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Fire test: recursive factorial

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Release： 2024-08-14 10:45:05

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Teste de fogo: fatorial recursivo

For those who are not following POJ (Pascal on the JVM) it is a compiler that transforms a subset from Pascal to JASM (Java Assembly) so that we can use the JVM as an execution environment.

In the last post, support for read/readln from Pascal was implemented, functions that allow reading data from standard input (stdin). In this publication we will complete one of the objectives of POJ: reading a number from standard input and calculating the factorial recursively.

As we are compiling for the JVM, it is necessary to detail the functioning of various points of this incredible virtual machine. Therefore, at various times I detail the internal functioning of the JVM as well as some of its instructions (opcodes).

Recursive factorial, the icing on the cake :-)

As mentioned at the beginning of the project, one of the objectives was to be able to calculate the factorial recursively, reading the number to be calculated from the standard input. The implementations in POJ to date have made this objective possible:

Hello world: created the project's code base with ANTLR, parser and code generation Java Assembly;
Addition, subtraction, multiplication, division operators and the beginning of conditional sentences - if: support for operators began as well as initial support for conditional sentences;
Repeat, while and for: support for repetition structures;
Functions in Pascal: implementation of recursive functions.

Now the time has come to validate what has been developed so far. That said, from the Pascal program below:

program fatorial;

var numero : integer;

function fatorial(n : integer) : integer;
begin
    if n<0 then fatorial := 0
    else begin
        if n<=1 then fatorial := 1
        else fatorial := n * fatorial(n-1);
    end;
end;

begin
    write('Introduza numero inteiro: ');
    readln(numero);
    writeln;
    writeln('O fatorial de ', numero, ' e: ', fatorial(numero));
end.

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POJ correctly generates the following assembly:

// Code generated by POJ 0.1
public class fatorial {
    public static numero I

    static fatorial(I)I {
        iload 0
        sipush 0
        if_icmpge L4
        iconst 1 
        goto L5
    L4: iconst 0 
    L5: ifeq L1
        sipush 0
        istore 100
        goto L2
    L1: iload 0
        sipush 1
        if_icmpgt L9
        iconst 1 
        goto L10
    L9: iconst 0 
    L10:ifeq L6
        sipush 1
        istore 100
        goto L7
    L6: iload 0
        iload 0
        sipush 1
        isub 
        invokestatic fatorial.fatorial(I)I 
        imul 
        istore 100
L7: L2: iload 100
        ireturn 
    }

    public static main([java/lang/String)V {
        getstatic java/lang/System.out java/io/PrintStream
        ldc "Introduza numero inteiro: "
        invokevirtual java/io/PrintStream.print(java/lang/String)V

        invokestatic java/lang/System.console()java/io/Console
        invokevirtual java/io/Console.readLine()java/lang/String
        invokestatic java/lang/Integer.parseInt(java/lang/String)I
        putstatic fatorial.numero I

        getstatic java/lang/System.out java/io/PrintStream
        invokevirtual java/io/PrintStream.println()V

        getstatic java/lang/System.out java/io/PrintStream
        ldc "O fatorial de "
        invokevirtual java/io/PrintStream.print(java/lang/String)V

        getstatic java/lang/System.out java/io/PrintStream
        getstatic fatorial.numero I
        invokevirtual java/io/PrintStream.print(I)V

        getstatic java/lang/System.out java/io/PrintStream
        ldc " e: "
        invokevirtual java/io/PrintStream.print(java/lang/String)V

        getstatic java/lang/System.out java/io/PrintStream
        getstatic fatorial.numero I
        invokestatic fatorial.fatorial(I)I 
        invokevirtual java/io/PrintStream.print(I)V

        getstatic java/lang/System.out java/io/PrintStream
        invokevirtual java/io/PrintStream.println()V

        return
    }
}

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Finally, the end

Here we come to the end of this learning journey with this project.

Some interesting things in the area of compilers can be explored with this project (such as optimization of generated code). Who knows, maybe in the near future we will start a new series with optimizations :-)

Complete project code

The repository with the project's complete code and documentation is here.

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