From 7eb9e1e47278577013e9b813ca1edb28d7e53aef Mon Sep 17 00:00:00 2001
From: Chris Dzombak <chris@dzombak.com>
Date: Sun, 12 Jan 2025 13:55:07 -0500
Subject: [PATCH] implement circular mean & stddev directly

---
 go.mod   |  2 --
 libwx.go |  8 +++-----
 stat.go  | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 7 deletions(-)
 create mode 100644 stat.go

diff --git a/go.mod b/go.mod
index 80db251..1bed671 100644
--- a/go.mod
+++ b/go.mod
@@ -4,11 +4,9 @@ go 1.22
 
 toolchain go1.23.4
 
-replace gonum.org/v1/gonum => github.com/cdzombak/gonum v0.0.0-20250111220929-90b1f7503766
 
 require (
 	github.com/stretchr/testify v1.9.0
-	gonum.org/v1/gonum v0.0.0-00010101000000-000000000000
 )
 
 require (
diff --git a/libwx.go b/libwx.go
index 5087f1a..1321394 100644
--- a/libwx.go
+++ b/libwx.go
@@ -8,8 +8,6 @@ package libwx
 import (
 	"errors"
 	"math"
-
-	"gonum.org/v1/gonum/stat"
 )
 
 var ErrInputRange = errors.New("one or more input values are outside the calculation's supported range")
@@ -284,7 +282,7 @@ func HeatIndexWarningC(heatIndex TempC) HeatIndexWarning {
 // AvgDirectionDeg calculates the circular mean of the given set of angles (in degrees).
 // This is useful to find e.g. the average wind direction.
 func AvgDirectionDeg(degrees []Degree) Degree {
-	return radToDeg(stat.CircularMean(degToRadSlice(clampedDegSlice(degrees)), nil))
+	return radToDeg(circularMean(degToRadSlice(clampedDegSlice(degrees)), nil))
 }
 
 // WeightedAvgDirectionDeg calculates the weighted circular mean of the given set of angles (in degrees).
@@ -293,11 +291,11 @@ func WeightedAvgDirectionDeg(degrees []Degree, weights []float64) (Degree, error
 	if len(degrees) != len(weights) {
 		return 0.0, ErrMismatchedInputLength
 	}
-	return radToDeg(stat.CircularMean(degToRadSlice(clampedDegSlice(degrees)), weights)), nil
+	return radToDeg(circularMean(degToRadSlice(clampedDegSlice(degrees)), weights)), nil
 }
 
 // StdDevDeg calculates the circular standard deviation of the given set of angles (in degrees).
 // This is useful to find e.g. the variability of wind direction.
 func StdDevDeg(degrees []Degree) Degree {
-	return radToDeg(stat.CircularStdDev(degToRadSlice(clampedDegSlice(degrees))))
+	return radToDeg(circularStdDev(degToRadSlice(clampedDegSlice(degrees))))
 }
diff --git a/stat.go b/stat.go
new file mode 100644
index 0000000..3f4d295
--- /dev/null
+++ b/stat.go
@@ -0,0 +1,35 @@
+package libwx
+
+import "math"
+
+// borrowed from gonum:
+func circularMean(x, weights []float64) float64 {
+	if weights != nil && len(x) != len(weights) {
+		panic("stat: slice length mismatch")
+	}
+
+	var aX, aY float64
+	if weights != nil {
+		for i, v := range x {
+			aX += weights[i] * math.Cos(v)
+			aY += weights[i] * math.Sin(v)
+		}
+	} else {
+		for _, v := range x {
+			aX += math.Cos(v)
+			aY += math.Sin(v)
+		}
+	}
+
+	return math.Atan2(aY, aX)
+}
+
+// from my (open) gonum PR which doesn't (yet) support weights:
+func circularStdDev(x []float64) float64 {
+	var aX, aY float64
+	for _, v := range x {
+		aX += math.Cos(v)
+		aY += math.Sin(v)
+	}
+	return math.Sqrt(-2 * math.Log(math.Sqrt(aY*aY+aX*aX)/float64(len(x))))
+}