DEV 1247: Add a new map generator interface (#71)

* reorganize code

* first draft of map generator interfaces

* add explicit random interface to board helpers

* implement standard map

* rename Generator to GameMap

* allow initializing snakes separately from placing them

* add random number generator to Settings

* updates to GameMap interface

* add helpers for creating and updating BoardState with maps

maps/standard.go

@@ -0,0 +1,80 @@
+package maps
+
+import (
+	"github.com/BattlesnakeOfficial/rules"
+)
+
+type StandardMap struct{}
+
+func init() {
+	globalRegistry.RegisterMap("standard", StandardMap{})
+}
+
+func (m StandardMap) ID() string {
+	return "standard"
+}
+
+func (m StandardMap) Meta() Metadata {
+	return Metadata{
+		Name:        "Standard",
+		Description: "Standard snake placement and food spawning",
+		Author:      "Battlesnake",
+	}
+}
+
+func (m StandardMap) SetupBoard(initialBoardState *rules.BoardState, settings rules.Settings, editor Editor) error {
+	snakeIDs := make([]string, 0, len(initialBoardState.Snakes))
+	for _, snake := range initialBoardState.Snakes {
+		snakeIDs = append(snakeIDs, snake.ID)
+	}
+
+	tempBoardState, err := rules.CreateDefaultBoardState(editor.Random(), initialBoardState.Width, initialBoardState.Height, snakeIDs)
+	if err != nil {
+		return err
+	}
+
+	// Copy food from temp board state
+	for _, food := range tempBoardState.Food {
+		editor.AddFood(food)
+	}
+
+	// Copy snakes from temp board state
+	for _, snake := range tempBoardState.Snakes {
+		editor.PlaceSnake(snake.ID, snake.Body, snake.Health)
+	}
+
+	return nil
+}
+
+func (m StandardMap) UpdateBoard(lastBoardState *rules.BoardState, settings rules.Settings, editor Editor) error {
+	minFood := int(settings.MinimumFood)
+	foodSpawnChance := int(settings.FoodSpawnChance)
+	numCurrentFood := len(lastBoardState.Food)
+
+	if numCurrentFood < minFood {
+		placeFoodRandomly(lastBoardState, editor, minFood-numCurrentFood)
+		return nil
+	}
+	if foodSpawnChance > 0 && (100-editor.Random().Intn(100)) < foodSpawnChance {
+		placeFoodRandomly(lastBoardState, editor, 1)
+		return nil
+	}
+
+	return nil
+}
+
+func placeFoodRandomly(b *rules.BoardState, editor Editor, n int) {
+	unoccupiedPoints := rules.GetUnoccupiedPoints(b, false)
+
+	if len(unoccupiedPoints) < n {
+		n = len(unoccupiedPoints)
+	}
+
+	editor.Random().Shuffle(len(unoccupiedPoints), func(i int, j int) {
+		unoccupiedPoints[i], unoccupiedPoints[j] = unoccupiedPoints[j], unoccupiedPoints[i]
+	})
+
+	for i := 0; i < n; i++ {
+		editor.AddFood(unoccupiedPoints[i])
+	}
+}