pxt-calliope/docs/projects/red-light-green-light.md

# Red Light Green Light

This is the classic "Red Light, Green Light" game where one person is a virtual stoplight and gives commands to the other players to either stop or go.

## Rules of play

The player chosen as the current the stoplight says "Green Light!" and turns away from the other players. The other players move toward the stoplight player, from a distance set at the beginning of the game, and try to touch them. The stoplight player can at any time say "Red light!" and then turn around to face the other players. If the stop light player sees anyone still moving, they call them out and they are finished playing until a new game is started. The stoplight player repeats the red light, green light cycle. If one of the other players happens to touch the stop light player before they can turn around when saying "Red Light!", then the current stoplight player moves to the beginning of the course and the other player becomes the stoplight. The game continues until only the the stoplight player remains.

In this remake of the game, we will use a @boardname@, its radio, and the accelerometer to enforce these rules!

## Creating the stoplight

Let's start with the code running on the stoplight's @boardname@. Don't use this code for the other players!

### States

We define two _states_, or game conditions, called ``GREENLIGHT`` and ``REDLIGHT``. A variable named ``state`` will store the current game state. When the stoplight player presses ``A``, the game goes into "green light" mode. When they press ``B``, the state goes into "red light" mode.

```blocks
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
GREENLIGHT = 1
REDLIGHT = 2
```

### Communication

The radio group for all game players is set to ``1``. We will set the same group in the player's code too. The game state is streamed in a ``||basic:forever||`` loop so that players continuously receive it.

```blocks
let state = 0;
radio.setGroup(1)
basic.forever(function () {
    radio.sendNumber(state)
})
```

### Red light, green light

Use the ``||input:on button pressed||`` block to run code when button ``A`` and ``B`` are pressed.
When ``A`` is pressed, the game goes into ``GREENLIGHT`` mode. When ``B`` is pressed, the game
goes into ``REDLIGHT`` mode. We also use ``||basic:show icon||`` to display the current game state.

```blocks
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
input.onButtonPressed(Button.A, function () {
    state = GREENLIGHT
    basic.showIcon(IconNames.Yes)
})
input.onButtonPressed(Button.B, function () {
    state = REDLIGHT
    basic.showIcon(IconNames.No)
})
GREENLIGHT = 1
REDLIGHT = 2
```

### Stoplight code

All together the stoplight code looks like this:

```blocks
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
input.onButtonPressed(Button.A, function () {
    state = GREENLIGHT
    basic.showIcon(IconNames.Yes)
})
input.onButtonPressed(Button.B, function () {
    state = REDLIGHT
    basic.showIcon(IconNames.No)
})
GREENLIGHT = 1
REDLIGHT = 2
radio.setGroup(1)
basic.forever(function () {
    radio.sendNumber(state)
})
```

### ~ hint

Make sure to rename this program to ``stoplight`` or something similar so that you don't confuse it with the player program!

### ~

### Improve the game

* Use ``||music:ring tone||`` to play a sound while the game is in ``GREENLIGHT`` mode.
* Attach a servo and move the arm based on the game state.

## The players

The code for the other players needs to listen for the stoplight's state.

### States

First, we again define the state constants ``GREENLIGHT``, ``REDLIGHT``, and set the radio group to ``1``. 
We also add a ``state`` variable that will store the state of the game.

```blocks
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
GREENLIGHT = 1
REDLIGHT = 2
radio.setGroup(1)
```

### Communication

We use the ``||radio:on received number||`` block to store the stoplight state into the ``state`` variable.

```blocks
let state = 0
radio.onReceivedNumber(function (receivedNumber) {
    state = receivedNumber
})
```

### Display

In a ``||basic:forever||`` loop, we display different icons based on the game state. Use a ``||logic:if||`` and
``||basic:show icon||`` blocks to display the game state.

```blocks
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
basic.forever(function () {
    if (state == GREENLIGHT) {
        basic.showIcon(IconNames.Yes)
    } else if (state == REDLIGHT) {
        basic.showIcon(IconNames.No)
    }
})
```

### Movement check

If the ``state`` is equal to ``REDLIGHT``, we need to check that the player is not moving. 
This is where the accelerometer comes into play. The accelerometer measures forces applied to the @boardname@.
If the player moves, it's likely that the accelerometer will detect any small forces applied to the @boardname@.
At all times, gravity is applied to the @boardname@, so the acceleration strength at rest is always near ``1000`` mg.
If the acceleration strength is far from that value, say ``1100`` or ``900``, we can assume that the player is moving. To compute this we use the formula:

```
moving = | acc strength - 1000 | > 100
```

Now that we know the math for it, we can turn this into code.

```blocks
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
let moving = false
if (state == REDLIGHT) {
    moving = Math.abs(input.acceleration(Dimension.Strength) - 1000) > 100
    if (moving) {
        game.gameOver()
    }
}
```

### Player code

All together, the code for the players is:

```blocks
let moving = false
let REDLIGHT = 0
let state = 0
let GREENLIGHT = 0
radio.onReceivedNumber(function (receivedNumber) {
    state = receivedNumber
})
GREENLIGHT = 1
REDLIGHT = 2
radio.setGroup(1)
basic.forever(function () {
    if (state == GREENLIGHT) {
        basic.showIcon(IconNames.Yes)
    } else if (state == REDLIGHT) {
        basic.showIcon(IconNames.No)
    }
    if (state == REDLIGHT) {
        moving = Math.abs(input.acceleration(Dimension.Strength) - 1000) > 100
        if (state == REDLIGHT && moving) {
            game.gameOver()
        }
    }
})
```

### ~ hint

Make sure to rename this program to ``player`` or something like that so you don't confuse it with the stoplight program!

### ~


### Tuning

Does the movement check work? Try changing the ``100`` value to tune the detection sensitivity. Try ``64`` maybe.

### Improve the game

* Use ``||music:ring tone||`` to play a tone while in green mode.
* Use the packet signal strength to detect that you've reached the stoplight.


```package
radio
```