dreddit-dapp/instructions/4 Coding: Smart Contract.md

Coding our contract

Let's start by modifying ./contracts/DReddit.sol and see how Embark recompiles our contract whenever we update and save or file.

Functions

Create a post

The create function is used to publish our posts. We need to add a new post to posts[] and emit an event indicating that a new post was created:

1. Add a Post to the posts[] array, asigning values to its attributes. For the creationDate use block.timestamp

uint postId = posts.length++;
posts[postId] = Post({
    creationDate: block.timestamp,
    description: _description,
    owner: msg.sender,
    upvotes: 0,
    downvotes: 0
});

2. Emit the NewPost event

emit NewPost(postId, msg.sender, _description);

Upvote/Downvote a post

We want to be able to asign a score to the posts. For that, let's work on the function vote() from which the users will be able to upvote or downvote each post. NOTE: this voting mechanism is very naive and not recomended to be used on real life due to not being Sybil resistant:

1. Determine if the post exists, and if we have already voted

Post storage p = posts[_postId];
require(p.creationDate != 0, "Post does not exist");
require(p.voters[msg.sender] == Ballot.NONE, "You already voted on this post");

2. Store the vote and update the post score

Ballot b = Ballot(_vote);
if (b == Ballot.UPVOTE) {
    p.upvotes++;
} else {
    p.downvotes++;
}
p.voters[msg.sender] = b;

3. Emit the event

emit Vote(_postId, msg.sender, _vote);

Building a Sybil resistant voting mechanism using a Proof of Identity that takes in account Privacy is a great challenge . How would you solve this problem?

Determine if an user can vote for a post

The users need to know somehow if they're able to vote on a post or not. There are two scenarios where a user should not be able to vote: a) The post doesn't exist, and b) The user already voted. This logic should be developed inside canVote():

1. Determine if the posts exists

if(_postId > posts.length - 1) return false;

2. If the post exists, we will only allow voting if the user hasn't voted yet

Post storage p = posts[_postId];    
return (p.voters[msg.sender] == Ballot.NONE);

Determine the user's vote

It's possible an user would like to know what was his choice on a previously voted post. Implement this on getVote().

Post storage p = posts[_postId];
return uint8(p.voters[msg.sender]);

Notice that on the last two functions, the input and output values related to votes are uin8 instead of an enum. Enum are not a valid input/output value for functions at least on the current solidity version.