// Bosch BMP280 compensation maths + calibration/ADC decoding — PURE LOGIC.
//
// ESP-IDF-free by design: no I2C, no esp_err_t, no FreeRTOS, no logging.
// Only <stdint.h>/<stddef.h>, so it compiles and is testable on a host
// with plain gcc (see ../test/). `bmp280.c` does the I2C traffic and calls
// in here for every byte-order decision and every line of arithmetic.
//
// The formulas are Bosch datasheet (rev 1.23) §3.11.3's double-precision
// reference implementation, transcribed near-verbatim, with the variable
// names kept close to the original so it's checkable against the datasheet
// PDF side-by-side. The register map decoded below is §3.11.1.
//
// What host tests can prove here: byte-order/packing of the calibration
// block and the 20-bit ADC words, and that the arithmetic behaves
// sanely and monotonically. What they CANNOT prove: that this exact
// silicon returns the register contents we assume.

#pragma once

#include <stddef.h>
#include <stdint.h>

#ifdef __cplusplus
extern "C" {
#endif

// Calibration block is 24 bytes at 0x88..0x9F (dig_T1..dig_P9).
#define BMP280_CALIB_LEN 24
// Burst measurement read is 6 bytes from 0xF7: press(3) + temp(3).
#define BMP280_RAW_LEN 6

typedef struct {
    uint16_t dig_T1;
    int16_t  dig_T2;
    int16_t  dig_T3;
    uint16_t dig_P1;
    int16_t  dig_P2;
    int16_t  dig_P3;
    int16_t  dig_P4;
    int16_t  dig_P5;
    int16_t  dig_P6;
    int16_t  dig_P7;
    int16_t  dig_P8;
    int16_t  dig_P9;
} bmp280_calib_t;

// Decode the 24-byte calibration block. Each coefficient is stored
// little-endian (LSB first) in the register map.
void bmp280_calib_from_regs(const uint8_t buf[BMP280_CALIB_LEN], bmp280_calib_t *out);

// Decode the 6-byte burst read into the two 20-bit ADC words. Pressure
// comes first (0xF7..0xF9), then temperature (0xFA..0xFC); each is
// MSB/LSB/XLSB with the XLSB's top nibble carrying the low 4 bits.
void bmp280_adc_from_regs(const uint8_t raw[BMP280_RAW_LEN], int32_t *out_adc_P, int32_t *out_adc_T);

// Returns degrees C, and writes the shared `t_fine` intermediate that the
// pressure compensation needs.
double bmp280_compensate_temperature(const bmp280_calib_t *c, int32_t adc_T, double *out_t_fine);

// Returns Pa. Returns exactly 0.0 when the datasheet's own divide-by-zero
// guard trips (var1 == 0, i.e. an all-zero / unread calibration block).
double bmp280_compensate_pressure(const bmp280_calib_t *c, int32_t adc_P, double t_fine);

#ifdef __cplusplus
}
#endif