**************************************************************
*                           DB_ENGINE                        *
*                           ---------                        *
*                README: INSTALLATION INSTRUCTIONS           *
*                ---------------------------------           *
*                          Shivam Garg                       *
*                           14074017                         *
*	         ([email protected])           *
**************************************************************

NOTE: This program is not robust to incorrect syntax and will often
crash due to incorrect syntax.

Installation Instructions:

1. Unzip the folder db_engine.tar to a folder of your choice.
2. Go inside the extracted folder.
3. Install by running "make" inside the folder.
4. Now the program is installed and may be run by "./main.out".
5. To uninstall, simply type "make clean".
6. The archive can be recreated by "make tar".


Running the queries:

1. Check the data.txt for examples and syntax information.


A note on working:
      
 1.  The program is split into multiple files, each file containing
     a group of related functions.
 2.  There is a main.cpp file, that begins the execution of the
     program. 
 3.  It initially calls read_db_dat() function from db_rw.hh to 
     read prior DB sessions.
 4.  It then runs loop, in each iteration it reads a query and
     sends it to parser() function from support_fn.hh.
 5.  parser check if the query is of INSERT, CREATE, EXIT, 
     SHOW_TABLES, or RA query.
 6.  For first type it calls appropriate functions from table_fn.hh.
 7.  For RA queries, it calls the ra() function.
 8.  ra() converts the infix query to reverse polish by calling
     shunting_yard_nest() from expr_eval.hh.
 9.  It then evaluates this reverse polish query by calling 
     eval_nest() from expr_eval.hh.
10.  eval_nest(), calls the corresponding functions, such as
     select(), project(), rename(), etc. from ra_op.hh and aggregate
     function from aggregate.hh.
11.  select() in particular can have a big infix query. This query
     is evaluated by conversion into reverse polish and its evaluation
     by calling shunting_yard_adv() and eval() function from 
     expr_eval.hh respectively. 
12.  eval_nest() after evaluating the reverse polish of the query
     returns the final table containg results of the query to ra().
13.  ra() then prints these queries on the screen by calling
     print_table() function from table_fn.hh.
14.  Finally the control passess back to main.cpp, which calls 
     the write_db_dat() function from db_rw.hh which stores the query
     if it was of INSERT or CREATE type, for future sessions.
15.  The program ends its execution via the query "EXIT;".


16.  All the data structures used are stored in data_str.hh and the
     functions of expr_eval.hh, ra_op.hh, support_fn.hh, table_fn.hh
     and aggregate.hh use it.
17.  EXPLANATORY COMMENTS OF data_str.hh are given below for quick
     reference:

// PICTORIAL REPRESENTATION OF A TABLE
// -----------------------------------
//                   Columns
//                      ^
//           ___________|____________
//          /                        \     // for multiline comment warning
// Table -> Col1  Col2  Col3 ...  Coln  
//           |     |     |         |    
//           v     v     v         v    
//          Val1  Val1  Val1 ...  Val1  \  // for multiline comment warning
//          Val2  Val2  Val2 ...  Val2  |
//          Val3  Val3  Val3 ...  Val3  |
//           .     .     .         .    -->Attribute
//           .     .     .         .    |
//           .     .     .         .    |
//          Valm  Valm  Valm ...  Valm  /
//
// NOTE: Indexing is from 1->n in diagram, and 0->(n-1) in the program.
// ----
// 
// struct attribute {
//   string name, type;
//   vector < string > values;
// };
//
// struct table {
//   string name;
//   vector < attribute > columns;
// };



                            ~~ END ~~