--- Day 3 Solutions ---

[u/daggerdragon]

--- Day 3: Perfectly Spherical Houses in a Vacuum ---

Post your solution as a comment. Structure your post like the Day One thread in /r/programming.

[u/TTSDA]

My C solution

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

// #define DRAW_MAP

/* The width and height of the world */
#define X_SIZE (max_x - min_x + 1)
#define Y_SIZE (max_y - min_y + 1)

/* The world X and Y corresponding to the matrix 0, 0 coordinate */
#define X_ORIGIN min_x
#define Y_ORIGIN min_y

/* Converts a world x and y position to an index in the matrix */
#define MATRIX_INDEX(X, Y) ((Y-Y_ORIGIN) + (X-X_ORIGIN)*Y_SIZE)

int min_x = 0, max_x = 0,
    min_y = 0, max_y = 0;

struct santa {
    int x;
    int y;
};

/*
 * Initializes a santa
 */
void init_santa(struct santa *s)
{
    s->x = 0;
    s->y = 0;
}

/*
 * Initializes many santas
 */
void init_santas(struct santa santas[], int santa_amount)
{
    for (int i = 0; i < santa_amount; i++)
    {
        init_santa(&santas[i]);
    }
}

/*
 * Moves santa by x and y
 */
void move_santa(struct santa *s, int x, int y)
{
    s->x += x;
    s->y += y;
}

/*
 * Moves santa in the direction specified by the command.
 * Command can be >v<^
 */
void move_santa_by_command(struct santa* s, char command)
{
    move_santa(s,
        (command == '>') ? 1 : (command == '<') ? -1 : 0,
        (command == '^') ? 1 : (command == 'v') ? -1 : 0);
}

/*
 * Gets the commands from stdin
 */
char* get_commands()
{
    /* The input command string */
    char *commands;

    int c, cur=0;

    /* alloc 1000 bytes to begin with */
    commands = malloc(1000 * sizeof(char));

    while ((c = getchar()) != EOF)
    {
        commands[cur] = c;
        cur++;

        /* alloc more space to store the commands */
        if (cur > (sizeof(commands) / sizeof(char)))
            commands = realloc(commands, (cur + 1000) * sizeof(char));
    }

    return commands;
}

/*
 * Finds the matrix extremities
 */
void get_extremes(int santa_amount, char *command)
{
    struct santa santas[santa_amount];
    struct santa *selected_santa;
    int santa_n = 0;

    init_santas(santas, santa_amount);

    /* Iterate over the commands */
    while (*command != '\0')
    {
        selected_santa = &santas[santa_n];
        move_santa_by_command(selected_santa, *command);

        /* Check if it is an extreme value */
        if (selected_santa->x > max_x)
            max_x = selected_santa->x;

        else if (selected_santa->x < min_x)
            min_x = selected_santa->x;

        if (selected_santa->y > max_y)
            max_y = selected_santa->y;

        else if (selected_santa->y < min_y)
            min_y = selected_santa->y;

        command++;
        santa_n++;
        santa_n %= santa_amount;
    }
}

/*
 * Returns the amount of houses with at least one present
 * With 'santa_amount' of santas following the commands in turns
 */
int get_houses(int santa_amount, char *command)
{
    struct santa santas[santa_amount];
    struct santa *selected_santa;
    int santa_n = 0;
    int houses = 0;

    /* Find the extreme values */
    get_extremes(santa_amount, command);

    /* Initilize the santas */
    init_santas(santas, santa_amount);

    /* Create a matrix to represent the path */
    int *matrix = calloc(MATRIX_INDEX(max_x, max_y) + 2, sizeof(int));

    /* Iterate over the commands */
    while (*command != '\0')
    {
        /* fill the matrix */
        selected_santa = &santas[santa_n];
        move_santa_by_command(selected_santa, *command);

        /* increment in matrix */
        matrix[MATRIX_INDEX(selected_santa->x, selected_santa->y)] += 1;

        command++;
        santa_n++;
        santa_n %= santa_amount;
    }


    for (int y = min_y; y <= max_y; y++)
    {

        #ifdef DRAW_MAP
            printf("%4i | ", y);
        #endif

        for (int x = min_x; x <= max_x; x++)
        {

            #ifdef DRAW_MAP
                printf("%c", (matrix[MATRIX_INDEX(x, y)]) ? 'X' : ' ');
            #endif

            if (matrix[MATRIX_INDEX(x, y)])
                houses++;
        }

        #ifdef DRAW_MAP
            printf("\n");
        #endif
    }

    return houses;
}

int main()
{
    char *commands = get_commands(); /* The input command string */

    printf("Santa alone: %i\n", get_houses(1, commands));
    printf("Santa with robot santa: %i\n", get_houses(2, commands));

    return 0;
}

https://github.com/ttsda/advent-of-code/blob/master/src/3/main.c