spektrum_rssi.h

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#pragma once

#include <climits>

/*
min_rssi_dbm = -92.0
max_rssi_dbm = -42.0

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,
};

#define MASK(n) (n >> (sizeof(n) * CHAR_BIT - 1))

/* constexpr-compatible version of abs() */
constexpr unsigned c_abs(int n)
{
    return (n + MASK(n)) ^ MASK(n);
}

/* 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)];
}

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%");