board: MapStructure refactor, extract function listing adjacent structure parts

detecting adjacent parts of a structure and lack thereof will be used
to tell whether a structure is completed

board.go

@@ -127,6 +127,44 @@ type MapCursor struct {
 	id byte
 }
 
+func (b *Board) GetNeighbourStructures(c MapCursor, includeMissing bool) (res []MapCursor) {
+	res = make([]MapCursor, 0)
+	place, _ := b.Get(c.x, c.y)
+	nCs := getAdjacentCoords(c.x, c.y)
+	for _, nCoord := range nCs {
+		nPlace, nOk := b.Get(nCoord.x, nCoord.y)
+		dirx := nCoord.x - c.x
+		diry := nCoord.y - c.y
+		fmidx  := 2 + (2 * dirx)
+		fmidy  := 1 + diry
+		nfmidx := 2 - (2 * dirx)
+		nfmidy := 1 - diry
+		// iterate left middle right edge fields
+		for _, lmr := range []int{-1, 0, 1} {
+			fx := fmidx + lmr * diry
+			fy := fmidy + lmr * dirx
+			nfx := nfmidx + lmr * diry
+			nfy := nfmidy + lmr * dirx
+			// check if this edge field is part of our structure
+			if place.piece.f[place.rot][fy][fx].id != c.id {
+				continue
+			}
+			nc := MapCursor{nCoord.x, nCoord.y, 0}
+			if !nOk {
+				// cursor with 0 id represents missing neighbour / open structure edge
+				if includeMissing {
+					res = append(res, nc)
+				}
+				break
+			}
+			nid := nPlace.piece.f[nPlace.rot][nfy][nfx].id
+			nc.id = nid
+			res = append(res, nc)
+		}
+	}
+	return
+}
+
 func (b *Board) MapStructure(f func(interface{}, MapCursor)interface{}, acc interface{}, x, y int, fieldId byte) interface{} {
 	visited := make(map[MapCursor]struct{})
 	queue := []MapCursor{MapCursor{x, y, fieldId}}
@@ -138,32 +176,9 @@ func (b *Board) MapStructure(f func(interface{}, MapCursor)interface{}, acc inte
 		}
 		acc = f(acc, c)
 		visited[c] = struct{}{}
-		place, _ := b.Get(c.x, c.y)
-		nCs, nPs := b.GetNeighbours(c.x, c.y)
-		for i, nCoord := range nCs {
-			nPlace := nPs[i]
-			dirx := nCoord.x - c.x
-			diry := nCoord.y - c.y
-			fmidx  := 2 + (2 * dirx)
-			fmidy  := 1 + diry
-			nfmidx := 2 - (2 * dirx)
-			nfmidy := 1 - diry
-			// iterate left middle right edge fields
-			for _, lmr := range []int{-1, 0, 1} {
-				fx := fmidx + lmr * diry
-				fy := fmidy + lmr * dirx
-				nfx := nfmidx + lmr * diry
-				nfy := nfmidy + lmr * dirx
-				// check if this edge field is part of our structure
-				if place.piece.f[place.rot][fy][fx].id != c.id {
-					continue
-				}
-				nid := nPlace.piece.f[nPlace.rot][nfy][nfx].id
-				nc := MapCursor{nCoord.x, nCoord.y, nid}
-				if _, ok := visited[nc]; ok {
-					continue
-				}
-				queue = append(queue, nc)
+		for _, n := range b.GetNeighbourStructures(c, false) {
+			if _, ok := visited[n]; !ok {
+				queue = append(queue, n)
 			}
 		}
 	}