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Recursion Done Right - blog post examples
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module Collections | |
def self.maximum((head, *tail)) | |
return 0 if tail.empty? | |
max head, (maximum tail) | |
end | |
def self.max(a, b) | |
a > b ? a : b | |
end | |
private_class_method :max | |
def self.map(f, (head, *tail)) | |
return [] unless head | |
[f.(head)] + map(f, tail) | |
end | |
def self.filter(f, (head, *tail)) | |
return [] unless head | |
if f.(head) | |
[head] + filter(f, tail) | |
else | |
filter(f, tail) | |
end | |
end | |
def self.quicksort((head, *tail)) | |
return [] unless head | |
smaller_sorted = quicksort(Collections.filter(->(x) { x <= head }, tail)) | |
bigger_sorted = quicksort(Collections.filter(->(x) { x > head }, tail)) | |
smaller_sorted + [head] + bigger_sorted | |
end | |
end | |
RSpec.describe 'Recursion done right' do | |
context 'maximum' do | |
it 'returns an empty array as max of empty list' do | |
expect(Collections.maximum([])).to eq(0) | |
end | |
it 'returns the maximum of a list' do | |
expect(Collections.maximum([1,3,2])).to eq(3) | |
end | |
end | |
context 'map' do | |
it 'mapping [] with (*3) gives []' do | |
expect(Collections.map(->(x){ x*3 }, [])).to be_empty | |
end | |
it 'mapping [1,2,3] with (*3) gives [1,6,9]' do | |
expect(Collections.map(->(x){ x*3 }, [1,2,3])).to eq([3,6,9]) | |
end | |
end | |
context 'quicksort' do | |
it 'returns an empty list for empty list' do | |
expect(Collections.quicksort([])).to be_empty | |
end | |
it 'sorts a list of items' do | |
expect(Collections.quicksort([2,5,3])).to eq([2,3,5]) | |
end | |
end | |
context 'filter' do | |
specify 'filter (>2) [] returns an empty list' do | |
expect(Collections.filter(->(x){ x > 2 }, [])).to be_empty | |
end | |
specify 'filter (>2) [1,3,5] returns [3,5]' do | |
expect(Collections.filter(->(x){ x > 2 }, [1,3,5])).to eq([3,5]) | |
end | |
end | |
end |
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