day6.c

#include <stdio.h>
#include <inttypes.h>
#include <time.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

char* read_file(const char *path) {
    FILE *f = fopen(path, "rb");
    if (f == NULL)
        return NULL;
    fseek(f, 0, SEEK_END);
    size_t fsz = (size_t) ftell(f);
    fseek(f, 0, SEEK_SET);
    char *input = (char*)malloc((fsz+1)*sizeof(char));
    input[fsz] = 0;
    fread(input, sizeof(char), fsz, f);
    fclose(f);
    return input;
}

long next_long(char **src) {
    char *endptr = NULL;
    long value = strtol(*src, &endptr, 10);
    *src = endptr;
    return value;
}

void ignore_seq(char **src, char *seq) {
    size_t len = strlen(seq);

    if (strncmp(*src, seq, len) == 0) {
        *src += len;
    }
}

bool iseof(char *src) {
    while (*src == '\n' || *src == ' ' || *src == '\t')
        src += 1;
    return *src == 0;
}

typedef int64_t spawner_t;

typedef struct {
	spawner_t spawners[10];
} fish_list_t;


void spawn(fish_list_t *list) {
	list->spawners[8] += 1;
}

void append(fish_list_t *list, spawner_t fish) {
	list->spawners[fish] += 1;
}	

void iteration(fish_list_t *list) {
	spawner_t new_spawners[10] = { 0 };
	int64_t newfishes = list->spawners[0];

	for (int i = 1; i < 10; i += 1) {
		new_spawners[i-1] = list->spawners[i];
	}
	new_spawners[6] += newfishes;
	new_spawners[8] += newfishes;
	
	memcpy(list->spawners, new_spawners, sizeof(int64_t)*10);
}

void print(fish_list_t *list) {
	int64_t total = 0;
	for (int i = 0; i < 10; i += 1) {
		total += list->spawners[i];
	}
	printf(" (%ld total)\n", total);
}

fish_list_t load_input(char *path) {
	char *file = read_file(path);
	char *orig_file = file;
	if (file == NULL) {
		fprintf(stderr, "Couldn't load %s\n", path);
		exit(-1);
	}
	
	fish_list_t list = { 0 };
		
	while (!iseof(file)) {
		append(&list, (int8_t)next_long(&file));
		ignore_seq(&file, ",");
	}
	
	free(orig_file);
	return list;
}


int main() {
	fish_list_t list = load_input("day6/test_input.txt");

	for (int i = 0; i < 80; i += 1) {
		iteration(&list);
	}	
	
	printf("Part 1: ");
	print(&list);

	for (int i = 0; i < 256-80; i += 1) {
		iteration(&list);
	}		

	printf("Part 2: ");
	print(&list);
	return 0;
}