feat(xyMedianYAtXs): add symmetric option to shrink the window at edges

src/xy/__tests__/xyMedianYAtXs.test.ts

@@ -50,6 +50,36 @@ test('window at edges is truncated', () => {
   expect(result.y).toStrictEqual(new Float64Array([5, 5]));
 });
 
+test('symmetric window shrinks at edges', () => {
+  const data = {
+    x: [1, 2, 3, 4, 5],
+    y: [10, 1, 5, 3, 8],
+  };
+
+  const result = xyMedianYAtXs(data, [1, 2, 3, 4, 5], {
+    windowSize: 5,
+    symmetric: true,
+  });
+
+  // index 0: window [10] -> 10
+  // index 1: window [10,1,5] -> 5
+  // index 2: window [10,1,5,3,8] -> 5
+  // index 3: window [5,3,8] -> 5
+  // index 4: window [8] -> 8
+  expect(result.y).toStrictEqual(new Float64Array([10, 5, 5, 5, 8]));
+});
+
+test('symmetric window keeps full size away from edges', () => {
+  const data = {
+    x: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
+    y: [2, 8, 3, 100, 5, 6, 1, 9, 4, 7],
+  };
+
+  const result = xyMedianYAtXs(data, [3, 7], { symmetric: true });
+
+  expect(result.y).toStrictEqual(new Float64Array([5, 5]));
+});
+
 test('does not mutate original Float64Array y data', () => {
   const y = new Float64Array([10, 1, 5, 3, 8]);
   const yOriginal = new Float64Array(y);

src/xy/xyMedianYAtXs.ts

@@ -6,6 +6,12 @@ import { xMedian } from '../x/xMedian.ts';
 export interface XYMedianYAtXsOptions {
   /** Number of points in the sliding window. Must be odd. Defaults to `5`. */
   windowSize?: number;
+  /**
+   * If true, the window shrinks on both sides so that it stays centered on the
+   * closest index when points are missing on one side (near an edge).
+   * @default false
+   */
+  symmetric?: boolean;
 }
 
 /**
@@ -22,16 +28,24 @@ export function xyMedianYAtXs(
   xValues: NumberArray,
   options: XYMedianYAtXsOptions = {},
 ): DataXY {
-  const { windowSize = 5 } = options;
+  const { windowSize = 5, symmetric = false } = options;
   const { x, y } = data;
 
   const halfWindow = Math.floor(windowSize / 2);
+  const lastIndex = y.length - 1;
   const result = new Float64Array(xValues.length);
 
   for (let i = 0; i < xValues.length; i++) {
     const centerIndex = xFindClosestIndex(x, xValues[i]);
-    const fromIndex = Math.max(0, centerIndex - halfWindow);
-    const toIndex = Math.min(y.length, centerIndex + halfWindow + 1);
+    let currentHalfWindow = halfWindow;
+    if (symmetric) {
+      if (centerIndex < currentHalfWindow) currentHalfWindow = centerIndex;
+      if (lastIndex - centerIndex < currentHalfWindow) {
+        currentHalfWindow = lastIndex - centerIndex;
+      }
+    }
+    const fromIndex = Math.max(0, centerIndex - currentHalfWindow);
+    const toIndex = Math.min(y.length, centerIndex + currentHalfWindow + 1);
     result[i] = xMedian(y, { exact: false, fromIndex, toIndex });
   }