Program 1 from Micro Adventure No. 9: Dead Ringer in its original AppleSoft BASIC and converted to C

decoder.bas

100  REM  DECODER
110  GOSUB 900
120  PRINT "(TYPE 'STOP' TO END)" :PRINT
130  PRINT "ENTER MESSAGE:"
140  INPUT M$
150  IF M$="STOP" THEN END
160  FOR I=1 to LEN(M$)
170  A$=M$:SB=I:SE=1:GOSUB 800
180  L$=XC$
190  IF ((L$<"A")+(L$>"Z")) THEN 250
200  KT=KT+1:IF KT>26 THEN KT=1
210  T=ASC(L$):T=T-KT
220  IF T< ASC("A") THEN T=T+26
230  PRINT CHR$(T);
240  GOTO 260
250  PRINT L$;
260  NEXT I
270  PRINT:PRINT:GOTO 130
800  XC$=MID$(A$,SB,SE):RETURN
900  HOME:RETURN

decoder.c

/*
To compile this code, run:

cc -o decoder decoder.c

To run it, run:

./decoder
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[]) {
    char character, done = 0, kt = 0, *message = NULL;
    size_t buffer_size = 0, index;
    ssize_t length = 0;

    printf("(TYPE 'STOP' TO END)\n\n");
    while (!done) {
        printf("ENTER MESSAGE:\n");
        putchar('?');
        length = getline(&message, &buffer_size, stdin);
        if (length == -1 || strncmp(message, "STOP\n", buffer_size) == 0) {
            done = 1;
        } else {
            for (index = 0; index < length; ++index) {
                character = message[index];
                if (character >= 'A' && character <= 'Z') {
                    ++kt;
                    if (kt > 26) kt = 1;
                    character -= kt;
                    if (character < 'A') character += 26;
                }
                putchar(character);
            }
        }
        putchar('\n');
    }
    if (message) {
        free(message);
        message = NULL;
    }
    return 0;
}