Reject malformed spline counts and unsorted knots

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

1 files changed, 7 insertions, 1 deletions

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