# Source code for tryalgo.convex_hull

```#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""\
Convex hull by Andrew

jill-jênn vie et christoph dürr - 2014-2019
"""

# pylint: disable=redefined-outer-name

from random import randint

__all__ = ["andrew"]

# snip{ left-turn
def left_turn(a, b, c):
"""function left-turn"""
return ((a[0] - c[0]) * (b[1] - c[1]) -
(a[1] - c[1]) * (b[0] - c[0]) > 0)
# snip}

# snip{

[docs]
def andrew(S):
"""Convex hull by Andrew

:param S: list of points as coordinate pairs
:requires: S has at least 2 points
:returns: list of points of the convex hull
:complexity: `O(n log n)`
"""
S.sort()
top = []
bot = []
for p in S:
while len(top) >= 2 and not left_turn(p, top[-1], top[-2]):
top.pop()
top.append(p)
while len(bot) >= 2 and not left_turn(bot[-2], bot[-1], p):
bot.pop()
bot.append(p)
return bot[:-1] + top[:0:-1]

# snip}

# pylint: disable=missing-docstring
if __name__ == "__main__":

def gnuplot(L):
for x, y in L:
print(x, y)

def tikz_points(S):
for p in S:
print('\\filldraw[black] (%f, %f) circle (1pt);' % p)

def tikz_polygone(S):
for i, _ in enumerate(S):
print('\\draw[blue] (%f, %f) -- (%f, %f);' % (S[i - 1] + S[i]))

S = [(randint(0, 25)/10., randint(0, 25)/10.) for _ in range(32)]
tikz_points(S)
tikz_polygone(andrew(S))
```