OS Grid: Remove magic number instances.

geofurlong · Apr 23, 2024 · 9dfa793 · 9dfa793
1 parent 159b416
commit 9dfa793
Showing 1 changed file with 10 additions and 7 deletions.
diff --git a/pkg/geocode/osgrid.go b/pkg/geocode/osgrid.go
@@ -22,6 +22,9 @@ const (
 
  // Order of the lettered OSGR tiles.
  TileLetterOrder = "VWXYZQRSTULMNOPFGHJKABCDE"
+
+ // Number of tiles per row in the lettered OSGR grid.
+ TilesPerRow = 5
 )
 
 // letterOrigin represents an OSGR tile letter and associated tile origin co-ordinates.
@@ -30,17 +33,17 @@ type letterOrigin struct {
  origin orb.Point
 }
 
-// tileAndOrigin returns the OSGR tile letter and associated tile origin co-ordinates.
+// tileAndOrigin returns the OSGR tile letter and associated tile origin co-ordinates for an Easting/Northing point.
 func tileAndOrigin(pt orb.Point, tileSize int) letterOrigin {
  tilePosX := math.Floor(pt.X() / float64(tileSize))
  tilePosY := math.Floor(pt.Y() / float64(tileSize))
- letter := string(TileLetterOrder[int(tilePosX)+5*int(tilePosY)])
+ letter := string(TileLetterOrder[int(tilePosX)+TilesPerRow*int(tilePosY)])
  originX := tilePosX * float64(tileSize)
  originY := tilePosY * float64(tileSize)
  return letterOrigin{letter, orb.Point{originX, originY}}
 }
 
-// tile100kmAndOrigin returns the OSGR tile letters and origin co-ordinates.
+// tile100kmAndOrigin returns the OSGR tile letters and origin co-ordinates for an Easting/Northing point.
 func tile100kmAndOrigin(pt orb.Point) letterOrigin {
  lo1 := tileAndOrigin(orb.Point{pt.X() - GridOriginSWEasting, pt.Y() - GridOriginSWNorthing}, PrimaryTileSize)
  lo2 := tileAndOrigin(orb.Point{pt.X() - lo1.origin.X() - GridOriginSWEasting, pt.Y() - lo1.origin.Y() - GridOriginSWNorthing}, SecondaryTileSize)
@@ -75,12 +78,12 @@ func OSGRToPoint(osgr string) (orb.Point, error) {
  }
 
  ixLetter1 := strings.Index(TileLetterOrder, string(prefix[0]))
- originX := PrimaryTileSize*(ixLetter1%5) + GridOriginSWEasting
- originY := PrimaryTileSize*(ixLetter1/5) + GridOriginSWNorthing
+ originX := PrimaryTileSize*(ixLetter1%TilesPerRow) + GridOriginSWEasting
+ originY := PrimaryTileSize*(ixLetter1/TilesPerRow) + GridOriginSWNorthing
 
  ixLetter2 := strings.Index(TileLetterOrder, string(prefix[1]))
- originX += SecondaryTileSize * (ixLetter2 % 5)
- originY += SecondaryTileSize * (ixLetter2 / 5)
+ originX += SecondaryTileSize * (ixLetter2 % TilesPerRow)
+ originY += SecondaryTileSize * (ixLetter2 / TilesPerRow)
 
  depth := (len(osgr) - 2) / 2
  cellSize := SecondaryTileSize / int(math.Pow10(depth))