cfitsio: optimize ImageHDU.load

- simplified the code by making use of the type switch
- removed extra memory buffer allocation and copy
- removed unused counting of 0 elements

Benchmarks show that Read()/load() now runs ~16% faster

Author: lenartj

image.go

@@ -66,7 +66,6 @@ func (hdu *ImageHDU) Data(data interface{}) error {
 
 // load loads the image data associated with this HDU into v.
 func (hdu *ImageHDU) load(v reflect.Value) error {
-	var err error
 	hdr := hdu.Header()
 	naxes := len(hdr.Axes())
 	if naxes == 0 {
@@ -76,78 +75,65 @@ func (hdu *ImageHDU) load(v reflect.Value) error {
 	for _, dim := range hdr.Axes() {
 		nelmts *= int(dim)
 	}
-	rv := reflect.MakeSlice(v.Type(), nelmts, nelmts)
+	if v.Len() != nelmts {
+		return fmt.Errorf("cfitsio: slice length [%v] is not as expected [%v]", v.Len(), nelmts)
+	}
 
 	c_start := C.LONGLONG(0)
 	c_nelmts := C.LONGLONG(nelmts)
-	c_anynull := C.int(0)
 	c_status := C.int(0)
 	c_imgtype := C.int(0)
 	var c_ptr unsafe.Pointer
-	switch rv.Interface().(type) {
+	switch data := v.Interface().(type) {
 	case []byte:
 		c_imgtype = C.TBYTE
-		data := rv.Interface().([]byte)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int8:
 		c_imgtype = C.TBYTE
-		data := rv.Interface().([]int8)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int16:
 		c_imgtype = C.TSHORT
-		data := rv.Interface().([]int16)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []uint16:
 		c_imgtype = C.TUSHORT
-		data := rv.Interface().([]uint16)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int32:
 		c_imgtype = C.TINT
-		data := rv.Interface().([]int32)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []uint32:
 		c_imgtype = C.TUINT
-		data := rv.Interface().([]uint32)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int64:
 		c_imgtype = C.TLONGLONG
-		data := rv.Interface().([]int64)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []uint64:
 		c_imgtype = C.TULONGLONG
-		data := rv.Interface().([]uint64)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []float32:
 		c_imgtype = C.TFLOAT
-		data := rv.Interface().([]float32)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []float64:
 		c_imgtype = C.TDOUBLE
-		data := rv.Interface().([]float64)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	default:
-		panic(fmt.Errorf("invalid image type [%T]", rv.Interface()))
+		panic(fmt.Errorf("invalid image type [%T]", v.Interface()))
 	}
-	C.fits_read_img(hdu.f.c, c_imgtype, c_start+1, c_nelmts, c_ptr, c_ptr, &c_anynull, &c_status)
+	C.fits_read_img(hdu.f.c, c_imgtype, c_start+1, c_nelmts, nil, c_ptr, nil, &c_status)
 	if c_status > 0 {
 		return to_err(c_status)
 	}
 
-	n := reflect.Copy(v, rv)
-	if n != nelmts {
-		err = fmt.Errorf("cfitsio: copied [%v] elements. expected [%v]", n, nelmts)
-	}
-	return err
+	return nil
 }
 
 // Write writes the image to disk
@@ -175,55 +161,45 @@ func (hdu *ImageHDU) Write(data interface{}) error {
 	c_imgtype := C.int(0)
 	var c_ptr unsafe.Pointer
 
-	switch rv.Interface().(type) {
+	switch data := rv.Interface().(type) {
 	case []byte:
 		c_imgtype = C.TBYTE
-		data := rv.Interface().([]byte)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int8:
 		c_imgtype = C.TBYTE
-		data := rv.Interface().([]int8)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int16:
 		c_imgtype = C.TSHORT
-		data := rv.Interface().([]int16)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []uint16:
 		c_imgtype = C.TUSHORT
-		data := rv.Interface().([]uint16)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int32:
 		c_imgtype = C.TINT
-		data := rv.Interface().([]int32)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []uint32:
 		c_imgtype = C.TUINT
-		data := rv.Interface().([]uint32)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []int64:
 		c_imgtype = C.TLONGLONG
-		data := rv.Interface().([]int64)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []uint64:
 		c_imgtype = C.TULONGLONG
-		data := rv.Interface().([]uint64)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []float32:
 		c_imgtype = C.TFLOAT
-		data := rv.Interface().([]float32)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	case []float64:
 		c_imgtype = C.TDOUBLE
-		data := rv.Interface().([]float64)
 		c_ptr = unsafe.Pointer(&data[0])
 
 	default: