animals.rs/src/animals.rs

use std::cell::Cell;
use std::cell::Ref;
use std::cell::RefCell;
use std::cell::RefMut;

use crate::slot::Slot;
use std::rc::Rc;

#[derive(Debug)]
enum NodeType {
    Question,
    Animal,
}

#[derive(Debug)]
struct NodeStruct {
    kind: NodeType,
    text: String,
    branch_true: Slot<NodeStruct>,
    branch_false: Slot<NodeStruct>,
}

pub struct AnimalDB {
    root: Slot<NodeStruct>,
    enable_debug: bool,
}

pub trait UserAPI {
    fn notify_new_game(&self);
    fn notify_game_ended(&self);
    fn notify_new_animal(&self, animal: &str);
    fn notify_victory(&self);
    fn answer_yes_no(&self, question: &str) -> bool;
    fn is_it_a(&self, animal: &str) -> bool;
    fn what_is_it(&self) -> String;
    fn how_to_tell_apart(&self, other: &str) -> (String, bool);
}

impl AnimalDB {
    pub fn new() -> AnimalDB {
        AnimalDB {
            root: Slot::new(),
            enable_debug: false,
        }
    }

    pub fn enable_debug(&mut self, yes: bool) {
        self.enable_debug = yes;
    }

    fn debug(&self, s: &str) {
        if self.enable_debug {
            println!("{}", s);
        }
    }

    fn insert_new_animal(
        &self,
        user: &dyn UserAPI,
        following: Rc<NodeStruct>,
        updated_branch: &Slot<NodeStruct>,
    ) {
        let new_name = user.what_is_it();
        let (question, answer_for_new) = user.how_to_tell_apart(&following.text);

        self.debug(&format!(
            "Updating DB with: ({}, y:{}, n:{})",
            question,
            if answer_for_new {
                &new_name
            } else {
                &following.text
            },
            if answer_for_new {
                &following.text
            } else {
                &new_name
            }
        ));

        // we have to insert a new Question node between the parent node and the
        // Animal node we just found

        drop(following); // must drop this lease, or .take() will panic
        let old_animal = Rc::try_unwrap(updated_branch.take()).unwrap();

        let mut new_q = NodeStruct {
            kind: NodeType::Question,
            text: question,
            branch_true: Slot::new(),
            branch_false: Slot::new(),
        };

        let (new_animal_branch, old_animal_branch) = match answer_for_new {
            true => (&new_q.branch_true, &new_q.branch_false),
            false => (&new_q.branch_false, &new_q.branch_true),
        };

        user.notify_new_animal(&new_name);

        let mut new_animal = NodeStruct {
            kind: NodeType::Animal,
            text: new_name,
            branch_true: Slot::new(),
            branch_false: Slot::new(),
        };

        new_animal_branch.put(new_animal);
        old_animal_branch.put(old_animal);

        assert!(updated_branch.is_empty());
        updated_branch.put(new_q);
    }

    fn play_game(&self, user: &dyn UserAPI) {
        // question node for the upcoming iteration of the loop
        let mut next = self.root.lease();

        loop {
            let user_answer;
            let updated_branch;
            let following;

            if let NodeType::Animal = &next.kind {
                // this is a leaf node (happens only when a leaf is in root).
                // root is used as a parent reference for inserting a new
                // in-between question if needed.
                updated_branch = &self.root;

                // move from next instead of another lease
                // we can do this here, because updated_branch is borrowed
                // from root directly, so there would be no orphaned reference
                // like in the 'else' branch
                following = next;
            } else {
                user_answer = user.answer_yes_no(&next.text);
                // find which branch will be updated if this is a new animal
                updated_branch = match user_answer {
                    true => &next.branch_true,
                    false => &next.branch_false,
                };
                // cannot overwrite next and use it in the rest of the function,
                // because updated_branch is borrowed from it and the references
                // would become invalid
                following = updated_branch.lease();
            }

            if let NodeType::Animal = &following.kind {
                if user.is_it_a(&following.text) {
                    user.notify_victory();
                } else {
                    self.insert_new_animal(user, following, updated_branch);
                }

                break;
            } else {
                // we found another Question node, proceed with questions.
                next = following; // move lease
            }
        }
    }

    //noinspection RsBorrowChecker
    pub fn start_game(&self, user: &dyn UserAPI) {
        user.notify_new_game();

        if self.root.is_empty() {
            let name = user.what_is_it();
            self.debug(&format!("Initializing empty DB with: ({})", name));

            self.root.put(NodeStruct {
                kind: NodeType::Animal,
                text: name,
                branch_true: Slot::new(),
                branch_false: Slot::new(),
            });
        } else {
            self.play_game(user);
        }

        user.notify_game_ended();
    }
}