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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.
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‎extmod/utime_mphal.c‎

Lines changed: 7 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -89,7 +89,13 @@ STATIC mp_obj_t time_ticks_diff(mp_obj_t end_in, mp_obj_t start_in) {
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// we assume that the arguments come from ticks_xx so are small ints
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uint32_t start = MP_OBJ_SMALL_INT_VALUE(start_in);
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uint32_t end = MP_OBJ_SMALL_INT_VALUE(end_in);
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return MP_OBJ_NEW_SMALL_INT((int32_t)(end - start));
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int32_t diff = end - start;
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if (diff < (signed)-(MICROPY_PY_UTIME_TICKS_PERIOD / 2)) {
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diff += MICROPY_PY_UTIME_TICKS_PERIOD;
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} else if (diff >= (signed)(MICROPY_PY_UTIME_TICKS_PERIOD / 2)) {
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diff -= MICROPY_PY_UTIME_TICKS_PERIOD;
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}
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return MP_OBJ_NEW_SMALL_INT(diff);
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}
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MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_diff_obj, time_ticks_diff);
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