1. Introduction

I will start with a high-level description of the project and its tasks in this section and then go into a detailed description on how to go about achieving these tasks in subsequent sections. The goal of this project is to implement a simple compiler for a simple programming language. By implementing this simple compiler, you will do basic parsing and use some simple data structures which would be useful for the other projects.

The input to your program will have two parts:

1. The first part of the input is a program which is a list of procedure declarations followed by a main procedure.
2. The second part of the input is a sequence of integers which will be used as the input to the program given in the first part.

Your compiler will read the program, represent it internally in appropriate data structures, and then it will execute the program using the second part of the input as the input to the program. The output of the compiler is the output produced by the program execution. More details about the input format and the expected output of your program are given in subsequent sections. The following is a high-level illustration of what your compiler should do for a particular example program

The remainder of this document is organized as follows.

1. The second section describes the input format.
2. The third section describes the expected output.
3. The fourth section describes the requirements on your solution.
4. The fifth section gives instructions for this programming assignment and additional instructions that apply to ALL programming assignments in this course.

2 Input Format

2.1 Grammar and Tokens

The input of your program is specified by the following context-free grammar:

input		programinputs
program		main
program		proc_decl_sectionmain
proc_decl_section		proc_decl
proc_decl_section		proc_declproc_decl_section
proc_decl		PROCprocedure_nameprocedure_bodyENDPROC
procedure_name		ID
procedure_name		NUM
procedure_body		statement_list
statement_list		statement
statement_list		statementstatement_list
statement		input_statement
statement		output_statement
statement		procedure_invocation
statement		do_statement
statement		assign_statement
input_statement		INPUT ID SEMICOLON
output_statement		OUTPUT ID SEMICOLON
procedure_invocation		procedure_name SEMICOLON
do_statement		DO ID procedure_invocation
assign_statement		ID EQUAL expr SEMICOLON
expr		primary
expr		primary operator primary
operator		PLUS
operator		MINUS
operator		MULT
operator		DIV
primary		ID
primary		NUM
main		MAIN procedure_body
inputs		NUM
inputs		NUM inputs

The code that we provided has a class LexicalAnalyzer with methods getToken() and ungetToken(). Your parser will use the provided functions to get or unget tokens as needed. You do not need to change the functions getToken() and ungetToken(); you just use them as provided. To use the methods, you should first instantiate a lexer object of the class LexicalAnalyzer and call the methods on this instance. The definition of the tokens is given below for completeness (you can ignore it for the most part if you want).

char =	a | b | | z | A | B | | Z | 0 | 1 | | 9
letter =	a | b | | z | A | B | | Z
pdigit =	1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
digit =	0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
SEMICOLON	= ;
PLUS	= +
MINUS	=
MULT	= *
DIV	= /
EQUAL	= =
MAIN	= (M).(A).(I).(N)
PROC		= (P).(R).(O).(C)
ENDPROC		= (E).(N).(D).(P).(R).(O).(C)
INPUT		= (I).(N).(P).(U).(T)
OUTPUT		= (O).(U).(T).(P).(U).(T)
DO		= (D).(O)
NUM		= 0 | pdigit . digit*
ID		= letter . char*

What you need to do is to write a parser to parse the input according to the grammar and store the information being parsed by your parser in appropriate data structures to allow your program to _execute the input program on the inputs. For now do not worry how that is achieved. I will explain that in details.

2.2 Examples

The following are examples of inputs (to your compiler) with corresponding outputs. The output will be explained in more details in the next section.

1. MAIN

X = 1; Y = X;

OUTPUT X;

OUTPUT Y;

1 2 3 18 19

This example shows a program with no procedure declarations (PROC) and a MAIN procedure that does not read any input. The output of the program will be

1 1

The sequence of numbers at the end (the input to the program) does not affect the output of the program.

2. MAIN

INPUT X; INPUT Y;

• = X + Y;
• = X+Y;

OUTPUT X; OUTPUT Y;

3 7 18 19

This is similar to the previous example, but here we have two input statements. The first input statement reads a value for X from the sequence of numbers and X gets the value 3. The second input statement reads a value for Y which gets the value 7. Here the output will be

10 17

Note that the values 18 and 19 are not read and do not affect the execution of the program.

3. PROC INCX
   • = X + 1;

ENDPROC MAIN

INPUT X;

INPUT Y;

INCX; INCX;

• = X+Y;

OUTPUT Y;

OUTPUT X;

3 18 19

In this example, we have a procedure called INCX. When the procedure is invoked, the code of the procedure is executed. In this case, X is incremented by 1. The second invocation also increments X again so the final value of X is 5. The output is the following.

23 5

4. PROC INCX
   • = X + 1;

ENDPROC MAIN

INPUT X; INPUT Y;

INPUT Z;

DO Z INCX;

Y = X+Y;

OUTPUT Y; OUTPUT X;

3 18 2 19

This is similar to the previous example, but instead of invoking INCX two separate times, we achieve the same result with a do_statement. The statement INPUT Z; assigns the value 2 to Z and DO Z INCX; will invoke INCXZ times (with the value of Z equal to 2). The output of this program is as before

23 5

For your parser, the parse function does not necessarily return true or false. They can return other quantities. For example, it might be useful for parse_primary() to return the token of the primary.

3 Semantics

In this section I give a precise definition of the meaning of the input and the output that your compiler should generate.

3.1 Variables and Locations

The program uses names to refer to variables. For each variable name, we associate a unique locations that will hold the value of the variable. This association between a variable name and its location is assumed to be implemented with a function location that takes a string as input and returns an integer value. We assume that there is a variable mem which is an array with each entry corresponding to one variable. All variables should be initialized to 0.

To support allocation of variables to mem entries, you can have simple table or map (which I will call the location table) that associates a variable name with a location. As your parser parses the input program, if it encounters a variable name, it needs to determine if this name has been previously encountered or not by looking it up in the location table. If the name is a new variable name, a new location needs to be associated with it and the mapping from the variable name to the location needs to be added to the location table. To associate a location with a variable, you can simply keep a counter that tells you how many locations have been used (associated to variable names). Initially the counter is 0. The first variable to be associated a location will get the location whose index is 0 (mem[0]) and the counter will be incremented to become 1. The next variable to be associated a location will get the location whose index is 1 and the counter will be incremented to become 2 and so on. For example, if the input program is

MAIN

INPUT X; INPUT Y;

• = X + Y;
• = X+Y;
• = X+Y;

W = Z;

OUTPUT X; OUTPUT Y;

3 7 18 19

Then the locations of variables will be

X 0 Y 1

Z 2

W3.2 Statements 3

We explain the semantics of each statement in terms of an implementation model that assigns locations to variables and that was described in the previous section.

3.2.1 Assignment Statement

We consider an assignment of the form

ID = primary1 operator primary2

This has the following effect mem[location(t1.lexeme)] = value(primary1) operator value(primary2)

where t1 is the ID token for the lhs for the assignment and value(primary) is equal to atoi(primary.lexeme) or mem[location(primary.lexeme)] depending on whether or not primary is NUM or ID respectively. For example, if the input program is

MAIN

INPUT X; INPUT Y;

• = Y + 3;
• = X+Y;
• = X+Y;

W = Z;

OUTPUT X; OUTPUT Y;

3 7 18 19

the statement X = Y+3 will be equivalent to mem[0] = mem[1] + 3.

3.2.2 Input and Output

Input and output statements are relatively straightforward OUTPUT X is equivalent to cout mem[location(X)];

• INPUT X is equivalent to cin mem[location(X)];.

3.2.3 Procedure Invocation

A procedure invocation

ID ;

is equivalent to executing every statement in the procedure at the point of the call. The execution of

stmt1 stmt2 stmtk

P; stmtk+1 stmtk+2 stmtm

where P is the name of a procedure declared as

PROC P stmt1 stmt2

stmtm

ENDPROC

is equivalent to

stmt1 stmt2 stmtk stmt1 stmt2

stmtm stmtk+1 stmtk+2 stmtm

3.2.4 DO Statememt

The statement DO ID1 ID2; where the first identifier is the name of a variable and the second identifier is the name of a procedure is equivalent to _n invocations of ID2 where _n is the value of ID1 at the point the do_statement is executed. If the variable that determine the number of invocations is modified in the procedure body, that does not affect the number of invocations. For example

PROC P

X = X+1;

ENDPROC MAIN

INPUT X;

DO X P;

OUTPUT X;

3

The statement DO X P; is equivalent to P; P; P;. The output of the program will be 6.

3.3 Assumptions

You can assume that the following semantic errors are not going to be tested

1. Two procedures declared with the same name. You can assume that all procedure names are unique. So, an invocation of a procedure cannot be ambiguous.
2. You can assume that if there is an invocation with a given procedure name, there must be a procedure declaration with the same name.
3. You can assume that if there is an invocation with a given procedure name, then there is no variable with the same name in the program.
4. You can assume that if there is a variable with a given name in the program, then there is no procedure declaration for a procedure with the same name as the variable.
5. If you want to use an array for the mem variable, you can use an array of size 1000 which should be enough for all test cases, but make sure that your code handles overflow (more than 1000 variables in the program) because that is good programming practice.

In addition, you can assume that the input will not have recursive procedure invocations.

• Requirements

You should write a program to generate the correct output for a given input as described above. You should start by writing the parser and make sure that it correctly parses the input before attempting to implement the rest of the project.

You will be provided with a number of test cases. Since this is the first project, the number of test cases provided with the project will be relatively large, but it will not be complete. It is still your responsibility to make sure that your implementation satisfies the requirements given in this document.

• Instructions

Follow these steps:

• Download the lexer.cc, lexer.h, inputbuf.cc and inputbuf.h files accompanying this project description.
• Design a solution before you start coding. It is really very important that you have a clear overall picture of what the project will require before starting coding. Deciding on data structures and how you will use them is crucial. One possible exception is the parser, which you can and should write first before the rest of the solution.
• Write your code and make sure to compile your code using GCC (7.4.0) on _{Ubuntu 18.04} (Ubuntu). These are the versions available on the second floor computer lab in the Brickyard. If you want to test your code on your personal machine, you should install a virtual machine with Ubuntu 18.04 and the correct version of GCC on it. You will need to use the g++ command to compile your code in a terminal window. See section 4 for more details on how to compile using GCC. _{You are required to}

compile and test your code on Ubuntu using the GCC compiler, but you are free to use any IDE or text editor on any platform while developing your code as long as you compile it and test it on Ubuntu/GCC before submitting it.

• Test your code to see if it passes the provided test cases. You will need to extract the test cases from the zip file and run the provided test script test1.sh. See section 5 for more details.
• Submit your code through canvas.
• Only the last version you submit is graded. There are no exceptions to this.

Keep in mind that

• You should use C++11, no other programming languages or versions of C++ are allowed.
• All programming assignments in this course are individual assignments. Students must complete the assignments on their own.
• You should submit your code through canvas, no other submission forms will be accepted.
• You should familiarize yourself with the Ubuntu environment and the GCC compiler. Programming assignments in this course might be very different from what you are used to in other classes.

5.0.1 Evaluation

The submissions are evaluated based on the automated test cases on the submission website. Your grade will be proportional to the number of test cases that your submission passes. If your code does not compile when you submit it, you will not receive any points even if it runs correctly on a different environment (Windows/Visual Studio for example).

NOTE: The next two sections apply to all programming assignments.

You should use the instructions in the following sections to compile and test your programs for all programming assignments in this course.

5.0.2 Compiling your code with GCC

You should compile your programs with the GCC compilers. GCC is a collection of compilers for many programming languages. There are separate commands for compiling C and C++ programs. Use the g++ command to compile C++ programs

Here is an example of how to compile a simple C++ program:

$ g++ test_program.cpp

If the compilation is successful, it will generate an executable file named a.out in the same directory (folder) as the program. You can change the executable file name by using the -o option. For example, if you want the executable name to be hello.out, you can execute

$ g++ test_program.cpp -o hello.out

To enable C++11, with g++, which you should do for projects in this class, use the -std=c++11 option:

$ g++ -std=c++11 test_program.cpp -o hello.out

The following table summarizes some useful compiler options for g++:

Option	Description
-o path	Change the filename of the generated artifact
-g	Generate debugging information
-ggdb	Generate debugging information for use by GDB
-Wall	Enable most warning messages
-std=c++11	Compile C++ code using 2011 C++ standard

Compiling projects with multiple files

If your program is written in multiple source files that should be linked together, you can compile and link all files together with one command:

$ g++ file1.cpp file2.cpp file3.cpp

Or you can compile them separately and then link:

$ g++ -c file1.cpp $ g++ -c file2.cpp $ g++ -c file3.cpp

$ g++ file1.o file2.o file3.o

The files with the .o extension are object files but are not executable. They are linked together with the last statement (g++ file1.o file2.o file3.o) and the final executable will be a.out.

5.0.3 Testing your code on Ubuntu

Your programs should not explicitly open any file. You can only use the _{standard input} and _standard output in C++. The provided lexical analyzer already reads the input from standard input and you should not modify it. In C++, standard input is std::cin and standard output is std::cout. In C++, any output that your program produces should be done with cout. To read input from a file or produce output to a file, we use IO redirection outside the program. The following illustrates the concept.

Suppose we have an executable program a.out, we can run it by issuing the following command in a terminal (the dollar sign is not part of the command):

$ ./a.out

If the program expects any input, it waits for it to be typed on the keyboard and any output generated by the program will be displayed on the terminal screen.

To get the input to the program from a file, we can redirect the standard input to a file:

$ ./a.out < input_data.txt

Now, the program will not wait for keyboard input, but rather read its input from the specified file as if the file input_data.txt is standard input. We can redirect the output of the program as well:

$ ./a.out > output_file.txt

In this way, no output will be shown in the terminal window, but rather it will be saved to the specified file^[1].

Finally, its possible to do redirection for standard input and standard output simultaneously. For example,

$ ./a.out < input_data.txt > output_file.txt will read standard input from input_data.txt and produces standard output to output_file.txt.

Now that we know how to use standard IO redirection, we are ready to test the program with test cases.

Test Cases

For a given input to your program, there is an _expected output which is the correct output that should be produced for the given input. So, a test case is represented by two files:

• txt
• txt.expected

The input is given in test_name.txt and the expected output is given in test_name.txt.expected. To test a program against a single test case, first we execute the program with the test input data:

$ ./a.out < test_name.txt > program_output.txt

With this command, the output generated by the program will be stored in program_output.txt. To see if the program generated the correct expected output, we need to compare program_output.txt and test_name.txt.expected. We do that using the diff command which is a command to determine differences between two files:

$ diff -Bw program_output.txt test_name.txt.expected

If the two files are the same, there should be no difference between them. The options -Bw tell diff to ignore whitespace differences between the two files. If the files are the same (ignoring the whitespace differences), we should see no output from diff, otherwise, diff will produce a report showing the differences between the two files.

We consider that the test _passed if diff could not find any differences, otherwise we consider that the test failed.

Our grading system uses this method to test your submissions against multiple test cases. In order to avoid having to type the commands shown above for running and comparing outputs for each test case manually, we provide you with aa script that automates this process. The script name is test1.sh. test1.sh will make your life easier by allowing you to test your code against multiple test cases with one command.

Here is how to use test1.sh to test your program:

• Store the provided test cases zip file in the same directory as your project source files
• Open a terminal window and navigate to your project directory
• Unzip the test archive using the unzip command: bash $ unzip tests.zip

This will create a directory called tests

• Store the test1.sh script in your project directory as well
• Make the script executable: bash $ chmod +x test1.sh
• Compile your program. The test script assumes your executable is called a.out
• Run the script to test your code: bash $ ./test1.sh

The output of the script should be self explanatory. To test your code after you make changes, you will just perform the last two steps (compile and run test1.sh).

[1] Programs have access to another standard stream which is called standard error e.g. std::cerr in C++. Any such output is still displayed on the terminal screen. It is possible to redirect standard error to a file as well, but we will not discuss that here