Added Percentile function to calculate the p-th percentile of a slice of float64 numbers.

src/math/stats.go

@@ -12,7 +12,6 @@ import (
 // Mean returns the mean (average) of a slice of float64 numbers.
 //
 // Special cases are:
-//
 //   Mean([]float64{}) = 0
 func Mean(numbers []float64) float64 {
 	if len(numbers) == 0 {
@@ -30,28 +29,29 @@ func Mean(numbers []float64) float64 {
 // Median returns the median of a slice of float64 numbers.
 //
 // Special cases are:
-//
 //   Median([]float64{}) = 0
 func Median(numbers []float64) float64 {
 	if len(numbers) == 0 {
 		return 0
 	}
 
+	// Create a sorted copy of the input slice
 	sorted := make([]float64, len(numbers))
 	copy(sorted, numbers)
 	sort.Float64s(sorted)
 
 	n := len(sorted)
 	if n%2 == 0 {
+		// If even, return the average of the two middle numbers
 		return (sorted[n/2-1] + sorted[n/2]) / 2
 	}
+	// If odd, return the middle number
 	return sorted[n/2]
 }
 
 // Mode returns the mode(s) of a slice of float64 numbers.
 //
 // Special cases are:
-//
 //   Mode([]float64{}) = nil
 func Mode(numbers []float64) []float64 {
 	if len(numbers) == 0 {
@@ -83,7 +83,6 @@ func Mode(numbers []float64) []float64 {
 // Variance returns the variance of a slice of float64 numbers.
 //
 // Special cases are:
-//
 //   Variance([]float64{}) = 0
 func Variance(numbers []float64) float64 {
 	if len(numbers) == 0 {
@@ -103,8 +102,34 @@ func Variance(numbers []float64) float64 {
 // StdDev returns the standard deviation of a slice of float64 numbers.
 //
 // Special cases are:
-//
 //   StdDev([]float64{}) = 0
 func StdDev(numbers []float64) float64 {
 	return math.Sqrt(Variance(numbers))
 }
+
+// Percentile returns the p-th percentile of a slice of float64 numbers.
+// The percentile is the value below which a given percentage of observations fall.
+//
+// Special cases are:
+//   Percentile([]float64{}, p) = 0 for any p
+func Percentile(numbers []float64, p float64) float64 {
+	if len(numbers) == 0 || p < 0 || p > 100 {
+		return 0
+	}
+
+	// Sort the slice
+	sorted := make([]float64, len(numbers))
+	copy(sorted, numbers)
+	sort.Float64s(sorted)
+
+	index := (p / 100) * float64(len(sorted)-1)
+	i := int(index)
+
+	// Handle the case for interpolation between two values if not an integer
+	if index == float64(i) {
+		return sorted[i]
+	}
+
+	// Linear interpolation
+	return sorted[i]*(float64(i+1)-index) + sorted[i+1]*(index-float64(i))
+}