Add sdr.euclidean()

Closes #64

docs/api/measurement.rst

@@ -16,6 +16,11 @@ Voltage
 
 .. python-apigen-group:: measurement-voltage
 
+Distance
+--------
+
+.. python-apigen-group:: measurement-distance
+
 Modulation
 ----------
 

src/sdr/_measurement/__init__.py

@@ -1,6 +1,7 @@
 """
 A subpackage for various measurement routines.
 """
+from ._distance import *
 from ._energy import *
 from ._errors import *
 from ._modulation import *

src/sdr/_measurement/_distance.py

@@ -0,0 +1,38 @@
+"""
+A module containing various distance measurement functions.
+"""
+from __future__ import annotations
+
+import numpy as np
+import numpy.typing as npt
+
+from .._helper import export
+
+
+@export
+def euclidean(
+    x: npt.NDArray,
+    y: npt.NDArray,
+    axis: int | tuple[int, ...] | None = None,
+) -> npt.NDArray:
+    r"""
+    Measures the Euclidean distance between two signals $x[n]$ and $y[n]$.
+
+    $$d = \sqrt{\sum_{n=0}^{N-1} \left| x[n] - y[n] \right|^2}$$
+
+    Arguments:
+        x: The time-domain signal $x[n]$.
+        y: The time-domain signal $y[n]$.
+        axis: Axis or axes along which to compute the distance. The default is `None`, which computes the distance
+            across the entire array.
+
+    Returns:
+        The Euclidean distance between $x[n]$ and $y[n]$.
+
+    Group:
+        measurement-distance
+    """
+    x = np.asarray(x)
+    y = np.asarray(y)
+    d = np.sqrt(np.sum(np.abs(x - y) ** 2, axis=axis))
+    return d