sim-modem/api/modem/gps/nmea.go

package gps

import (
	"fmt"
	"log"
	"strings"
	"time"
)

type nmeaFlags int

const (
	gga     nmeaFlags = 1 << iota // global positioning systemfix data
	rmc                           // recommended minimumspecific	GPS/TRANSIT data
	gpgsv                         // GPS satellites in view
	gpgsa                         // GPS DOP and active satellites
	vtg                           // track made good and ground speed
	xfi                           // Global Positioning SystemExtended FixData.)Bit 6 – GLGSV (GLONASS satellites in view GLONASSfixesonly
	glgsa                         // 1. GPS/2. Glonass/3. GALILE DOPandActiveSatellites.
	gns                           // fix data for GNSS receivers; output for GPS, GLONASS, GALILEO
	_                             // Reserved
	gagsv                         // GALILEO satellites in view
	_                             // Reserved
	_                             // Reserved
	_                             // Reserved
	_                             // Reserved
	_                             // Reserved
	bdpqgsa                       // BEIDOU/QZSS DOP and activesatellites
	bdpqgsv                       // BEIDOUQZSS satellites in view

	nmeaFlagsMask = (1 << 18) - 1

	collectTimeout = 1 * time.Second
)

func secondCountDownTimer(title string, logger *log.Logger, t time.Duration) {
	counter := 0
	for {
		if counter > int(t.Seconds()) {
			break
		}
		logger.Printf("%s: %d/%f\n", title, counter, t.Seconds())
		time.Sleep(time.Second)
		counter += 1
	}
}

// Go... otherwise will throw warning
var nmeaFlagsAll = gga | rmc | gpgsv | gpgsa | vtg | xfi | glgsa | gns | gagsv | bdpqgsa | bdpqgsv

func (g *gps) rawCollect(flags nmeaFlags) (string, error) {
	// Need to omplement low level write/read here because collect must be atomic operation
	// If other command is executed(write/read) it will read a part of nmea report and cause an error

	// Setup gps

	// Set output rate to 10Hz
	if resp, err := g.port.Send("AT+CGPSNMEARATE=1"); err != nil {
		return "", fmt.Errorf("AT+CGPSNMEARATE= request: %w", err)
	} else {
		if !resp.Check() {
			return "", fmt.Errorf("set output rate: error response: %s", resp)
		}
	}

	g.switchGpsMode(true)
	g.port.Mutex().Lock()
	s := g.port.SerialPort()
	defer func() {
		g.port.Mutex().Unlock()
		g.switchGpsMode(false)
	}()

	// Send AT+CGPSINFOCFG=255, flags
	flags &= nmeaFlagsMask
	if _, err := s.Write([]byte("AT+CGPSINFOCFG=1,31\r\n")); err != nil {
		return "", fmt.Errorf("serial port write 1: %w", err)
	}
	// Do I need to read answer

	// Wait
	secondCountDownTimer("Collecting NMEA data", g.logger, collectTimeout)

	// Send AT+CGPSINFOCFG=0, flags
	if _, err := s.Write([]byte("AT+CGPSINFOCFG=0,31\r\n")); err != nil {
		return "", fmt.Errorf("serial port write 2: %w", err)
	}
	if _, err := s.Write([]byte("AT+CGPSFTM=0\r\n")); err != nil { // For sure because sometimes it cannot stop...
		return "", fmt.Errorf("serial port write 2: %w", err)
	}

	time.Sleep(100 * time.Millisecond) // To enshure

	// Read
	outBuf := make([]byte, 0)
	buf := make([]byte, 128)

readLoop:
	for {
		n, err := s.Read(buf)
		if err != nil {
			return string(outBuf), fmt.Errorf("serial port read: %w", err)
		}
		if n == 0 {
			break readLoop
		}
		outBuf = append(outBuf, buf[:n]...)
	}

	return string(outBuf), nil
}

func (g *gps) collectNmeaReports(flags nmeaFlags) ([]string, error) {
	// Raw collect
	resp, err := g.rawCollect(flags)
	if err != nil {
		return nil, fmt.Errorf("raw collect: %w", err)
	}

	// DEBUG
	// g.logger.Println("NMEA raw collect:", resp)

	// Right responce struct:
	// \r\n
	// OK
	// \r\n
	// (NMEA sentence)...
	// \r\n
	// OK
	// \r\n
	strs := strings.Split(strings.Replace(resp, "\r", "", -1), "\n")

	// Check
	// Now wait for:
	// OK
	// (NMEA sentence)...
	// OK
	// if len(strs) < 2 {
	// 	return nil, fmt.Errorf("responce too few rows: %d", len(strs))
	// }
	// if !(strs[0] == "OK" && strs[len(strs)-1] == "OK") {
	// 	return nil, fmt.Errorf("not OK responce: [%  s]", strs)
	// }

	// This... response is not stable
	// Every time it gives one or two OK and in ramdom order
	// So I will not check gor it
	return strs, nil
}