game.h

#ifndef GAME_H
#define GAME_H

#include <stdint.h>
#include <stdlib.h>

#if defined(__EMSCRIPTEN__)

const int FIELD_WIDTH = 64;
const int FIELD_HEIGHT = 64;

#elif defined(__AVR__)

const int FIELD_WIDTH = 32;
const int FIELD_HEIGHT = 8;

#else

#error Unsupported platform.

#endif

/**
 * Game field cell type.
 */
typedef enum {
    CELL_TYPE_EMPTY,
    CELL_TYPE_SNAKE,
    CELL_TYPE_FOOD,
    CELL_TYPE_BLOCK
} CELL_TYPE;

/**
 * Direction to snake next cell.
 */
typedef enum {
    DIRECTION_LEFT,
    DIRECTION_RIGHT,
    DIRECTION_UP,
    DIRECTION_DOWN
} DIRECTION;

/**
 * The result of game loop step.
 */
typedef enum {
    STEP_RESULT_SUCCESS,
    STEP_RESULT_FAIL
 } STEP_RESULT;

/**
 * Point coordinates.
 */
typedef struct Point {
    /**
     * X-coordinate: [0..FIELD_WIDTH].
     */
    uint8_t x;
    /**
     * Y-coordinate: [0..FIELD_HEIGHT].
     */
    uint8_t y;
} Point;

#define POINT(x, y) ((Point) {(uint8_t) x, (uint8_t) y})

#define POINT_IS_EQUAL(p1, p2) (p1.x == p2.x && p1.y == p2.y)

/**
 * Snake data container.
 */
typedef struct Snake {
    Point first_point;
    Point last_point;;
} Snake;

/**
 * Cell data will be encoded in one byte by the following bit schema:
 * 2 bits - type of the cell, one of CELL_TYPE value.
 * Next bits depends on type of the cell.
 * - Snake cell:
 *  2 bits - next cell direction, one of DIRECTION value.
 *  The first snake cell contains direction for future step.
 */
typedef uint8_t Cell;

#define GAME_CELL_MAKE_EMPTY ((Cell) CELL_TYPE_EMPTY)
#define GAME_CELL_MAKE_SNAKE(next_direction) ((Cell) (CELL_TYPE_SNAKE | ((next_direction) << 2)))
#define GAME_CELL_MAKE_FOOD ((Cell) CELL_TYPE_FOOD)
#define GAME_CELL_MAKE_BLOCK ((Cell) CELL_TYPE_BLOCK)

#define GAME_CELL_GET_TYPE(cell) ((CELL_TYPE) ((cell) & 0x03))
#define GAME_CELL_IS_TYPE(cell, type) (GAME_CELL_GET_TYPE(cell) == (type))

#define GAME_CELL_GET_SNAKE_NEXT(cell) ((DIRECTION) (((cell) >> 2) & 0x03))

/**
 * Game field, contains only cells.
 */
typedef struct Field {
    Cell cells[FIELD_WIDTH][FIELD_HEIGHT];
} Field;

/**
 * Game root struct.
 */
typedef struct Game {
    Field field;
    Snake snake;
    Point food_location;
    short score;
} Game;

/**
 * User input.
 */
typedef struct UserInput {
    DIRECTION next_move;
} UserInput;

/**
 * Initialize game struct.
 */
void game_init(Game *game);

/**
 * Initialize game for next level.
 */
void game_prepare_level(Game *game);

/**
 * Handle next game loop step.
 */
STEP_RESULT game_handle_next_step(Game *game, UserInput *input);

#endif