[PATCH v3 3/5] decimal: fix string formatting on construction from double
Serge Petrenko
sergepetrenko at tarantool.org
Tue Jul 2 20:27:50 MSK 2019
Use printf "%g" option instead of "%f" to trim traling zeros in such
cases:
decimal_from_double(1) -> '1.000000000000000' -> decimal_from_string()
Now it should be
decimal_from_double(1) -> '1' ->decimal_from_string()
Follow-up 6d62c6c19ed418932ead1bba44fcd7cd84c78a19
---
src/lib/core/decimal.c | 10 +++++++++-
test/unit/decimal.c | 3 ++-
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/src/lib/core/decimal.c b/src/lib/core/decimal.c
index 91ee3f307..1b4f44362 100644
--- a/src/lib/core/decimal.c
+++ b/src/lib/core/decimal.c
@@ -121,7 +121,15 @@ decimal_from_double(decimal_t *dec, double d)
char buf[DECIMAL_MAX_DIGITS + 3];
if (isinf(d) || isnan(d))
return NULL;
- snprintf(buf, sizeof(buf), "%.*f", DBL_DIG, d);
+ /*
+ * DBL_DIG is 15, it is the guaranteed amount of
+ * correct significant decimal digits in a double
+ * value. There is no point in using higher precision,
+ * since every non-representable number has a long
+ * tail of erroneous digits:
+ * `23.42` -> `23.420000000000001705302565824240446091`
+ */
+ snprintf(buf, sizeof(buf), "%.*g", DBL_DIG, d);
return decimal_from_string(dec, buf);
}
diff --git a/test/unit/decimal.c b/test/unit/decimal.c
index ae806a106..b587e1f14 100644
--- a/test/unit/decimal.c
+++ b/test/unit/decimal.c
@@ -27,7 +27,8 @@
\
is(decimal_div(&t, &u, &v), &t, "decimal("#a") / decimal("#b")");\
is(decimal_from_double(&w, (double)((a)) / (b)), &w, "decimal(("#a") / ("#b"))");\
- is(decimal_round(&t, DBL_DIG), &t, "decimal_round(("#a")/("#b"), %d)", DBL_DIG);\
+ is(decimal_round(&t, DBL_DIG - decimal_precision(&t) + decimal_scale(&t)), &t,\
+ "decimal_round(("#a")/("#b"), %d)", DBL_DIG);\
is(decimal_compare(&t, &w), 0, "decimal("#a") / decimal("#b") == ("#a") / ("#b")");\
})
--
2.20.1 (Apple Git-117)
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