hilbert.go

/*
Copyright 2014 Workiva, LLC

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package hilbert

import (
	"runtime"
	"sync"

	h "github.com/Workiva/go-datastructures/numerics/hilbert"
	"github.com/Workiva/go-datastructures/rtree"
)

func getCenter(rect rtree.Rectangle) (int32, int32) {
	xlow, ylow := rect.LowerLeft()
	xhigh, yhigh := rect.UpperRight()

	return (xhigh + xlow) / 2, (yhigh + ylow) / 2
}

type hilbertBundle struct {
	hilbert hilbert
	rect    rtree.Rectangle
}

func bundlesFromRects(rects ...rtree.Rectangle) []*hilbertBundle {
	chunks := chunkRectangles(rects, int64(runtime.NumCPU()))
	bundleChunks := make([][]*hilbertBundle, len(chunks))
	var wg sync.WaitGroup
	wg.Add(len(chunks))

	for i := 0; i < runtime.NumCPU(); i++ {
		if len(chunks[i]) == 0 {
			bundleChunks[i] = []*hilbertBundle{}
			wg.Done()
			continue
		}
		go func(i int) {
			bundles := make([]*hilbertBundle, 0, len(chunks[i]))
			for _, r := range chunks[i] {
				h := h.Encode(getCenter(r))
				bundles = append(bundles, &hilbertBundle{hilbert(h), r})
			}
			bundleChunks[i] = bundles
			wg.Done()
		}(i)
	}

	wg.Wait()

	bundles := make([]*hilbertBundle, 0, len(rects))
	for _, bc := range bundleChunks {
		bundles = append(bundles, bc...)
	}

	return bundles
}

// chunkRectangles takes a slice of rtree.Rectangle values and chunks it into `numParts` subslices.
func chunkRectangles(slice rtree.Rectangles, numParts int64) []rtree.Rectangles {
	parts := make([]rtree.Rectangles, numParts)
	for i := int64(0); i < numParts; i++ {
		parts[i] = slice[i*int64(len(slice))/numParts : (i+1)*int64(len(slice))/numParts]
	}
	return parts
}