sys/analog_util: Refactor adc_map, fix compilation

Change the API to use int32_t instead of int, to allow for greater
flexibility on 8- and 16-bit platforms. Removed limitation on input
arguments that min < max. Times where it can be useful to have min > max
is when measuring a sensor where a higher measured voltage means a lower
physical value. For example a thermistor can be connected so that the
measured voltage goes down when the temperature goes up.

sys/analog_util/adc_util.c

 /*
+ * Copyright (C) 2018 Eistec AB
  * Copyright (C) 2015 Freie Universität Berlin
  *
  * This file is subject to the terms and conditions of the GNU Lesser
@@ -14,28 +15,58 @@
  * @brief       ADC utility function implementation
  *
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ * @author      Joakim Nohlgård <joakim.nohlgard@eistec.se>
  *
  * @}
  */
 
+#include <stdint.h>
+#include <inttypes.h>
+
+#include "cpu.h"
+#include "periph/adc.h"
 #include "analog_util.h"
+#include "assert.h"
+
+#define ENABLE_DEBUG    (0)
+#include "debug.h"
 
-/* keep a max value to ADC resolution mapping for quick access in the ROM */
-static const int val_max[] = {
-    [ADC_RES_6BIT] =    0x003f,
-    [ADC_RES_8BIT] =    0x00ff,
-    [ADC_RES_10BIT] =   0x03ff,
-    [ADC_RES_12BIT] =   0x0fff,
-    [ADC_RES_14BIT] =   0x3fff,
-    [ADC_RES_16BIT] =   0xffff
-};
+/**
+ * @brief Convert adc_res_t resolution setting into numeric bit count
+ */
+static unsigned int _adc_res_bits(adc_res_t res)
+{
+    switch (res) {
+        case ADC_RES_6BIT:
+            return 6;
+        case ADC_RES_8BIT:
+            return 8;
+        case ADC_RES_10BIT:
+            return 10;
+        case ADC_RES_12BIT:
+            return 12;
+        case ADC_RES_14BIT:
+            return 14;
+        case ADC_RES_16BIT:
+            return 16;
+        default:
+            /* Unsupported ADC resolution, modify your application to use a
+             * different resolution, or add it above */
+            assert(0 == 1);
+            return 0;
+    }
+}
 
-int adc_util_map(int sample, adc_res_t res, int min, int max)
+int32_t adc_util_map(int sample, adc_res_t res, int32_t min, int32_t max)
 {
-    return ((((max - min) * sample) / val_max[res]) + min);
+    /* Using 64 bit signed int as intermediate to prevent overflow when range
+     * multiplied by sample requires more than 32 bits */
+    int32_t scaled = (((int64_t)(max - min) * sample) >> _adc_res_bits(res));
+    DEBUG("scaled: %" PRId32 "\n", scaled);
+    return (min + scaled);
 }
 
 float adc_util_mapf(int sample, adc_res_t res, float min, float max)
 {
-    return ((((max - min) * sample) / val_max[res]) + min);
+    return ((((max - min) * sample) / ((int32_t)1L << _adc_res_bits(res))) + min);
 }

sys/include/analog_util.h

@@ -22,6 +22,7 @@
 #define ANALOG_UTIL_H
 
 #include <stdint.h>
+
 #include "periph/adc.h"
 
 #ifdef __cplusplus
@@ -34,8 +35,6 @@ extern "C" {
  * This function is useful for converting sampled ADC values into their physical
  * representation.
  *
- * The min value is asserted to be smaller than the max value.
- *
  * @param[in] sample        sampled ADC value
  * @param[in] res           ADC resolution
  * @param[in] min           the lower bound of the target interval
@@ -43,7 +42,7 @@ extern "C" {
  *
  * @return                  the mapped value
  */
-int adc_util_map(int sample, adc_res_t res, int min, int max);
+int32_t adc_util_map(int sample, adc_res_t res, int32_t min, int32_t max);
 
 /**
  * @brief   Map a sampled ADC value to a given range (using floating point