Snail mode (#98)

* Add snail-mode map

* snail-mode: cap max hazards to 7

- Ensure that no more than 7 hazards are added to a square. This
  fixes a bug where some squares were getting way too many hazards
  applied to them.  There must be some other bug at work here as
  well.
- Change author names to be github usernames instead of first names

* snail-mode: fix bug with eliminated snakes

- Ensure that hazard snail-trail is not added for eliminated snakes

* Update from Stream July 31

Added comments to most functions and important bits of code

Also changed the map so that instead of a fixed number of 7 hazards,
we add hazards equal to the length of the snake.

* snail-mode: add TAG_EXPERIMENTAL and TAG_HAZARD_PLACEMENT

* snail-mode: use Point as map key

Co-authored-by: Corey Alexander <coreyja@gmail.com>

Author: jlafayette

maps/snail_mode.go

@@ -0,0 +1,179 @@
+package maps
+
+import (
+	"github.com/BattlesnakeOfficial/rules"
+)
+
+type SnailModeMap struct{}
+
+// init registers this map in the global registry.
+func init() {
+	globalRegistry.RegisterMap("snail_mode", SnailModeMap{})
+}
+
+// ID returns a unique identifier for this map.
+func (m SnailModeMap) ID() string {
+	return "snail_mode"
+}
+
+// Meta returns the non-functional metadata about this map.
+func (m SnailModeMap) Meta() Metadata {
+	return Metadata{
+		Name:        "Snail Mode",
+		Description: "Snakes leave behind a trail of hazards",
+		Author:      "coreyja and jlafayette",
+		Version:     1,
+		MinPlayers:  1,
+		MaxPlayers:  16,
+		BoardSizes:  OddSizes(rules.BoardSizeSmall, rules.BoardSizeXXLarge),
+		Tags:        []string{TAG_EXPERIMENTAL, TAG_HAZARD_PLACEMENT},
+	}
+}
+
+// SetupBoard here is pretty 'standard' and doesn't do any special setup for this game mode
+func (m SnailModeMap) SetupBoard(initialBoardState *rules.BoardState, settings rules.Settings, editor Editor) error {
+	rand := settings.GetRand(0)
+
+	if len(initialBoardState.Snakes) > int(m.Meta().MaxPlayers) {
+		return rules.ErrorTooManySnakes
+	}
+
+	snakeIDs := make([]string, 0, len(initialBoardState.Snakes))
+	for _, snake := range initialBoardState.Snakes {
+		snakeIDs = append(snakeIDs, snake.ID)
+	}
+
+	tempBoardState := rules.NewBoardState(initialBoardState.Width, initialBoardState.Height)
+	err := rules.PlaceSnakesAutomatically(rand, tempBoardState, snakeIDs)
+	if err != nil {
+		return err
+	}
+
+	// Copy snakes from temp board state
+	for _, snake := range tempBoardState.Snakes {
+		editor.PlaceSnake(snake.ID, snake.Body, snake.Health)
+	}
+
+	return nil
+}
+
+// storeTailLocation returns an offboard point that corresponds to the given point.
+// This is useful for storing state that can be accessed next turn.
+func storeTailLocation(point rules.Point, height int) rules.Point {
+	return rules.Point{X: point.X, Y: point.Y + height}
+}
+
+// getPrevTailLocation returns the onboard point that corresponds to an offboard point.
+// This is useful for restoring state that was stored last turn.
+func getPrevTailLocation(point rules.Point, height int) rules.Point {
+	return rules.Point{X: point.X, Y: point.Y - height}
+}
+
+// outOfBounds determines if the given point is out of bounds for the current board size
+func outOfBounds(p rules.Point, w, h int) bool {
+	return p.X < 0 || p.Y < 0 || p.X >= w || p.Y >= h
+}
+
+// doubleTail determine if the snake has a double stacked tail currently
+func doubleTail(snake *rules.Snake) bool {
+	almostTail := snake.Body[len(snake.Body)-2]
+	tail := snake.Body[len(snake.Body)-1]
+	return almostTail.X == tail.X && almostTail.Y == tail.Y
+}
+
+// isEliminated determines if the snake is already eliminated
+func isEliminated(s *rules.Snake) bool {
+	return s.EliminatedCause != rules.NotEliminated
+}
+
+// UpdateBoard does the work of placing the hazards along the 'snail tail' of snakes
+// This is responsible for saving the current tail location off the board
+// and restoring the previous tail position. This also handles removing one hazards from
+// the current stacks so the hazards tails fade as the snake moves away.
+func (m SnailModeMap) UpdateBoard(lastBoardState *rules.BoardState, settings rules.Settings, editor Editor) error {
+	err := StandardMap{}.UpdateBoard(lastBoardState, settings, editor)
+	if err != nil {
+		return err
+	}
+
+	// This map decrements the stack of hazards on a point each turn, so they
+	// need to be cleared first.
+	editor.ClearHazards()
+
+	// This is a list of all the hazards we want to add for the previous tails
+	// These were stored off board in the previous turn as a way to save state
+	// When we add the locations to this list we have already converted the off-board
+	// points to on-board points
+	tailLocations := make([]rules.Point, 0, len(lastBoardState.Snakes))
+
+	// Count the number of hazards for a given position
+	// Add non-double tail locations to a slice
+	hazardCounts := map[rules.Point]int{}
+	for _, hazard := range lastBoardState.Hazards {
+
+		// discard out of bound
+		if outOfBounds(hazard, lastBoardState.Width, lastBoardState.Height) {
+			onBoardTail := getPrevTailLocation(hazard, lastBoardState.Height)
+			tailLocations = append(tailLocations, onBoardTail)
+		} else {
+			hazardCounts[hazard]++
+		}
+	}
+
+	// Add back existing hazards, but with a stack of 1 less than before.
+	// This has the effect of making the snail-trail disappear over time.
+	for hazard, count := range hazardCounts {
+
+		for i := 0; i < count-1; i++ {
+			editor.AddHazard(hazard)
+		}
+	}
+
+	// Store a stack of hazards for the tail of each snake.  This is stored out
+	// of bounds and then applied on the next turn.  The stack count is equal
+	// the lenght of the snake.
+	for _, snake := range lastBoardState.Snakes {
+		if isEliminated(&snake) {
+			continue
+		}
+
+		// Double tail means that the tail will stay on the same square for more
+		// than one turn, so we don't want to spawn hazards
+		if doubleTail(&snake) {
+			continue
+		}
+
+		tail := snake.Body[len(snake.Body)-1]
+		offBoardTail := storeTailLocation(tail, lastBoardState.Height)
+		for i := 0; i < len(snake.Body); i++ {
+			editor.AddHazard(offBoardTail)
+		}
+	}
+
+	// Read offboard tails and move them to the board. The offboard tails are
+	// stacked based on the length of the snake
+	for _, p := range tailLocations {
+
+		// Skip position if a snakes head occupies it.
+		// Otherwise hazard shows up in the viewer on top of a snake head, but
+		// does not damage the snake, which is visually confusing.
+		isHead := false
+		for _, snake := range lastBoardState.Snakes {
+			if isEliminated(&snake) {
+				continue
+			}
+			head := snake.Body[0]
+			if p.X == head.X && p.Y == head.Y {
+				isHead = true
+				break
+			}
+		}
+		if isHead {
+			continue
+		}
+
+		editor.AddHazard(p)
+	}
+
+	return nil
+}