mvt.go

package mvt

// See https://www.mapbox.com/vector-tiles/specification/
// and https://github.com/mapbox/vector-tile-spec/tree/master/2.1
// Zero value is ready to use.
type GeomBuffer struct {
	geom  []uint32
	begun bool   // True if a command sequence has begun.
	start int    // Index of the command header.
	count uint32 // Number of commands written so far in this sequence.
}

func (buf *GeomBuffer) begin(cmd uint32) {
	if buf.begun {
		panic("already inside command sequence")
	}
	start := len(buf.geom)
	buf.geom = append(buf.geom, cmd&0x7)
	buf.begun = true
	buf.start = start
	buf.count = 0
}

func (buf *GeomBuffer) MoveTo(x, y int32) {
	buf.BeginMoves()
	buf.Call(x, y)
	buf.End()
}

func (buf *GeomBuffer) BeginMoves() {
	buf.begin(1)
}

func (buf *GeomBuffer) BeginLines() {
	buf.begin(2)
}

func (buf *GeomBuffer) Call(x, y int32) {
	if !buf.begun {
		panic("arguments outside of command sequence")
	}
	buf.count++
	buf.geom = append(buf.geom, zigZag(x), zigZag(y))
}

func (buf *GeomBuffer) ClosePath() {
	buf.begin(7)
	buf.count = 1 // No parameters, but still one command to execute.
	buf.End()
}

func (buf *GeomBuffer) End() {
	if !buf.begun {
		panic("end outside of command sequence")
	}
	buf.geom[buf.start] |= (buf.count << 3)
	buf.begun = false
}

// Finish extracts the encoded data and resets the buffer.
func (buf *GeomBuffer) Finish() []uint32 {
	if buf.begun {
		panic("finish inside command sequence")
	}
	res := buf.geom
	*buf = GeomBuffer{}
	return res
}

// ZigZag-encodes a signed integer so that small absolute
// values have small, positive varint encodings.
// See https://developers.google.com/protocol-buffers/docs/encoding#types
func zigZag(i int32) uint32 {
	return uint32((i << 1) ^ (i >> 31))
}