Merge two arrays avoiding O(n^2) complexity - arrays

Question

Given two arrays, farmers and collections, I want to be able to merge the farmer information to each collection when farmer_id in the collection is equal to id in farmers. if there is no id of the farmer that matches farmer_id in the collection then that collection should have a an empty farmer object
const farmers = [{
id: 10,
name: 'John Doe',
email: 'jdoe#gmail.com'
},
{
id: 11,
name: 'James Bond',
email: 'james#gmail.com'
}
]
const collections = [{
id: 9,
name: 'Book',
farmer_id: 10,
date: 'June'
},
{
id: 10,
name: 'Game',
farmer_id: 11,
date: 'July'
},
{
id: 13,
name: 'Car',
farmer_id: 10,
date: 'August'
},
{
id: 11,
name: 'Wristwatches',
farmer_id: 20,
date: 'August'
}
]
The result should be in this format below
const result = [{
id: 9,
name: 'Book',
farmer_id: 10,
date: 'June',
farmer: {
id: 10,
name: 'John Doe',
email: 'jdoe#gmail.com'
}
},
{
id: 10,
name: 'Game',
farmer_id: 11,
date: 'July',
farmer: {
id: 11,
name: 'James Bond',
email: 'james#gmail.com'
}
},
{
id: 13,
name: 'Car',
farmer_id: 10,
date: 'August',
farmer: {
id: 10,
name: 'John Doe',
email: 'jdoe#gmail.com'
}
},
{
id: 11,
name: 'Wristwatches',
farmer_id: 20,
date: 'August',
farmer: {}
}
]
This is what i have been able to come up with but am stuck right now
function mapper(farmers, collectors) {
for (let k = 0; k < farmers.length; k++) {
const idToFarmerInfo = {};
idToFarmerInfo[farmers[k].id] = farmers[k];
for (let j = 0; j < collectors.length; j++) {
let mapper = idToFarmerInfo[collectors[j].farmer_id];
farmers[mapper] = collectors[j]
}
}
return farmers
}
i followed this link as am trying to avoid O of N squared but O of N complexity

Answer 1

For a better performance you could create a hash of farmers where the complexity is O(N) because we're iterating the farmers list only once.
const farmers = [{ id: 10, name: 'John Doe', email: 'jdoe#gmail.com' }, { id: 11, name: 'James Bond', email: 'james#gmail.com' } ]; const collections = [{ id: 9, name: 'Book', farmer_id: 10, date: 'June' }, { id: 10, name: 'Game', farmer_id: 11, date: 'July' }, { id: 13, name: 'Car', farmer_id: 10, date: 'August' }, { id: 11, name: 'Wristwatches', farmer_id: 20, date: 'August' } ]
var farmers_hash = farmers.reduce((hash, item) => {
hash[item.id] = item;
return hash;
}, {});
console.log(farmers_hash);
The following step is to build the desired output by assigning one farmer using hash keys.
This can be achieved using map method in combination with Object.assign.
const farmers = [{ id: 10, name: 'John Doe', email: 'jdoe#gmail.com' }, { id: 11, name: 'James Bond', email: 'james#gmail.com' } ]; const collections = [{ id: 9, name: 'Book', farmer_id: 10, date: 'June' }, { id: 10, name: 'Game', farmer_id: 11, date: 'July' }, { id: 13, name: 'Car', farmer_id: 10, date: 'August' }, { id: 11, name: 'Wristwatches', farmer_id: 20, date: 'August' } ]
var farmers_hash = farmers.reduce((hash, item) => {
hash[item.id] = item;
return hash;
}, {});
var result = collections.map((item) => {
item.farmer = Object.assign({}, farmers_hash[item.farmer_id])
return item;
});
console.log(result);
As you can see the final complexity is O(N) + O(M) where N is the length of farmers array and M is the length of collections array.

Answer 2

Demo on stackblitz
You can use a more declarative approach and use Array.map and Array.find
const result = collections.map(collection => {
return {
...collection,
farmer: farmers.find(farmer => collection.farmer_id == farmer.id) || {}
};
});
console.log(result);

Answer 3

You can create a Map collection to have O(N) of access to desired farmer by id. Then mapping becomes faster in terms of performance:
const unique = new Map(farmers.map(f=> [f.id, f]));
const result = collections.map(s => ({
...s, farmer_id: unique.get(s.farmer_id) || s.farmer_id
}))
Now mapping of collections has complexity O(N). However, do not forget to sum complexity of making unique farmers. The overall complexity is O(N) + O(M).
An example:
const farmers = [{
id: 10,
name: 'John Doe',
email: 'jdoe#gmail.com'
},
{
id: 11,
name: 'James Bond',
email: 'james#gmail.com'
}
];
const collections = [{
id: 9,
name: 'Book',
farmer_id: 10,
date: 'June'
},
{
id: 10,
name: 'Game',
farmer_id: 11,
date: 'July'
},
{
id: 13,
name: 'Car',
farmer_id: 10,
date: 'August'
},
{
id: 11,
name: 'Wristwatches',
farmer_id: 20,
date: 'August'
}
];
const unique = new Map(farmers.map(f=> [f.id, f]));
const result = collections.map(s => ({
...s, farmer_id: unique.get(s.farmer_id) || s.farmer_id
}))
console.log(result);