analog: improve output readability by using SI prefix

[libsigrok.git] / src / analog.c

diff --git a/src/analog.c b/src/analog.c
index 7343b481e610d409d51bb45dd5f2be1a31138c77..634e9a674ba8ada7817bd489d68bedbe7853ed59 100644 (file)
--- a/src/analog.c
+++ b/src/analog.c
@@ -22,6 +22,7 @@
 #include <stdint.h>
 #include <string.h>
 #include <ctype.h>
+#include <math.h>
 #include <libsigrok/libsigrok.h>
 #include "libsigrok-internal.h"
 
@@ -292,6 +293,37 @@ SR_API int sr_analog_to_float(const struct sr_datafeed_analog *analog,
        return SR_OK;
 }
 
+/**
+ * Scale a float value to the appropriate SI prefix.
+ *
+ * @param[in,out] value The float value to convert to appropriate SI prefix.
+ * @param[in,out] digits The number of significant decimal digits in value.
+ *
+ * @return The SI prefix to which value was scaled, as a printable string.
+ *
+ * @since 0.5.0
+ */
+SR_API const char *sr_analog_si_prefix(float *value, int *digits)
+{
+#define NEG_PREFIX_COUNT 5  /* number of prefixes below unity */
+#define POS_PREFIX_COUNT (int)(ARRAY_SIZE(prefixes) - NEG_PREFIX_COUNT - 1)
+       static const char *prefixes[] = { "f","p","n","µ","m","","k","M","G","T" };
+
+       if (value == NULL || digits == NULL || isnan(*value))
+               return prefixes[NEG_PREFIX_COUNT];
+
+       float logval = log10f(fabsf(*value));
+       int prefix = (logval / 3) - (logval < 1);
+
+       if (prefix < -NEG_PREFIX_COUNT) prefix = -NEG_PREFIX_COUNT;
+       if (3 * prefix < -*digits)      prefix = (-*digits + 2 * (*digits < 0)) / 3;
+       if (prefix > POS_PREFIX_COUNT)  prefix = POS_PREFIX_COUNT;
+
+       *value *= powf(10, -3 * prefix);
+       *digits += 3 * prefix;
+       return prefixes[prefix + NEG_PREFIX_COUNT];
+}
+
 /**
  * Convert the unit/MQ/MQ flags in the analog struct to a string.
  *