Optimize parsing JSON number.

Author: Barenboim

Makefile

@@ -1,5 +1,4 @@
 CFLAGS = -g -Wall -std=c99
-LDFLAGS = -lm
 ifeq ($(DEBUG), y)
 	CFLAGS += -O0
 else
@@ -13,10 +12,10 @@ all: test_speed parse_json
 json_parser.o: json_parser.c json_parser.h list.h rbtree.h
 
 test_speed: json_parser.o rbtree.o test_speed.o
-	$(LD) -o test_speed $^ $(LDFLAGS)
+	$(LD) -o test_speed $^
 
 parse_json: json_parser.o rbtree.o test.o
-	$(LD) -o parse_json $^ $(LDFLAGS)
+	$(LD) -o parse_json $^
 
 clean:
 	rm -f parse_json test_speed *.o

json_parser.c

@@ -252,6 +252,71 @@ static int __parse_json_string(const char *cursor, const char **end,
 	return 0;
 }
 
+static const double __power_of_10[309] = {
+	1.0e0,   1.0e1,   1.0e2,   1.0e3,   1.0e4,
+	1.0e5,   1.0e6,   1.0e7,   1.0e8,   1.0e9,
+	1.0e10,  1.0e11,  1.0e12,  1.0e13,  1.0e14,
+	1.0e15,  1.0e16,  1.0e17,  1.0e18,  1.0e19,
+	1.0e20,  1.0e21,  1.0e22,  1.0e23,  1.0e24,
+	1.0e25,  1.0e26,  1.0e27,  1.0e28,  1.0e29,
+	1.0e30,  1.0e31,  1.0e32,  1.0e33,  1.0e34,
+	1.0e35,  1.0e36,  1.0e37,  1.0e38,  1.0e39,
+	1.0e40,  1.0e41,  1.0e42,  1.0e43,  1.0e44,
+	1.0e45,  1.0e46,  1.0e47,  1.0e48,  1.0e49,
+	1.0e50,  1.0e51,  1.0e52,  1.0e53,  1.0e54,
+	1.0e55,  1.0e56,  1.0e57,  1.0e58,  1.0e59,
+	1.0e60,  1.0e61,  1.0e62,  1.0e63,  1.0e64,
+	1.0e65,  1.0e66,  1.0e67,  1.0e68,  1.0e69,
+	1.0e70,  1.0e71,  1.0e72,  1.0e73,  1.0e74,
+	1.0e75,  1.0e76,  1.0e77,  1.0e78,  1.0e79,
+	1.0e80,  1.0e81,  1.0e82,  1.0e83,  1.0e84,
+	1.0e85,  1.0e86,  1.0e87,  1.0e88,  1.0e89,
+	1.0e90,  1.0e91,  1.0e92,  1.0e93,  1.0e94,
+	1.0e95,  1.0e96,  1.0e97,  1.0e98,  1.0e99,
+	1.0e100, 1.0e101, 1.0e102, 1.0e103, 1.0e104,
+	1.0e105, 1.0e106, 1.0e107, 1.0e108, 1.0e109,
+	1.0e110, 1.0e111, 1.0e112, 1.0e113, 1.0e114,
+	1.0e115, 1.0e116, 1.0e117, 1.0e118, 1.0e119,
+	1.0e120, 1.0e121, 1.0e122, 1.0e123, 1.0e124,
+	1.0e125, 1.0e126, 1.0e127, 1.0e128, 1.0e129,
+	1.0e130, 1.0e131, 1.0e132, 1.0e133, 1.0e134,
+	1.0e135, 1.0e136, 1.0e137, 1.0e138, 1.0e139,
+	1.0e140, 1.0e141, 1.0e142, 1.0e143, 1.0e144,
+	1.0e145, 1.0e146, 1.0e147, 1.0e148, 1.0e149,
+	1.0e150, 1.0e151, 1.0e152, 1.0e153, 1.0e154,
+	1.0e155, 1.0e156, 1.0e157, 1.0e158, 1.0e159,
+	1.0e160, 1.0e161, 1.0e162, 1.0e163, 1.0e164,
+	1.0e165, 1.0e166, 1.0e167, 1.0e168, 1.0e169,
+	1.0e170, 1.0e171, 1.0e172, 1.0e173, 1.0e174,
+	1.0e175, 1.0e176, 1.0e177, 1.0e178, 1.0e179,
+	1.0e180, 1.0e181, 1.0e182, 1.0e183, 1.0e184,
+	1.0e185, 1.0e186, 1.0e187, 1.0e188, 1.0e189,
+	1.0e190, 1.0e191, 1.0e192, 1.0e193, 1.0e194,
+	1.0e195, 1.0e196, 1.0e197, 1.0e198, 1.0e199,
+	1.0e200, 1.0e201, 1.0e202, 1.0e203, 1.0e204,
+	1.0e205, 1.0e206, 1.0e207, 1.0e208, 1.0e209,
+	1.0e210, 1.0e211, 1.0e212, 1.0e213, 1.0e214,
+	1.0e215, 1.0e216, 1.0e217, 1.0e218, 1.0e219,
+	1.0e220, 1.0e221, 1.0e222, 1.0e223, 1.0e224,
+	1.0e225, 1.0e226, 1.0e227, 1.0e228, 1.0e229,
+	1.0e230, 1.0e231, 1.0e232, 1.0e233, 1.0e234,
+	1.0e235, 1.0e236, 1.0e237, 1.0e238, 1.0e239,
+	1.0e240, 1.0e241, 1.0e242, 1.0e243, 1.0e244,
+	1.0e245, 1.0e246, 1.0e247, 1.0e248, 1.0e249,
+	1.0e250, 1.0e251, 1.0e252, 1.0e253, 1.0e254,
+	1.0e255, 1.0e256, 1.0e257, 1.0e258, 1.0e259,
+	1.0e260, 1.0e261, 1.0e262, 1.0e263, 1.0e264,
+	1.0e265, 1.0e266, 1.0e267, 1.0e268, 1.0e269,
+	1.0e270, 1.0e271, 1.0e272, 1.0e273, 1.0e274,
+	1.0e275, 1.0e276, 1.0e277, 1.0e278, 1.0e279,
+	1.0e280, 1.0e281, 1.0e282, 1.0e283, 1.0e284,
+	1.0e285, 1.0e286, 1.0e287, 1.0e288, 1.0e289,
+	1.0e290, 1.0e291, 1.0e292, 1.0e293, 1.0e294,
+	1.0e295, 1.0e296, 1.0e297, 1.0e298, 1.0e299,
+	1.0e300, 1.0e301, 1.0e302, 1.0e303, 1.0e304,
+	1.0e305, 1.0e306, 1.0e307, 1.0e308
+};
+
 static double __evaluate_json_number(const char *integer,
 									 const char *fraction,
 									 int exp)
@@ -269,16 +334,17 @@ static double __evaluate_json_number(const char *integer,
 	{
 		figures++;
 		mant = *integer - '0';
-		while (isdigit(*++integer))
+		while (isdigit(*++integer) && figures < 18)
 		{
-			if (figures < 18)
-			{
-				figures++;
-				mant *= 10;
-				mant += *integer - '0';
-			}
-			else
-				exp++;
+			figures++;
+			mant *= 10;
+			mant += *integer - '0';
+		}
+
+		while (isdigit(*integer))
+		{
+			exp++;
+			integer++;
 		}
 	}
 	else
@@ -303,15 +369,19 @@ static double __evaluate_json_number(const char *integer,
 	num = mant;
 	if (exp != 0 && figures != 0)
 	{
-		if (exp > 0)
-			num *= pow(10, exp);
-		else if (exp >= -308)
-			num /= pow(10, -exp);
-		else
+		if (exp > 309 - figures)
+			num = INFINITY;
+		else if (exp > 0)
+			num *= __power_of_10[exp];
+		else if (exp > -309)
+			num /= __power_of_10[-exp];
+		else if (exp > -324 - figures)
 		{
-			num /= pow(10, -exp - 308);
-			num /= 1.0e308;
+			num /= __power_of_10[-exp - 308];
+			num /= __power_of_10[308];
 		}
+		else
+			num = 0.0;
 	}
 
 	return sign ? -num : num;