use test_bit to determining if a device is a touchscreen (#1389)

selfdrive/common/touch.c

@@ -10,6 +10,13 @@
 
 #include "touch.h"
 
+/* this macro is used to tell if "bit" is set in "array"
+ * it selects a byte from the array, and does a boolean AND
+ * operation with a byte that only has the relevant bit set.
+ * eg. to check for the 12th bit, we do (array[1] & 1<<4)
+ */
+#define test_bit(bit, array)    (array[bit/8] & (1<<(bit%8)))
+
 static int find_dev() {
   int err;
 
@@ -28,10 +35,7 @@ static int find_dev() {
     err = ioctl(fd, EVIOCGBIT(EV_ABS, sizeof(ev_bits)), ev_bits);
     assert(err >= 0);
 
-    const int x_key = ABS_MT_POSITION_X / 8;
-    const int y_key = ABS_MT_POSITION_Y / 8;
-    if ((ev_bits[x_key] & (ABS_MT_POSITION_X - x_key)) &&
-        (ev_bits[y_key] & (ABS_MT_POSITION_Y - y_key))) {
+    if (test_bit(ABS_MT_POSITION_X, ev_bits) && test_bit(ABS_MT_POSITION_Y, ev_bits)) {
       ret = fd;
       break;
     }