alokkiitd1729/project-3

A publish-subscribe Social Platform

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Project-3

"A publish-subscribe Social Platform"

we are going to build the basic data structures underlying publish-subscribe social platforms like Twitter, Instagram etc.

Overview

A publish-subscribe system typically has a set of users, U. Each user has two roles: (a) as a publisher the user is allowed to publish media and as a (b) reader the user is allowed to view media posted by certain other users. To keep this assignment manageable we will make the following simplify- ing assumptions:

1. Time is discrete with no subdivisions, i.e., every action we describe below will take place at a time t 2 f0; 1; : : :g.
2. The only media publishers are allowed to post are short text of up to 30 characters. To implement this assignment you will have to assign each user a unique user id. You will also have to assign each short text a unique text id. Each short text will also contain a eld which will have a ag indicating whether it is a new text, a republished (reposted) text or a reply to an earlier text. Now let us see the details of the working of the system.

The publishing process.

Each publisher (user) is allowed to publish only one short text of up to 30 characters at every time step. This is done through a PUBLISH action.

The reading process

Each user subscribes to a set of publishers (who are also users). At any time t the user may perform a SUBSCRIBE action to subscribe to a publisher or an UNSUBSCRIBE action to unsubscribe. To read text posted on the platform the user may perform a READ action. Let us say the current time at which the user, v, performs a READ action is t0 and the last time the user performed a read was at time t1 < t0 then a list of texts are returned. A text x is included in the list

1. If the user who posted x is ux and v subscribed to ux at time ts < t0 and did not unsubscribe from ux between the ts and t0 and

2. if the timestamp tx of x is such that max{t1; ts}<= tx < t0.

3. Or if t1 <=tx < t0 and x is a reply to some text posted by v.

In other words the text should either be a reply to one of v's text from any user or it should be an unseen text from a publisher that v is currently subscribed to. The texts should be output sorted by time. Before a user has performed any READ we can assume that t1 has some default value like -1 which is smaller than any allowed timestamp.

Actions:

� PUBLISH,t,uid,NEW,text,tid: User uid publish new text at time t having text id tid.

� PUBLISH,t,uid,REPOST(ptid),tid: User uid repost existing text represented by text id ptid at time t. The repost text is assigned a new text id tid.

� PUBLISH,t,uid,REPLY(ptid),text,tid: User uid reply to an exist- ing text represented by text id ptid at time t. The reply text is assigned a new text id tid.

� SUBSCRIBE,t,uid,pid: User uid subscribe to a publisher pid at time t. � UNSUBSCRIBE,t,uid,pid: User uid unsubscribe to a publisher pid at time t.

� READ,t,uid: User uid reads text at time t.