Ensure xyShape is ordered by x coordinates when multiplying by negatives

Value: [0.00001 to 0.0005]

packages/squiggle-lang/__tests__/Distributions/AlgebraicShapeCombination_test.res

@@ -0,0 +1,16 @@
+open Jest
+open TestHelpers
+
+describe("combine with discrete", () => {
+  makeTest(
+    "keep order when multiplying by negative number",
+    AlgebraicShapeCombination.checkOrdered(
+      AlgebraicShapeCombination.combineShapesContinuousDiscrete(
+        #Multiply,
+        {xs: [0., 1.], ys: [1., 1.]},
+        {xs: [-1.], ys: [1.]},
+      ),
+    ), // Multiply distribution by -1
+    true,
+  )
+})

packages/squiggle-lang/jest.config.js

@@ -2,7 +2,6 @@
 module.exports = {
   preset: "ts-jest",
   testEnvironment: "node",
-  bail: true,
   setupFilesAfterEnv: [
     "<rootdir>/../../node_modules/bisect_ppx/src/runtime/js/jest.bs.js",
   ],

packages/squiggle-lang/src/rescript/Distributions/PointSetDist/AlgebraicShapeCombination.res

@@ -215,7 +215,6 @@ let combineShapesContinuousDiscrete = (
             continuousShape.ys[i] *. discreteShape.ys[j],
           ),
         ) |> ignore
-        ()
       }
       Belt.Array.set(outXYShapes, j, dxyShape) |> ignore
       ()
@@ -225,9 +224,13 @@ let combineShapesContinuousDiscrete = (
       // creates a new continuous shape for each one of the discrete points, and collects them in outXYShapes.
       let dxyShape: array<(float, float)> = Belt.Array.makeUninitializedUnsafe(t1n)
       for i in 0 to t1n - 1 {
+        // If this operation would flip the x axis (such as -1 * normal(5, 2)),
+        // then we want to fill the shape in backwards to ensure all the points
+        // are still in the right order
+        let index = discreteShape.xs[j] > 0.0 ? i : t1n - 1 - i
         Belt.Array.set(
           dxyShape,
-          i,
+          index,
           (
             fn(continuousShape.xs[i], discreteShape.xs[j]),
             continuousShape.ys[i] *. discreteShape.ys[j] /. Js.Math.abs_float(discreteShape.xs[j]),
@@ -251,3 +254,5 @@ let combineShapesContinuousDiscrete = (
     XYShape.T.empty,
   )
 }
+
+let checkOrdered = (a: XYShape.T.t): bool => a.xs[0] < a.xs[1]