Method - 1
1. Find the other users who have the same items with user_A to build friend relationship
2. Calculate the Jaccard distance of user_A and his friends
3. Calculate the score of each item of user_A based on friend community
4. Ranking the score of all items which the user may consider to buy them
Example:
User A : (Item a，Item c,Item d)
User B : (Item a，Item b,Item e)
User C : (Item b，Item c,Item f)
User D : (Item a，Item c,Item g)
User E : (Item e，Item f,Item g)
Purpose: What items will User_ A buy ?
Firstly, finding A's friends who have one of the same item, as a result, user B, C and D are user A's friends.
① A's friends = {B,C,D}
Secondly, calculating the score of each items of user A, using Jaccard distance between friends.
②Jaccard(A,B) = {A.items ∩ B.items} / {A.items ∪ B.items} = {a} / {a,c,d,b,e} = 1/5
②Jaccard(A,C) = {A.items ∩ C.items} / {A.items ∪ C.items} = {c} / {a,c,d,b,f} = 1/5
②Jaccard(A,D) = {A.items ∩ D.items} / {A.items ∪ D.items} = {a,c} / {a,c,d,g} = 1/2
Next, calculating the score of each item of user A.
③Score(a) += {if a in friends'items} * Jaccard(A,friends) => Score(a) = Jaccard(A,B) + Jaccard(A,D) = 1/5 + 1/2 = 7/10
③Score(c) += {if c in friends'items} * Jaccard(A,friends) => Score(c) = Jaccard(A,C) + Jaccard(A,D) = 1/5 + 1/2 = 7/10
③Score(d) += {if d in friends'items} * Jaccard(A,friends) => Score(d) = 0
Finally, ranking the score of user A's all itemms.
④ a = c > d > b = e = f =g