From 5fc806bd394ba5e0661efdb88240fdc542d40cab Mon Sep 17 00:00:00 2001
From: Pim Nelissen <pim.nelissen@protonmail.com>
Date: Fri, 20 Feb 2026 11:43:13 +0100
Subject: [PATCH] Add function to go from relative source position to absolute
 position.

---
 src/pg_rad/utils/__init__.py   |  0
 src/pg_rad/utils/positional.py | 50 ++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 src/pg_rad/utils/__init__.py
 create mode 100644 src/pg_rad/utils/positional.py

diff --git a/src/pg_rad/utils/__init__.py b/src/pg_rad/utils/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/pg_rad/utils/positional.py b/src/pg_rad/utils/positional.py
new file mode 100644
index 0000000..e8b2ddd
--- /dev/null
+++ b/src/pg_rad/utils/positional.py
@@ -0,0 +1,50 @@
+from typing import List
+
+import numpy as np
+
+from pg_rad.exceptions.exceptions import OutOfBoundsError
+
+
+def rel_to_abs_source_position(
+    x_list: List,
+    y_list: List,
+    path_z: int | float,
+    along_path: int | float,
+    side: str,
+    dist_from_path: int | float,
+    z_rel: int | float = 0
+):
+    path = np.column_stack((x_list, y_list))
+    segments = np.diff(path, axis=0)
+    segment_lengths = np.hypot(segments[:, 0], segments[:, 1])
+
+    arc_length = np.cumsum(segment_lengths)
+    arc_length = np.insert(arc_length, 0, 0)
+
+    # find index of the segment where the source should be placed
+    s_i = np.searchsorted(arc_length, along_path, side='right') - 1
+    if not 0 <= s_i < len(segments):
+        raise OutOfBoundsError(
+            "along_path must be less than the length of the path!"
+            )
+
+    # fraction of segment length where to place the point source
+    segment_fraction = (
+        (along_path - arc_length[s_i])
+        / segment_lengths[s_i]
+    )
+
+    # tangential vector at point where point source to be placed
+    point_on_path = path[s_i] + segment_fraction * segments[s_i]
+    tangent_vec = segments[s_i] / segment_lengths[s_i]
+
+    if side not in {"left", "right"}:
+        raise ValueError("side must be 'left' or 'right'")
+
+    orientation = 1 if side == "left" else -1
+    perp_vec = orientation * np.array([-tangent_vec[1], tangent_vec[0]])
+
+    x_abs, y_abs = point_on_path + dist_from_path * perp_vec
+    z_abs = path_z + z_rel
+
+    return x_abs, y_abs, z_abs