"Hello world" demo program for gdb talk

transforming_hello.c

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

const int BUFSIZE = 1024;
const int ARBITRARY_THRESHOLD = 3;

int is_uppercase(char ch) {
    return ch >= 'A' && ch <= 'Z';
}

int is_lowercase(char ch) {
    return ch >= 'a' && ch <= 'z';
}

char flip_case(char ch) {
    /* If ch is an uppercase letter, return it in lowercase.
     * If ch is a lowercase letter, return it in uppercase.
     * Otherwise, return ch. */
    if (is_uppercase(ch)) {
        return ch - 'A' + 'a';
    } else if (is_lowercase(ch)) {
        return ch - 'a' + 'A';
    } else {
        return ch;
    }
}

char rot13(char ch) {
    /* If ch is a letter, offset it by 13 (wrap back to start of alphabet
     * if necessary). */
    int offset = 0;
    if (is_uppercase(ch)) {
        offset = 'A';
    } else if (is_lowercase(ch)) {
        offset = 'a';
    } else {
        return ch;
    }

    return (ch - offset + 13) % 26 + offset;
}

int main(int argc, char **argv) {
    int intuitive_arg_count = argc - 1;
    char *buf = (char *) malloc(BUFSIZE);  // sizeof(char) is defined as 1

    snprintf(buf, BUFSIZE,
             "Hello, world! I was called with %d argument%s.\n",
             intuitive_arg_count,
             (intuitive_arg_count) == 1 ? "" : "s");
    
    char (*fn)(char);  // fn: pointer to function that takes and returns a char
    if (intuitive_arg_count > ARBITRARY_THRESHOLD) {
        fn = &flip_case;
    } else {
        fn = &rot13;
    }

    for (int i = 0; i < strlen(buf); i++) {
        buf[i] = (*fn)(buf[i]);
    }

    printf("%s", buf);
    free(buf);  // Not strictly necessary since we're exiting, but hygenic
    return 0;
}