A step by step tutorial for a Snake game in Codea. Aimed at absolute beginners (though would recommend starting with the Noob Lander tutorial first). Each step is commented and in its own separate tab- move the tab you wish to run to the far right

Snake

--# Main
-- Snake tutorial
-- by West

--Graphics used are those supplied with Codea, particularly those of Kenney.nl and twolivesleft.com

-- This is a very simple example snake game.  It is intended for use as an introduction into creating your second game in codea and is aimed at absolute beginners. Try the lander tutorial before this one

--ONLY THE FURTHEST RIGHT HAND TAB WILL BE EXECUTED. DRAG THE TAB FOR THE STEP YOU WISH TO RUN TO THE FAR RIGHT THEN PRESS RUN
--The following steps build up the game tab by tab
--1. Place a sprite on the screen
--2. Add x and y variables to control the position
--3. Rotate the sprite
--4. Add a variable, angle, to control the rotation of the sprite
--5. Control rotation through screen tapping. Also restrict the game to landscape only
--6. More advanced control to allow rotation in both directions
--7. Add a variable turning to control the rate of turning
--8. Add movement
--9. Add a body part
--10. Add a delay to the position of the body part
--11. Add a second body part
--12. Add a third body part
--13. Handle body parts as a loop
--14. Reverse the loop to draw from the tail forward
--15. Create "edible" mushrooms at a random screen position
--16. Add collision detection to allow the mushrooms to be picked up. Start with a smaller snake (5) and each mushroom will make the snake longer. Make the snake turn tighter by increasing the turning value
--17. Add collision detection with itself and the walls

--END OF TUTORIAL BUT NOT END OF THE GAME. The rest is up to you.  Some general stuff, such as controlling the states of the game through a finite state machine are covered in the lunar lander tutorial.

--Some suggestions/challenges. Some are straightforward, others not so.
--1. Add in a finite state machine to control the win/lose/restart/menu screens and functions (see lunar lander)
--2. Add sound effects (see lunar lander)
--3. Add in a score system
--4. Make the snake faster (watch! simply adjusting the speed variable will cause the snake to stretch out - do you know why and how to stop it from happening?)
--5. Add in random bonuses (speed up, increase turning, decrease turning, reverse controls, add two body segments instead of 1, double snake size!, shrink back to orginal size,...)
--6. Add in walls. First round the edges, but then to form a non-rectangle arena - perhaps have a block in the centre? Moving walls?
--7. Add different levels, perhaps with an objective to complete each level (say collect 20 mushrooms, survive for 180s, avoid mushrooms for a minute)
--8. As the game progreses spawn more mushrooms at once
--9. Add a pretty background
--10. Customise the graphices
--11. Add an enemy snake!
--12. Use your imagination to create whatever you want! 

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    saveProjectInfo("Description", "Step by step tutorial for writing a Snake game")
    saveProjectInfo("Author", "Graeme West")
    x=300
    y=500    
    angle=90 
    turning=4 
    speed=2.5
    body={}
    numBodyParts=5
    mushx=50+math.random(WIDTH-100)
    mushy=50+math.random(HEIGHT-100)
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    sprite("Platformer Art:Mushroom",mushx,mushy)
    
    if math.abs(x-mushx)<40 and math.abs(y-mushy)<40 then
        numBodyParts = numBodyParts + 1 
        mushx=50+math.random(WIDTH-100) 
        mushy=50+math.random(HEIGHT-100)
    end
    
    if x<0 or x>WIDTH or y<0 or y>HEIGHT then
        background(math.random(255),math.random(255),math.random(255))
    end
    
    for i=4,numBodyParts do
        local reqFrame=4+(i*8)
        if #body>reqFrame and math.abs(x-body[reqFrame].prevx)<25 and math.abs(y-body[reqFrame].prevy)<25 then
            background(math.random(255),math.random(255),math.random(255))
        end
    end
    
    for i=numBodyParts,1,-1 do
        local reqFrame=4+(i*8)
        if #body>reqFrame then
            pushMatrix()
                translate(body[reqFrame].prevx,body[reqFrame].prevy)
                rotate(body[reqFrame].prevangle) 
                sprite("Space Art:Part Green Hull 5",0,0) 
            popMatrix()
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
    --bit of tidying up to limit the size of the body table and by extension the size of the snake. without it I guess a memory limit could be reached
    for i=1200,#body do
        table.remove(body)
    end
end

--# Step1
-- Snake tutorial
-- by West

--1. Place a sprite on the screen

function setup()
    displayMode(FULLSCREEN)
end

function draw()
    background(40, 40, 50)
    --place the sprite at position 400,400 on the screen.  Chose this sprite as it sort of looks like a snakes head
    sprite("Space Art:Part Green Hull 4",400,400)
end


--# Step2
-- Snake tutorial
-- by West

--2. Add x and y variables to control the position

function setup()
    displayMode(FULLSCREEN)
    --variable x to control the horizontal position of the snake head
    x=300
    --variable y to control the vertical position of the snake head
    y=500    
end

function draw()
    background(40, 40, 50)
    --place the sprite at position x,y - you can change these in setup and later we will be changing these values in the draw loop to generate movement
    sprite("Space Art:Part Green Hull 4",x,y)
end

--# Step3
-- Snake tutorial
-- by West

--3. Rotate the sprite

function setup()
    displayMode(FULLSCREEN)
    x=300
    y=500    
end

function draw()
    background(40, 40, 50)

--Unfortunately there is not an easy angle function included in sprite like the x and y position so a slightly more complex set of steps is required.  This is a powerful approach for complex movements or transformations particularly when working in 3D but does mean the simple task of rotating a sprite is a bit more complex.
--The basics are:
--1.that the sets of "transforms" or movements in the x and y direction, along with any rotations should start with the pushMatrix() statement and end with the popMatrix() statement.
--2. The order in which the statements are executed is reversed between the popMatrix and pushMatrix commands.  That is to say that bottom most statement is executed first, followed by the second bottom and so on
--3. The statements stack. For example rotating by 90 degrees then rotating by another 90 degrees means the sprite will be rotated by 180 degrees
--4. Rotate requires and angle in degrees and rotates ANTICLOCKWISE as a default (you can specify axis of rotation but this is too complex for just now)

--the following three examples show the effect of the different orders

--Example 1  - Green sprite.  The sprite is drawn at 0,0 (bottom left) then rotated 20 degrees anticlockwise then moved by 300 along the x axis then 500 along the y axis
    pushMatrix()
        translate(x,y) --executed 3rd -move sprite by x along x axis and y along y axis
        rotate(20) --executed 2nd - rotate sprite by 20 degrees anticlockwise
        sprite("Space Art:Part Green Hull 4",0,0) --executed 1st - draw sprite at 0,0
    popMatrix()
    
--Example 2 - Red sprite. The sprite is drawn at 0,0 (bottom left) then moved 300 along the x axis and 500 along the y axis, then rotated by 20 degrees ABOUT THE ORIGIN OR BOTTOM LEFT CORNER. The sprite effectively follows the circumference of a circle.
--if you change the rotate(20) to rotate(ElapsedTime*25) you will see the path of the sprite
    pushMatrix()
        rotate(20) --executed 3rd - rotate sprite by 20 degrees anticlockwise
        translate(x,y) --executed 2nd -move sprite by x along x axis and y along y axis
        sprite("Space Art:Part Red Hull 4",0,0) --executed 1st - draw sprite at 0,0
    popMatrix()

-- Example 3 - Gray sprite. the first two commands have no sprite to act on so do nothing
    pushMatrix()
        sprite("Space Art:Part Gray Hull 4",0,0) --executed 3rd - draw sprite at 0,0
        translate(x,y) --executed 2nd - does nothing
        rotate(20) --executed 1st - does nothing
    popMatrix()
end

--# Step4
-- Snake tutorial
-- by West

--4. Add a variable, angle, to control the rotation of the sprite

function setup()
    displayMode(FULLSCREEN)
    x=300
    y=500    
    angle=90 --variable to control the orientation of the sprite
end

function draw()
    background(40, 40, 50)
    pushMatrix()
        translate(x,y)
        rotate(angle) --rotation controlled by variable angle
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
end

--# Step5
-- Snake tutorial
-- by West

--5. Control rotation through screen tapping. Also restrict the game to landscape only

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) --locks the screen to use only landscape
    x=300
    y=500    
    angle=90 
end

function draw()
    background(40, 40, 50)
    
    --this simple example will use the CurrentTouch command. A more advanced (and better) way would be to handle touches separately.  This may be explored later.
    
    -- check to see if the screen has been touched
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        --the state of the current touch is active then
        angle = angle + 1 --increase the angle
    end

    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
end

--# Step6
-- Snake tutorial
-- by West

--6. More advanced control to allow rotation in both directions

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        --test to see if the touch occured on the left hand side of the screen
        if CurrentTouch.x<WIDTH/2 then --if the x position of the touch is less the half the screen width (WIDTH)
            angle = angle + 2 --increase the angle which turns anticlockwise
        else --if it isn't then the touch must have happened on the right
            angle = angle -2 --decrease the angle which turns clockwise
        end
    end

    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
end

--# Step7
-- Snake tutorial
-- by West

--7. Add a variable turning to control the rate of turning

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning --use the turning variable
        else 
            angle = angle - turning --use the turning variable
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
end

--# Step8
-- Snake tutorial
-- by West

--8. Add movement

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 --a variable to hold the current speed of the snake
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning --use the turning variable
        else 
            angle = angle - turning --use the turning variable
        end
    end

--change the x and y coordinates based on the current direction. The direction needs to be resolved into its x and y components using simple trigonometry.  The variable speed will be used to control the magnitude of the components.

--math.sin is the sine function - I assume you know what this is - you should have learned trigonometry at school. If you are still at school - ask your teacher.
--in codea, the trig functions require the angles to be provided in radians not degrees.  Fortunately there is an in built function to convert from degrees to radians called math.rad().  Again I will assume you know what radians are, but if you don't either use the function blindly or find out!

--Finally, the angle is negative.  I'm not sure why but reckon it's linked back to the rotate function being anticlockwise
x = x + speed*math.sin(math.rad(-angle))
y = y + speed*math.cos(math.rad(-angle))

    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
end

--# Step9
-- Snake tutorial
-- by West

--9. Add a body part

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    --three variables to hold the x,y and angle of the previous position of the head
    prevx=x
    prevy=y
    prevangle=angle

end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning --use the turning variable
        else 
            angle = angle - turning --use the turning variable
        end
    end

x = x + speed*math.sin(math.rad(-angle))
y = y + speed*math.cos(math.rad(-angle))

--draw the body befor the head so the head will appear on top
--use the same routine but use the prev variables

    pushMatrix()
        translate(prevx,prevy)
        rotate(prevangle) 
        sprite("Space Art:Part Green Hull 5",0,0) 
    popMatrix()

    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    
    --update the prev variables with the current ones (which will be the old ones next time the draw loop executes)
    prevx=x
    prevy=y
    prevangle=angle
end

--# Step10
-- Snake tutorial
-- by West

--10. Add a delay to the position of the body part

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    --turn the three variables into a table called body
    body={}

end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning --use the turning variable
        else 
            angle = angle - turning --use the turning variable
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    --do a check to see if the table size is bigger than 12. The # symbol in front of the table name gives its size 
    --Position 12 in the table will indicate where the head was 12 cycles of the draw function ago. this is where we want to put the first body piece
    if #body>12 then
        pushMatrix()
            translate(body[12].prevx,body[12].prevy)
            rotate(body[12].prevangle) 
            sprite("Space Art:Part Green Hull 5",0,0) 
        popMatrix()
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    
--add the current head position into the table at the start.  this will shove everything along one so the old position
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step11
-- Snake tutorial
-- by West

--11. Add a second body part

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    body={}
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    --Exactly the same as the first body part except the position is taken from 24 frames (cycles of draw) ago. Again a check to make sure there at least 24 previous positions recorded
    if #body>20 then
        pushMatrix()
            translate(body[20].prevx,body[20].prevy)
            rotate(body[20].prevangle) 
            sprite("Space Art:Part Green Hull 5",0,0) 
        popMatrix()
    end
    
    if #body>12 then
        pushMatrix()
            translate(body[12].prevx,body[12].prevy)
            rotate(body[12].prevangle) 
            sprite("Space Art:Part Green Hull 5",0,0) 
        popMatrix()
    end

    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step12
-- Snake tutorial
-- by West

--12. Add a third body part

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    body={}
end

function draw()
    background(40, 40, 50)
    
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))

    --same again just with a different frame position
    if #body>28 then
        pushMatrix()
            translate(body[28].prevx,body[28].prevy)
            rotate(body[28].prevangle) 
            sprite("Space Art:Part Green Hull 5",0,0) 
        popMatrix()
    end
    
    if #body>20 then
        pushMatrix()
            translate(body[20].prevx,body[20].prevy)
            rotate(body[20].prevangle) 
            sprite("Space Art:Part Green Hull 5",0,0) 
        popMatrix()
    end
    
    if #body>12 then
        pushMatrix()
            translate(body[12].prevx,body[12].prevy)
            rotate(body[12].prevangle) 
            sprite("Space Art:Part Green Hull 5",0,0) 
        popMatrix()
    end

    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step13
-- Snake tutorial
-- by West

--13. Handle body parts as a loop

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    body={}
    numBodyParts=25 -- variable to hold the current number of body parts
end

function draw()
    background(40, 40, 50)
    
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    -- create a loop where we repeat a bit of code as many times as we specify in the loop. In this case we are repeating it 25 times because numBodyParts is 25. If we had continued with the approach shown in steps 11 and 12, the code would quickly become long and repetitive
    for i=1,numBodyParts do
        local reqFrame=4+(i*8) -- this is a local variable - only used inside this loop and gives us the number of the frame based ona simple formula.
        --the first time round the loop, the value of i is 1 and therefore reqFrame is 12. 
        --Second time round the loop i is 2 and reqFrame is 4+(2*8)=20
        --Third time round it i is 3 and reqFrame is 4+(3*8)=28 
        --looking back to the previous steps this number is the only thing that changed in the block of code that prints out the body part.
        --Therefore we can replace thenumber with this local variable
        if #body>reqFrame then
            pushMatrix()
                translate(body[reqFrame].prevx,body[reqFrame].prevy)
                rotate(body[reqFrame].prevangle) 
                sprite("Space Art:Part Green Hull 5",0,0) 
            popMatrix()
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step14
-- Snake tutorial
-- by West

--14. Reverse the loop to draw from the tail forward

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    body={}
    numBodyParts=25 -- variable to hold the current number of body parts
end

-- This function gets called once every frame
function draw()
    -- This sets a dark background color 
    background(40, 40, 50)
    
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    --Instead of counting up from 1 to 25 (numBodyParts) we want to count down. normally with loops, the step size is 1, but we can alter this by specifying a third parameter,
    --this now basically reads as: go from 25 (numBodyParts) to 1 in steps of 1 (downwards - in other words -1) and use i to store which position you are currently at
    for i=numBodyParts,1,-1 do
        local reqFrame=4+(i*8)
        if #body>reqFrame then
            pushMatrix()
                translate(body[reqFrame].prevx,body[reqFrame].prevy)
                rotate(body[reqFrame].prevangle) 
                sprite("Space Art:Part Green Hull 5",0,0) 
            popMatrix()
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step15
-- Snake tutorial
-- by West

--15. Create "edible" mushrooms at a random screen position

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=2
    speed=2.5 
    body={}
    numBodyParts=25 
    mushx=50+math.random(WIDTH-100)--pick a random screen x location which is at least 50 pixels away from both the left and right sides
    mushy=50+math.random(HEIGHT-100)--pick a random screen y location which is at least 50 pixels away from the top and bottom edges
end

function draw()
    background(40, 40, 50)

    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    --draw a sprite for the mushroom at the appropriate position
    sprite("Platformer Art:Mushroom",mushx,mushy)
    
    for i=numBodyParts,1,-1 do
        local reqFrame=4+(i*8)
        if #body>reqFrame then
            pushMatrix()
                translate(body[reqFrame].prevx,body[reqFrame].prevy)
                rotate(body[reqFrame].prevangle) 
                sprite("Space Art:Part Green Hull 5",0,0) 
            popMatrix()
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step16
-- Snake tutorial
-- by West

--16. Add collision detection to allow the mushrooms to be picked up. Start with a smaller snake (5) and each mushroom will make the snake longer. Make the snake turn tighter by increasing the turning value

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=4 --increase the turning rate
    speed=2.5
    body={}
    numBodyParts=5 --set snake initial size to 5
    mushx=50+math.random(WIDTH-100)
    mushy=50+math.random(HEIGHT-100)
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    sprite("Platformer Art:Mushroom",mushx,mushy)
    
    --check to see if the snake head and the mushroom overlap
    --this compares the magnitude of the difference between the x value of the snake head and the x vale of the mushroom and if it is close enough (along with a similar comparison of the y values) then a collision is detected
    if math.abs(x-mushx)<40 and math.abs(y-mushy)<40 then
        numBodyParts = numBodyParts + 1 --increase the length of the snake
        mushx=50+math.random(WIDTH-100) --create new position for next mushroom
        mushy=50+math.random(HEIGHT-100)
    end
    
    for i=numBodyParts,1,-1 do
        local reqFrame=4+(i*8)
        if #body>reqFrame then
            pushMatrix()
                translate(body[reqFrame].prevx,body[reqFrame].prevy)
                rotate(body[reqFrame].prevangle) 
                sprite("Space Art:Part Green Hull 5",0,0) 
            popMatrix()
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end

--# Step17
-- Snake tutorial
-- by West

--17. Add collision detection with itself and the walls

function setup()
    displayMode(FULLSCREEN)
    supportedOrientations(LANDSCAPE_ANY) 
    x=300
    y=500    
    angle=90 
    turning=4 
    speed=2.5
    body={}
    numBodyParts=5
    mushx=50+math.random(WIDTH-100)
    mushy=50+math.random(HEIGHT-100)
end

function draw()
    background(40, 40, 50)
    if CurrentTouch.state==BEGAN or CurrentTouch.state==MOVING then
        if CurrentTouch.x<WIDTH/2 then 
            angle = angle + turning 
        else 
            angle = angle - turning 
        end
    end

    x = x + speed*math.sin(math.rad(-angle))
    y = y + speed*math.cos(math.rad(-angle))
    
    sprite("Platformer Art:Mushroom",mushx,mushy)
    
    if math.abs(x-mushx)<40 and math.abs(y-mushy)<40 then
        numBodyParts = numBodyParts + 1 
        mushx=50+math.random(WIDTH-100) 
        mushy=50+math.random(HEIGHT-100)
    end
    
    --check to see if the snake has gone outside the screen boundaries
    if x<0 or x>WIDTH or y<0 or y>HEIGHT then
        --add a flash
        background(math.random(255),math.random(255),math.random(255))
    end
    
    --check to see if the snake has collided with itself. Don't check the first few segments as they will naturally be near the head but the turning circle shouldn't be tight enough to allow the head to "eat" these first sections
    for i=4,numBodyParts do
        local reqFrame=4+(i*8)
        if #body>reqFrame and math.abs(x-body[reqFrame].prevx)<25 and math.abs(y-body[reqFrame].prevy)<25 then
            --add a flash
            background(math.random(255),math.random(255),math.random(255))
        end
    end
    
    for i=numBodyParts,1,-1 do
        local reqFrame=4+(i*8)
        if #body>reqFrame then
            pushMatrix()
                translate(body[reqFrame].prevx,body[reqFrame].prevy)
                rotate(body[reqFrame].prevangle) 
                sprite("Space Art:Part Green Hull 5",0,0) 
            popMatrix()
        end
    end
    pushMatrix()
        translate(x,y)
        rotate(angle) 
        sprite("Space Art:Part Green Hull 4",0,0) 
    popMatrix()
    table.insert(body,1,{prevx=x,prevy=y,prevangle=angle})
end