day5.c

/* solution to advent of code 2020, day 5, by pv2b */
#include <stdio.h>
#include <string.h>
#include <limits.h>

/* based on the seat codes, we know there are a maximum of 1024 rows */
#define NUM_ROWS 1024

/* gets the row from a seatID */
#define ROW(id) ((id) >> 3)

/* gets the column from a seatID */
#define COL(id) ((id) & 0x07)

/* constructs a seatID from a row and column */
#define SEAT_ID(r, c) (((r) << 3) | ((c) & 0x07))

/* reads and decodes a single seat ID from STDIN,
 * or returns -1 if EOF or error
 */
int read_one_seat(void) {
    int seatID = 0;

    for (int bit = 0x200; bit != 0x04; bit >>= 1) {
        switch (getchar()) {
            case 'F': break;
            case 'B': seatID |= bit; break;
            default: return -1;
        }
    }

    for (int bit = 0x04; bit != 0x00; bit >>= 1) {
        switch (getchar()) {
            case 'L': break;
            case 'R': seatID |= bit; break;
            default: return -1;
        }
    }

    if (getchar() != '\n') return -1;

    return seatID;
}

int main (int argc, char *argv[]) {
    /* based on the seat codes, we know there are exactly 8 seats per row.
     * So every char represents a row, with each bit in thie char
     * representing one seat, with 1 for occupied, and 0 for free.
     */

    char seatMap[NUM_ROWS];

    int seatID;
    int minSeatID = INT_MAX;
    int maxSeatID = INT_MIN;
    int emptySeatID;
    
    /* fills in seatMap after initializing it, determining the minimum
     * and maximum seat ID as we go
     */

    memset(seatMap, 0, NUM_ROWS);
    while ((seatID = read_one_seat()) != -1) {
        seatMap[ROW(seatID)] |= 1 << COL(seatID);
        if (seatID < minSeatID) minSeatID = seatID;
        if (seatID > maxSeatID) maxSeatID = seatID;
    }

    printf("part 1: %d\n", maxSeatID);

    /* the puzzle tells us that we're not sitting on the first or last
     * occupied row. so.... let's find the first occupied row, and start
     * looking at the first seat of the row after that.
     */

    for (int row = ROW(minSeatID) + 1; row < NUM_ROWS; row++) {
        char currentRow = seatMap[row];

        /* skip this row if it's full */
        if (currentRow == 0xFF) continue;

        /* look for empty seats */
        for (int col = 0; col < 7; col++) {
            if ((currentRow & (1 << col)) == 0) {
                /* found empty seat */
                emptySeatID = SEAT_ID(row, col);
                printf("part 2: %d\n", emptySeatID);
                return 0;
            }
        }
    }
    printf("No empty seat found!\n");
    return 1;
}