MINIBASIC

A tiny but powerful interpreter that allows a user to code, save and run programs in BASIC.

1. Overview

1.1 Supported usage

Required COMMAND in the document and add support for function.
Immediately print the comstant expression and expression only contain constant number and defined varivables.

HINT: Each token should split by a blankspace. -1 can either prepresent as -1 or - 1. support expression like 1 * - 2
SYNTAX ERROR will be printed on the console during writing program. RUNTIME ERROR will be printed and stop the running at once.

1.2 Files

basic: Export a QTextEdit class to implement a console to interact with the user. Process the command and print information.
tokenizer: Provide support to scan strings and divide it into a list of tokens.
parser: Provide support to scan statements (create different statements according to the command scaned) and to scan expressions (create a binary expression tree according to the tokens scanned).
statement: Export class Statement and its concrete subclasses.
expression: Export class Expression and its three concrete subclasses: ConstantExp, IdentifierExp, CompoundExp.
evaluation: Export a class called Evaluation. A space storing all additional information required by the evaluator(e.g. the declared functions and variables). Act like %rip, we use it to change the execution order of programs.

2. Main Procedure

2.1 process the input line

The keyPressEvent function receives the line input by the user and trigger the processLine function.
If this line begins with a number, it trigger parseStatement function and store it in the program.
Else, it trigger the processCommand function to process the command.

2.2 process the code

First store the program, then use parseStatement function exported in the parser class to parse this line.
According to the command, the parseStatement function new different Subclass of Statement.
Each constructor of Statement check the expression of the program and stored the information (like the variable name, the GOTO line number) it needed when being executed. If the expression is valid, it call setParsed function implemented by program.h file to decide whelter to remove this line of code.

2.3 process the command

According to the command, the processCommand function call different functions.
listProgram print the lines stored in program out on the window, clear delete all the previous information and printHelpMsg print all the help information.

run has the following structures:

int index = first line number of this program
while(index != -1){
    execute the statment
    if (eval->getNextLineNum() != -1){
        index = eval->getNextLineNum();
    }else {
        index = program->getNextLineNum(index);
    }
}

eval is used to change the line of the program to be executed, which supports the GOTO and IF THEN command.

2.4 details of how `execute()` work

2.4.1 evaluation

Expression class create a Expression tree, each node is a IdentifierExp class, a CompoundExp class or a ConstantExp class.
the evalution method use recursive algorithm to return the value of a expression tree.
the value of variables are stored in evaluation class.

2.4.2 input

Use RUNNING and UNRUNNING flag in console to mark whelter the program is running. When running, if get a value inputted, store it.
Check whelter the value is inputted every 0.1s by implementing waitSec method.
Check the validity of the input, if it is invalid, repeat again.

2.4.3 function

Store the function name, their starting line and end line in evaluation class.
When meet CALL command, push the return line in the stack. When reach the end of the function again, pop the line and set next line to it.

3. Error Handling

process SYNTAX ERROR

  try{
      processLine(Line);
  }catch (const QString error) {
      this->write(error);
  }

process RUNTIME ERROR

   try{
       getParsedStatement(index)->execute(eval);
   }catch(int){
       break;
   }catch(const QString error) {
       this->write(error);
       break;
   }

promisivia/Mini-Basic-Interpreter