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diff --git a/vendor/modernc.org/mathutil/envelope.go b/vendor/modernc.org/mathutil/envelope.go
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+// Copyright (c) 2014 The mathutil Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mathutil // import "modernc.org/mathutil"
+
+import (
+	"math"
+)
+
+// Approximation type determines approximation methods used by e.g. Envelope.
+type Approximation int
+
+// Specific approximation method tags
+const (
+	_          Approximation = iota
+	Linear                   // As named
+	Sinusoidal               // Smooth for all derivations
+)
+
+// Envelope is an utility for defining simple curves using a small (usually)
+// set of data points.  Envelope returns a value defined by x, points and
+// approximation.  The value of x must be in [0,1) otherwise the result is
+// undefined or the function may panic. Points are interpreted as dividing the
+// [0,1) interval in len(points)-1 sections, so len(points) must be > 1 or the
+// function may panic. According to the left and right points closing/adjacent
+// to the section the resulting value is interpolated using the chosen
+// approximation method.  Unsupported values of approximation are silently
+// interpreted as 'Linear'.
+func Envelope(x float64, points []float64, approximation Approximation) float64 {
+	step := 1 / float64(len(points)-1)
+	fslot := math.Floor(x / step)
+	mod := x - fslot*step
+	slot := int(fslot)
+	l, r := points[slot], points[slot+1]
+	rmod := mod / step
+	switch approximation {
+	case Sinusoidal:
+		k := (math.Sin(math.Pi*(rmod-0.5)) + 1) / 2
+		return l + (r-l)*k
+	case Linear:
+		fallthrough
+	default:
+		return l + (r-l)*rmod
+	}
+}