extmod/utime_mphal: Fix implementation of new semantics of ticks_diff().

Now the function properly uses ring arithmetic to return signed value
in range (inclusive):
[-MICROPY_PY_UTIME_TICKS_PERIOD/2, MICROPY_PY_UTIME_TICKS_PERIOD/2-1].

That means that function can properly process 2 time values away from
each other within MICROPY_PY_UTIME_TICKS_PERIOD/2 ticks, but away in
both directions. For example, if tick value 'a' predates tick value 'b',
ticks_diff(a, b) will return negative value, and positive value otherwise.
But at positive value of MICROPY_PY_UTIME_TICKS_PERIOD/2-1, the result
of the function will wrap around to negative -MICROPY_PY_UTIME_TICKS_PERIOD/2,
in other words, if a follows b in more than MICROPY_PY_UTIME_TICKS_PERIOD/2 - 1
ticks, the function will "consider" a to actually predate b.

1 files changed, 7 insertions, 1 deletions

diff --git a/extmod/utime_mphal.c b/extmod/utime_mphal.c
index 1a32180cf..9aa2f1324 100644
--- a/extmod/utime_mphal.c
+++ b/extmod/utime_mphal.c
@@ -89,7 +89,13 @@ STATIC mp_obj_t time_ticks_diff(mp_obj_t end_in, mp_obj_t start_in) {
     // we assume that the arguments come from ticks_xx so are small ints
     uint32_t start = MP_OBJ_SMALL_INT_VALUE(start_in);
     uint32_t end = MP_OBJ_SMALL_INT_VALUE(end_in);
-    return MP_OBJ_NEW_SMALL_INT((int32_t)(end - start));
+    int32_t diff = end - start;
+    if (diff < (signed)-(MICROPY_PY_UTIME_TICKS_PERIOD / 2)) {
+        diff += MICROPY_PY_UTIME_TICKS_PERIOD;
+    } else if (diff >= (signed)(MICROPY_PY_UTIME_TICKS_PERIOD / 2)) {
+        diff -= MICROPY_PY_UTIME_TICKS_PERIOD;
+    }
+    return MP_OBJ_NEW_SMALL_INT(diff);
 }
 MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_diff_obj, time_ticks_diff);