Mapping with custom function

def build_grid(%Identicon.Image{hex: hex} = image) do
	grid =
		hex
			|> Enum.chunk(3)
			**|> Enum.map(&mirror_row/1)** 
			# &: call the reference of function, 
			# /1: number of argument to differentiate different function with same name.
			**|> List.flatten** # no nested logic
			**|> Enum.with_index** 
			# every element turns into Tuple of {value, index}
			# Elixir doesn't provide the index so it's Elixir-style of indexing

	%Identicon.Image{image | grid: grid}
end

def mirror_row(row) do
	[first, second | _tail] = row

	row ++ [second, first]
end

Filter

One meets the condition, it passes

iex> Enum.filter([1,2,3], fn(x) -> rem(x, 2) == 0 end)
[2]

Enum.filter(grid, fn(square) -> end)
# is often written in the form of below
Enum.filter grid, fn(square) -> end
# and also
Enum.filter grid, fn(square) ->
    # you can write everything you want here
end

# Just for the context
defmodule Identicon.Image do
	defstruct hex: nil, color: nil
end

defmodule Identicon do
	def main(input) do
		input
		|> hash_input
		|> pick_color
		|> build_grid
		**|> filter_odd_squares**
	end

	**def filter_odd_squares(%Identicon.Image{grid: grid} = image) do
		Enum.filter grid, fn({code, _index}) ->
			rem(code, 2) == 0 # divisible by 2, then it's 0

	end**

	def build_grid(%Identicon.Image{hex: hex} = image) do
		grid =
			hex
				|> Enum.chunk(3)
				|> Enum.map(&mirror_row/1) 
				|> List.flatten # no nested logic
				|> Enum.with_index 
	
		%Identicon.Image{image | grid: grid}
	end

def mirror_row(row) do
	[first, second | _tail] = row

	row ++ [second, first]
end

	def pick_color(%Identicon.Image{hex: [r,g,b | _tail} = image ) do
		%Identicon.Image{image | color: {r, g, b}}
	end

	def hash_input(input) do
		hex = :crypto.hash(:md5, input)
		|> :binary.bin_to_list

		%Identicon.Image{hex: hex}
	end
end

def build_grid(%Identicon.Image{hex: hex} = image) do
	grid =
		hex
			|> Enum.chunk(3)
			|> Enum.map(&mirror_row/1) 
			|> List.flatten 
			|> Enum.with_index 

	%Identicon.Image{image | grid: grid}
end

def mirror_row(row) do
	[first, second | _tail] = row

	row ++ [second, first]
end