Pytensor-native interpolation functions (#1141)

* add interpolate.py

* Add jax dispatch for `searchsorted`

* Import user-facing functions in `tensor.__init__`

pytensor/link/jax/dispatch/extra_ops.py

@@ -10,6 +10,7 @@
     FillDiagonalOffset,
     RavelMultiIndex,
     Repeat,
+    SearchsortedOp,
     Unique,
     UnravelIndex,
 )
@@ -130,3 +131,13 @@ def jax_funcify_FillDiagonalOffset(op, **kwargs):
     # return filldiagonaloffset
 
     raise NotImplementedError("flatiter not implemented in JAX")
+
+
+@jax_funcify.register(SearchsortedOp)
+def jax_funcify_SearchsortedOp(op, **kwargs):
+    side = op.side
+
+    def searchsorted(a, v, side=side, sorter=None):
+        return jnp.searchsorted(a=a, v=v, side=side, sorter=sorter)
+
+    return searchsorted

pytensor/tensor/__init__.py

@@ -128,6 +128,7 @@ def _get_vector_length_Constant(op: Op | Variable, var: Constant) -> int:
 from pytensor.tensor.basic import *
 from pytensor.tensor.blas import batched_dot, batched_tensordot
 from pytensor.tensor.extra_ops import *
+from pytensor.tensor.interpolate import interp, interpolate1d
 from pytensor.tensor.io import *
 from pytensor.tensor.math import *
 from pytensor.tensor.pad import pad

pytensor/tensor/interpolate.py

@@ -0,0 +1,200 @@
+from collections.abc import Callable
+from difflib import get_close_matches
+from typing import Literal, get_args
+
+from pytensor import Variable
+from pytensor.tensor.basic import as_tensor_variable, switch
+from pytensor.tensor.extra_ops import searchsorted
+from pytensor.tensor.functional import vectorize
+from pytensor.tensor.math import clip, eq, le
+from pytensor.tensor.sort import argsort
+
+
+InterpolationMethod = Literal["linear", "nearest", "first", "last", "mean"]
+valid_methods = get_args(InterpolationMethod)
+
+
+def pad_or_return(x, idx, output, left_pad, right_pad, extrapolate):
+    if extrapolate:
+        return output
+
+    n = x.shape[0]
+
+    return switch(eq(idx, 0), left_pad, switch(eq(idx, n), right_pad, output))
+
+
+def _linear_interp1d(x, y, x_hat, idx, left_pad, right_pad, extrapolate=True):
+    clip_idx = clip(idx, 1, x.shape[0] - 1)
+
+    slope = (x_hat - x[clip_idx - 1]) / (x[clip_idx] - x[clip_idx - 1])
+    y_hat = y[clip_idx - 1] + slope * (y[clip_idx] - y[clip_idx - 1])
+
+    return pad_or_return(x, idx, y_hat, left_pad, right_pad, extrapolate)
+
+
+def _nearest_neighbor_interp1d(x, y, x_hat, idx, left_pad, right_pad, extrapolate=True):
+    clip_idx = clip(idx, 1, x.shape[0] - 1)
+
+    left_distance = x_hat - x[clip_idx - 1]
+    right_distance = x[clip_idx] - x_hat
+    y_hat = switch(le(left_distance, right_distance), y[clip_idx - 1], y[clip_idx])
+
+    return pad_or_return(x, idx, y_hat, left_pad, right_pad, extrapolate)
+
+
+def _stepwise_first_interp1d(x, y, x_hat, idx, left_pad, right_pad, extrapolate=True):
+    clip_idx = clip(idx - 1, 0, x.shape[0] - 1)
+    y_hat = y[clip_idx]
+
+    return pad_or_return(x, idx, y_hat, left_pad, right_pad, extrapolate)
+
+
+def _stepwise_last_interp1d(x, y, x_hat, idx, left_pad, right_pad, extrapolate=True):
+    clip_idx = clip(idx, 0, x.shape[0] - 1)
+    y_hat = y[clip_idx]
+
+    return pad_or_return(x, idx, y_hat, left_pad, right_pad, extrapolate)
+
+
+def _stepwise_mean_interp1d(x, y, x_hat, idx, left_pad, right_pad, extrapolate=True):
+    clip_idx = clip(idx, 1, x.shape[0] - 1)
+    y_hat = (y[clip_idx - 1] + y[clip_idx]) / 2
+
+    return pad_or_return(x, idx, y_hat, left_pad, right_pad, extrapolate)
+
+
+def interpolate1d(
+    x: Variable,
+    y: Variable,
+    method: InterpolationMethod = "linear",
+    left_pad: Variable | None = None,
+    right_pad: Variable | None = None,
+    extrapolate: bool = True,
+) -> Callable[[Variable], Variable]:
+    """
+    Create a function to interpolate one-dimensional data.
+
+    Parameters
+    ----------
+    x : TensorLike
+        Input data used to create an interpolation function. Data will be sorted to be monotonically increasing.
+    y: TensorLike
+        Output data used to create an interpolation function. Must have the same shape as `x`.
+    method : InterpolationMethod, optional
+        Method for interpolation. The following methods are available:
+        - 'linear': Linear interpolation
+        - 'nearest': Nearest neighbor interpolation
+        - 'first': Stepwise interpolation using the closest value to the left of the query point
+        - 'last': Stepwise interpolation using the closest value to the right of the query point
+        - 'mean': Stepwise interpolation using the mean of the two closest values to the query point
+    left_pad: TensorLike, optional
+        Value to return inputs `x_hat < x[0]`. Default is `y[0]`. Ignored if ``extrapolate == True``; in this
+        case, values `x_hat < x[0]` will be extrapolated from the endpoints of `x` and `y`.
+    right_pad: TensorLike, optional
+        Value to return for inputs `x_hat > x[-1]`. Default is `y[-1]`. Ignored if ``extrapolate == True``; in this
+        case, values `x_hat > x[-1]` will be extrapolated from the endpoints of `x` and `y`.
+    extrapolate: bool
+        Whether to extend the request interpolation function beyond the range of the input-output pairs specified in
+        `x` and `y.` If False, constant values will be returned for such inputs.
+
+    Returns
+    -------
+    interpolation_func: OpFromGraph
+        A function that can be used to interpolate new data. The function takes a single input `x_hat` and returns
+        the interpolated value `y_hat`. The input `x_hat` must be a 1d array.
+
+    """
+    x = as_tensor_variable(x)
+    y = as_tensor_variable(y)
+
+    sort_idx = argsort(x)
+    x = x[sort_idx]
+    y = y[sort_idx]
+
+    if left_pad is None:
+        left_pad = y[0]  #  type: ignore
+    else:
+        left_pad = as_tensor_variable(left_pad)
+    if right_pad is None:
+        right_pad = y[-1]  # type: ignore
+    else:
+        right_pad = as_tensor_variable(right_pad)
+
+    def _scalar_interpolate1d(x_hat):
+        idx = searchsorted(x, x_hat)
+
+        if x.ndim != 1 or y.ndim != 1:
+            raise ValueError("Inputs must be 1d")
+
+        if method == "linear":
+            y_hat = _linear_interp1d(
+                x, y, x_hat, idx, left_pad, right_pad, extrapolate=extrapolate
+            )
+        elif method == "nearest":
+            y_hat = _nearest_neighbor_interp1d(
+                x, y, x_hat, idx, left_pad, right_pad, extrapolate=extrapolate
+            )
+        elif method == "first":
+            y_hat = _stepwise_first_interp1d(
+                x, y, x_hat, idx, left_pad, right_pad, extrapolate=extrapolate
+            )
+        elif method == "mean":
+            y_hat = _stepwise_mean_interp1d(
+                x, y, x_hat, idx, left_pad, right_pad, extrapolate=extrapolate
+            )
+        elif method == "last":
+            y_hat = _stepwise_last_interp1d(
+                x, y, x_hat, idx, left_pad, right_pad, extrapolate=extrapolate
+            )
+        else:
+            raise NotImplementedError(
+                f"Unknown interpolation method: {method}. "
+                f"Did you mean {get_close_matches(method, valid_methods)}?"
+            )
+
+        return y_hat
+
+    return vectorize(_scalar_interpolate1d, signature="()->()")
+
+
+def interp(x, xp, fp, left=None, right=None, period=None):
+    """
+    One-dimensional linear interpolation. Similar to ``pytensor.interpolate.interpolate1d``, but with a signature that
+    matches ``np.interp``
+
+    Parameters
+    ----------
+    x : TensorLike
+        The x-coordinates at which to evaluate the interpolated values.
+
+    xp : TensorLike
+        The x-coordinates of the data points, must be increasing if argument `period` is not specified. Otherwise,
+        `xp` is internally sorted after normalizing the periodic boundaries with ``xp = xp % period``.
+
+    fp : TensorLike
+        The y-coordinates of the data points, same length as `xp`.
+
+    left : float, optional
+        Value to return for `x < xp[0]`. Default is `fp[0]`.
+
+    right : float, optional
+        Value to return for `x > xp[-1]`. Default is `fp[-1]`.
+
+    period : None
+        Not supported. Included to ensure the signature of this function matches ``numpy.interp``.
+
+    Returns
+    -------
+    y : Variable
+        The interpolated values, same shape as `x`.
+    """
+
+    xp = as_tensor_variable(xp)
+    fp = as_tensor_variable(fp)
+    x = as_tensor_variable(x)
+
+    f = interpolate1d(
+        xp, fp, method="linear", left_pad=left, right_pad=right, extrapolate=False
+    )
+
+    return f(x)

tests/link/jax/test_extra_ops.py

@@ -6,6 +6,7 @@
 from pytensor.graph.fg import FunctionGraph
 from pytensor.graph.op import get_test_value
 from pytensor.tensor import extra_ops as pt_extra_ops
+from pytensor.tensor.sort import argsort
 from pytensor.tensor.type import matrix, tensor
 from tests.link.jax.test_basic import compare_jax_and_py
 
@@ -55,6 +56,13 @@ def test_extra_ops():
         fgraph, [get_test_value(i) for i in fgraph.inputs], must_be_device_array=False
     )
 
+    v = ptb.as_tensor_variable(6.0)
+    sorted_idx = argsort(a.ravel())
+
+    out = pt_extra_ops.searchsorted(a.ravel()[sorted_idx], v)
+    fgraph = FunctionGraph([a], [out])
+    compare_jax_and_py(fgraph, [a_test])
+
 
 @pytest.mark.xfail(reason="Jitted JAX does not support dynamic shapes")
 def test_bartlett_dynamic_shape():

tests/tensor/test_interpolate.py

@@ -0,0 +1,107 @@
+import numpy as np
+import pytest
+from numpy.testing import assert_allclose
+
+import pytensor
+import pytensor.tensor as pt
+from pytensor.tensor.interpolate import (
+    InterpolationMethod,
+    interp,
+    interpolate1d,
+    valid_methods,
+)
+
+
+floatX = pytensor.config.floatX
+
+
+def test_interp():
+    xp = [1.0, 2.0, 3.0]
+    fp = [3.0, 2.0, 0.0]
+
+    x = [0, 1, 1.5, 2.72, 3.14]
+
+    out = interp(x, xp, fp).eval()
+    np_out = np.interp(x, xp, fp)
+
+    assert_allclose(out, np_out)
+
+
+def test_interp_padded():
+    xp = [1.0, 2.0, 3.0]
+    fp = [3.0, 2.0, 0.0]
+
+    assert interp(3.14, xp, fp, right=-99.0).eval() == -99.0
+    assert_allclose(
+        interp([-1.0, -2.0, -3.0], xp, fp, left=1000.0).eval(), [1000.0, 1000.0, 1000.0]
+    )
+    assert_allclose(
+        interp([-1.0, 10.0], xp, fp, left=-10, right=10).eval(), [-10, 10.0]
+    )
+
+
+@pytest.mark.parametrize("method", valid_methods, ids=str)
+@pytest.mark.parametrize(
+    "left_pad, right_pad", [(None, None), (None, 100), (-100, None), (-100, 100)]
+)
+def test_interpolate_scalar_no_extrapolate(
+    method: InterpolationMethod, left_pad, right_pad
+):
+    x = np.linspace(-2, 6, 10)
+    y = np.sin(x)
+
+    f_op = interpolate1d(
+        x, y, method, extrapolate=False, left_pad=left_pad, right_pad=right_pad
+    )
+    x_hat_pt = pt.dscalar("x_hat")
+    f = pytensor.function([x_hat_pt], f_op(x_hat_pt), mode="FAST_RUN")
+
+    # Data points should be returned exactly, except when method == mean
+    if method not in ["mean", "first"]:
+        assert f(x[3]) == y[3]
+    elif method == "first":
+        assert f(x[3]) == y[2]
+    else:
+        # method == 'mean
+        assert f(x[3]) == (y[2] + y[3]) / 2
+
+    # When extrapolate=False, points beyond the data envelope should be constant
+    left_pad = y[0] if left_pad is None else left_pad
+    right_pad = y[-1] if right_pad is None else right_pad
+
+    assert f(-10) == left_pad
+    assert f(100) == right_pad
+
+
+@pytest.mark.parametrize("method", valid_methods, ids=str)
+def test_interpolate_scalar_extrapolate(method: InterpolationMethod):
+    x = np.linspace(-2, 6, 10)
+    y = np.sin(x)
+
+    f_op = interpolate1d(x, y, method)
+    x_hat_pt = pt.dscalar("x_hat")
+    f = pytensor.function([x_hat_pt], f_op(x_hat_pt), mode="FAST_RUN")
+
+    left_test_point = -5
+    right_test_point = 100
+    if method == "linear":
+        # Linear will compute a slope from the endpoints and continue it
+        left_slope = (left_test_point - x[0]) / (x[1] - x[0])
+        right_slope = (right_test_point - x[-2]) / (x[-1] - x[-2])
+        assert f(left_test_point) == y[0] + left_slope * (y[1] - y[0])
+        assert f(right_test_point) == y[-2] + right_slope * (y[-1] - y[-2])
+
+    elif method == "mean":
+        left_expected = (y[0] + y[1]) / 2
+        right_expected = (y[-1] + y[-2]) / 2
+        assert f(left_test_point) == left_expected
+        assert f(right_test_point) == right_expected
+
+    else:
+        assert f(left_test_point) == y[0]
+        assert f(right_test_point) == y[-1]
+
+        # For interior points, "first" and "last" should disagree. First should take the left side of the interval,
+        # and last should take the right.
+        interior_point = x[3] + 0.1
+        assert f(interior_point) == (y[4] if method == "last" else y[3])