Due date: Sunday 1/31/2016 @ 11:59pm

- Please remember to answer the questions in REPORT.txt

Objective:
~~~~~~~~~~

- Improve your C programming skills
- Work with pointers, structs, and the dynamic heap

Assignment:
~~~~~~~~~~~

(1) Answer the questions in REPORT.txt

(2) Finish an interpreter for a more realistic language that the one we defined
in p1.

The interpreter generally has 4 concerns:

- Source character encoding. A string of ASCII characters in our case.

- The lexical rules: how those characters are converted to a sequence of
  tokens. Our language has the following tokens.

  * keywords: if else while

  * special characters: = == ( ) { } + * ;

  * identifiers: start with lower case letters followed by a sequence of
                 lower case letters and numbers.

         examples: x hello h123 xy3yt

  * immediate values: sequences of digits representing integers. They always
                 start with a digit but could contain '_' characters that are
                 ignored

         examples: 12 0 12_234

- The syntax: rules for combining tokens into meaningful constructs. Our syntax
  looks like familiar syntax for popular languages like C and Java. You're given
  most of that code but you need to change it in order to implement the "if" and
  "while" statements. Things to keep in mind about our language:

  * ';' is optional

  * we only have assignment, if, and while statements

  * the following operators are supported '*', '+', '=='

- Semantics: assigning meaning to syntactically correct programs

What you need to do:
~~~~~~~~~~~~~~~~~~~~

- Finish the implementation of the lexer. In order to keep the implementation
  simple, the code assumes that there is a global variable that represents
  the current valid token, starting with the first token in the program.

  In order to finish the lexer, you need to implement the following functions:

    char *remaining(); // returns the program text starting to the current token
                       // used for error reporting

    void consume();    // consume the current token, move the cursor to the
                       // next valid token. Must handle end of string.

    int isWhile();     // current token is "while"

    int isIf();        // current token is "if"

    int isElse();      // current token is "else"

    int isSemi();      // current token is ';'

    int isLeftBlock(); // current token is '{'

    int isRightBlock();// current token is '}'

    int isEq();        // current token is '='

    int isEqEq();      // current token is "=="

    int isLeft();      // current token is '('

    int isRight();     // current token is ')'

    int isEnd();       // current token is end of string (ASCII 0)

    int isId();        // current token is identifier

    int isInt();       // current token is an immediate value

    int isMul();       // current token is '*'

    int isPlus();      // current token is '+'

    char *getId();     // if current token is an idenifier, return it as a
                          string

    int getInt();     // if current token is an immediate value, return the value

- The language syntax is expressed using recursive functions starting with
  the "program()" function. You need to understand what it's doing and you'll
  need to make some changes to it in order to implement the "if" and "while"
  statements.

- You'll need to implement a symbol table, exposed using those two functions:

    int get(char *id);             // return the value of the given identifier

    void set(char *id, int value); // set the value of the given identifier
                                   // and print it

Examples:
~~~~~~~~~

./p2 "x = 10;"
x:10

./p2 "x = 2_09 ; a = 17;"
x:209
a:17

./p2 "x = 2_09 ; a = 17; x = 3 + 12"
x:209
a:17
x:15

Files you're allowed to change:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   p2.c

Files you're supposed to leave alone:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   Everything else
   
To compile:
~~~~~~~~~~~

    make

To run test:
~~~~~~~~~~~~

    make clean test

To run one test (e.g. t1):
~~~~~~~~~~~~~~~~~~~~~~~~~~

    make clean t1.run

To make the output less noisy:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    make -s clean test

To add your own tests:
~~~~~~~~~~~~~~~~~~~~~~

Let's say you want to create a test called "bob".

   - create a file bob.test that contains a program in the first line
   - create a file bob.ok that contains the expected output

To debug with gdb
~~~~~~~~~~~~~~~~~

    make
    gdb ./p2
    (gdb) run "x = 100; a = 30;"