snail_sort_solution1.py

"""
Description:

Snail Sort

Given an n x n array, return the array elements arranged from outermost elements to the middle element, traveling clockwise.

array = [[1,2,3],
         [4,5,6],
         [7,8,9]]
snail(array) #=> [1,2,3,6,9,8,7,4,5]
For better understanding, please follow the numbers of the next array consecutively:

array = [[1,2,3],
         [8,9,4],
         [7,6,5]]
snail(array) #=> [1,2,3,4,5,6,7,8,9]

NOTE: The idea is not sort the elements from the lowest value to the highest; the idea is to traverse the 2-d array in a clockwise snailshell pattern.

NOTE 2: The 0x0 (empty matrix) is represented as [[]]
"""

def direction_vector(direction):
    return {
        0: (0, 1), # LEFT 
        1: (1, 0), # DOWN
        2: (0, -1), # RIGHT
        3: (-1, 0) # UP
    }[direction]    

def snail(array):
    n = len(array[0])
    result = []
    x, y = 0, -1 # start point
    direction = 0 # start direction
    
    offset = 0
    while n - offset > 0:
        dx, dy = direction_vector(direction)
        for _ in range(n - offset):    
            x, y = x + dx, y + dy
            result.append(array[x][y])
            
        direction = (direction + 1) % 4
        if direction == 1 or direction == 3:
            offset += 1
    return result
        

# Tests 

array = [[]]
expected = []
test.assert_equals(snail(array), expected)

array = [[1,2,3],
         [4,5,6],
         [7,8,9]]
expected = [1,2,3,6,9,8,7,4,5]
test.assert_equals(snail(array), expected)


array = [[1,2,3],
         [8,9,4],
         [7,6,5]]
expected = [1,2,3,4,5,6,7,8,9]
test.assert_equals(snail(array), expected)

array = [[1,2,3,4],
         [5,6,7,8],
         [9,10,11,12],
         [13,14,15,16]]
expected = [1,2,3,4,8,12,16,15,14,13,9,5,6,7,11,10]
test.assert_equals(snail(array), expected)