Make Float#floor with ndigits argument handle error

The previous implementation could result in a returned
float that is 1/(10**ndigits) too low.  First try adding
one before dividing, and if that results in a value that is
greater than the initial number, then try the original
calculation.

Spec added for ciel, but the issue doesn't appear to affect
ciel, at least not for the same number.  If the issue does
effect ciel, a similar fix could probably work for it.

Fixes [Bug #18018]

1 files changed, 7 insertions, 3 deletions

diff --git a/numeric.c b/numeric.c
index 345067c6a5..5564a6eceb 100644
--- a/numeric.c
+++ b/numeric.c
@@ -1829,20 +1829,24 @@ flo_prev_float(VALUE vx)
 VALUE
 rb_float_floor(VALUE num, int ndigits)
 {
-    double number, f;
+    double number;
     number = RFLOAT_VALUE(num);
     if (number == 0.0) {
 	return ndigits > 0 ? DBL2NUM(number) : INT2FIX(0);
     }
     if (ndigits > 0) {
 	int binexp;
+        double f, mul, res;
 	frexp(number, &binexp);
 	if (float_round_overflow(ndigits, binexp)) return num;
 	if (number > 0.0 && float_round_underflow(ndigits, binexp))
 	    return DBL2NUM(0.0);
 	f = pow(10, ndigits);
-	f = floor(number * f) / f;
-	return DBL2NUM(f);
+        mul = floor(number * f);
+        res = (mul + 1) / f;
+        if (res > number)
+            res = mul / f;
+        return DBL2NUM(res);
     }
     else {
 	num = dbl2ival(floor(number));