CODE100 - GSM-7 or UCS-2

challenge.c

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

#define GSM7_BITSET_SIZE 32 // 256 / 8
unsigned char gsm7_bitset[GSM7_BITSET_SIZE] = {0};

void init_gsm7_bitset() {
    const char *gsm7_chars = "@£$¥èéùìòÇ\nØø\rÅåΔ_ΦΓΛΩΠΨΣΘΞ\x1BÆæßÉ "
                             "!\"#¤%&'()*+,-./0123456789:;<=>?"
                             "¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑܧ¿"
                             "abcdefghijklmnopqrstuvwxyzäöñüà";
    while (*gsm7_chars) gsm7_bitset[(unsigned char)*gsm7_chars / 8] |= 1 << ((unsigned char)*gsm7_chars++ % 8);
}

void calc_msg_bits(const char *msg, const char **enc, int *bits) {
    int len = 0;
    while (msg[len]) {
        if (!(gsm7_bitset[(unsigned char)msg[len] / 8] & (1 << ((unsigned char)msg[len] % 8)))) {
            *enc = "UCS-2";
            *bits = (len + strlen(msg + len)) * 16;
            return;
        }
        len++;
    }
    *enc = "GSM-7";
    *bits = len * 7;
}

void test() {
    const struct {
        const char *text;
        const char *expected_enc;
        int expected_len;
    } messages[] = {
        {"Ahoy World", "GSM-7", 70},
        {"This is a test message with special characters: ñáéíóú", "UCS-2", 880},
        {"Visit the Twilio booth at Hall A 03 during WeAreDeveloper World Congress", "GSM-7", 504},
        {"Rumors say Twilio will provide healthy smoothies 🥤🍓🍍", "UCS-2", 1184}
    };

    for (int i = 0; i < 4; i++) {
        const char *enc;
        int bits;
        calc_msg_bits(messages[i].text, &enc, &bits);
        printf("\"%s\" => Expected: %s, %d bits, Got: %s, %d bits\n",
               messages[i].text, messages[i].expected_enc, messages[i].expected_len, enc, bits);
    }
}

int main() {
    init_gsm7_bitset();
    test();
    return 0;
}