crocparty/game/game_collision.c

#include <assert.h>
#include "sys/sys.h"
#include "game.h"
#include "map/game_map.h"

bool game_collision_is_occlusive(sys_maptile tile) {
    if (tile >= 0 && tile < 30) { return true; }
    if (tile >= 60 && tile < 90) { return true; }
    return false;
}

// c's modulus can produce a negative result. don't allow that!
#define modulus(x, mod) ((x) % (mod) + (mod)) % (mod)
#define round_down(x, incr) (x) - modulus(x, incr)
#define round_up(x, incr) round_down((x) + (incr) - 1, incr)

bool game_collision_can_move(game_bbox body, sys_i32 dx, sys_i32 dy) {
    // First: place the body in space.
    // Align its top-left corner to the pixel boundary
    sys_i32 x = body.x + dx;
    sys_i32 y = body.y + dy;

    // this is extremely similar to the logic in move_to_contact
    // but has different rounding behavior
    // (because a body at a fractional position must not actually collide
    //  with fractional pixels)
    sys_i32 body_x0 = round_down(x, PIXEL_SZ_MICROPIXEL);
    sys_i32 body_y0 = round_down(y, PIXEL_SZ_MICROPIXEL);
    sys_i32 body_x1 = body_x0 + body.w;
    sys_i32 body_y1 = body_y0 + body.h;

    sys_i32 tile_x0 = round_down(body_x0, TILE_SZ_MICROPIXEL);
    sys_i32 tile_y0 = round_down(body_y0, TILE_SZ_MICROPIXEL);
    sys_i32 tile_x1 = round_up(body_x1, TILE_SZ_MICROPIXEL);
    sys_i32 tile_y1 = round_up(body_y1, TILE_SZ_MICROPIXEL);

    // now convert to tile coords instead of pixels
    tile_x0 /= TILE_SZ_MICROPIXEL;
    tile_y0 /= TILE_SZ_MICROPIXEL;
    tile_x1 /= TILE_SZ_MICROPIXEL;
    tile_y1 /= TILE_SZ_MICROPIXEL;

    for (sys_i32 y = tile_y0; y < tile_y1; y++) {
        for (sys_i32 x = tile_x0; x < tile_x1; x++) {
            bool invalid;
            sys_maptile tile = sys_map_get(map_game_map, x, y, &invalid);
            if (invalid || game_collision_is_occlusive(tile)) {
                return false;
            }
        }
    }

    return true;
}

game_collision game_collision_move_to_contact(
    game_bbox* body, sys_i32 total_dx, sys_i32 total_dy
) {
    sys_i32 src_x = body->x;
    sys_i32 src_y = body->y;
    game_collision collision = {
        .collided_x=false,
        .collided_y=false,
        .distance_x=0,
        .distance_y=0,
    };
    sys_i32 tar_x = body->x + total_dx;
    sys_i32 tar_y = body->y + total_dy;

    while (true) {
        // calculate how much we still need to move
        int32_t remaining_dx = tar_x - body->x;
        int32_t remaining_dy = tar_y - body->y;

        // if we don't need to move, stop
        if (remaining_dx == 0 && remaining_dy == 0) { 
            collision.distance_x = body->x - src_x;
            collision.distance_y = body->x - src_y;
            return collision; 
        }

        // figure out where we are
        sys_i32 body_x0 = body->x;
        sys_i32 body_y0 = body->y;
        sys_i32 body_x1 = body->x + body->w;
        sys_i32 body_y1 = body->y + body->h;

        // figure out what tile we're still in
        sys_i32 tile_x0 = round_down(body_x0, TILE_SZ_MICROPIXEL);
        sys_i32 tile_y0 = round_down(body_y0, TILE_SZ_MICROPIXEL);
        sys_i32 tile_x1 = round_up(body_x1, TILE_SZ_MICROPIXEL);
        sys_i32 tile_y1 = round_up(body_y1, TILE_SZ_MICROPIXEL);

        // but we want the _next_ tile
        if (tile_x0 == body_x0) { tile_x0 -= TILE_SZ_MICROPIXEL; }
        if (tile_y0 == body_y0) { tile_y0 -= TILE_SZ_MICROPIXEL; }
        if (tile_x1 == body_x1) { tile_x1 += TILE_SZ_MICROPIXEL; }
        if (tile_y1 == body_y1) { tile_y1 += TILE_SZ_MICROPIXEL; }

        // length of an x axis move
        int step_dx = 
            remaining_dx < 0 ? 
                // line up our left side with the tile to our left
                // not exceeding dx
                sys_max_i32(remaining_dx, tile_x0 - body_x0) :
            remaining_dx > 0 ?  
                // line up our right side with the tile to our right
                // not exceeding dx
                sys_min_i32(remaining_dx, tile_x1 - body_x1) :
            // dx == 0 
            INT32_MAX ;

        int step_dy = 
            remaining_dy < 0 ?
                // line up our top side with the tile above us
                // not exceeding dy
                sys_max_i32(remaining_dy, tile_y0 - body_y0) :
            remaining_dy > 0 ?
                // line up our bottom side with the tile below us
                // not exceeding dy
                sys_min_i32(remaining_dy, tile_y1 - body_y1) :
            INT32_MAX;

        // there must be a viable move in some direction
        assert(step_dx != INT32_MAX || step_dy != INT32_MAX);

        if (sys_abs_i32(step_dx) < sys_abs_i32(step_dy)) {
            if (game_collision_can_move(*body, step_dx, 0)) {
                body->x += step_dx; // take the tiny move
            } else {
                tar_x = body->x;  // stop trying to move
                collision.collided_x = true;
            }
        } else {
            if (game_collision_can_move(*body, 0, step_dy)) {
                body->y += step_dy; // take the tiny move
            } else {
                tar_y = body->y;  // stop trying to move
                collision.collided_y = true;
            }
        }
    }
}