chanevents: add quantile function

Add Quantile, a generic linear-interpolation q-quantile over a slice of
sortable numeric values. The forwarding-ability analyzer needs to
characterise the distribution of historical forwarded amounts and uses a
configurable percentile as the headline statistic; lifting the
computation into its own helper keeps the analyzer focused on forwarding
logic and gives the quantile contract its own table-driven test that
covers the interpolation rule and the empty/out-of-bounds error paths.

chanevents/quantile.go

@@ -0,0 +1,55 @@
+package chanevents
+
+import (
+	"errors"
+	"sort"
+
+	"golang.org/x/exp/constraints"
+)
+
+// number is the type constraint Quantile accepts: any sortable numeric type.
+type number interface {
+	constraints.Integer | constraints.Float
+}
+
+// Quantile computes the q-quantile of a slice of comparable values. This can be
+// used to compute the median (q=0.5) or the min (q=0) or max (q=1).
+func Quantile[T number](xs []T, q float64) (float64, error) {
+	if q < 0 || q > 1 {
+		return 0, errors.New("quantile must be between 0 and 1")
+	}
+
+	if len(xs) == 0 {
+		return 0, errors.New("cannot compute quantile of empty slice")
+	}
+
+	if len(xs) == 1 {
+		return float64(xs[0]), nil
+	}
+
+	// Create a copy of the slice to avoid mutating the original.
+	ys := make([]T, len(xs))
+	copy(ys, xs)
+
+	sort.Slice(ys, func(i, j int) bool {
+		return ys[i] < ys[j]
+	})
+
+	// Compute fractional index of q-quantile.
+	if q == 1.0 {
+		return float64(ys[len(ys)-1]), nil
+	}
+	i := q * float64(len(ys)-1)
+
+	// Interpolate between the two consecutive values, depending on the
+	// fractional index position in between.
+	lowerIdx := int(i)
+	upperIdx := lowerIdx + 1
+
+	lowerVal := float64(ys[lowerIdx])
+	upperVal := float64(ys[upperIdx])
+
+	indexDiff := i - float64(lowerIdx)
+
+	return lowerVal + (upperVal-lowerVal)*indexDiff, nil
+}

chanevents/quantile_test.go

@@ -0,0 +1,162 @@
+package chanevents
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestQuantile pins the interpolation contract and the error paths Quantile
+// surfaces to callers.
+func TestQuantile(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name      string
+		q         float64
+		xs        []float64
+		want      float64
+		expectErr bool
+	}{
+		{
+			name:      "empty slice",
+			xs:        []float64{},
+			expectErr: true,
+		},
+		{
+			name: "single value",
+			xs: []float64{
+				1,
+			},
+			want: 1.0,
+		},
+		{
+			name: "single value median",
+			xs: []float64{
+				1,
+			},
+			q:    0.5,
+			want: 1.0,
+		},
+		{
+			name:      "quantile out of bound below",
+			xs:        []float64{},
+			q:         -0.1,
+			expectErr: true,
+		},
+		{
+			name:      "quantile out of bound above",
+			xs:        []float64{},
+			q:         1.1,
+			expectErr: true,
+		},
+		{
+			name: "median odd values",
+			q:    0.5,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+				5,
+			},
+			want: 3.0,
+		},
+		{
+			name: "median even values",
+			q:    0.5,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+			},
+			want: 2.5,
+		},
+		{
+			name: "median unsorted",
+			q:    0.5,
+			xs: []float64{
+				1,
+				3,
+				2,
+				4,
+			},
+			want: 2.5,
+		},
+		{
+			name: "0 percentile",
+			q:    0,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+				5,
+			},
+			want: 1.0,
+		},
+		{
+			name: "25 percentile",
+			q:    0.25,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+				5,
+			},
+			want: 2.0,
+		},
+		{
+			name: "75 percentile",
+			q:    0.75,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+				5,
+			},
+			want: 4.0,
+		},
+		{
+			name: "0.875 percentile",
+			q:    0.875,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+				5,
+			},
+			want: 4.5,
+		},
+		{
+			name: "100 percentile",
+			q:    1.0,
+			xs: []float64{
+				1,
+				2,
+				3,
+				4,
+				5,
+			},
+			want: 5.0,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(tt *testing.T) {
+			tt.Parallel()
+
+			got, err := Quantile(tc.xs, tc.q)
+			if tc.expectErr {
+				require.Error(tt, err)
+				return
+			}
+
+			require.InDelta(tt, tc.want, got, 1e-6)
+		})
+	}
+}

go.mod

@@ -20,6 +20,7 @@ require (
 	github.com/shopspring/decimal v1.2.0
 	github.com/stretchr/testify v1.10.0
 	github.com/urfave/cli v1.22.14
+	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b
 	google.golang.org/grpc v1.65.0
 	google.golang.org/protobuf v1.34.2
 	gopkg.in/macaroon-bakery.v2 v2.0.1
@@ -163,7 +164,6 @@ require (
 	go.uber.org/multierr v1.6.0 // indirect
 	go.uber.org/zap v1.17.0 // indirect
 	golang.org/x/crypto v0.39.0 // indirect
-	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b // indirect
 	golang.org/x/net v0.41.0 // indirect
 	golang.org/x/sync v0.15.0 // indirect
 	golang.org/x/sys v0.34.0 // indirect