/* Homework 3, see http://cs.fit.edu/~mmahoney/cse1502/
Matt Mahoney

This program reads a time from the user of the form
hh:mm:ss in the range 00:00:00 to 99:59:59 and a file
containing a list of times, elevations, and temperatures
(as doubles), with times in ascending order, for example:

  00:03:00 500 16.5
  00:04:00 550 17.5
  00:04:30 570 17
  01:44:30 870 -3

If the user enters a time that matches a line of the
file, then the program prints the elevation and temperature
from that line. If the user time is between two lines
then it interpolates the value. If it is before the first
line or after the last, then it prints an error, for
example, if the above file is named balloon3.txt, then:

  Enter time: 00:04:00
  Enter file name: balloon3.txt
  Elevation is 550
  Temperature is 17.5

  Enter time: 00:03:30
  Enter file name: balloon3.txt
  Elevation is 525
  Temperature is 17

  Enter time: 00:04:20
  Enter file name: balloon3.txt
  Elevation is 563.333
  Temperature is 17.1667

  Enter time: 01:34:30
  Enter file name: balloon3.txt
  Elevation is 840
  Temperature is -1

  Enter time: 00:02:59
  Enter file name: balloon3.txt
  Error: time is before first reading.

  Enter time: 01:45:00
  Enter file name: balloon3.txt
  Error: time is after last reading.

  Enter time: 00:04:00
  Enter file name: balloon4.txt
  Error: file balloon4.txt not found.
*/

#include <iostream> // cin, cout, endl
#include <fstream>  // ifstream
#include <string>   // string
#include <cstdlib>  // exit
using namespace std;

// Returns interpolated value f(b) given a, b, c
// on x axis and f(a), f(c) on y axis.

double interpolate(double a, double b, double c,
                   double fa, double fc)
{
  return fa + (fc-fa)/(c-a) * (b-a);
}

int main()
{

  // Read user time
  int hr, min, sec;
  char colon;
  cout << "Enter time (hh:mm:ss) : ";
  cin >> hr >> colon >> min >> colon >> sec;
  int b = hr*3600 + min*60 + sec; // user time

  // Get file name from user
  cout << "Please enter file name: ";
  string filename;
  cin >> filename;

  // Open file and check for file not found
  ifstream in(filename.c_str());
  if (!in)
  {
    cout << "File not found: " << filename
         << endl;
    exit(0);
  }

  double elev=0, temp=0;  // from current line of file
  double last_elev, last_temp; // from previous line
  int a=0, c=0;  // previous and current time in sec.
  bool first=true;  // first line of file?
  while (true)
  {
    // Save previous line of file
    a = c;
    last_elev = elev;
    last_temp = temp;

    // Read current line of file
    if (!(in >> hr >> colon >> min >> colon >> sec
       >> elev >> temp))
    {
      // error (too late)
      cout << "Error: time is after last reading\n";
      break;
    }
    c = hr*3600 + min*60 + sec; // current time
    if (c > b)
    {
      if (first)
      {
         // too soon
         cout << "Error: time is before first reading\n";
         break;
      }
      else 
      {
        // interpolate
        cout << "Elevation is " <<
          interpolate(a, b, c, last_elev, elev) << "\n";
        cout << "Temperature is " <<
          interpolate(a, b, c, last_temp, temp) << "\n";
        break;
      }
    }
    else if (c == b)
    {
      // match
      cout << "Elevation is " << elev << "\n"
           << "Temperature is " << temp << "\n";
      break;
    }
    first=false;
  }
  return 0;
}