AoC18/day5p1/main.c

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

void *read_file_contents(const char *fname, size_t *len)
{
    FILE *f = fopen(fname, "r");
    if (!f) {
        perror(fname);
        return NULL;
    }
    fseek(f, 0, SEEK_END);
    *len = ftell(f);
    rewind(f);
    void *contents = malloc(*len);
    if (!contents) {
        perror("malloc");
        fclose(f);
        return NULL;
    }
    fread(contents, *len, 1, f);
    fclose(f);
    return contents;
}

void remove_dupes(char *mem, size_t len)
{
    int found;
    char last_char;
    size_t last_char_pos;

    do {
        found = 0;
        last_char = '\0';
        last_char_pos = 0;

        for (size_t i = 0; i < len; i++) {
            if (!mem[i]) continue;

            if (tolower(mem[i]) == tolower(last_char) &&
                    mem[i] != last_char) {
                mem[i] = mem[last_char_pos] = '\0';
                found = 1;
                last_char_pos = 0;
                last_char = '\0';
            } else {
                last_char_pos = i;
                last_char = mem[last_char_pos];
            }
        }
    } while (found);
}

size_t strip_zeroes(char *mem, size_t len)
{
    size_t new_len = len;

    char *current = mem + len - 1;
    while (len) {
        while (len && *current) {len--; current--;}
        if (!len) break;

        char *start = current;
        while (len && !*current) {len--; current--;}
        size_t diff = new_len - len - (start - current);
        memmove(current + 1, start + 1, diff);
        new_len = len + diff;
    }

    return new_len;
}

int main()
{
    size_t string_len;
    char *string = read_file_contents("input", &string_len);
    if (!string) return 0;

    while (isspace(string[string_len - 1])) string[--string_len] = '\0';

    puts("Removing dupes...");
    remove_dupes(string, string_len);

    puts("Stripping zeroes...");
    string_len = strip_zeroes(string, string_len);
    printf("Remaining polymers: %lu\n", string_len);

    free(string);
    return 0;
}