59 lines
1.8 KiB
Go
59 lines
1.8 KiB
Go
// Copyright (c) 2017 Couchbase, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package geo
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import (
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"math"
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)
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var earthDiameterPerLatitude []float64
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const (
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radiusTabsSize = (1 << 10) + 1
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radiusDelta = (math.Pi / 2) / (radiusTabsSize - 1)
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radiusIndexer = 1 / radiusDelta
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)
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func init() {
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// initializes the tables used for the sloppy math functions
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// earth radius
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a := 6378137.0
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b := 6356752.31420
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a2 := a * a
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b2 := b * b
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earthDiameterPerLatitude = make([]float64, radiusTabsSize)
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earthDiameterPerLatitude[0] = 2.0 * a / 1000
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earthDiameterPerLatitude[radiusTabsSize-1] = 2.0 * b / 1000
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for i := 1; i < radiusTabsSize-1; i++ {
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lat := math.Pi * float64(i) / (2*radiusTabsSize - 1)
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one := math.Pow(a2*math.Cos(lat), 2)
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two := math.Pow(b2*math.Sin(lat), 2)
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three := math.Pow(float64(a)*math.Cos(lat), 2)
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four := math.Pow(b*math.Sin(lat), 2)
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radius := math.Sqrt((one + two) / (three + four))
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earthDiameterPerLatitude[i] = 2 * radius / 1000
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}
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}
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// earthDiameter returns an estimation of the earth's diameter at the specified
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// latitude in kilometers
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func earthDiameter(lat float64) float64 {
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index := math.Mod(math.Abs(lat)*radiusIndexer+0.5, float64(len(earthDiameterPerLatitude)))
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if math.IsNaN(index) {
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return 0
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}
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return earthDiameterPerLatitude[int(index)]
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}
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