/*
  Ultrasonic Tape Measure
  Adapted by Kurt Kara and Nathan Young

  created 3 Nov 2008
  by David A. Mellis
  modified 30 Aug 2011
  by Tom Igoe

  This example code is in the public domain.

  https://www.arduino.cc/en/Tutorial/BuiltInExamples/Ping
*/

// Constant input and output required brcause of 4-pin ultrasonic sensor. Also we need a separate input and output sensor in order to stop ONLY the output, with a switch, so that the last measurement is held, like a tape measure. 
// Because of this, instead of pingPin, we will have to use trigPin for the output from the Arduino board, and echoPin as the input to the Arduino board:
const int echoPin = 1;
const int trigPin = 2;
#include <SD.h>
#include <SPI.h>
#define CS_PIN 53
File root;

void save_distance() {
  root = SD.open("Dist.txt", FILE_WRITE);
  root.print("Measurement (cm): ");
  root.print(microsecondsToCentimeters(delayMicroseconds));
  root.close();
}

void setup() {
  // initialize serial communication:
  Serial.begin(9600);
  Serial.print("Initializing SD card...");

  if (!SD.begin(CS_PIN)) {
    Serial.println("Card initialization failed!");
    while(true);
  }

  Serial.println("Initialization done.");

  Serial.println("Files in the card:");

  File textFile = SD.open("Dist.txt");
  if (textFile) {
    Serial.print("Dist.txt: ");
    while (textFile.available()) {
      Serial.write(textFile.read());
    }
    textFile.close();
  } else {
    Serial.println("Error in opening Dist.txt!");
  }
}

void loop() {
  // establishs the variables for duration of the ping, and the distance result in centimeters. we are using only cm because in Engineering, we prefer base 10 measurement:
  long duration, cm;

  // The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
  // This Ping goes throught the Trig pin, while the return signal goes through the Echo pin because of the previously mentioned 4-pin setup
  pinMode(trigPin, OUTPUT);
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(trigPin, LOW);

  // Because of the 4-pin setup, the Echo pin is used to read the signal from the PING))): a HIGH pulse
  // whose duration is the time (in microseconds) from the sending of the ping
  // to the reception of its echo off of an object.
  // We need to define the echoPin as the input of the Arduino board, since we have separate input and output pins
  pinMode(echoPin, INPUT);
  duration = pulseIn(echoPin, HIGH);

  // convert the time into a distance
  cm = microsecondsToCentimeters(duration);

  Serial.print(cm);
  Serial.print("cm");
  Serial.println();
  // Delay pauses the function for a period of time
// Smaller number means more measurements in a shorter period of time, delay is in milliseconds. 
// Making the delay shorter allows for measuring while moving, for instance along a curve, since it will return values fairly quickly, and at the end of the curve you can flip the switch to turn it off, retaining the values.
  delay(50);
}

// Function that converts microseconds to cm
long microsecondsToCentimeters(long microseconds) {
  // The speed of sound is 340 m/s or 29 microseconds per centimeter.
  // The ping travels out and back, so to find the distance of the object we
  // take half of the distance travelled.
  return microseconds / 29 / 2;
}