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258
README.md
258
README.md
@@ -1,21 +1,26 @@
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# c_state_pattern
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A simple example of how to implement the state pattern in c. check [`main.c`](src/main.c) for the implementation.
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A simple example of how to implement the state pattern in c. Check [`main.c`](src/main.c) for the implementation.
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the state pattern is a pattern that aims to get rid of the mess that a state machine causes within your code. So i propose a problem for you, you are trying to write an state machine for a phone, sounds easy your phone only has 3 inputs that change the current state of the phone.
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The state pattern is a pattern allows for greater flexibility, maintainability as well as reducing decoupling between states over using a state machine using conditional statements. So i propose a problem for you, you are trying to write an state machine for a phone, sounds easy your phone only has 3 inputs that change the current state of the phone.
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This
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#### inputs:
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- power button
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- lock button
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- string enter button
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the power button does what it says either turns the phone off or on, the lock button too does what it says on the tin locks the phone, lastly the string enter button enters what every string is in the phones buffer this is used for entering things like passwords to unlock the phone.
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using these buttons you determin you need 4 states for your phone these are:
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The power button does what it says either turns the phone off or on, the lock button too does what it says `on the tin locks the phone, lastly the string enter button enters what every string is in the phones buffer this is used for entering things like passwords to unlock the phone. Using these buttons you determine you need 4 states for your phone these are:
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#### states:
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- off
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- unlock
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- locked
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- debug
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and you want the inter-action between these states to work as follows:
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And you want the inter-action between these states to work as follows:
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#### state diagram
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```
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pwr : power button pressed
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lck : lock button pressed
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@@ -35,8 +40,8 @@ str=x : string enter button press with string x
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│ │state │ │state │
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│ └──────┬─┘◄───┬───────┴──┬─────┘
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│ ▲ │ lck │
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│ lck ┤ ├ str=dbg ├ str=dbg
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│ │ │ │
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│ lck ┤ │ ├ str=dbg
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│ │ ├ str=dbg │
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│ │ │ │
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│ │ ▼ │
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│ ┌─┴──────┐ │
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@@ -47,95 +52,204 @@ str=x : string enter button press with string x
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----------- \ state diagram -----------
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```
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now you may read these requirements for the device and think ill just implement this with a state machine how bad can it be. Well your tech lead tells you that this systems needs to scaled to over 100 different states as they make the system more complex. you ask him "why do we need this" they reply back with managment said so. And so between the idea of getting fired from throwing a brick at the managment team and implementing this state diagram you decide you need this job and to implement state pattern.
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this pattern allows for greater flexablity when creating new states since you dont have to add to an ever growning state machine (lets do some simple math if we assume that you end up with 100 states in the end and each state takes up 20 odd line that would be a 2k line state machine and a nightmare to debug (i should know ive seen them in the wild))
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now you may read these requirements for the device and think ill just implement this with a state machine how bad can it be. Well your tech lead tells you that this systems needs to scaled to over 100 different states as they make the system more complex. You ask him "why do we need this" they reply back with management said so. And so between the idea of getting fired from throwing a brick at the management team and implementing this state diagram you decide you need this job and to implement state diagram. To start you beginning to implement this state diagram with a bog standard conditional state machine pattern as can be seen below.
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so to start implement this pattern we first draw up a uml diagram of how it should go togeather. we can see we have the device itself and which is composed of a device state that has the pointer to the device that it is in
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```c
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```mermaid
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classDiagram
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class Device_t{
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+DeviceState_t state
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+String entered_string
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void changeState(Device_t* device) {
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+pressPwrButton() pressPwrMethod
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+pressStrInputButton() pressStrInputMethod
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+pressLockButton() pressLockMethod
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}
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State_e state = device->state;
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class DeviceInterface_s {
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+pressPwrMethod()
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+pressStrInputMethod()
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+pressLockMethod()
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}
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switch (state) {
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class DeviceState_t{
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+String state_name
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+DeviceInterface_s methods
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}
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Device_t *-- DeviceState_t
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%% DeviceInterface_s --o Device_t
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Device_t o-- DeviceInterface_s
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case OFF:
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if (offButtonPressed()) {
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device->state = LOCK;
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}
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break;
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DeviceInterface_s --o DeviceState_t
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case LOCK:
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if (offButtonPressed()){
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device->state = OFF:
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}
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if (strcmp(device->entered_string, "dbg") == 0) {
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device->state = DEBUG:
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}
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if (strcmp(device->entered_string, "pwd") == 0) {
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device->state = UNLOCKED:
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}
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break;
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...
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```
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However like a good engineer you think back to what your tech lead said this system is going to have to scale to over 100 state. Currently you have implemented 2 states and the function is already over 20 lines of code (LOC) long. Assuming that as you add more states each state grows a little in lines of code and ends up somewhere with 20 LOC per state. We can do some simple math to see how out of hand this can get:
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$$
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100_{\text{states}} \times 20_{\text{LOC}} = 2000_{\text{LOC}}
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$$
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That would be a 2k LOC state machine and a nightmare to debug and maintain (i should know ive seen them in the wild, and let me tell you some LSP dont like it).
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Therefor due to this mess of unmaintainable and heavily coupled code, you decide you need a different solution, and there for you decide to implement the namesake of this article **The state pattern**. Using this pattern all your states become singular easily maintainable [c files](/src/states/) as well as decoupling some states from the dependencies that other states need.
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So to start implement this pattern we first draw up a uml diagram of how it should go together. We can see we have the device itself and which is composed of a device state, and the entered string. This "class" also has 3 methods pressPwrButton, pressStrInputButton, and pressLockButton. These methods are used to create an abstraction from the device state so the underlying state can change while using the same input functions. This means that the state field will change its self when one of the transition functions is called. These transition functions are the ones defined within the interface and the ones defined within the `Device_t` struct with the ones in the `Device_t` struct just calling the functions that are implemented by the `DeviceState_t`. To give and example when the devices `pressPwrButton` method is called this calls the `DeviceState_t` `pressPwrMethod`.
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#### uml diagram
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```
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---------------------------------- UML Diagram ----------------------------------
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┌──────────────────────────────────────────────┐ ┌────────────────────────┐
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│ Device_t │<>────┤ «interface» │
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├──────────────────────────────────────────────┤ │ DeviceInterface_t │
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│ +DeviceState_t state │ ├────────────────────────┤
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│ +String entered_string │ │ │
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├──────────────────────────────────────────────┤ ├────────────────────────┤
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│ +pressPwrButton() : pressPwrMethod │ │ +pressPwrMethod() │
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│ +pressStrInputButton() : pressStrInputMethod │ │ +pressStrInputMethod() │
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│ +pressLockButton() : pressLockMethod │ │ +pressLockMethod() │
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└──────────────────────────────────────────────┘ └─────┬──────────────────┘
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▲ │
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▼ │
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│ │
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│ │
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┌────────────┴───────────────┐ │
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│ DeviceState_t │<>────────────────────────────┘
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├────────────────────────────┤
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│ +String state_name │
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│ +DeviceInterface_s methods │
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├────────────────────────────┤
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│ +setDeviceStateToStateX() │
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└────────────────────────────┘
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--------------------------------- \ UML Diagram ---------------------------------
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```
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(note not real uml diagram because no one knows how to read them)
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1. So to implement this we will first start off with the [device struct](/src/device.h#L64)
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```c
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// device.h
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typedef struct Device_s{
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DeviceState_t state,
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char* entered_string,
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} Device_t;
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```
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2. Next we will implement the [device interface](/src/device.h#L34) which as the name implies is an [interface](https://en.wikipedia.org/wiki/Interface_(object-oriented_programming)) bascially a struct that holds methods that creates a common set of functions that both structs need to implement. This means the [device interface](/src/device.h#L34) will defined 3 methods all relating to these transition functions which are all the changes that can be made to the device i.e. Your [inputs](#inputs).
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```c
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// device.h
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typedef struct DeviceInterface_s{
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void (*pressPwr)(Device_t*);
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void (*pressStrInput)(Device_t*);
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void (*pressLock)(Device_t*);
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} DeviceInterface_t;
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```
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so to implement this we will first create a struct for the device well call this [Device_t](/src/device.h#L67) this device holds its current state ([DeviceState_s](/src/device.h#L52)) and the string that was entered. The [DeviceState_s](/src/device.h#L52) actually holds all the [functions](/src/device.h#L34) or [methods](/src/device.h#L34) that this device uses, with the pressPwrButton, pressStrInputButton, and pressLockButton methods just being alias to these methods as can be seen in [device.c](/src/device.c). these method are defined within the [DeviceInterface_t](/src/device.h#L34) struct which really just acts as a [vtable](https://en.wikipedia.org/wiki/Virtual_method_table) for the given inputs (power button, lock button, string enter button) that will effect the device.
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This whole interface allows for the underlying state to change with out having to use differnt function calls depending on the current state.
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3. And finally we will implement the [struct to hold the actual state](/src/device.h#L52) of the device. (note there is some funkiness in c with implementing this since you will need forward declaration)
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```c
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// device.h
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typedef struct DeviceState_s{
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const char* state_name;
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DeviceInterface_t methods;
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}DeviceState_t;
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```
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4. Now we need to implement the 3 transition functions for device inputs, ive implemented one here as the rest are very [similar](/src/device.c#L14). So this function takes the given device and then uses the methods defined within the device state to change the current state of the device as defined within the [uml diagram](#uml-diagram).
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```c
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// device.c
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void pressPwrButton(Device_t *device) {
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device->state.methods.pressPwr(device);
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}
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```
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5. Now we will create a new header file called state, this file will contain the functions that sets the state of the given device to a new state. So we will implement a function per out defined [states](#states).
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```c
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//state.h
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void setDeviceStateToUnlock(Device_t *device);
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void setDeviceStateToDebug(Device_t *device);
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void setDeviceStateToLock(Device_t *device);
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void setDeviceStateToOff(Device_t *device);
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```
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6. Now we can implement one of these states as the rest of them should be fairly self explanatory. The state we will be implementing is the [lock state](/src/states/lock_state.c) as defined within the [state diagram](#state-diagram) since this should show off most of the different transitions. We will first start out with implementing the [set state function](/src/states/lock_state.c#L36). This function takes the given device and sets that devices state to this new state.
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```c
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// lock_state.h
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void setDeviceStateToLock(Device_t *device) {
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device->state = (DeviceState_t){
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.state_name = lock_state_name,
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.methods = (DeviceInterface_t){
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.pressPwr = &pressPwrMethod,
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.pressStrInput = &pressStrInputMethod,
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.pressLock = &pressLockMethod,
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},
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};
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}
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```
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7. Now that we have the function for setting this state we can now implement the functions defined within the [device interface](/src/device.h#L34). To start we will implement the simplest transition function, the [function](/src/states/lock_state.c#L10) called when the power button is press i.e. When the function is called we set the devices state to off.
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```c
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// lock_state.c
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static void pressPwrMethod(Device_t *device) {
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printf("turning off device\n");
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setDeviceStateToOff(device);
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}
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now to actually create the meat of this device the logic that changes the state, to do this we will first create a [header file](/src/states/state.h) to store all the functions that change the state of the device. now we will implement and state so we will start [off state](/src/states/off_state.c) with the easest one the off state (no bother having the rest if the device cant turn on). to create this state we will implement the functions that are defined within the [interface](/src/device.h#L34) we created, this functions will define what happens to the current state based on the given input. so `pressPwrMethod` is what happens when the power button is pressed when its in the off state (to make the code [grep-able](https://morizbuesing.com/blog/greppability-code-metric/) these functions should probably be prefixed with the given state like `OffStatePressPwrMethod`). these function are prefix with the static keyword as they should only ever be used here and not exposed. the other methods should be self explanatory. lastly we must implement the function that actually changes the state of the device to this given state, so to change the state we just set the devices state to a new struct with the given methods that we defined within this file.
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```
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8. Now we will implement the next transition function, this time the [function](/src/states/lock_state.c#L31) is called when the lock button is pressed, which in the lock state will do nothing.
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```c
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// lock_state.c
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static void pressLockMethod(Device_t *device) {
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(void) device;
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printf("nothing happens\n");
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}
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```
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```c
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9. Finally we create the transition function for handling the text input, this [function](/src/states/lock_state.c#L15) will check the entered string to see if its either "dbg", or "pwd" and if it is enter debug or unlock respectively.
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```c
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static void pressStrInputMethod(Device_t *device) {
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if (strcmp(device->entered_string, "dbg") == 0) {
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printf("entering debug state\n");
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setDeviceStateToDebug(device);
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return;
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}
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if (strcmp(device->entered_string, "pwd") == 0) {
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printf("entering unlock state\n");
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setDeviceStateToUnlock(device);
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return;
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}
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printf("unknown string %s\n", device->entered_string);
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}
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```
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10. Now we just need to implement the other [states](#states) in the same way we implemented this first one.
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11. Finally we need to test our device. Firstly within [main](/src/main.c) we create a new device and set its initial state to off.
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```c
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// arrange:
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Device_t device;
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initDevice(&device);
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setDeviceStateToOff(&device);
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```
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12. Now we can test that the device correctly travels through each [state](#state-diagram), by pressing one of the buttons ([invoking the input method functions](/src/device.h#L34)) then checking that the `state_name` is equal to the set state name.
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```c
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// act :
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// turn on the device
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pressPwrButton(&device);
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static void pressPwrMethod(Device_t *device) {
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printf("turning on device\n");
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setDeviceStateToLock(device);
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}
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// assert :
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// check that the device entered the right state
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if (strcmp(device.state.state_name, "lock_state.c") != 0) return 0;
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```
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static void pressStrInputMethod(Device_t *device) {
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// cast it to void since its unused
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(void) device;
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printf("nothing happens\n");
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}
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static void pressLockMethod(Device_t *device) {
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// cast it to void since its unused
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(void) device;
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printf("nothing happens\n");
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}
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void setDeviceStateToOff(Device_t *device) {
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device->state = (DeviceState_t){
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.state_name = off_state_name,
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.device = device,
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.methods = (DeviceInterface_t){
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.pressPwr = &pressPwrMethod,
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.pressStrInput = &pressStrInputMethod,
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.pressLock = &pressLockMethod,
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},
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};
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}
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```
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and finally we expose the set method in our [header file](/src/states/state.h) so the other states can set to this state. next we just implement the rest of the state Lock, Unlock, and Debug and can finally test our creation in [`main.c`](/src/main.c).
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now that you have implement this basic state pattern your tech lead comes to you and tells you to implement one more state then they will let you throw a brick at managment as a treat this state is:
|
||||
Now that you have implement this basic state pattern your tech lead comes to you and tells you to implement one more state then they will let you throw a brick at management as a treat this state is:
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- A calling state where:
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- when the phone is unlocked you can type in a number in the the string input and it will call it
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- you can also turn off the phone from this state
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- the lock button ends the current call and returns back to the unlocked state
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so try implementing this yourself.
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So try implementing this state yourself.
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Other good resources for learning how this patterns works is [bob nystrom's game programming patterns book](https://gameprogrammingpatterns.com/state.html) as well as [refactoring Guru](https://refactoring.guru/design-patterns/state) however both of these implement this in the fun languages with object and interfaces.
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# how to build and run it
|
||||
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this demo uses a simple build system called [nob](https://github.com/tsoding/nob.h) a header only build system for c projects.
|
||||
This demo uses a simple build system called [nob](https://github.com/tsoding/nob.h) a header only build system for c projects.
|
||||
|
||||
to build run these command in the root of the project:
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||||
To build run these command in the root of the project:
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||||
```bash
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|
||||
# bootstraps the build system
|
||||
@@ -148,4 +262,4 @@ to build run these command in the root of the project:
|
||||
> ./build/main
|
||||
|
||||
```
|
||||
and if you have a differnt complier you want to use that is posix compliant just change the `CC` macro in the `nob.c` with your one.
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||||
And if you have a different compiler you want to use that is posix compliant just change the `CC` macro in the `nob.c` with your one.
|
||||
Reference in New Issue
Block a user