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

rand.go

@@ -0,0 +1,56 @@
+package rules
+
+import "math/rand"
+
+type Rand interface {
+	Intn(n int) int
+	Shuffle(n int, swap func(i, j int))
+}
+
+// A Rand implementation that just uses the global math/rand generator.
+var GlobalRand globalRand
+
+type globalRand struct{}
+
+func (globalRand) Intn(n int) int {
+	return rand.Intn(n)
+}
+
+func (globalRand) Shuffle(n int, swap func(i, j int)) {
+	rand.Shuffle(n, swap)
+}
+
+// For testing purposes
+
+// A Rand implementation that always returns the minimum value for any method.
+var MinRand minRand
+
+type minRand struct{}
+
+func (minRand) Intn(n int) int {
+	return 0
+}
+
+func (minRand) Shuffle(n int, swap func(i, j int)) {
+	// no shuffling
+}
+
+// A Rand implementation that always returns the maximum value for any method.
+var MaxRand maxRand
+
+type maxRand struct{}
+
+func (maxRand) Intn(n int) int {
+	return n - 1
+}
+
+func (maxRand) Shuffle(n int, swap func(i, j int)) {
+	// rotate by one element so every element is moved
+	if n < 2 {
+		return
+	}
+	for i := 0; i < n-2; i++ {
+		swap(i, i+1)
+	}
+	swap(n-2, n-1)
+}