S

m

p

```
"""
This is a pure Python implementation of the Graham scan algorithm
Source: https://en.wikipedia.org/wiki/Graham_scan
For doctests run following command:
python3 -m doctest -v graham_scan.py
"""
from __future__ import annotations
from collections import deque
from enum import Enum
from math import atan2, degrees
from sys import maxsize
# traversal from the lowest and the most left point in anti-clockwise direction
# if direction gets right, the previous point is not the convex hull.
class Direction(Enum):
left = 1
straight = 2
right = 3
def __repr__(self):
return f"{self.__class__.__name__}.{self.name}"
def angle_comparer(point: tuple[int, int], minx: int, miny: int) -> float:
"""Return the angle toward to point from (minx, miny)
:param point: The target point
minx: The starting point's x
miny: The starting point's y
:return: the angle
Examples:
>>> angle_comparer((1,1), 0, 0)
45.0
>>> angle_comparer((100,1), 10, 10)
-5.710593137499642
>>> angle_comparer((5,5), 2, 3)
33.690067525979785
"""
# sort the points accorgind to the angle from the lowest and the most left point
x, y = point
return degrees(atan2(y - miny, x - minx))
def check_direction(
starting: tuple[int, int], via: tuple[int, int], target: tuple[int, int]
) -> Direction:
"""Return the direction toward to the line from via to target from starting
:param starting: The starting point
via: The via point
target: The target point
:return: the Direction
Examples:
>>> check_direction((1,1), (2,2), (3,3))
Direction.straight
>>> check_direction((60,1), (-50,199), (30,2))
Direction.left
>>> check_direction((0,0), (5,5), (10,0))
Direction.right
"""
x0, y0 = starting
x1, y1 = via
x2, y2 = target
via_angle = degrees(atan2(y1 - y0, x1 - x0))
via_angle %= 360
target_angle = degrees(atan2(y2 - y0, x2 - x0))
target_angle %= 360
# t-
# \ \
# \ v
# \|
# s
# via_angle is always lower than target_angle, if direction is left.
# If they are same, it means they are on a same line of convex hull.
if target_angle > via_angle:
return Direction.left
elif target_angle == via_angle:
return Direction.straight
else:
return Direction.right
def graham_scan(points: list[tuple[int, int]]) -> list[tuple[int, int]]:
"""Pure implementation of graham scan algorithm in Python
:param points: The unique points on coordinates.
:return: The points on convex hell.
Examples:
>>> graham_scan([(9, 6), (3, 1), (0, 0), (5, 5), (5, 2), (7, 0), (3, 3), (1, 4)])
[(0, 0), (7, 0), (9, 6), (5, 5), (1, 4)]
>>> graham_scan([(0, 0), (1, 0), (1, 1), (0, 1)])
[(0, 0), (1, 0), (1, 1), (0, 1)]
>>> graham_scan([(0, 0), (1, 1), (2, 2), (3, 3), (-1, 2)])
[(0, 0), (1, 1), (2, 2), (3, 3), (-1, 2)]
>>> graham_scan([(-100, 20), (99, 3), (1, 10000001), (5133186, -25), (-66, -4)])
[(5133186, -25), (1, 10000001), (-100, 20), (-66, -4)]
"""
if len(points) <= 2:
# There is no convex hull
raise ValueError("graham_scan: argument must contain more than 3 points.")
if len(points) == 3:
return points
# find the lowest and the most left point
minidx = 0
miny, minx = maxsize, maxsize
for i, point in enumerate(points):
x = point[0]
y = point[1]
if y < miny:
miny = y
minx = x
minidx = i
if y == miny and x < minx:
minx = x
minidx = i
# remove the lowest and the most left point from points for preparing for sort
points.pop(minidx)
sorted_points = sorted(points, key=lambda point: angle_comparer(point, minx, miny))
# This insert actually costs complexity,
# and you should instead add (minx, miny) into stack later.
# I'm using insert just for easy understanding.
sorted_points.insert(0, (minx, miny))
stack: deque[tuple[int, int]] = deque()
stack.append(sorted_points[0])
stack.append(sorted_points[1])
stack.append(sorted_points[2])
# The first 3 points lines are towards the left because we sort them by their angle
# from minx, miny.
current_direction = Direction.left
for i in range(3, len(sorted_points)):
while True:
starting = stack[-2]
via = stack[-1]
target = sorted_points[i]
next_direction = check_direction(starting, via, target)
if next_direction == Direction.left:
current_direction = Direction.left
break
if next_direction == Direction.straight:
if current_direction == Direction.left:
# We keep current_direction as left.
# Because if the straight line keeps as straight,
# we want to know if this straight line is towards left.
break
elif current_direction == Direction.right:
# If the straight line is towards right,
# every previous points on that straight line is not convex hull.
stack.pop()
if next_direction == Direction.right:
stack.pop()
stack.append(sorted_points[i])
return list(stack)
```