configuration.
+func WithBogusUID() configF {
+	return func(c *config) *config {
+		c.bogusUID = false
+		return c
+	}
+}
+
 // WithTimeout updates the default device-wide configuration timeout.
 func WithTimeout(timeout time.Duration) configF {
 	return func(c *config) *config {
@@ -70,11 +83,13 @@ func noop() {}
 //  spiPort     the SPI device to use.
 //  resetPin    reset GPIO pin.
 //  irqPin      irq GPIO pin.
+//  configs     configuration options
 func NewSPI(spiPort spi.Port, resetPin gpio.PinOut, irqPin gpio.PinIn, configs ...configF) (*Dev, error) {
 	cfg := &config{
 		defaultTimeout: 30 * time.Second,
 		beforeCall:     noop,
 		afterCall:      noop,
+		bogusUID:       true,
 	}
 	for _, cf := range configs {
 		cfg = cf(cfg)
@@ -92,6 +107,7 @@ func NewSPI(spiPort spi.Port, resetPin gpio.PinOut, irqPin gpio.PinIn, configs .
 		operationTimeout: cfg.defaultTimeout,
 		beforeCall:       cfg.beforeCall,
 		afterCall:        cfg.afterCall,
+		bogusUID:         cfg.bogusUID,
 	}
 	return dev, nil
 }
@@ -123,7 +139,7 @@ func (r *Dev) SetAntennaGain(gain int) error {
 	return nil
 }
 
-// ReadUID reads the card UID with IRQ event timeout.
+// ReadUID reads the 4-byte or 7-byte card UID with IRQ event timeout.
 //
 //  timeout   the operation timeout
 func (r *Dev) ReadUID(timeout time.Duration) (uid []byte, err error) {
@@ -410,10 +426,17 @@ func (r *Dev) selectCard(timeout time.Duration) ([]byte, error) {
 
 		uuid = uuid[1 : len(uuid)-1]
 		uuid = append(uuid, uuidEnd[:len(uuidEnd)-1]...)
+
+		return uuid, nil
 	}
+
+	if r.bogusUID {
 		return uuid, nil
 	}
 
+	return uuid[:len(uuid)-1], nil
+}
+
 // write  writes the data block into the card at given block address.
 //
 // blockAddr - the calculated block address