From 571f6ffe3cce7ecd97c7dfe649a2ce4b25beca18 Mon Sep 17 00:00:00 2001
From: Dan Kortschak <dan@kortschak.io>
Date: Fri, 21 May 2021 17:50:37 +0930
Subject: [PATCH] stat: explicitly panic when calling CDF with empty x

---
 stat/stat.go      |  4 ++++
 stat/stat_test.go | 10 ++++++++++
 2 files changed, 14 insertions(+)

diff --git a/stat/stat.go b/stat/stat.go
index 994dfc30..f17d317a 100644
--- a/stat/stat.go
+++ b/stat/stat.go
@@ -56,6 +56,7 @@ func Bhattacharyya(p, q []float64) float64 {
 //
 // The x data must be sorted in increasing order. If weights is nil then all
 // of the weights are 1. If weights is not nil, then len(x) must equal len(weights).
+// CDF will panic if the length of x is zero.
 //
 // CumulantKind behaviors:
 //  - Empirical: Returns the lowest fraction for which q is greater than or equal
@@ -67,6 +68,9 @@ func CDF(q float64, c CumulantKind, x, weights []float64) float64 {
 	if floats.HasNaN(x) {
 		return math.NaN()
 	}
+	if len(x) == 0 {
+		panic("stat: zero length slice")
+	}
 	if !sort.Float64sAreSorted(x) {
 		panic("x data are not sorted")
 	}
diff --git a/stat/stat_test.go b/stat/stat_test.go
index 275d8492..9a29e3b8 100644
--- a/stat/stat_test.go
+++ b/stat/stat_test.go
@@ -1396,6 +1396,16 @@ func TestCDF(t *testing.T) {
 		x       []float64
 		weights []float64
 	}{
+		{
+			name: "x == nil",
+			kind: Empirical,
+			x:    nil,
+		},
+		{
+			name: "len(x) == 0",
+			kind: Empirical,
+			x:    []float64{},
+		},
 		{
 			name:    "len(x) != len(weights)",
 			q:       1.5,