slippy.NewTile should check tile if x and y are valid for given z

[ear7h]

geom/slippy/tile.go

Lines 15 to 21 in 2760310

	func NewTile(z, x, y uint) *Tile {
	return &Tile{
	Z: z,
	X: x,
	Y: y,
	}
	}

The NewTile function simply fills in the fields but it should do input validation, for example the tile {Z:0, X: 99, Y:99} does not exist.

I can see two solutions, changing the function signature to func NewTile(z, x, y uint) (*Tile, error) or returning nil from the current function if the tile does not exist.

New Signature

pros:

• more explicit error state
• error state is a compile error
  • tile := slippy.NewTile(...); tile.Dereference is caught by the compiler
• the returned error can be created in geom

cons:

• breaking change

Return nil

pros:

• non-breaking
• error state would give a runtime error (better than going un-noticed)
  • tile := slippy.NewTile(...); tile.Dereference is a nil pointer dereference at runtime

cons:

• error state is would give a runtime error (not as helpful as a compiler error)