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

#if 0
#define PART_1
#else
#define PART_2
#endif

#if 0
#define FILE_PATH "example.txt"
#define IDS 4
#else
#define FILE_PATH "input.txt"
#define IDS 908
#endif

#define LOWER(min, max) ((max) - (((max) - (min))/2 + 1))
#define UPPER(min, max) (((min) + (max))/2 + 1)

#define ROWS  128
#define COLUMNS 8

int highest_id = 0;
int ids[IDS] = {0};

void process_pass(char *pass, int index)
{
    int min_r = 0; int max_r = ROWS - 1;
    int min_c = 0; int max_c = COLUMNS - 1;

    for(int i = 0; i < 10; i++)
    {
        switch(pass[i])
        {
        case 'F':
            max_r = LOWER(min_r, max_r);
            break;
        case 'B':
            min_r = UPPER(min_r, max_r);
            break;
        case 'L':
            max_c = LOWER(min_c, max_c);
            break;
        case 'R':
            min_c = UPPER(min_c, max_c);
            break;
        }
    }

    int id = min_r * 8 + min_c;

#ifdef PART_1
    (void)index;
    if(id > highest_id) highest_id = id;
#endif

#ifdef PART_2
    assert(index < IDS);
    ids[index] = id;
#endif

}

void parse()
{
    FILE *fp = fopen(FILE_PATH, "r");
    if(!fp) {
        fprintf(stderr, "ERROR: Could not open file: %s\n", FILE_PATH);
        exit(EXIT_FAILURE);
    }

    int i = 0;
    char line[256] = {0};
    while(fgets(line, sizeof(line), fp) != NULL)
    {
        if(line[10] == '\n') line[10] = '\0';
        process_pass(line, i);
        i++;
    }

    fclose(fp);
}

int find_id()
{
    int your_seat;
    int seat_missing;

    for(int i = 0; i < IDS; i++)
    {
        your_seat    = -1;
        seat_missing = 0;

        for(int j = 0; j < IDS; j++)
        {
            if(ids[i] == (ids[j] + 2))
            {
                your_seat = ids[i] - 1;
                seat_missing = 1;

                for(int k = 0; k < IDS; k++)
                    if(your_seat == ids[k]) seat_missing = 0;
            }
        }

        if(your_seat!= -1 && seat_missing != 0)
            return your_seat;

    }
    return -1;
}

void part_1()
{
    printf("Highest boarding ID is %d\n", highest_id);
}

void part_2()
{
    printf("Your id is: %d\n", find_id());
}

int main(void)
{
    parse();

#ifdef PART_1
    part_1();
#endif
#ifdef PART_2
    part_2();
#endif

    return 0;
}