Replace implementation of arrow_score with generic calculation from face specification

arrow_score no longer dispatches on scoring system and does not have magic numbers

Author: TomHall2020

archeryutils/handicaps/handicap_scheme.py

@@ -34,6 +34,7 @@
 import warnings
 from abc import ABC, abstractmethod
 from typing import Optional, TypeVar, Union, overload
+import itertools as itr
 
 import numpy as np
 import numpy.typing as npt
@@ -42,6 +43,17 @@
 
 FloatArray = TypeVar("FloatArray", float, npt.NDArray[np.float_])
 
+# itertools.pairwise not available until python 3.10
+# workaround can be removed when support for 3.9 is dropped
+# ignore for coverage (runner is > 3.10, ci shows this works on 3.9)
+if not hasattr(itr, "pairwise"):  # pragma: no cover
+
+    def _pairwise(iterable):
+        a, b = itr.tee(iterable)
+        next(b, None)
+        return zip(a, b)
+
+    setattr(itr, "pairwise", _pairwise)
 
 class HandicapScheme(ABC):
     r"""
@@ -216,104 +228,48 @@ def arrow_score(
                 arw_d = self.arw_d_out
 
         arw_rad = arw_d / 2.0
-
-        tar_dia = target.diameter
+        spec = target.get_face_spec()
         sig_r = self.sigma_r(handicap, target.distance)
+        s_bar = self._s_bar(spec, arw_rad, sig_r)
 
-        if target.scoring_system == "5_zone":
-            s_bar = (
-                9.0
-                - 2.0
-                * sum(
-                    np.exp(-((((n * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-                    for n in range(1, 5)
-                )
-                - np.exp(-((((5.0 * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-            )
-
-        elif target.scoring_system == "10_zone":
-            s_bar = 10.0 - sum(
-                np.exp(-((((n * tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-                for n in range(1, 11)
-            )
-
-        elif target.scoring_system == "10_zone_6_ring":
-            s_bar = (
-                10.0
-                - sum(
-                    np.exp(-((((n * tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-                    for n in range(1, 6)
-                )
-                - 5.0 * np.exp(-((((6.0 * tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-            )
-
-        elif target.scoring_system == "10_zone_compound":
-            s_bar = (
-                10.0
-                - np.exp(-((((tar_dia / 40.0) + arw_rad) / sig_r) ** 2))
-                - sum(
-                    np.exp(-((((n * tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-                    for n in range(2, 11)
-                )
-            )
-
-        elif target.scoring_system == "10_zone_5_ring":
-            s_bar = (
-                10.0
-                - sum(
-                    np.exp(-((((n * tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-                    for n in range(1, 5)
-                )
-                - 6.0 * np.exp(-((((5.0 * tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-            )
-
-        elif target.scoring_system == "10_zone_5_ring_compound":
-            s_bar = (
-                10.0
-                - np.exp(-((((tar_dia / 40) + arw_rad) / sig_r) ** 2))
-                - sum(
-                    np.exp(-((((n * tar_dia / 20) + arw_rad) / sig_r) ** 2))
-                    for n in range(2, 5)
-                )
-                - 6.0 * np.exp(-((((5 * tar_dia / 20) + arw_rad) / sig_r) ** 2))
-            )
-
-        elif target.scoring_system == "WA_field":
-            s_bar = (
-                6.0
-                - np.exp(-((((tar_dia / 20.0) + arw_rad) / sig_r) ** 2))
-                - sum(
-                    np.exp(-((((n * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-                    for n in range(2, 7)
-                )
-            )
+        return s_bar
 
-        elif target.scoring_system == "IFAA_field":
-            s_bar = (
-                5.0
-                - np.exp(-((((tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-                - np.exp(-((((3.0 * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-                - 3.0 * np.exp(-((((5.0 * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-            )
+    def _s_bar(
+        self, target_specs: dict[float, int], arw_rad: float, sig_r: FloatArray
+    ) -> FloatArray:
+        """Calculate expected score directly from target ring sizes.
 
-        elif target.scoring_system == "Beiter_hit_miss":
-            s_bar = 1.0 - np.exp(-((((tar_dia / 2.0) + arw_rad) / sig_r) ** 2))
+        Parameters
+        ----------
+        target_specs : dict[float, int]
+            Mapping of target ring *diameters* in [metres], to points scored
+        arw_d : float
+            arrow diameter in [metres]
+        sig_r : float
+            standard deviation of group size [metres]
 
-        elif target.scoring_system in ("Worcester", "IFAA_field_expert"):
-            s_bar = 5.0 - sum(
-                np.exp(-((((n * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-                for n in range(1, 6)
-            )
+        Returns
+        -------
+        s_bar : float
+            expected average score per arrow
+
+        Notes
+        -----
+        Assumes that:
+        - target rings are concentric
+        - score decreases monotonically as ring sizes increase
+        """
+        target_specs = dict(sorted(target_specs.items()))
+        ring_sizes = target_specs.keys()
+        ring_scores = list(itr.chain(target_specs.values(), [0]))
 
-        elif target.scoring_system == "Worcester_2_ring":
-            s_bar = (
-                5.0
-                - np.exp(-((((tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-                - 4.0 * np.exp(-((((2 * tar_dia / 10.0) + arw_rad) / sig_r) ** 2))
-            )
-        # No else raising error needed as invalid scoring systems handled in Target class
+        score_drops = (inner - outer for inner, outer in itr.pairwise(ring_scores))
+        max_score = max(ring_scores)
 
-        return s_bar
+        return max_score - sum(
+            score_drop * np.exp(-(((arw_rad + (ring_diam / 2)) / sig_r) ** 2))
+            for ring_diam, score_drop in zip(ring_sizes, score_drops)
+        )
 
     def score_for_passes(
         self,

archeryutils/handicaps/tests/test_handicaps.py

@@ -340,7 +340,7 @@ def test_different_handicap_systems(
             ("10_zone_6_ring", 7.397557278),
             ("10_zone_5_ring", 7.059965360),
             ("10_zone_5_ring_compound", 6.993772436),
-            ("WA_field", 4.807397627),
+            ("WA_field", 4.115600784),
             ("IFAA_field", 4.265744101),
             ("IFAA_field_expert", 4.021942762),
             ("Beiter_hit_miss", 0.9998380401),