tether-map 0.1.0

use core::hash::BuildHasher;
use core::hash::Hash;

use crate::Ptr;
use crate::arena::ActiveSlotRef;
use crate::linked_hash_map::Entry;
use crate::linked_hash_map::Iter;
use crate::linked_hash_map::LinkedHashMap;
use crate::linked_hash_map::RemovedEntry;

#[derive(Debug)]
/// A cursor for navigating and modifying a linked hash map.
///
/// A `CursorMut` is like an iterator, except that it can freely seek
/// back-and-forth and can safely mutate the map during iteration. This is
/// because the lifetime is tied to the map, not the cursor itself.
///
/// Cursors always point to an element in the map. The `CursorMut` is positioned
/// at a specific entry and allows for insertion and removal operations relative
/// to that position.
///
/// # Examples
///
/// ```
/// use tether_map::LinkedHashMap;
///
/// let mut map = LinkedHashMap::new();
/// map.insert("a", 1);
/// map.insert("b", 2);
/// map.insert("c", 3);
///
/// let mut cursor = map.head_cursor_mut();
/// if let Some((key, value)) = cursor.current_mut() {
///     *value *= 10;
/// }
/// assert_eq!(map.get(&"a"), Some(&10));
/// ```
pub struct CursorMut<'m, K, T, S> {
    pub(crate) ptr: Option<ActiveSlotRef<K, T>>,
    pub(crate) map: &'m mut LinkedHashMap<K, T, S>,
}

impl<'m, K: Hash + Eq, T, S: BuildHasher> CursorMut<'m, K, T, S> {
    /// Inserts a key-value pair after the cursor's current position and
    /// moves the cursor to the inserted or updated entry.
    pub fn insert_after_move_to(&mut self, key: K, value: T) -> Option<T> {
        let ptr = if self.ptr.is_none() {
            self.map.head_tail.as_ref().map(|ht| ht.tail)
        } else {
            self.ptr
        };

        match self.map.entry(key) {
            Entry::Occupied(occupied_entry) => {
                let mut map_ptr = *occupied_entry.entry.get();
                if Some(map_ptr) != ptr {
                    self.map.move_after_internal(map_ptr, ptr.unwrap());
                }
                self.ptr = Some(map_ptr);
                Some(core::mem::replace(
                    &mut map_ptr.data_mut(&mut self.map.nodes).value,
                    value,
                ))
            }
            Entry::Vacant(vacant_entry) => {
                self.ptr = Some(vacant_entry.insert_after_internal(value, ptr).0);
                None
            }
        }
    }

    /// Inserts a key-value pair before the cursor's current position and
    /// moves the cursor to the inserted or updated entry.
    pub fn insert_before_move_to(&mut self, key: K, value: T) -> Option<T> {
        let ptr = if self.ptr.is_none() {
            self.map.head_tail.as_ref().map(|ht| ht.head)
        } else {
            self.ptr
        };

        match self.map.entry(key) {
            Entry::Occupied(occupied_entry) => {
                let mut map_ptr = *occupied_entry.entry.get();
                if Some(map_ptr) != ptr {
                    self.map.move_before_internal(map_ptr, ptr.unwrap());
                }
                self.ptr = Some(map_ptr);
                Some(core::mem::replace(
                    &mut map_ptr.data_mut(&mut self.map.nodes).value,
                    value,
                ))
            }
            Entry::Vacant(vacant_entry) => {
                self.ptr = Some(vacant_entry.insert_before_internal(value, ptr).0);
                None
            }
        }
    }

    /// Returns the pointer to the entry with the given key.
    ///
    /// This is a convenience method that delegates to the underlying map's
    /// `get_ptr` method.
    ///
    /// # Arguments
    ///
    /// * `key` - The key to search for
    ///
    /// # Returns
    ///
    /// * `Some(ptr)` if the key exists in the map
    /// * `None` if the key is not found
    pub fn get_ptr(&self, key: &K) -> Option<Ptr> {
        self.map.get_ptr(key)
    }
}

impl<'m, K: Hash + Eq, T, S: BuildHasher> CursorMut<'m, K, T, S> {
    /// Removes the entry before the cursor's current position and returns it.
    pub fn remove_prev(&mut self) -> Option<(Ptr, RemovedEntry<K, T>)> {
        let prev = self.ptr.map(|slot| slot.prev(&self.map.nodes))?;
        Some(self.map.remove_ptr_internal(prev))
    }

    /// Removes the entry after the cursor's current position and returns it.
    pub fn remove_next(&mut self) -> Option<(Ptr, RemovedEntry<K, T>)> {
        let next = self.ptr.map(|slot| slot.next(&self.map.nodes))?;
        Some(self.map.remove_ptr_internal(next))
    }

    /// Removes the entry at the cursor's current position and returns it.
    pub fn remove(self) -> Option<(Ptr, RemovedEntry<K, T>)> {
        let ptr = self.ptr?;
        Some(self.map.remove_ptr_internal(ptr))
    }
}

impl<'m, K, T, S> CursorMut<'m, K, T, S> {
    /// Returns an iterator starting from the cursor's current position.
    pub fn iter(&self) -> Iter<'_, K, T> {
        Iter {
            forward_ptr: self.ptr,
            reverse_ptr: self.ptr.map(|slot| slot.prev(&self.map.nodes)),
            nodes: &self.map.nodes,
        }
    }

    /// Moves the cursor to the next entry in the linked list. The internal
    /// linked list is **circular**, so moving next from the tail wraps around
    /// to the head.
    pub fn move_next(&mut self) {
        self.ptr = self.ptr.map(|slot| slot.next(&self.map.nodes));
    }

    /// Moves the cursor to the previous entry in the linked list. The internal
    /// linked list is **circular**, so moving previous from the head wraps
    /// around to the tail.
    pub fn move_prev(&mut self) {
        self.ptr = self.ptr.map(|slot| slot.prev(&self.map.nodes));
    }

    /// Gets the current pointer of the cursor.
    pub fn ptr(&self) -> Option<Ptr> {
        self.ptr.map(|slot| slot.this(&self.map.nodes))
    }

    /// Checks if the cursor is currently at the tail of the linked list.
    pub fn at_tail(&self) -> bool {
        self.ptr == self.map.head_tail.as_ref().map(|ht| ht.tail)
    }

    /// Checks if the cursor is currently at the head of the linked list.
    pub fn at_head(&self) -> bool {
        self.ptr == self.map.head_tail.as_ref().map(|ht| ht.head)
    }

    /// Returns the entry at the cursor's current position.
    pub fn current(&self) -> Option<(&K, &T)> {
        let ptr = self.ptr?;
        let data = ptr.data(&self.map.nodes);
        Some((&data.key, &data.value))
    }

    /// Returns a mutable reference to the key-value pair at the cursor's
    /// current position.
    ///
    /// The key reference is immutable while the value reference is mutable,
    /// allowing modification of the value while preserving the key's
    /// integrity.
    ///
    /// # Returns
    ///
    /// * `Some((&K, &mut T))` if the cursor is positioned at a valid entry
    /// * `None` if the cursor is positioned at a null entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert("key", 42);
    ///
    /// let mut cursor = map.head_cursor_mut();
    /// if let Some((key, value)) = cursor.current_mut() {
    ///     assert_eq!(key, &"key");
    ///     *value = 100;
    /// }
    /// assert_eq!(map.get(&"key"), Some(&100));
    /// ```
    pub fn current_mut(&mut self) -> Option<(&K, &mut T)> {
        let mut ptr = self.ptr?;
        let data = ptr.data_mut(&mut self.map.nodes);
        Some((&data.key, &mut data.value))
    }

    /// Returns the pointer to the next entry in the linked list from the
    /// cursor's position.
    ///
    /// # Returns
    ///
    /// * `Some(next_ptr)` if there is a next entry
    /// * `None` if the cursor is at the last entry or positioned at a null
    ///   entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert_tail("a", 1);
    /// map.insert_tail("b", 2);
    ///
    /// let cursor = map.head_cursor_mut();
    /// if let Some(next_ptr) = cursor.next_ptr() {
    ///     assert_eq!(map.ptr_get(next_ptr), Some(&2));
    /// }
    /// ```
    pub fn next_ptr(&self) -> Option<Ptr> {
        self.ptr
            .map(|slot| slot.next(&self.map.nodes).this(&self.map.nodes))
    }

    /// Returns a reference to the key-value pair of the next entry in the
    /// linked list.
    ///
    /// This is a convenience method that combines `next_ptr()` and accessing
    /// the entry.
    ///
    /// # Returns
    ///
    /// * `Some((&K, &T))` if there is a next entry
    /// * `None` if the cursor is at the last entry or positioned at a null
    ///   entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert_tail("a", 1);
    /// map.insert_tail("b", 2);
    ///
    /// let cursor = map.head_cursor_mut();
    /// if let Some((key, value)) = cursor.next() {
    ///     assert_eq!(key, &"b");
    ///     assert_eq!(value, &2);
    /// }
    /// ```
    pub fn next(&self) -> Option<(&K, &T)> {
        let ptr = self.next_ptr()?;
        self.map.ptr_get_entry(ptr)
    }

    /// Returns a mutable reference to the key-value pair of the next entry in
    /// the linked list.
    ///
    /// This is a convenience method that combines `next_ptr()` and accessing
    /// the entry mutably. The key reference is immutable while the value
    /// reference is mutable.
    ///
    /// # Returns
    ///
    /// * `Some((&K, &mut T))` if there is a next entry
    /// * `None` if the cursor is at the last entry or positioned at a null
    ///   entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert_tail("a", 1);
    /// map.insert_tail("b", 2);
    ///
    /// let mut cursor = map.head_cursor_mut();
    /// if let Some((key, value)) = cursor.next_mut() {
    ///     assert_eq!(key, &"b");
    ///     *value = 20;
    /// }
    /// assert_eq!(map.get(&"b"), Some(&20));
    /// ```
    pub fn next_mut(&mut self) -> Option<(&K, &mut T)> {
        let ptr = self.next_ptr()?;
        self.map.ptr_get_entry_mut(ptr)
    }

    /// Returns the pointer to the previous entry in the linked list from the
    /// cursor's position.
    ///
    /// # Returns
    ///
    /// * `Some(prev_ptr)` if there is a previous entry
    /// * `None` if the cursor is at the first entry or positioned at a null
    ///   entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert_tail("a", 1);
    /// map.insert_tail("b", 2);
    ///
    /// let cursor = map.tail_cursor_mut();
    /// if let Some(prev_ptr) = cursor.prev_ptr() {
    ///     assert_eq!(map.ptr_get(prev_ptr), Some(&1));
    /// }
    /// ```
    pub fn prev_ptr(&self) -> Option<Ptr> {
        self.ptr
            .map(|slot| slot.prev(&self.map.nodes).this(&self.map.nodes))
    }

    /// Returns a reference to the key-value pair of the previous entry in the
    /// linked list.
    ///
    /// This is a convenience method that combines `prev_ptr()` and accessing
    /// the entry.
    ///
    /// # Returns
    ///
    /// * `Some((&K, &T))` if there is a previous entry
    /// * `None` if the cursor is at the first entry or positioned at a null
    ///   entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert_tail("a", 1);
    /// map.insert_tail("b", 2);
    ///
    /// let cursor = map.tail_cursor_mut();
    /// if let Some((key, value)) = cursor.prev() {
    ///     assert_eq!(key, &"a");
    ///     assert_eq!(value, &1);
    /// }
    /// ```
    pub fn prev(&self) -> Option<(&K, &T)> {
        let ptr = self.prev_ptr()?;
        self.map.ptr_get_entry(ptr)
    }

    /// Returns a mutable reference to the key-value pair of the previous entry
    /// in the linked list.
    ///
    /// This is a convenience method that combines `prev_ptr()` and accessing
    /// the entry mutably. The key reference is immutable while the value
    /// reference is mutable.
    ///
    /// # Returns
    ///
    /// * `Some((&K, &mut T))` if there is a previous entry
    /// * `None` if the cursor is at the first entry or positioned at a null
    ///   entry
    ///
    /// # Examples
    ///
    /// ```
    /// use tether_map::LinkedHashMap;
    ///
    /// let mut map = LinkedHashMap::new();
    /// map.insert_tail("a", 1);
    /// map.insert_tail("b", 2);
    ///
    /// let mut cursor = map.tail_cursor_mut();
    /// if let Some((key, value)) = cursor.prev_mut() {
    ///     assert_eq!(key, &"a");
    ///     *value = 10;
    /// }
    /// assert_eq!(map.get(&"a"), Some(&10));
    /// ```
    pub fn prev_mut(&mut self) -> Option<(&K, &mut T)> {
        let ptr = self.prev_ptr()?;
        self.map.ptr_get_entry_mut(ptr)
    }
}