state_machine.c

/* A simple state machine in C: The enemy moves left, then to the
 * right, and finally stops. */

#include <stdio.h>

/* Enemy forward declaration, so that EnemyState can refer to it */
struct Enemy;

/* Enemy state type */
typedef struct EnemyState {
    void (*update)(struct Enemy* enemy);
    void (*shout)(struct Enemy* enemy);
} EnemyState;

/* Enemy type */
typedef struct Enemy {
    int position;
    const EnemyState* state;
} Enemy;

/* Enemy state forward declarations, so that the states can refer to
 * each other. */
const EnemyState enemy_moving_left;
const EnemyState enemy_moving_right;
const EnemyState enemy_stopped;

/* Enemy state 1: Moving Left */
void enemy_moving_left_update(Enemy* enemy) {
    enemy->position -= 1;
    if (enemy->position <= -3) {
        enemy->state = &enemy_moving_right;
    }
}
void enemy_moving_left_shout(Enemy* enemy) {
    printf("I'm moving to the left! Yarrr!\n");
}
const EnemyState enemy_moving_left = {
    enemy_moving_left_update,
    enemy_moving_left_shout
};

/* Enemy state 2: Moving Right */
void enemy_moving_right_update(Enemy* enemy) {
    enemy->position += 1;
    if (enemy->position >= 3) {
        enemy->state = &enemy_stopped;
    }
}
void enemy_moving_right_shout(Enemy* enemy) {
    printf("I'm moving to the right! Whee!\n");
}
const EnemyState enemy_moving_right = {
    enemy_moving_right_update,
    enemy_moving_right_shout
};

/* Enemy state 3: Stopped */
void enemy_stopped_update(Enemy* enemy) {
}
void enemy_stopped_shout(Enemy* enemy) {
    printf("I'm not moving anywhere!\n");
}
const EnemyState enemy_stopped = {
    enemy_stopped_update,
    enemy_stopped_shout
};

/* Finally we can use the state machine. */
int main(int argc, char* argv[]) {
    /* Create an enemy in the middle, moving left */
    Enemy enemy = {0, &enemy_moving_left};

    /* Let the enemy update itself and shout, until it reaches the
     * stopped state. */
    do {
        enemy.state->update(&enemy);
        enemy.state->shout(&enemy);
    } while (enemy.state != &enemy_stopped);
    
    return 0;
}