Implement index_distance()

lib/xmet/geo.py

@@ -1,4 +1,5 @@
 import re
+import enum
 import math
 import shapely
 
@@ -14,6 +15,11 @@ def each_intermediate_point(p1: shapely.Point,
     elif typeof is shapely.MultiLineString:
         pass
 
+class PointDirection(enum.Enum):
+    EQUAL = 0
+    LEFT  = enum.auto()
+    RIGHT = enum.auto()
+
 class PointSequence(list):
     linestring: shapely.LineString
     polygon:    shapely.Polygon
@@ -73,6 +79,23 @@ class PointSequence(list):
 
         return indices[0][0]
 
+    def index_distance(self, i1: int, i2: int) -> int:
+        """
+        Returns the index distance of i1 relative to i2, and whether i1 is
+        considered left of, equal to, or right of i2.
+        """
+        count = len(self)
+        value = count - ((i1 - i2) % count)
+
+        if value == 0:
+            direction = PointDirection.EQUAL
+        elif value > count / 2:
+            direction = PointDirection.RIGHT
+        else:
+            direction = PointDirection.LEFT
+
+        return value, direction
+
 class PolygonBuilder():
     point_first: shapely.Point
     point_last:  shapely.Point