Rivers and Bridges map refactor (#103)

* Separated out rivers and bridges into its own file with three map variants

* fixing tags

* removed extra 4 starting positions from the medium map since it only supports 8 players

* update GetUnoccupiedPoints to consider hazards with a flag

* use new utility method to fine unoccupied points and enforce map sizes

* changed up casting to make IsAllowable() more usable

maps/rivers_and_bridges_test.go

@@ -0,0 +1,42 @@
+package maps_test
+
+import (
+	"testing"
+
+	"github.com/BattlesnakeOfficial/rules"
+	"github.com/BattlesnakeOfficial/rules/maps"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRiversAndBridgetsHazardsMap(t *testing.T) {
+	// check error handling
+	m := maps.RiverAndBridgesMediumHazardsMap{}
+	settings := rules.Settings{}
+
+	// check error for unsupported board sizes
+	state := rules.NewBoardState(9, 9)
+	editor := maps.NewBoardStateEditor(state)
+	err := m.SetupBoard(state, settings, editor)
+	require.Error(t, err)
+
+	tests := []struct {
+		Map    maps.GameMap
+		Width  uint
+		Height uint
+	}{
+		{maps.RiverAndBridgesMediumHazardsMap{}, 11, 11},
+		{maps.RiverAndBridgesLargeHazardsMap{}, 19, 19},
+		{maps.RiverAndBridgesExtraLargeHazardsMap{}, 25, 25},
+	}
+
+	// check all the supported sizes
+	for _, test := range tests {
+		state = rules.NewBoardState(int(test.Width), int(test.Height))
+		state.Snakes = append(state.Snakes, rules.Snake{ID: "1", Body: []rules.Point{}})
+		editor = maps.NewBoardStateEditor(state)
+		require.Empty(t, state.Hazards)
+		err = test.Map.SetupBoard(state, settings, editor)
+		require.NoError(t, err)
+		require.NotEmpty(t, state.Hazards)
+	}
+}