single diff measurement question (intersection of two structs via prns)

sdiff_empty.c

// Compiles with gcc -O0 single_diff_libswiftnav.c

#include <stdint.h>
#include <stdbool.h>
#include <string.h>
#include <stdio.h>
#include <inttypes.h>

typedef uint8_t u8;
typedef uint16_t u16;

typedef struct {
  double pseudorange;
  double carrier_phase;
  double doppler;
  double snr;
  u8 prn;
} navigation_measurement_t;

typedef struct {
  double pseudorange;
  double carrier_phase;
  double doppler;
  double snr;
  u8 prn;
} sdiff_t;

/** Given given two lists of undifferenced input observations
 * (navigation_measurement_t), nav_meas_1 and nav_meas_2,
 * constructs the pseudorange, carrier_phase, and
 * Doppler sdiffs (single difference observations) using the
 * intersection of their PRNs.
 *
 * Also:
 *  0. SNR in the output is the min of the SNRs of inputs 1 and 2.
 *  1. `sat_pos` and `sat_vel` are taken from input 1.
 *
 * Returns the number of sdiffs in the intersection.
 *
 * \param nav_meas1_len  Number of measurements in `nav_meas1`
 * \oaram nav_meas1      Array of undifferenced observations
 * \param nav_meas2_len  Number of measurements in `nav_meas2`
 * \oaram nav_meas2      Array of navigation measurements
 * \param sdiff          Single difference observations
 *
 * \return               The number of observations written to `sdiff`
 *
 */
u8 single_diff(u8 nav_meas1_len,
               navigation_measurement_t *nav_meas1,
               u8 nav_meas2_len,
               navigation_measurement_t *nav_meas2,
               sdiff_t *sdiff)
{
    return 0;
}

int main() {
  // Fake measurements
  navigation_measurement_t nm1 = {.prn = 1};
  navigation_measurement_t nm2 = {.prn = 2};
  navigation_measurement_t nm3 = {.prn = 3};
  navigation_measurement_t nm4 = {.prn = 4};
    
  // Array of measurements.
  navigation_measurement_t nms_no_match1[2];
  navigation_measurement_t nms_no_match2[2];
    
  // Single differences
  sdiff_t sds_out[3];
  memcpy(&nms_no_match1[0], &nm1, sizeof(navigation_measurement_t));
  memcpy(&nms_no_match1[1], &nm2, sizeof(navigation_measurement_t));
  memcpy(&nms_no_match2[0], &nm1, sizeof(navigation_measurement_t));
  memcpy(&nms_no_match2[1], &nm3, sizeof(navigation_measurement_t));
  
  u8 num_match = single_diff(2, nms_no_match1,
                             2, nms_no_match2,
                             sds_out);
  printf("%d", num_match);
}