Enumerate all cases in the overflow detection strategy in mpd_qlog10().
diff --git a/Modules/_decimal/libmpdec/mpdecimal.c b/Modules/_decimal/libmpdec/mpdecimal.c
index b21563a..1fc3cb9 100644
--- a/Modules/_decimal/libmpdec/mpdecimal.c
+++ b/Modules/_decimal/libmpdec/mpdecimal.c
@@ -4793,12 +4793,25 @@
mpd_qfinalize(result, &workctx, status);
return;
}
- /* Check if the result will overflow.
+ /*
+ * Check if the result will overflow (0 < x, x != 1):
+ * 1) log10(x) < 0 iff adjexp(x) < 0
+ * 2) 0 < x /\ x <= y ==> adjexp(x) <= adjexp(y)
+ * 3) adjexp(x) <= log10(x) < adjexp(x) + 1
*
- * 1) adjexp(a) + 1 > log10(a) >= adjexp(a)
+ * Case adjexp(x) >= 0:
+ * 4) adjexp(x) <= abs(log10(x))
+ * Case adjexp(x) > 0:
+ * 5) adjexp(adjexp(x)) <= adjexp(abs(log10(x)))
+ * Case adjexp(x) == 0:
+ * mpd_exp_digits(t)-1 == 0 <= emax (the shortcut is not triggered)
*
- * 2) |log10(a)| >= adjexp(a), if adjexp(a) >= 0
- * |log10(a)| > -adjexp(a)-1, if adjexp(a) < 0
+ * Case adjexp(x) < 0:
+ * 6) -adjexp(x) - 1 < abs(log10(x))
+ * Case adjexp(x) < -1:
+ * 7) adjexp(-adjexp(x) - 1) <= adjexp(abs(log(x)))
+ * Case adjexp(x) == -1:
+ * mpd_exp_digits(t)-1 == 0 <= emax (the shortcut is not triggered)
*/
adjexp = mpd_adjexp(a);
t = (adjexp < 0) ? -adjexp-1 : adjexp;