/**
 * The application state is a list of Posts and every action is processed by the root reducer, i.e. PostsReducer,
 * which returns the new state.
 */
function PostsReducer(currentState: Post[] = [], action: Action){
    switch(action.type){
        case 'CREATE_POST':
            return createPost(currentState, action);
        case 'ADD_COMMENT':
            return addCommentToPost(currentState, action);
        default:
            return currentState;
    }
}

function createPost(currentState: Post[] = [], action: Action){
    const newPost: Post = {
        id: action.payload.id,
        title: action.payload.title,
        text: action.payload.text,
        createdBy: action.createdBy,
        createdAt: action.createdAt,
        comments: []
    }
    return [...currentState, newPost];
}

function addCommentToPost(currentState: Post[] = [], action: Action){
    return currentState.map(post => {
        if(post.id === action.payload.postId) {
            /* this creates a new object copying all atributes in 'post' but replacing
                posts.comments with the result of the commentsRecuder function call.
            */
            return {...post, comments: CommentsReducer(post.comments, action) };
        }else{
            return post;
        }
    });
}

function CommentsReducer(currentState: Comment[] = [], action: Action){
    switch(action.type){
        case 'ADD_COMMENT':
            return addComment(currentState, action);
        default:
            return currentState;
    }
}

function addComment(currentState: Comment[], action: Action){
    const newComment: Comment = {
        id: action.payload.id,
        text: action.payload.text,
        createdAt: action.createdAt,
        createdBy: action.createdBy
    }
    return [...currentState, newComment];
}

interface Post{
    id: string;
    title: string;
    text: string;
    createdBy: string;
    createdAt: number;
    comments: Comment[];
}

interface Comment{
    id: string;
    text: string;
    createdBy: string;
    createdAt: number;
}

interface Action{
    id: string;
    type: string;
    createdAt: number;
    createdBy: string;
    payload: any; // in a real application create a type for this as well.
}