strutil: add text to float conversion which also gets precision from text

Provide a common string helper routine which converts input text to a
double precision floating point number, and also gets its precision in
the process.

src/libsigrok-internal.h

@@ -1786,6 +1786,7 @@ SR_PRIV int sr_atoi(const char *str, int *ret);
 SR_PRIV int sr_atod(const char *str, double *ret);
 SR_PRIV int sr_atof(const char *str, float *ret);
 SR_PRIV int sr_atod_ascii(const char *str, double *ret);
+SR_PRIV int sr_atod_ascii_digits(const char *str, double *ret, int *digits);
 SR_PRIV int sr_atof_ascii(const char *str, float *ret);
 
 SR_PRIV GString *sr_hexdump_new(const uint8_t *data, const size_t len);

src/strutil.c

@@ -272,6 +272,77 @@ SR_PRIV int sr_atod_ascii(const char *str, double *ret)
 	return SR_OK;
 }
 
+/**
+ * Convert text to a floating point value, and get its precision.
+ *
+ * @param[in] str The input text to convert.
+ * @param[out] ret The conversion result, a double precision float number.
+ * @param[out] digits The number of significant decimals.
+ *
+ * @returns SR_OK in case of successful text to number conversion.
+ * @returns SR_ERR when conversion fails.
+ *
+ * @since 0.6.0
+ */
+SR_PRIV int sr_atod_ascii_digits(const char *str, double *ret, int *digits)
+{
+	const char *p;
+	int *dig_ref, m_dig, exp;
+	char c;
+	double f;
+
+	/*
+	 * Convert floating point text to the number value, _and_ get
+	 * the value's precision in the process. Steps taken to do it:
+	 * - Skip leading whitespace.
+	 * - Count the number of decimals after the mantissa's period.
+	 * - Get the exponent's signed value.
+	 *
+	 * This implementation still uses common code for the actual
+	 * conversion, but "violates API layers" by duplicating the
+	 * text scan, to get the number of significant digits.
+	 */
+	p = str;
+	while (*p && isspace(*p))
+		p++;
+	if (*p == '-' || *p == '+')
+		p++;
+	m_dig = 0;
+	exp = 0;
+	dig_ref = NULL;
+	while (*p) {
+		c = *p++;
+		if (toupper(c) == 'E') {
+			exp = strtol(p, NULL, 10);
+			break;
+		}
+		if (c == '.') {
+			m_dig = 0;
+			dig_ref = &m_dig;
+			continue;
+		}
+		if (isdigit(c)) {
+			if (dig_ref)
+				(*dig_ref)++;
+			continue;
+		}
+		/* Need not warn, conversion will fail. */
+		break;
+	}
+	sr_spew("atod digits: txt \"%s\" -> m %d, e %d -> digits %d",
+		str, m_dig, exp, m_dig + -exp);
+	m_dig += -exp;
+
+	if (sr_atod_ascii(str, &f) != SR_OK)
+		return SR_ERR;
+	if (ret)
+		*ret = f;
+	if (digits)
+		*digits = m_dig;
+
+	return SR_OK;
+}
+
 /**
  * Convert a string representation of a numeric value to a float. The
  * conversion is strict and will fail if the complete string does not represent