ListComp inside generator code too complex

Question

This is a piece of very cryptic code that I've read and cannot follow. I will post here and seek experts help. It's about to decide how many minimum meeting rooms will need if given some meeting schedule.

For example, if the meetings schedule is - [[1,10], [5,12],[10,15]] Then it will need minimum 2 rooms.

The code is following:

def mini_room(intervals):

    room, current = 0, 0
    for i, v in sorted(x for i, j in intervals for x in [[i,1], [j,-1]]):
        current += v
        room = max(room, current)

print(mini_room(meetings)) # 

Answer 1

The approach is simple. You need a counter, that increments whenever an event starts and decrements whenever an event ends. The maximum value of that counter will give you the required number of rooms.

The "confusing" statement for i, v in sorted(x for i, j in intervals for x in [[i,1], [j,-1]]) is essentially doing this:

1. For each interval i,j, it breaks it into [i,1] and [j,-1]
2. Create a list of said intervals
3. Sort them and traverse it

1 was added to i (in [i,1]) because like I explained above, the counter needs to be incremented when an event starts. 1 marks the new event, and this value gets added to the counter. Similarly, -1 in [j,-1] marks the end of the event and is subtracted from the counter.

Edit:
Here's a simplified version.

simple = []
for i in intervals:
    simple.append([i[0],1])
    simple.append([i[1],-1])
simple.sort()
counter = 0
ans = 0
for i in simple:
    counter += i[1]
    ans = max(ans, counter)
print(ans)