spektrum_rssi: initialize rssi lookup as constexpr

Rather than initializing the rssi percentage lookup table at runtime
on the heap, we would like it to be stored in flash.

This change pre-computes the rssi lookup table.

src/lib/rc/dsm.cpp

@@ -79,8 +79,6 @@ static unsigned dsm_channel_shift = 0;			/**< Channel resolution, 0=unknown, 1=1
 static unsigned dsm_frame_drops = 0;			/**< Count of incomplete DSM frames */
 static uint16_t dsm_chan_count = 0;         /**< DSM channel count */
 
-static const SpektrumRssi spektrum_rssi;
-
 static bool
 dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, bool *dsm_11_bit, unsigned max_values,
 	   int8_t *rssi_percent);
@@ -429,7 +427,7 @@ dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, bool
 				*rssi_percent = 0;
 
 			} else {
-				*rssi_percent = spektrum_rssi.dbm_to_percent(dbm);
+				*rssi_percent = spek_dbm_to_percent(dbm);
 			}
 
 		} else {

src/lib/rc/spektrum_rssi.h

@@ -41,38 +41,48 @@
 
 #pragma once
 
-#include <math.h>
+#include <climits>
 
-#define MIN_RSSI_DBM (-92.0f)
-#define MAX_RSSI_DBM (-42.0f)
+/*
+min_rssi_dbm = -92.0
+max_rssi_dbm = -42.0
 
-class SpektrumRssi
-{
-	int8_t lu_dbm_percent[129];
-
-public:
-	SpektrumRssi()
-	{
-		for (int i = 0; i <= 128; i++) {
-			float rssi_dbm = -1.0f * (float)i;
-			float percent;
-
-			if (rssi_dbm > MAX_RSSI_DBM) {
-				percent = 100.0f;
+def dbm_to_percent(abs_dbm):
+	if -abs_dbm < min_rssi_dbm:
+		return 0.0
+	elif -abs_dbm > max_rssi_dbm:
+		return 100.0
+	else:
+		return 100.0 * math.log10(1.0 + (float)(-abs_dbm - min_rssi_dbm) * (9.0 / (max_rssi_dbm - min_rssi_dbm)))
+*/
+constexpr int8_t lu_dbm_percent[] = {
+	100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100,
+	100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100,
+	100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 99,  98,  98,  97,  96,
+	95,  94,  93,  92,  91,  90,  89,  88,  87,  86,  85,  84,  83,  82,  81,  79,
+	78,  77,  75,  74,  73,  71,  70,  68,  66,  65,  63,  61,  59,  57,  55,  52,
+	50,  47,  45,  42,  39,  35,  32,  28,  24,  19,  13,  7,   0,   0,   0,   0,
+	0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+	0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+	0,
+};
 
-			} else if (rssi_dbm < MIN_RSSI_DBM) {
-				percent = 0.0f;
+#define MASK(n) (n >> (sizeof(n) * CHAR_BIT - 1))
 
-			} else {
-				percent = 100.0f * log10f(1 + (rssi_dbm - MIN_RSSI_DBM) * (9.0f / (MAX_RSSI_DBM - MIN_RSSI_DBM)));
-			}
+/* constexpr-compatible version of abs() */
+constexpr unsigned c_abs(int n)
+{
+	return (n + MASK(n)) ^ MASK(n);
+}
 
-			lu_dbm_percent[i] = (int8_t)roundf(percent);
-		}
-	}
+/* convert signed spektrum dbm (-92 to -42) to percentage */
+constexpr int8_t spek_dbm_to_percent(int8_t dbm)
+{
+	return lu_dbm_percent[c_abs(dbm)];
+}
 
-	int8_t dbm_to_percent(int8_t dbm) const
-	{
-		return lu_dbm_percent[abs(dbm)];
-	}
-};
+static_assert(spek_dbm_to_percent(0) == 100, "0 dbm should be 100%");
+static_assert(spek_dbm_to_percent(-42) == 100, "-42 dbm should be 100%");
+static_assert(spek_dbm_to_percent(-80) == 50, "-80 dbm should be 50%");
+static_assert(spek_dbm_to_percent(-92) == 0, "-92 dbm should be 0%");
+static_assert(spek_dbm_to_percent(-128) == 0, "-128 dbm should be 0%");