From 1402ca7d6c2bb0cea2cf52a4ecce7b809ebf31b1 Mon Sep 17 00:00:00 2001
From: "Danilo M." <danix@danix.xyz>
Date: Wed, 28 Oct 2026 10:31:32 +0100
Subject: Reject malformed spline counts and unsorted knots

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
---
 generat3d/formula.py  |  8 +++++++-
 tests/test_formula.py | 26 ++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/generat3d/formula.py b/generat3d/formula.py
index 2165e16..2434d89 100644
--- a/generat3d/formula.py
+++ b/generat3d/formula.py
@@ -123,7 +123,10 @@ class Graph:
 
 
 def _spline_op(name, stack):
-    n = int(stack.pop())
+    n = stack.pop()
+    if isinstance(n, bool) or not isinstance(n, (int, float)) or n != int(n):
+        raise ValueError
+    n = int(n)
     if n < 1 or n > len(stack) - 1:
         raise IndexError
     knots = stack[-n:]
@@ -135,6 +138,8 @@ def spline(kind, x, knots):
     """Evaluate a DSON spline at x. Knots are [x, y] or [x, y, t, c, b], sorted by x."""
     xs = [k[0] for k in knots]
     ys = [k[1] for k in knots]
+    if any(a > b for a, b in zip(xs, xs[1:])):
+        raise ValueError("spline knots not sorted by x")
     if x <= xs[0]:
         return ys[0]
     if x >= xs[-1]:
@@ -147,6 +152,7 @@ def spline(kind, x, knots):
         return ys[i] + s * (ys[i + 1] - ys[i])
     # ponytail: Kochanek-Bartels with uniform segment parameterization;
     # if the corpus shows Daz scales tangents by knot spacing, add that here.
+    # end tangents use a one-sided difference (halved), check endpoints against the corpus too.
     m0 = _tcb_tangent(knots, ys, i, outgoing=True)
     m1 = _tcb_tangent(knots, ys, i + 1, outgoing=False)
     s2, s3 = s * s, s * s * s
diff --git a/tests/test_formula.py b/tests/test_formula.py
index 7440681..10065d2 100644
--- a/tests/test_formula.py
+++ b/tests/test_formula.py
@@ -168,3 +168,29 @@ class SplineTest(unittest.TestCase):
         g.add_prop("b")
         g.add_formula("b", spline_ops("spline_linear", [[0, 0], [1, 10]]))
         self.assertAlmostEqual(g.evaluate()["b"], 5.0)
+
+    def test_fractional_count_is_fatal(self):
+        g = Graph()
+        g.add_prop("a", 0.5)
+        g.add_prop("b")
+        ops = [push("a"), val([0, 0]), val([1, 1]), val(2.5), {"op": "spline_linear"}]
+        g.add_formula("b", ops, source="f")
+        with self.assertRaisesRegex(Error, "malformed formula for b"):
+            g.evaluate()
+
+    def test_unsorted_knots_are_fatal(self):
+        g = Graph()
+        g.add_prop("a", 0.5)
+        g.add_prop("b")
+        g.add_formula("b", spline_ops("spline_linear", [[1, 1], [0, 0], [2, 5]]), source="f")
+        with self.assertRaisesRegex(Error, "malformed formula for b"):
+            g.evaluate()
+
+    def test_count_larger_than_stack_is_fatal(self):
+        g = Graph()
+        g.add_prop("a", 0.5)
+        g.add_prop("b")
+        ops = [push("a"), val(0), val(1), {"op": "spline_linear"}]
+        g.add_formula("b", ops, source="f")
+        with self.assertRaisesRegex(Error, "malformed formula for b"):
+            g.evaluate()
-- 
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