import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;

/* CSVQueryR.java
** Java program that allows the user to run "queries" on the records in
** a Comma Separated Values (CSV) file, the name of which is expected to
** be provided via a command line argument (aka a "run argument" in jGrasp's
** terminology).  The file is assumed to have a "header" record (itself in
** the form of a CSV-string) containing the titles of the fields of the
** remaining records.
** To run a query on a CSV-record is simply to enter a field title for the
** purpose of asking for the value of the named field of that record to be
** displayed.
** Initially, the record that is subject to being queried is the second one 
** in the file (i.e., the one immediately following the header record).  The
** user can advance from one record to the next by entering an
** appropriate response at the program's prompt.  The user can also quit.
**
** Authors: P.M.J. and R.W.M.
** Date: November 2024
** Last modified: December 2024
*/

public class CSVQueryR {

   // Constants 
   static final String QUIT = "Q";   // command to quit
   static final String NEXT = "N";   // command to advance to next record in file
   static final String TITLES = "T"; // command to display field titles

   static final String COMMA = ","; 
   static final String QUESTION_MARK = "?";

   static CSV_String fieldTitles;
   static CSV_String currentRec;
   
   /* The name of a correctly formatted "CSV file" is expected as the single 
   ** command-line argument (a.k.a. run argument, in jGrasp's terminology).
   */
   public static void main(String[] args) throws FileNotFoundException {
      // Establish a Scanner that is able to read from the file whose name
      // was provided via the command-line argument.
      String fileName = args[0];
      Scanner fileInput = new Scanner(new File(fileName));

      // Read the first line from the file, which is the header record
      // containing the titles of the fields of the remaining records..
      fieldTitles = new CSV_String(fileInput.nextLine());

      // Read the second line from the file and make a CSV_String object
      // from it.  It becomes the record initially subject to being queried.
      currentRec = new CSV_String(fileInput.nextLine());

      // Display the newly-established "current record".
      printCurrentRec();

      // Establish a Scanner to read input from the keyboard.
      Scanner keyboard = new Scanner(System.in);

      // Each iteration of the following loop prompts the user to enter a
      // command, which is expected to be either the code to advance to the
      // next record ('N'), the code to quit ('Q'), the code to ask for the
      // titles of the fields ('T'), or the title of one of the fields.  In 
      // the last case, the value of that field (within the current record)
      // is displayed.
      boolean done = false;
      do {
         String query = getString("\nEnter query:>",keyboard).trim();
         if (query.equals(QUIT)) {
            done = true;
         } 
         else if (query.equals(TITLES)) {
            System.out.println(fieldTitles);
         } 
         else if(query.equals(NEXT)) {
            if(fileInput.hasNext()) {
               // Read the next record and use it to make a CSV_String object
               currentRec = new CSV_String(fileInput.nextLine());
               // Display the newly-established "current" record.
               printCurrentRec();
            } 
            else {
               done = true;
               System.out.println("---End of file reached!!!");
            }
         } else {
            // The query is (probably) the title of a field;
            // find the position of that field.
            int fieldPos = title2Position(query);

            // Retrieve the value of the field from within the current record.
            String fieldVal = currentRec.elementAt(fieldPos);

            // Display the value of the retrieved field or, if appropriate,
            // report that the query was not valid
            if (fieldVal == null) {
               System.out.println("--->Invalid query; try again.");
            } else {
               System.out.println(fieldVal);
            }
         }
      }  while (!done);      
      System.out.println("\nDone!");
   }

   private static void printCurrentRec() {
      System.out.println("Currect record is now: " + currentRec);
   }

   /* Returns the position of the field within the 'fieldTitles' record
   ** that matches the given 'title', or -1 if there is no such field. 
   ** This version of the method utilizes the "iterator" methods of the 
   ** CSV_String class.
   */
   private static int title2Position(String title) {
      fieldTitles.reset();  
      int k = 0;
      boolean keepGoing = true;
      // Loop invariant: None among field titles 0, 1, ... , k-1 matches 'title'.
      //    If k == -1, no field title matches 'title'
      while (keepGoing) {
         if (!fieldTitles.hasNext()) {
            keepGoing = false; k = -1;
         }
         else if (fieldTitles.next().equals(title)) {
            keepGoing = false;
         }
         else {
            k = k+1;
         }
      }
      // Assert: If k == -1, no field title matches 'title'; 
      //         otherwise, field title #k matches 'title'.
      return k;
   }

   /* Alternative version of the method above.
   ** This version uses the CSV_String class's elementAt() method 
   ** rather than its "iterator" methods.
   */
   private static int title2PositionAlt(String title) {
      final int N = fieldTitles.numberOfFields();
      int k = -1;
      // Loop invariant:
      //    None among fields 0, 1, ..., k-1 of 'fieldTitles' matches 'title'
      do {
         k = k+1;
      } 
      while (k != N  &&  !title.equals(fieldTitles.elementAt(k)));

      // Assert: either 0 <= k < N and the k-th field of 'fieldTitles' matches
      //         'title', or else k == N and there is no such matching field.
      if (k == N) 
         { k = -1; }  // there is no matching title 
      return k;
   }



   /* Prints the given prompt and then reads and returns the full line of
   ** input (as a string) entered by the interactive user.
   */
   public static String getString(String prompt, Scanner input) {
      System.out.print(prompt);
      String line = input.nextLine();
      // System.out.println(line);
      return line;
   }
 
}