From aae8464ff3c8d2297f00c7bbb2d8fc941602cb54 Mon Sep 17 00:00:00 2001
From: PHJ2000 <105151063+PHJ2000@users.noreply.github.com>
Date: Mon, 5 Oct 2026 21:35:20 +0900
Subject: [PATCH] Fix loss of precision in standard deviation

---
 .../function/numeric/StandardDeviation.java   | 14 ++++++++------
 .../function/NumericPathFunctionTest.java     | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/json-path/src/main/java/com/jayway/jsonpath/internal/function/numeric/StandardDeviation.java b/json-path/src/main/java/com/jayway/jsonpath/internal/function/numeric/StandardDeviation.java
index 0a83d8a86..5e82a04ca 100644
--- a/json-path/src/main/java/com/jayway/jsonpath/internal/function/numeric/StandardDeviation.java
+++ b/json-path/src/main/java/com/jayway/jsonpath/internal/function/numeric/StandardDeviation.java
@@ -6,19 +6,21 @@
  * Created by mattg on 6/27/15.
  */
 public class StandardDeviation extends AbstractAggregation {
-    private Double sumSq = 0d;
-    private Double sum = 0d;
-    private Double count = 0d;
+    private double mean = 0d;
+    private double sumOfSquaredDifferences = 0d;
+    private long count = 0;
 
     @Override
     protected void next(Number value) {
-        sum += value.doubleValue();
-        sumSq += value.doubleValue() * value.doubleValue();
+        double number = value.doubleValue();
         count++;
+        double delta = number - mean;
+        mean += delta / count;
+        sumOfSquaredDifferences += delta * (number - mean);
     }
 
     @Override
     protected Number getValue() {
-        return Math.sqrt(sumSq/count - sum*sum/count/count);
+        return Math.sqrt(sumOfSquaredDifferences / count);
     }
 }
diff --git a/json-path/src/test/java/com/jayway/jsonpath/internal/function/NumericPathFunctionTest.java b/json-path/src/test/java/com/jayway/jsonpath/internal/function/NumericPathFunctionTest.java
index c271b9f2c..7152f18c0 100644
--- a/json-path/src/test/java/com/jayway/jsonpath/internal/function/NumericPathFunctionTest.java
+++ b/json-path/src/test/java/com/jayway/jsonpath/internal/function/NumericPathFunctionTest.java
@@ -120,4 +120,23 @@ public void testStddevOfEmptyListNegative(Configuration conf) {
 //        assertThat(using(conf).parse(NUMBER_SERIES).read("$.numbers.foo()")).isEqualTo(numberSeries);
 //    }
 
+
+    @ParameterizedTest
+    @MethodSource("configurations")
+    public void testStdDevWithLargeOffset(Configuration conf) {
+        Object result = com.jayway.jsonpath.JsonPath.using(conf)
+                .parse("[10000000000,10000000001,10000000002]").read("$.stddev()");
+        Number value = (Number) conf.jsonProvider().unwrap(result);
+        assertEquals(Math.sqrt(2.0 / 3.0), value.doubleValue(), 1e-12);
+    }
+
+    @ParameterizedTest
+    @MethodSource("configurations")
+    public void testStdDevOfIdenticalLargeValues(Configuration conf) {
+        Object result = com.jayway.jsonpath.JsonPath.using(conf)
+                .parse("[1e200,1e200]").read("$.stddev()");
+        Number value = (Number) conf.jsonProvider().unwrap(result);
+        assertEquals(0.0, value.doubleValue(), 0.0);
+    }
+
 }