notifications-admin/app/broadcast_areas/polygons.py

import itertools

from shapely.geometry import (
    JOIN_STYLE,
    GeometryCollection,
    MultiPolygon,
    Polygon,
)
from shapely.ops import unary_union
from werkzeug.utils import cached_property


class Polygons():

    approx_metres_to_degree = 111_320
    approx_square_metres_to_square_degree = approx_metres_to_degree ** 2
    square_degrees_to_square_miles = (
        approx_square_metres_to_square_degree / (1000 * 1000) * 0.386102
    )

    # Estimated amount of bleed into neigbouring areas based on typical
    # range/separation of cell towers.
    approx_bleed_in_degrees = 1_500 / approx_metres_to_degree

    # Controls how much buffer to add for a shape of a given perimeter.
    # Smaller number means more buffering and a smoother shape. For
    # example `1000` means 1m of buffer for every 1km of perimeter, or
    # 20m of buffer for a 5km square. This gives us control over how
    # much we fill in very concave features like channels, harbours and
    # zawns.
    perimeter_to_buffer_ratio = 360

    # Ratio of how much detail a shape of a given perimeter has once
    # simplified. Smaller number means less detail. For example `1000`
    # means that for a shape with a perimeter of 1000m, the simplified
    # line will never deviate more than 1m from the original.
    # Or for a 5km square, the line won’t deviate more than 20m. This
    # gives us approximate control over the total number of points.
    perimeter_to_simplification_ratio = 1_620

    # The threshold for removing very small areas from the map. These
    # areas are likely glitches in  the data where the shoreline hasn’t
    # been subtracted from the land properly
    minimum_area_size_square_metres = 6_500

    def __init__(self, polygons):
        if not polygons:
            self.polygons = []
        elif isinstance(polygons[0], list):
            self.polygons = [
                Polygon(polygon) for polygon in polygons
            ]
        else:
            self.polygons = polygons

    def __getitem__(self, index):
        return self.polygons[index]

    def __len__(self):
        return len(self.polygons)

    @cached_property
    def perimeter_length(self):
        return sum(
            polygon.length for polygon in self
        )

    @cached_property
    def buffer_outward_in_degrees(self):
        return (
            # If two areas are close enough that the distance between
            # them is less than the minimum bleed of a cell
            # broadcast then this joins them together. The aim is to
            # reduce the total number of polygons in areas with many
            # small shapes like Orkney or the Isles of Scilly.
            self.approx_bleed_in_degrees / 3
        ) + (
            self.perimeter_length / self.perimeter_to_buffer_ratio
        )

    @cached_property
    def buffer_inward_in_degrees(self):
        return self.buffer_outward_in_degrees - (
            # We should leave the shape expanded by at least the
            # simplification tolerance in all places, so the
            # simplification never moves a point inside the original
            # shape. In practice half of the tolerance is enough to
            # acheive this.
            self.simplification_tolerance_in_degrees / 2
        )

    @cached_property
    def simplification_tolerance_in_degrees(self):
        return self.perimeter_length / self.perimeter_to_simplification_ratio

    @cached_property
    def smooth(self):
        buffered = [
            polygon.buffer(
                self.buffer_outward_in_degrees,
                resolution=4,
                join_style=JOIN_STYLE.round,
            )
            for polygon in self
        ]
        unioned = union_polygons(buffered)
        debuffered = [
            polygon.buffer(
                -1 * self.buffer_inward_in_degrees,
                resolution=1,
                join_style=JOIN_STYLE.bevel,
            )
            for polygon in unioned
        ]
        flattened = list(itertools.chain(*[
            flatten_polygons(polygon) for polygon in debuffered
        ]))
        return Polygons(flattened)

    @cached_property
    def simplify(self):
        return Polygons([
            polygon.simplify(self.simplification_tolerance_in_degrees)
            for polygon in self
        ])

    @cached_property
    def bleed(self):
        return Polygons(union_polygons([
            polygon.buffer(
                self.approx_bleed_in_degrees,
                resolution=4,
                join_style=JOIN_STYLE.round,
            )
            for polygon in self
        ]))

    @cached_property
    def remove_too_small(self):
        return Polygons([
            polygon for polygon in self
            if (
                polygon.area * self.approx_square_metres_to_square_degree
            ) > (
                self.minimum_area_size_square_metres
            )
        ])

    @cached_property
    def as_coordinate_pairs_long_lat(self):
        return [
            [[x, y] for x, y in polygon.exterior.coords]
            for polygon in self
        ]

    @cached_property
    def as_coordinate_pairs_lat_long(self):
        return [
            [[y, x] for x, y in coordinate_pairs]
            for coordinate_pairs in self.as_coordinate_pairs_long_lat
        ]

    @cached_property
    def point_count(self):
        return len(list(itertools.chain(*self.as_coordinate_pairs_long_lat)))

    @property
    def estimated_area(self):
        return sum(
            polygon.area for polygon in self
        ) * self.square_degrees_to_square_miles


def flatten_polygons(polygons):
    if isinstance(polygons, GeometryCollection):
        return []
    if isinstance(polygons, MultiPolygon):
        return [
            p for p in polygons
        ]
    else:
        return [polygons]


def union_polygons(polygons):
    return flatten_polygons(unary_union(polygons))