aria-core 0.1.0

use std::collections::{HashMap, HashSet};

/// Trait that caller's item type must implement.
///
/// The engine only needs three things from any item:
///   - a stable unique ID
///   - a normalised score proxy (0.0–1.0) used by built-in factors
///     (difficulty in learning, price_ratio in ecommerce, remoteness in travel…)
///   - a category/topic string for coverage factor
///
/// All other domain data lives in `metadata()` — accessible to custom factors.
pub trait Scoreable: Send + Sync {
    fn id(&self) -> &str;

    /// Normalised difficulty/complexity proxy. Range [0.0, 1.0].
    /// Caller defines what this means in their domain.
    fn score_proxy(&self) -> f32;

    /// Category label used for coverage balancing.
    /// e.g. "algebra", "electronics", "beach", "thriller"
    fn category(&self) -> &str;

    /// Prerequisite item IDs. Engine will not suggest this item
    /// until all prereqs appear in the user's `resolved_set`.
    fn prerequisites(&self) -> &[String] {
        &[]
    }

    /// Arbitrary key-value metadata for custom factor logic.
    fn metadata(&self) -> &HashMap<String, String>;
}

/// Concrete generic item — callers can use this directly
/// or implement `Scoreable` on their own type.
#[derive(Debug, Clone)]
pub struct Item {
    pub id: String,
    pub score_proxy: f32,
    pub category: String,
    pub prerequisites: Vec<String>,
    pub metadata: HashMap<String, String>,
}

impl Item {
    pub fn new(id: impl Into<String>, score_proxy: f32, category: impl Into<String>) -> Self {
        Self {
            id: id.into(),
            score_proxy: score_proxy.clamp(0.0, 1.0),
            category: category.into(),
            prerequisites: Vec::new(),
            metadata: HashMap::new(),
        }
    }

    pub fn with_prereqs(mut self, prereqs: Vec<String>) -> Self {
        self.prerequisites = prereqs;
        self
    }

    pub fn with_metadata(mut self, metadata: HashMap<String, String>) -> Self {
        self.metadata = metadata;
        self
    }
}

impl Scoreable for Item {
    fn id(&self) -> &str {
        &self.id
    }

    fn score_proxy(&self) -> f32 {
        self.score_proxy
    }

    fn category(&self) -> &str {
        &self.category
    }

    fn prerequisites(&self) -> &[String] {
        &self.prerequisites
    }

    fn metadata(&self) -> &HashMap<String, String> {
        &self.metadata
    }
}

/// Item registry — owns all registered items, validates prereq graph.
pub struct ItemRegistry {
    items: HashMap<String, Item>,
}

impl ItemRegistry {
    pub fn new() -> Self {
        Self {
            items: HashMap::new(),
        }
    }

    /// Register items. Returns Err if a cyclic prerequisite is detected.
    pub fn register(&mut self, items: Vec<Item>) -> Result<(), crate::error::AriaError> {
        for item in items {
            self.items.insert(item.id.clone(), item);
        }
        self.validate_prereq_graph()
    }

    /// Returns items whose prerequisites are all satisfied by resolved_set.
    pub fn eligible<'a>(&'a self, resolved_set: &HashSet<String>) -> Vec<&'a Item> {
        self.items
            .values()
            .filter(|item| {
                item.prerequisites
                    .iter()
                    .all(|prereq| resolved_set.contains(prereq))
            })
            .collect()
    }

    pub fn get(&self, id: &str) -> Option<&Item> {
        self.items.get(id)
    }

    pub fn len(&self) -> usize {
        self.items.len()
    }

    pub fn is_empty(&self) -> bool {
        self.items.is_empty()
    }

    /// Topological sort to detect cycles in prerequisite graph.
    fn validate_prereq_graph(&self) -> Result<(), crate::error::AriaError> {
        let mut visited: HashSet<&str> = HashSet::new();
        let mut in_stack: HashSet<&str> = HashSet::new();

        for id in self.items.keys() {
            if !visited.contains(id.as_str()) {
                self.dfs(id, &mut visited, &mut in_stack)?;
            }
        }
        Ok(())
    }

    fn dfs<'a>(
        &'a self,
        id: &'a str,
        visited: &mut HashSet<&'a str>,
        in_stack: &mut HashSet<&'a str>,
    ) -> Result<(), crate::error::AriaError> {
        visited.insert(id);
        in_stack.insert(id);

        if let Some(item) = self.items.get(id) {
            for prereq in &item.prerequisites {
                if !visited.contains(prereq.as_str()) {
                    self.dfs(prereq, visited, in_stack)?;
                } else if in_stack.contains(prereq.as_str()) {
                    return Err(crate::error::AriaError::CyclicPrerequisite(prereq.clone()));
                }
            }
        }

        in_stack.remove(id);
        Ok(())
    }
}

impl Default for ItemRegistry {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn cycle_detection() {
        let mut registry = ItemRegistry::new();
        let items = vec![
            Item::new("a", 0.3, "cat").with_prereqs(vec!["b".into()]),
            Item::new("b", 0.5, "cat").with_prereqs(vec!["a".into()]),
        ];
        assert!(registry.register(items).is_err());
    }

    #[test]
    fn eligible_filters_unsatisfied_prereqs() {
        let mut registry = ItemRegistry::new();
        registry
            .register(vec![
                Item::new("a", 0.3, "cat"),
                Item::new("b", 0.5, "cat").with_prereqs(vec!["a".into()]),
            ])
            .unwrap();

        let resolved: HashSet<String> = HashSet::new();
        let eligible = registry.eligible(&resolved);
        assert_eq!(eligible.len(), 1);
        assert_eq!(eligible[0].id(), "a");
    }

    #[test]
    fn eligible_after_prereq_resolved() {
        let mut registry = ItemRegistry::new();
        registry
            .register(vec![
                Item::new("a", 0.3, "cat"),
                Item::new("b", 0.5, "cat").with_prereqs(vec!["a".into()]),
            ])
            .unwrap();

        let mut resolved: HashSet<String> = HashSet::new();
        resolved.insert("a".into());
        let eligible = registry.eligible(&resolved);
        assert_eq!(eligible.len(), 2);
    }
}