Challenge: find the longest time gap
Very occasionally I’m inspired by a code challenge, this time it’s finding the longest gap in minutes between consecutive timestamps (via @andycroll). Here’s my approach:
def find_longest_time_gap(times)
times
.map { _1.split(':', 2).map(&:to_i) }
.map { |(h, m)| h * 60 + m }
.sort
.each_cons(2)
.map { |(a, b)| b - a }
.max || 0
end
find_longest_time_gap(['14:00', '09:00', '15:00', '10:30'])
# => 210
Breaking it down.
I’m using a “numbered parameter” _1 (which I kinda like more than the newer it 😬 though I’m not sure I’d use either in “proper” code). This is split into two-and-only-two parts (it’s maybe a bit defensive but I like that it’s explicit) and turned into integers to get an array of hours/minutes tuples:
.map { _1.split(':', 2).map(&:to_i) }
# => [[14, 0], [9, 0], [15, 0], [10, 30]]
They’re converted to minutes. Here I’m expanding each two-element tuple/array block argument into two variables (I’m not sure what this is called) which is helpful in these simple cases because it turns it into a neat single line:
.map { |(h, m)| h * 60 + m }
# => [840, 540, 900, 630]
The original times may not be in order so we need to sort them:
.sort
# => [540, 630, 840, 900]
Ruby has a wealth of useful methods – I’m thinking of chunk_while – so I knew there would be something to help me out. I cheated a little and peeked at Andy’s solution 👀 which revealed that this time it’s each_cons (its name was bound to contain an underscore).
each_cons is one of those methods I find slightly weird. It’s called “each” which suggests a side-effect rather than something being returned, but calling it without a block returns an Enumerator which I think I’d want more often (map_cons?).
Here’s what each_cons(n) does:
# Original array
[1, 2, 3, 4, 5]
# each_cons(2)
[1, 2]
[2, 3]
[3, 4]
[4, 5]
# each_cons(3)
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
So here’s the each_cons(2) magic:
.each_cons(2)
# => [[540, 630], [630, 840], [840, 900]]
Now we can map over these to get the differences:
.map { |(a, b)| b - a }
# => [90, 210, 60]
Then get the maximum value. If the array has fewer items than the number passed to each_cons then the returned Enumerator will be empty and the call to max will return nil so we should default to zero:
.max || 0
# => 210
Update: I realised I hadn’t read the question fully and had an incorrect assumption that the times would always be sorted. I fixed that, and the case where there’s only one time (or zero) and therefore no gap, by adding some tests:
require 'minitest/autorun'
class TestMe < Minitest::Test
def test_cases
assert_equal 0, find_longest_time_gap(['12:00'])
assert_equal 120, find_longest_time_gap(['09:00', '11:00'])
assert_equal 210, find_longest_time_gap(['14:00', '09:00', '15:00', '10:30'])
assert_equal 240, find_longest_time_gap(['08:00', '10:00', '10:00', '14:00'])
end
end
