strutil: add method to get an sr_rational from a string

The method accepts strings with numbers in scientific or normal notation,
e.g. -1.25 or 3.37e-6. The numeric range is limited by the sr_rational
range, i.e +-9.2e18, resolution is ~19 digits.

include/libsigrok/proto.h

@@ -226,6 +226,7 @@ SR_API uint64_t sr_parse_timestring(const char *timestring);
 SR_API gboolean sr_parse_boolstring(const char *boolstring);
 SR_API int sr_parse_period(const char *periodstr, uint64_t *p, uint64_t *q);
 SR_API int sr_parse_voltage(const char *voltstr, uint64_t *p, uint64_t *q);
+SR_API int sr_parse_rational(const char *str, struct sr_rational *ret);
 
 /*--- version.c -------------------------------------------------------------*/
 

src/strutil.c

@@ -211,6 +211,77 @@ SR_PRIV int sr_atof_ascii(const char *str, float *ret)
 	return SR_OK;
 }
 
+/**
+ * Convert a string representation of a numeric value to a @sr_rational. The
+ * conversion is strict and will fail if the complete string does not represent
+ * a valid number. The function sets errno according to the details of the
+ * failure. This version ignores the locale.
+ *
+ * @param str The string representation to convert.
+ * @param ret Pointer to sr_rational where the result of the conversion will be stored.
+ *
+ * @retval SR_OK Conversion successful.
+ * @retval SR_ERR Failure.
+ *
+ * @since 0.5.0
+ */
+SR_API int sr_parse_rational(const char *str, struct sr_rational *ret)
+{
+	char *endptr = NULL;
+	int64_t integral;
+	int64_t fractional = 0;
+	int64_t denominator = 1;
+	int32_t fractional_len = 0;
+	int32_t exponent = 0;
+
+	errno = 0;
+	integral = g_ascii_strtoll(str, &endptr, 10);
+
+	if (errno)
+		return SR_ERR;
+
+	if (*endptr == '.') {
+		const char* start = endptr + 1;
+		fractional = g_ascii_strtoll(start, &endptr, 10);
+		if (errno)
+			return SR_ERR;
+		fractional_len = endptr - start;
+	}
+
+	if ((*endptr == 'E') || (*endptr == 'e')) {
+		exponent = g_ascii_strtoll(endptr + 1, &endptr, 10);
+		if (errno)
+			return SR_ERR;
+	}
+
+	if (*endptr != '\0')
+		return SR_ERR;
+
+	for (int i = 0; i < fractional_len; i++)
+		integral *= 10;
+	exponent -= fractional_len;
+
+	if (integral >= 0)
+		integral += fractional;
+	else
+		integral -= fractional;
+
+	while (exponent > 0) {
+		integral *= 10;
+		exponent--;
+	}
+
+	while (exponent < 0) {
+		denominator *= 10;
+		exponent++;
+	}
+
+	ret->p = integral;
+	ret->q = denominator;
+
+	return SR_OK;
+}
+
 /**
  * Convert a numeric value value to its "natural" string representation
  * in SI units.

tests/strutil.c

@@ -34,6 +34,18 @@ static void test_samplerate(uint64_t samplerate, const char *expected)
 	g_free(s);
 }
 
+static void test_rational(const char *input, struct sr_rational expected)
+{
+	int ret;
+	struct sr_rational rational;
+
+	ret = sr_parse_rational(input, &rational);
+	fail_unless(ret == SR_OK);
+	fail_unless((expected.p == rational.p) && (expected.q == rational.q),
+		    "Invalid result for '%s': %ld/%ld'.",
+		    input, rational.p, rational.q);
+}
+
 /*
  * Check various inputs for sr_samplerate_string():
  *
@@ -155,6 +167,41 @@ START_TEST(test_ghz)
 }
 END_TEST
 
+START_TEST(test_integral)
+{
+	test_rational("1", (struct sr_rational){1, 1});
+	test_rational("2", (struct sr_rational){2, 1});
+	test_rational("10", (struct sr_rational){10, 1});
+	test_rational("-255", (struct sr_rational){-255, 1});
+}
+END_TEST
+
+START_TEST(test_fractional)
+{
+	test_rational("0.1", (struct sr_rational){1, 10});
+	test_rational("1.0", (struct sr_rational){10, 10});
+	test_rational("1.2", (struct sr_rational){12, 10});
+	test_rational("12.34", (struct sr_rational){1234, 100});
+	test_rational("-12.34", (struct sr_rational){-1234, 100});
+	test_rational("10.00", (struct sr_rational){1000, 100});
+}
+END_TEST
+
+START_TEST(test_exponent)
+{
+	test_rational("1e0", (struct sr_rational){1, 1});
+	test_rational("1E0", (struct sr_rational){1, 1});
+	test_rational("1E1", (struct sr_rational){10, 1});
+	test_rational("1e-1", (struct sr_rational){1, 10});
+	test_rational("-1.234e-0", (struct sr_rational){-1234, 1000});
+	test_rational("-1.234e3", (struct sr_rational){-1234, 1});
+	test_rational("-1.234e-3", (struct sr_rational){-1234, 1000000});
+	test_rational("0.001e3", (struct sr_rational){1, 1});
+	test_rational("0.001e0", (struct sr_rational){1, 1000});
+	test_rational("0.001e-3", (struct sr_rational){1, 1000000});
+}
+END_TEST
+
 Suite *suite_strutil(void)
 {
 	Suite *s;
@@ -168,6 +215,9 @@ Suite *suite_strutil(void)
 	tcase_add_test(tc, test_khz);
 	tcase_add_test(tc, test_mhz);
 	tcase_add_test(tc, test_ghz);
+	tcase_add_test(tc, test_integral);
+	tcase_add_test(tc, test_fractional);
+	tcase_add_test(tc, test_exponent);
 	suite_add_tcase(s, tc);
 
 	return s;