State Machine Patterns

Toggle State

A tiny state machine can switch between off and on with each event.

state An enum names each possible state instead of using unexplained integers.
event Each button press applies the same transition rule to the current state.

Toggle State

presses
toggle_state.c
Replay: real traced execution (multi-file project)
#include <stdio.h>

enum SwitchState {
    SWITCH_OFF,
    SWITCH_ON
};

enum SwitchState toggle(enum SwitchState state) {
    if (state == SWITCH_OFF) {
        return SWITCH_ON;
    }
    return SWITCH_OFF;
}

int main(void) {
    int presses = 3;
    enum SwitchState state = SWITCH_OFF;

    for (int i = 0; i < presses; i++) {
        state = toggle(state);
    }

    printf("presses=%d state=%d\n", presses, state);
    return 0;
}

#include <stdio.h>

enum SwitchState {
    SWITCH_OFF,
    SWITCH_ON
};

enum SwitchState toggle(enum SwitchState state) {
    if (state == SWITCH_OFF) {
        return SWITCH_ON;
    }
    return SWITCH_OFF;
}

int main(void) {
    int presses = 1;
    enum SwitchState state = SWITCH_OFF;

    for (int i = 0; i < presses; i++) {
        state = toggle(state);
    }

    printf("presses=%d state=%d\n", presses, state);
    return 0;
}

#include <stdio.h>

enum SwitchState {
    SWITCH_OFF,
    SWITCH_ON
};

enum SwitchState toggle(enum SwitchState state) {
    if (state == SWITCH_OFF) {
        return SWITCH_ON;
    }
    return SWITCH_OFF;
}

int main(void) {
    int presses = 4;
    enum SwitchState state = SWITCH_OFF;

    for (int i = 0; i < presses; i++) {
        state = toggle(state);
    }

    printf("presses=%d state=%d\n", presses, state);
    return 0;
}

  1. presses ← 3

    15int main(void) {16    int presses→ 3 = 3; //@presses=1, 417    enum SwitchState state = SWITCH_OFF;

  2. for (int i = 0; i < presses; i++)

    pass 1 of 3
    19for (int i0 = 0; i < presses3; i++) {20    state0 = toggle(state);21}
    All 3 passes — pass 1 is the card above
    passistateSWITCH_OFFSWITCH_ON
    10001
    211——
    32001

  3. enum SwitchState toggle(enum SwitchState state)

    pass 1 of 3
    8enum SwitchState toggle(enum SwitchState state0) {9    if (state == SWITCH_OFF) {
    All 3 passes — pass 1 is the card above
    passstateSWITCH_ON
    101
    21—
    301

  4. if (state == SWITCH_OFF)

    pass 1 of 2
    8enum SwitchState toggle(enum SwitchState state) {9    if (state0 == SWITCH_OFF0) {10        return SWITCH_ON1;11    }

  5. state ← 1

    19for (int i = 0; i < presses; i++) {20    state→ 1 = toggle(state);21}

  6. state ← 0

    19for (int i = 0; i < presses; i++) {20    state→ 0 = toggle(state);21}

  7. if (state == SWITCH_OFF)

    pass 2 of 2
    8enum SwitchState toggle(enum SwitchState state) {9    if (state0 == SWITCH_OFF0) {10        return SWITCH_ON1;11    }

  8. state ← 1

    19for (int i = 0; i < presses; i++) {20    state→ 1 = toggle(state);21}

  9. printf("presses=%d state=%d ", presses, state);

    23    printf("presses=%d state=%d\n", presses3, state1);24    return 0;25}
    outputpresses=3 state=1

  1. presses ← 1

    15int main(void) {16    int presses→ 1 = 1;17    enum SwitchState state = SWITCH_OFF;

  2. for (int i = 0; i < presses; i++)

    19for (int i0 = 0; i < presses1; i++) {20    state0 = toggle(state);21}

  3. enum SwitchState toggle(enum SwitchState state)

    8enum SwitchState toggle(enum SwitchState state0) {9    if (state == SWITCH_OFF) {

  4. if (state == SWITCH_OFF)

    8enum SwitchState toggle(enum SwitchState state) {9    if (state0 == SWITCH_OFF0) {10        return SWITCH_ON1;11    }

  5. state ← 1

    19for (int i = 0; i < presses; i++) {20    state→ 1 = toggle(state);21}

  6. printf("presses=%d state=%d ", presses, state);

    23    printf("presses=%d state=%d\n", presses1, state1);24    return 0;25}
    outputpresses=1 state=1

  1. presses ← 4

    15int main(void) {16    int presses→ 4 = 4;17    enum SwitchState state = SWITCH_OFF;

  2. for (int i = 0; i < presses; i++)

    pass 1 of 4
    19for (int i0 = 0; i < presses4; i++) {20    state0 = toggle(state);21}
    All 4 passes — pass 1 is the card above
    passistateSWITCH_OFFSWITCH_ON
    10001
    211——
    32001
    431——

  3. enum SwitchState toggle(enum SwitchState state)

    pass 1 of 4
    8enum SwitchState toggle(enum SwitchState state0) {9    if (state == SWITCH_OFF) {
    All 4 passes — pass 1 is the card above
    passstateSWITCH_ON
    101
    21—
    301
    41—

  4. if (state == SWITCH_OFF)

    pass 1 of 2
    8enum SwitchState toggle(enum SwitchState state) {9    if (state0 == SWITCH_OFF0) {10        return SWITCH_ON1;11    }

  5. state ← 1

    19for (int i = 0; i < presses; i++) {20    state→ 1 = toggle(state);21}

  6. state ← 0

    19for (int i = 0; i < presses; i++) {20    state→ 0 = toggle(state);21}

  7. if (state == SWITCH_OFF)

    pass 2 of 2
    8enum SwitchState toggle(enum SwitchState state) {9    if (state0 == SWITCH_OFF0) {10        return SWITCH_ON1;11    }

  8. state ← 1

    19for (int i = 0; i < presses; i++) {20    state→ 1 = toggle(state);21}

  9. state ← 0

    19for (int i = 0; i < presses; i++) {20    state→ 0 = toggle(state);21}

  10. printf("presses=%d state=%d ", presses, state);

    23    printf("presses=%d state=%d\n", presses4, state0);24    return 0;25}
    outputpresses=4 state=0