Snake IV

Review — Snake III 5 min

• The tick runs on the timer, so the beat stays steady.
• Only two cells are drawn each tick: the old head becomes body, the old tail becomes grass.
• Walls come from a list of letters, exactly like Tile Maker’s levels.

Quick-fire

Two snakes move at once. Blue moves first, then checks whether it crashed. Is that fair to Orange?

Today’s Topic 3 min

• A second snake with its own pair of lists, on its own keys
• A referee on the Stage: move both, then judge both
• Head-on, body hits and draws
• A best score that survives the green flag, and a title screen

Learning outcome

By the end of this lesson you will be able to:

1. Run two copies of the same game logic on two sets of data.
2. Explain why “everyone moves, then we check” is fairer than checking as each one moves.
3. Finish a game with a front door and a result.

Learnt — a fair duel 28 min

1 · Two snakes, two pairs of lists

Blue keeps xs and ys. Orange gets its own: xs 2 and ys 2, with dx 2, dy 2, c 2 and r 2. Duplicate the Blue sprite, then change every list and variable in the copy.

Blue steers with W A S D; Orange with the arrow keys. Each sprite answers two messages: move (work out the new head, insert it, eat or delete the tail) and show (draw its two changed cells).

when I receive [move v]
move :: custom

when I receive [show v]
show :: custom

when [d v] key pressed
steer [R] :: custom
            

2 · A referee: move both, then judge

The Stage runs the game. Each tick it tells both snakes to move and waits, and only then asks what happened:

when flag clicked
erase all
set [result v] to []
broadcast [new game] and wait
repeat until <not <(result) = []>>
  wait (0.2) seconds
  broadcast [move v] and wait
  judge :: custom
  broadcast [show v] and wait
end
say (result)
            

define judge
set [out v] to (0)
set [out 2 v] to (0)
on blue (c) (r) (2) :: custom
if <(hit) = (1)> then
  set [out v] to (1)
end
on orange (c) (r) (2) :: custom
if <(hit) = (1)> then
  set [out v] to (1)
end
on orange (c 2) (r 2) (2) :: custom
if <(hit) = (1)> then
  set [out 2 v] to (1)
end
on blue (c 2) (r 2) (2) :: custom
if <(hit) = (1)> then
  set [out 2 v] to (1)
end
if <<(c) = (c 2)> and <(r) = (r 2)>> then
  set [out v] to (1)
  set [out 2 v] to (1)
end
if <<(out) = (1)> and <(out 2) = (1)>> then
  set [result v] to [A draw - both crashed.]
else
  if <(out) = (1)> then
    set [result v] to [Orange wins!]
  end
  if <(out 2) = (1)> then
    set [result v] to [Blue wins!]
  end
end
            

3 · A best score that survives the green flag

A variable keeps its value until a block changes it. So never reset best at the green flag — only raise it when a game beats it. It is still there next time you click the flag, and it is saved with the project.

if <(score) > (best)> then
  set [best v] to (score)
end
            

4 · A front door

A title card waits for the space bar before anything moves. Upload title.svg as a sprite.

when flag clicked
show
go to [front v] layer
wait until <key [space v] pressed?>
hide
broadcast [play v]
            

Basic — Snake Duel 25 min

Open your snake from Lesson 43. Remove the walls for now — the garden is crowded enough with two snakes.

Step 1 — the second snake

1. Duplicate the Snake sprite. Rename them Blue and Orange. Upload head-orange.svg and body-orange.svg to Orange.
2. Make xs 2, ys 2, dx 2, dy 2, dir 2, c 2, r 2 and score 2. Change every block on Orange to use them.
3. Blue starts at column 5, row 4, going right. Orange starts at column 12, row 9, going left.

Step 2 — the referee

1. Move the game loop to the Stage. The sprites only answer new game, move and show.
2. Build on blue, on orange and judge on the Stage.
3. The apple is eaten by whichever head reaches it. The referee places a new one, off both snakes.

Step 3 — the end

1. Upload game-over.svg. It shows the result and the best score ever.

Best of three

One crash should not decide everything. Play rounds until somebody has won two.

• Count blue wins and orange wins. A draw counts for nobody.
• Each round starts the snakes from scratch.
• Announce who took the match, and the score.

repeat until <<(blue wins) = (2)> or <(orange wins) = (2)>>
  change [round v] by (1)
  set [result v] to []
  broadcast [new game] and wait
  repeat until <not <(result) = []>>
    wait (0.2) seconds
    broadcast [move v] and wait
    judge :: custom
    broadcast [show v] and wait
  end
  if <(result) = [Blue wins!]> then
    change [blue wins v] by (1)
  end
  if <(result) = [Orange wins!]> then
    change [orange wins v] by (1)
  end
end
            

Swapping heads

Two snakes face each other, one cell apart. Next tick they swap cells — the heads never share a cell. Does your referee notice?

• Work out on paper where both snakes are after the swap.
• Find which check in judge catches it.
• Make sure the answer is a draw.

First to three apples

A second way to win: the first snake to eat three apples wins, crash or no crash.

• After judge, check both scores.
• The loop now stops when there is a result or somebody reaches 3.

repeat until <<not <(result) = []>> or <<(score) > (2)> or <(score 2) > (2)>>>
  wait (0.2) seconds
  broadcast [move v] and wait
  judge :: custom
  broadcast [show v] and wait
end
if <(score) > (2)> then
  set [result v] to [Blue wins - 3 apples!]
end
if <(score 2) > (2)> then
  set [result v] to [Orange wins - 3 apples!]
end
            

A computer player

Play Snake Duel alone: Orange steers itself, using Lesson 42’s hunting idea.

• Before each move the referee broadcasts think and waits.
• Orange tries the ways towards the apple first. A way is no good if it leads off the grid or onto either snake.
• If no way towards the apple is safe, any safe way will do.

Summary 5 min

• Two players are the same logic on two sets of lists and variables.
• A referee moves everyone first, then judges everyone — that is what makes it fair.
• Head-on, body hits and swaps all fall out of the same checks.
• A best score is a variable you never reset; a title is a sprite that waits for a key.

Referee

One script that runs the turns and decides what happened, so no player decides for itself.

Head-on

Both heads arriving in the same cell on the same tick.

Persist

To keep a value between runs — the best score persists.

Hand in

Save Snake Duel. The last arc of Level 4 starts next lesson: a classmate remixes it, and you learn to give — and get — credit properly.