A Zero-Overhead Macro for Unified Management of Floating-Point/Fixed-Point Resolution

In applications where floating-point and fixed-point values need to coexist, resolution conversion can be a real headache, so here is a macro-based approach.

The mapping looks like 2--0.01--100, covering the numeric relationships for both multiplying and dividing by a scale factor.


/* 辅助宏 */
#define LIB_DEC_FACTOR_TOKEN(dp) LIB_DEC_FACTOR_##dp
#define LIB_DEC_FACTOR_0         1
#define LIB_DEC_FACTOR_1         10
#define LIB_DEC_FACTOR_2         100
#define LIB_DEC_FACTOR_3         1000
#define LIB_DEC_FACTOR_4         10000
#define LIB_DEC_FACTOR_5         100000
#define LIB_DEC_FACTOR_6         1000000
#define LIB_DEC_FACTOR_7         10000000
#define LIB_DEC_FACTOR_8         100000000
#define LIB_DEC_FACTOR_9         1000000000
#define LIB_DECIMAL_FACTOR(dp)   LIB_DEC_FACTOR_TOKEN(dp)

#define LIB_DEC_STEP_TOKEN(dp) LIB_DEC_STEP_##dp
#define LIB_DEC_STEP_0         1.0f
#define LIB_DEC_STEP_1         1e-1f
#define LIB_DEC_STEP_2         1e-2f
#define LIB_DEC_STEP_3         1e-3f
#define LIB_DEC_STEP_4         1e-4f
#define LIB_DEC_STEP_5         1e-5f
#define LIB_DEC_STEP_6         1e-6f
#define LIB_DEC_STEP_7         1e-7f
#define LIB_DEC_STEP_8         1e-8f
#define LIB_DEC_STEP_9         1e-9f
#define LIB_DECIMAL_STEP(dp)   LIB_DEC_STEP_TOKEN(dp)

/* 统一管理 */
#define RES_DEC_ACVoltage       2 /* 交流电压 */
#define RES_DEC_LoadACCurrent   2 /* 交流电流 */
#define RES_DEC_OutputACFreq    2 /* 交流频率 */
#define RES_DEC_PowerFactors    3 /* 功率因数 */

#define RES_DEC(name)           (RES_DEC_##name) /* 小数位数 */
#define RES_FACTOR(name)        (LIB_DECIMAL_FACTOR(RES_DEC_##name)) /* 倍数 */
#define RES_STEP(name)          (LIB_DECIMAL_STEP(RES_DEC_##name))   /* 小数 */

Usage is like this:

int a = RES_DEC(ACVoltage);    // =2 获得小数位数
int b = RES_FACTOR(ACVoltage); // =100 获得对应倍数系数
int c = RES_STEP(ACVoltage);   // =0.01 获得对应小数系数

Helper Functions

Even with a scale factor, converting a floating-point value to a fixed-point integer still runs into the small rounding issue. This can be handled by combining the macros with the function below.

#include <stdint.h>
#include <limits.h>

/**
    * @brief  浮点值按十进制位缩放并四舍五入后转整型。
    * @param  value:          期望被转化的浮点值
    * @param  decimalPlaces:  缩放的小数位数(10^decimalPlaces)
    * @retval 四舍五入后的缩放结果，超限时按int32范围饱和
*/
int32_t LIB_FloatToScaledInt(float value, uint8_t decimalPlaces)
{
    double scale = 1.0;
    for(uint8_t i = 0; i < decimalPlaces; i++)
    {
        scale *= 10.0;
    }

    double scaled = (double)value * scale;
    if(scaled >= 0.0)
    {
        scaled += 0.5;
    }
    else
    {
        scaled -= 0.5;
    }

    if(scaled > (double)INT32_MAX)
    {
        return INT32_MAX;
    }
    if(scaled < (double)INT32_MIN)
    {
        return INT32_MIN;
    }

    return (int32_t)scaled;
}

A usage example:

SelfACRunSetting[0].ACVoltage = 123.45678;

int a = LIB_FloatToScaledInt(SelfACRunSetting[0].ACVoltage, RES_DEC(ACVoltage); // = 12347