[PATCH 1/5] decimal: fix decimal.round() when scale == 0

[Serge Petrenko <sergepetrenko@tarantool.org>]

Fixes decimal.round() with zero scale, fixes an error with
decimal.round() when rounding leads to a number with the same
precision, for example, decimal.round(9.9, 0) -> 10.

Follow-up #692
---
 src/lib/core/decimal.c    |  8 ++++----
 test/app/decimal.result   | 18 ++++++++++++++++++
 test/app/decimal.test.lua |  6 ++++++
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/lib/core/decimal.c b/src/lib/core/decimal.c
index 1b4f44362..1423ae418 100644
--- a/src/lib/core/decimal.c
+++ b/src/lib/core/decimal.c
@@ -172,14 +172,14 @@ decimal_round(decimal_t *dec, int scale)
 	if (scale < 0 || scale > DECIMAL_MAX_DIGITS)
 		return NULL;
 
-	if (scale > decimal_scale(dec))
+	if (scale >= decimal_scale(dec))
 		return dec;
 
-	int ndig = decimal_precision(dec) - decimal_scale(dec) + scale;
+	int ndig = MAX(decimal_precision(dec) - decimal_scale(dec) + scale, 1);
 	decContext context = {
 		ndig, /* Precision */
-		ndig - 1, /* emax */
-		-1, /* emin */
+		ndig, /* emax */
+		scale != 0 ? -1 : 0, /* emin */
 		DECIMAL_ROUNDING, /* rounding */
 		0, /* no traps */
 		0, /* zero status */
diff --git a/test/app/decimal.result b/test/app/decimal.result
index ecb189277..53ec73edc 100644
--- a/test/app/decimal.result
+++ b/test/app/decimal.result
@@ -458,3 +458,21 @@ a ^ 2.5
  | ---
  | - error: '[string "return a ^ 2.5 "]:1: decimal operation failed'
  | ...
+
+-- check correct rounding when scale = 0
+decimal.round(decimal.new(0.9), 0)
+ | ---
+ | - '1'
+ | ...
+decimal.round(decimal.new(9.9), 0)
+ | ---
+ | - '10'
+ | ...
+decimal.round(decimal.new(99.9), 0)
+ | ---
+ | - '100'
+ | ...
+decimal.round(decimal.new(99.4), 0)
+ | ---
+ | - '99'
+ | ...
diff --git a/test/app/decimal.test.lua b/test/app/decimal.test.lua
index 4aff0f2a4..20fd4d81a 100644
--- a/test/app/decimal.test.lua
+++ b/test/app/decimal.test.lua
@@ -128,3 +128,9 @@ a = decimal.new('-13')
 a ^ 2
 -- fractional powers are allowed only for positive numbers
 a ^ 2.5
+
+-- check correct rounding when scale = 0
+decimal.round(decimal.new(0.9), 0)
+decimal.round(decimal.new(9.9), 0)
+decimal.round(decimal.new(99.9), 0)
+decimal.round(decimal.new(99.4), 0)
-- 
2.20.1 (Apple Git-117)