Issue3 (#15)

* changed to match official API

* added example

Convert.go

@@ -0,0 +1,134 @@
+package proj
+
+import (
+	"fmt"
+
+	"github.com/go-spatial/proj/core"
+	"github.com/go-spatial/proj/support"
+
+	// need to pull in the operations table entries
+	_ "github.com/go-spatial/proj/operations"
+)
+
+// EPSGCode is the enum type for coordinate systems
+type EPSGCode int
+
+// Supported EPSG codes
+const (
+	EPSG3395      EPSGCode = 3395
+	WorldMercator          = EPSG3395
+	EPSG3857               = 3857
+	WebMercator            = EPSG3857
+)
+
+// Convert performs a conversion from a 4326 coordinate system (lon/lat
+// degrees, 2D) to the given projected system (x/y meters, 2D).
+//
+// The input is assumed to be an array of lon/lat points, e.g. [lon0, lat0,
+// lon1, lat1, lon2, lat2, ...]. The length of the array must, therefore, be
+// even.
+//
+// The returned output is a similar array of x/y points, e.g. [x0, y0, x1,
+// y1, x2, y2, ...].
+func Convert(dest EPSGCode, input []float64) ([]float64, error) {
+
+	conv, err := newConversion(dest)
+	if err != nil {
+		return nil, nil
+	}
+
+	return conv.convert(input)
+}
+
+//---------------------------------------------------------------------------
+
+// conversion holds the objects needed to perform a conversion
+type conversion struct {
+	dest       EPSGCode
+	projString *support.ProjString
+	system     *core.System
+	operation  core.IOperation
+	converter  core.IConvertLPToXY
+}
+
+var conversions = map[EPSGCode]*conversion{}
+
+var projStrings = map[EPSGCode]string{
+	EPSG3395: "+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84",                            // TODO: support +units=m +no_defs
+	EPSG3857: "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0", // TODO: support +units=m +nadgrids=@null +wktext +no_defs
+}
+
+// newConversion creates a conversion object for the destination systems. If
+// such a conversion already exists in the cache, use that.
+func newConversion(dest EPSGCode) (*conversion, error) {
+
+	str, ok := projStrings[dest]
+	if !ok {
+		return nil, fmt.Errorf("epsg code is not a supported projection")
+	}
+
+	conv, ok := conversions[dest]
+	if ok {
+		return conv, nil
+	}
+
+	// need to build it
+
+	ps, err := support.NewProjString(str)
+	if err != nil {
+		return nil, err
+	}
+
+	sys, opx, err := core.NewSystem(ps)
+	if err != nil {
+		return nil, err
+	}
+
+	if !opx.GetDescription().IsConvertLPToXY() {
+		return nil, fmt.Errorf("projection type is not supported")
+	}
+
+	conv = &conversion{
+		dest:       dest,
+		projString: ps,
+		system:     sys,
+		operation:  opx,
+		converter:  opx.(core.IConvertLPToXY),
+	}
+
+	// cache it
+	conversions[dest] = conv
+
+	return conv, nil
+}
+
+// convert performs the projection on the given input points
+func (conv *conversion) convert(input []float64) ([]float64, error) {
+
+	if conv == nil || conv.converter == nil {
+		return nil, fmt.Errorf("conversion not initialized")
+	}
+
+	if len(input)%2 != 0 {
+		return nil, fmt.Errorf("input array of lon/lat values must be an even number")
+	}
+
+	output := make([]float64, len(input))
+
+	lp := &core.CoordLP{}
+
+	for i := 0; i < len(input); i += 2 {
+		lp.Lam = support.DDToR(input[i])
+		lp.Phi = support.DDToR(input[i+1])
+
+		xy, err := conv.converter.Forward(lp)
+		if err != nil {
+			return nil, err
+		}
+
+		output[i] = xy.X
+		output[i+1] = xy.Y
+	}
+
+	return output, nil
+}

Convert_test.go

@@ -0,0 +1,90 @@
+package proj_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/go-spatial/proj"
+	"github.com/stretchr/testify/assert"
+)
+
+var inputA = []float64{
+	-0.127758, 51.507351, // London
+	2.352222, 48.856614, // Paris
+	12.496366, 41.902783, // Rome
+}
+var inputB = []float64{
+	-77.625583, 38.833846, // mpg
+}
+
+type testcase struct {
+	dest      proj.EPSGCode
+	expectedA []float64
+	expectedB []float64
+}
+
+var testcases = []testcase{
+	{
+		dest: proj.EPSG3395,
+		expectedA: []float64{
+			-14221.96, 6678068.96,
+			261848.16, 6218371.80,
+			1391089.10, 5117883.04,
+		},
+		expectedB: []float64{
+			-8641240.37, 4671101.60,
+		},
+	},
+	{
+		dest: proj.EPSG3857,
+		expectedA: []float64{
+			-14221.96, 6711533.71,
+			261848.16, 6250566.72,
+			1391089.10, 5146427.91,
+		},
+		expectedB: []float64{
+			-8641240.37, 4697899.31,
+		},
+	},
+}
+
+func TestConvert(t *testing.T) {
+	assert := assert.New(t)
+
+	for _, tc := range testcases {
+
+		outputA, err := proj.Convert(tc.dest, inputA)
+		assert.NoError(err)
+
+		outputB, err := proj.Convert(tc.dest, inputB)
+		assert.NoError(err)
+
+		const tol = 1.0e-2
+
+		for i := range tc.expectedA {
+			tag := fmt.Sprintf("epsg:%d, input=A.%d", int(tc.dest), i)
+			assert.InDelta(tc.expectedA[i], outputA[i], tol, tag)
+			assert.InDelta(tc.expectedA[i], outputA[i], tol, tag)
+		}
+		for i := range tc.expectedB {
+			tag := fmt.Sprintf("epsg:%d, input=B.%d", int(tc.dest), i)
+			assert.InDelta(tc.expectedB[i], outputB[i], tol, tag)
+			assert.InDelta(tc.expectedB[i], outputB[i], tol, tag)
+		}
+	}
+}
+
+func ExampleConvert() {
+
+	var dd = []float64{
+		-77.625583, 38.833846,
+	}
+
+	xy, err := proj.Convert(proj.EPSG3395, dd)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("%.2f, %.2f\n", xy[0], xy[1])
+	// Output: -8641240.37, 4671101.60
+}