get intersection between two ranges in javascript

Question

I'm trying to find the intersection between two ranges (of int values) and (if exists) return an array containing the start and the end of the intersection

example

range 1 : 2,5
range 2 : 4,7

result : 4,5

I found several other topics regarding intersections between arrays but no one helped me in finding the exact intersection (I just found an useful function that returns a 'true' if the intersection exists but doesn't tell what the intersection is)

I'm very bad at alghoritms so i'm finding some issues and i would really appreciate an hint

Thanks

the function that returns true if an intersection exists is a good starting point — Jaromanda X
– Jaromanda X, Commented Aug 17, 2015 at 12:56
get the min and max from the first range, and from the second range. Then you can determine the real min and max from both ranges. Everything > min and < max is an intersection — Jasper Seinhorst
– Jasper Seinhorst, Commented Aug 17, 2015 at 12:58
Get out a pencil and paper, and explicitly write down how you get the answer in your example. Then convert that to code. — Teepeemm
– Teepeemm, Commented Aug 17, 2015 at 13:58

user4668606 · Accepted Answer · 2015-08-17 13:17:48Z

18

This is just some basic logic.

struct range
     int start , end

range intersection(range a , range b)
    //get the range with the smaller starting point (min) and greater start (max)
    range min = (a.start < b.start  ? a : b)
    range max = (min == a ? b : a)

    //min ends before max starts -> no intersection
    if min.end < max.start
        return null //the ranges don't intersect

    return range(max.start , (min.end < max.end ? min.end : max.end))

answered Aug 17, 2015 at 13:17

user4668606

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1 Comment

Max Yari Over a year ago

despite the fact that its indeed a basic logic i was surprised by this way to check for intersetction (i.e comparing minend with maxstart), will come in handy, thanks for that!

Artem Fedotov · Accepted Answer · 2020-10-22 11:24:31Z

If I understand correctly - you want to get the intersection of pairs

const range = [
       [8, 12], [17, 22]],
       [[5, 11], [14, 18], [20, 23]];

and you expect the intersection would be like:

const result = [[8, 11], [17, 18], [20, 22]]

It can be done with two cycles by getting the startMax and endMin, also you can reduce complecity to O(n) using indices

Okey then, now we need to build the function to find these intersections:

        function intersection(user1, user2) {
            const targetUser = (user1.length >= user2.lengh ? user1 : user2);
            const restUser = (targetUser == user1 ? user2 : user1);
            const foundItersections = targetUser.map(tu=> { 
               const mwminmax = restUser.map(ru => {
                    const startMax = (tu[0]>=ru[0] ? tu[0] : ru[0])
                    const endMin =(tu[1]<=ru[1] ? tu[1] : ru[1])
                    if(startMax<endMin){
                        const retarr = [].concat(startMax,endMin)
                        return retarr;
                    }
                    else return;
                })
                const filteredmwminmax = mwminmax.filter(x=>x!==undefined)
                return filteredmwminmax; 
            })
            return foundItersections.flat();
        }
    console.log(intersection(
        [[8, 12], [17, 22]],
        [[5, 11], [14, 18], [20, 23]]
    ))
     // [[8, 11], [17, 18], [20, 22]]
    
    
    console.log(intersection(
        [[9, 15], [18, 21]],
         [[10, 14], [21, 22]]
    ))
    // [[10, 14]]

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