#include <iostream.h>
#include <fstream.h>
#include <conio.h>


void pause (void) {

  cout << "Hit any key to continue...\n";
  getch ();

}


bool read_in_table (int capacity,
                    double temperatures[], double pressures[],
                    double u_sub_f[], double u_sub_g[],
                    int &table_size) {

  ifstream fin;
  char file_name[60];
  double temperature_value;

  for (;;) {
    cout << "Enter name of input file: ";
    cin >> file_name;
    fin.open (file_name);
    if (!fin.fail()) {
      break;
    }
    cout << "Unable to open input file - please try again.\n";
  }

  table_size = 0; // no values so far

  for (;;) {

    fin >> temperature_value;

    if (fin.fail()) {

      if (!fin.eof()) {
        cout << "\nThe input file contains a bad temperature value.\n";
        return false;
      }

      if (table_size < 2) {
        cout << "The table contains fewer than two lines.\n";
        return false;
      }

      return true;

    }

    if (table_size == capacity) {
      cout << "\nThe table is too large to be read..\n";
      return false;
    }

    temperatures[table_size] = temperature_value;

    fin >> pressures[table_size] >> u_sub_f[table_size] >> u_sub_g[table_size];

    if (fin.fail()) {
      cout << "\nThe input files contains  bad data or a partial line.\n";
      return false;
    }

    table_size++;

  }

}


void output_results (double T,
                     double pressure, double u_sub_f, double u_sub_g) {

  cout << "\nAt " << T << " degrees C:\n"
       << "  Pressure = " << pressure << " bar\n"
       << "  u sub f = " << u_sub_f << " KJ/kg\n"
       << "  u sub f = " << u_sub_g << " KJ/kg\n";

}


void calculate_and_output (int table_size,
                           double temperatures[], double pressures[],
                           double u_sub_f[], double u_sub_g[],
                           double T) {

  int low, high, mid;
  double k;

  if ((T < temperatures[0]) || (T > temperatures[table_size - 1])) {
    cout << "Temperature is not within the range covered by the table.\n";
    return;
  }

  low = 0; high = table_size - 1;

  for (;;) {

    if (high == (low + 1)) {
      k = (T - temperatures[low]) / (temperatures[high] - temperatures[low]);
      output_results (T,
                      pressures[low] + (k * (pressures[high] - pressures[low])),
                      u_sub_f[low] + (k * (u_sub_f[high] - u_sub_f[low])),
                      u_sub_g[low] + (k * (u_sub_g[high] - u_sub_g[low])));
      return;
    }

    mid = (low + high) / 2;

    if (temperatures[mid] == T) {
      output_results (T, pressures[mid], u_sub_f[mid], u_sub_g[mid]);
      return;
    }

    if (temperatures[mid] > T) {
      high = mid;
    } else {
      low = mid;
    }

  }

}


void main (void) {

  const int capacity = 100;
  double temperatures[capacity], pressures[capacity],
         u_sub_f[capacity], u_sub_g[capacity],
         T;
  int table_size;

  if (!read_in_table(capacity, temperatures, pressures,
                               u_sub_f, u_sub_g, table_size)) {
    cout << "\nUnable for read table - program execution terminated.\n";
    pause ();
    return;
  }

  cout << "\nTable successfully read.\n"
       << "The table contains " << table_size << " entries.\n"
       << "Temperatures run from " << temperatures[0] << " degrees C to "
       << temperatures[table_size - 1] << " degrees C.\n";

  for (;;) {

    cout << "\nEnter temperature (-999 to end): ";
    cin >> T;

    if (T == -999) {
      return;
    }

    calculate_and_output
        (table_size, temperatures, pressures, u_sub_f, u_sub_g, T);

  }

}