From 93546d8a3d9d3fdaa51994302afb92b192b80e0d Mon Sep 17 00:00:00 2001
From: mloubout <mathias.louboutin@gmail.com>
Date: Tue, 14 Apr 2026 14:42:19 -0400
Subject: [PATCH] api: add support for no interp (interp_order=0)

---
 devito/types/basic.py        |  5 +++++
 devito/types/dense.py        |  4 +---
 tests/test_differentiable.py | 19 ++++++++++++++++---
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/devito/types/basic.py b/devito/types/basic.py
index 3239638a98..e6dea0e8e0 100644
--- a/devito/types/basic.py
+++ b/devito/types/basic.py
@@ -1074,6 +1074,11 @@ def _evaluate(self, **kwargs):
         io = self.interp_order
         retval = self.subs({i.subs(subs): self.indices_ref[d]
                             for d, i in mapper.items()})
+
+        if io == 0:
+            # No interpolation, just substitution (e.g nearest grid point)
+            return retval
+
         if self.is_harmonic:
             retval = retval._inv(retval, safe=self.is_harmonic_safe)
 
diff --git a/devito/types/dense.py b/devito/types/dense.py
index 5fcbe8d1d7..30205a9712 100644
--- a/devito/types/dense.py
+++ b/devito/types/dense.py
@@ -1095,8 +1095,6 @@ def __init_finalize__(self, *args, **kwargs):
         interp_order = kwargs.get('interp_order', 2)
         if not is_integer(interp_order):
             raise TypeError("`interp_order` must be an integer")
-        elif interp_order < 1:
-            raise ValueError("`interp_order` must be >= 2")
         elif interp_order > self._space_order and self._space_order > 1:
             raise ValueError("`interp_order` must be <= `space_order`")
         self._interp_order = interp_order
@@ -1121,7 +1119,7 @@ def _fd_priority(self):
         return 1 if self.staggered.on_node else 2
 
     def _eval_at(self, func):
-        if self.staggered == func.staggered:
+        if self.staggered == func.staggered or self.interp_order == 0:
             return self
 
         mapper = {}
diff --git a/tests/test_differentiable.py b/tests/test_differentiable.py
index 65554d4487..ab8d724b60 100644
--- a/tests/test_differentiable.py
+++ b/tests/test_differentiable.py
@@ -111,9 +111,6 @@ def test_avg_mode(ndim, io):
     with pytest.raises(ValueError):
         # interp_order > space_order
         Function(name="a", grid=grid, interp_order=8, space_order=4)
-    with pytest.raises(ValueError):
-        # interp_order < 1
-        Function(name="a", grid=grid, interp_order=0, space_order=4)
     with pytest.raises(TypeError):
         # interp_order not int
         Function(name="a", grid=grid, interp_order=2.5, space_order=4)
@@ -152,3 +149,19 @@ def test_avg_mode(ndim, io):
     assert sympy.simplify(b_avg.args[0] - expected) == 0
     assert isinstance(b_avg, SafeInv)
     assert b_avg.base == b
+
+
+def test_no_interp():
+    grid = Grid((10, 10))
+    x = grid.dimensions[0]
+    a = Function(name="a", grid=grid, staggered=NODE, interp_order=0)
+    sa = Function(name="as", grid=grid, staggered=x)
+
+    assert a._eval_at(sa) == a
+    assert sa._eval_at(a) == sa._subs(x, x - x.spacing/2)
+    assert (a*sa)._eval_at(sa) == a*sa
+    assert (a + sa)._eval_at(sa) == a + sa
+
+    a_shift = a._subs(x, x + x.spacing / 2)
+    # Should just do nearest grid point, so shift back to original
+    assert a_shift.evaluate == a