types: ensure double serializatiion with decimal places

The `%g` printf format used for serializing double values into strings
will not include any decimal place if the value happens to be integral,
leading to an unwanted double to integer conversion when serializing
and subsequently deserializing an integral double value as JSON.

Solve this issue by checking the serialized string result for a decimal
point or exponential notation and appending `.0` if neither is found.

Ref: #173
Suggested-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

2 files changed, 40 insertions, 1 deletions

diff --git a/tests/custom/99_bugs/43_types_json_double_format b/tests/custom/99_bugs/43_types_json_double_format
new file mode 100644
index 0000000..9b90e37
--- /dev/null
+++ b/tests/custom/99_bugs/43_types_json_double_format
@@ -0,0 +1,24 @@
+When formatting integral double values as JSON, ensure that at least one
+decimal place is retained.
+
+-- Testcase --
+printf("%.J\n", [
+	1e100,
+	1.23,
+	4.00,
+	1.0/3*3,
+]);
+-- End --
+
+-- Args --
+-R
+-- End --
+
+-- Expect stdout --
+[
+	1e+100,
+	1.23,
+	4.0,
+	1.0
+]
+-- End --
diff --git a/types.c b/types.c
index c9472f7..eeac8c7 100644
--- a/types.c
+++ b/types.c
@@ -1561,6 +1561,21 @@ ucv_to_stringbuf_add_padding(uc_stringbuf_t *pb, char pad_char, size_t pad_size)
 	}
 }
 
+static void
+ucv_to_stringbuf_add_double(uc_stringbuf_t *pb, double val, bool json)
+{
+	int len = ucv_stringbuf_printf(pb, "%.14g", val);
+
+	if (!json)
+		return;
+
+	for (char *p = pb->buf + pb->bpos - len; len > 0; len--, p++)
+		if (*p == '.' || *p == 'e')
+			return;
+
+	ucv_stringbuf_append(pb, ".0");
+}
+
 void
 ucv_to_stringbuf_formatted(uc_vm_t *vm, uc_stringbuf_t *pb, uc_value_t *uv, size_t depth, char pad_char, size_t pad_size)
 {
@@ -1624,7 +1639,7 @@ ucv_to_stringbuf_formatted(uc_vm_t *vm, uc_stringbuf_t *pb, uc_value_t *uv, size
 		else if (d == -INFINITY)
 			ucv_stringbuf_append(pb, "-Infinity");
 		else
-			ucv_stringbuf_printf(pb, "%.14g", d);
+			ucv_to_stringbuf_add_double(pb, d, json);
 
 		break;