auto merge of #10211 : ktt3ja/rust/add-lrucache, r=brson

There's an open issue ([Issue #4988](#4988)) for adding an LRU Cache to the standard library. I'm new to this so I hope I didn't miss anything I'm supposed to do.

src/libextra/lib.rs

@@ -70,6 +70,7 @@ pub mod sort;
 pub mod dlist;
 pub mod treemap;
 pub mod btree;
+pub mod lru_cache;
 
 // And ... other stuff
 

src/libextra/lru_cache.rs

@@ -0,0 +1,365 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+//! A cache that holds a limited number of key-value pairs. When the
+//! capacity of the cache is exceeded, the least-recently-used
+//! (where "used" means a look-up or putting the pair into the cache)
+//! pair is automatically removed.
+//!
+//! # Example
+//!
+//! ```rust
+//! let mut cache: LruCache<int, int> = LruCache::new(2);
+//! cache.put(1, 10);
+//! cache.put(2, 20);
+//! cache.put(3, 30);
+//! assert!(cache.get(&1).is_none());
+//! assert_eq!(*cache.get(&2).unwrap(), 20);
+//! assert_eq!(*cache.get(&3).unwrap(), 30);
+//!
+//! cache.put(2, 22);
+//! assert_eq!(*cache.get(&2).unwrap(), 22);
+//!
+//! cache.put(6, 60);
+//! assert!(cache.get(&3).is_none());
+//!
+//! cache.change_capacity(1);
+//! assert!(cache.get(&2).is_none());
+//! ```
+
+use std::container::Container;
+use std::hashmap::HashMap;
+use std::to_bytes::Cb;
+use std::ptr;
+use std::cast;
+
+struct KeyRef<K> { priv k: *K }
+
+struct LruEntry<K, V> {
+    priv key: Option<K>,
+    priv value: Option<V>,
+    priv next: *mut LruEntry<K, V>,
+    priv prev: *mut LruEntry<K, V>,
+}
+
+/// An LRU Cache.
+pub struct LruCache<K, V> {
+    priv map: HashMap<KeyRef<K>, ~LruEntry<K, V>>,
+    priv max_size: uint,
+    priv head: *mut LruEntry<K, V>,
+    priv tail: *mut LruEntry<K, V>,
+}
+
+impl<K: IterBytes> IterBytes for KeyRef<K> {
+    fn iter_bytes(&self, lsb0: bool, f: Cb) -> bool {
+        unsafe{ (*self.k).iter_bytes(lsb0, f) }
+    }
+}
+
+impl<K: Eq> Eq for KeyRef<K> {
+    fn eq(&self, other: &KeyRef<K>) -> bool {
+        unsafe{ (*self.k).eq(&*other.k) }
+    }
+}
+
+impl<K, V> LruEntry<K, V> {
+    fn new() -> LruEntry<K, V> {
+        LruEntry {
+            key: None,
+            value: None,
+            next: ptr::mut_null(),
+            prev: ptr::mut_null(),
+        }
+    }
+
+    fn with_key_value(k: K, v: V) -> LruEntry<K, V> {
+        LruEntry {
+            key: Some(k),
+            value: Some(v),
+            next: ptr::mut_null(),
+            prev: ptr::mut_null(),
+        }
+    }
+}
+
+impl<K: IterBytes + Eq, V> LruCache<K, V> {
+    /// Create an LRU Cache that holds at most `capacity` items.
+    pub fn new(capacity: uint) -> LruCache<K, V> {
+        let cache = LruCache {
+            map: HashMap::new(),
+            max_size: capacity,
+            head: unsafe{ cast::transmute(~LruEntry::<K, V>::new()) },
+            tail: unsafe{ cast::transmute(~LruEntry::<K, V>::new()) },
+        };
+        unsafe {
+            (*cache.head).next = cache.tail;
+            (*cache.tail).prev = cache.head;
+        }
+        return cache;
+    }
+
+    /// Put a key-value pair into cache.
+    pub fn put(&mut self, k: K, v: V) {
+        let mut key_existed = false;
+        let (node_ptr, node_opt) = match self.map.find_mut(&KeyRef{k: &k}) {
+            Some(node) => {
+                key_existed = true;
+                node.value = Some(v);
+                let node_ptr: *mut LruEntry<K, V> = &mut **node;
+                (node_ptr, None)
+            }
+            None => {
+                let mut node = ~LruEntry::with_key_value(k, v);
+                let node_ptr: *mut LruEntry<K, V> = &mut *node;
+                (node_ptr, Some(node))
+            }
+        };
+        if key_existed {
+            self.detach(node_ptr);
+            self.attach(node_ptr);
+        } else {
+            let keyref = unsafe { (*node_ptr).key.as_ref().unwrap() };
+            self.map.swap(KeyRef{k: keyref}, node_opt.unwrap());
+            self.attach(node_ptr);
+            if self.len() > self.capacity() {
+                self.remove_lru();
+            }
+        }
+    }
+
+    /// Return a value corresponding to the key in the cache.
+    pub fn get<'a>(&'a mut self, k: &K) -> Option<&'a V> {
+        let (value, node_ptr_opt) = match self.map.find_mut(&KeyRef{k: k}) {
+            None => (None, None),
+            Some(node) => {
+                let node_ptr: *mut LruEntry<K, V> = &mut **node;
+                unsafe {
+                    match (*node_ptr).value {
+                        None => (None, None),
+                        Some(ref value) => (Some(value), Some(node_ptr))
+                    }
+                }
+            }
+        };
+        match node_ptr_opt {
+            None => (),
+            Some(node_ptr) => {
+                self.detach(node_ptr);
+                self.attach(node_ptr);
+            }
+        }
+        return value;
+    }
+
+    /// Remove and return a value corresponding to the key from the cache.
+    pub fn pop(&mut self, k: &K) -> Option<V> {
+        match self.map.pop(&KeyRef{k: k}) {
+            None => None,
+            Some(lru_entry) => lru_entry.value
+        }
+    }
+
+    /// Return the maximum number of key-value pairs the cache can hold.
+    pub fn capacity(&self) -> uint {
+        self.max_size
+    }
+
+    /// Change the number of key-value pairs the cache can hold. Remove
+    /// least-recently-used key-value pairs if necessary.
+    pub fn change_capacity(&mut self, capacity: uint) {
+        for _ in range(capacity, self.len()) {
+            self.remove_lru();
+        }
+        self.max_size = capacity;
+    }
+
+    #[inline]
+    fn remove_lru(&mut self) {
+        if self.len() > 0 {
+            let lru = unsafe { (*self.tail).prev };
+            self.detach(lru);
+            unsafe {
+                match (*lru).key {
+                    None => (),
+                    Some(ref k) => { self.map.pop(&KeyRef{k: k}); }
+                }
+            }
+        }
+    }
+
+    #[inline]
+    fn detach(&mut self, node: *mut LruEntry<K, V>) {
+        unsafe {
+            (*(*node).prev).next = (*node).next;
+            (*(*node).next).prev = (*node).prev;
+        }
+    }
+
+    #[inline]
+    fn attach(&mut self, node: *mut LruEntry<K, V>) {
+        unsafe {
+            (*node).next = (*self.head).next;
+            (*node).prev = self.head;
+            (*self.head).next = node;
+            (*(*node).next).prev = node;
+        }
+    }
+}
+
+impl<A: ToStr + IterBytes + Eq, B: ToStr> ToStr for LruCache<A, B> {
+    /// Return a string that lists the key-value pairs from most-recently
+    /// used to least-recently used.
+    #[inline]
+    fn to_str(&self) -> ~str {
+        let mut acc = ~"{";
+        let mut cur = self.head;
+        for i in range(0, self.len()) {
+            if i > 0 {
+                acc.push_str(", ");
+            }
+            unsafe {
+                cur = (*cur).next;
+                match (*cur).key {
+                    // should never print nil
+                    None => acc.push_str("nil"),
+                    Some(ref k) => acc.push_str(k.to_str())
+                }
+            }
+            acc.push_str(": ");
+            unsafe {
+                match (*cur).value {
+                    // should never print nil
+                    None => acc.push_str("nil"),
+                    Some(ref value) => acc.push_str(value.to_str())
+                }
+            }
+        }
+        acc.push_char('}');
+        acc
+    }
+}
+
+impl<K: IterBytes + Eq, V> Container for LruCache<K, V> {
+    /// Return the number of key-value pairs in the cache.
+    fn len(&self) -> uint {
+        self.map.len()
+    }
+}
+
+impl<K: IterBytes + Eq, V> Mutable for LruCache<K, V> {
+    /// Clear the cache of all key-value pairs.
+    fn clear(&mut self) {
+        self.map.clear();
+    }
+}
+
+#[unsafe_destructor]
+impl<K, V> Drop for LruCache<K, V> {
+    fn drop(&mut self) {
+        unsafe {
+            let _: ~LruEntry<K, V> = cast::transmute(self.head);
+            let _: ~LruEntry<K, V> = cast::transmute(self.tail);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::LruCache;
+
+    fn assert_opt_eq<V: Eq>(opt: Option<&V>, v: V) {
+        assert!(opt.is_some());
+        assert_eq!(opt.unwrap(), &v);
+    }
+
+    #[test]
+    fn test_put_and_get() {
+        let mut cache: LruCache<int, int> = LruCache::new(2);
+        cache.put(1, 10);
+        cache.put(2, 20);
+        assert_opt_eq(cache.get(&1), 10);
+        assert_opt_eq(cache.get(&2), 20);
+        assert_eq!(cache.len(), 2);
+    }
+
+    #[test]
+    fn test_put_update() {
+        let mut cache: LruCache<~str, ~[u8]> = LruCache::new(1);
+        cache.put(~"1", ~[10, 10]);
+        cache.put(~"1", ~[10, 19]);
+        assert_opt_eq(cache.get(&~"1"), ~[10, 19]);
+        assert_eq!(cache.len(), 1);
+    }
+
+    #[test]
+    fn test_expire_lru() {
+        let mut cache: LruCache<~str, ~str> = LruCache::new(2);
+        cache.put(~"foo1", ~"bar1");
+        cache.put(~"foo2", ~"bar2");
+        cache.put(~"foo3", ~"bar3");
+        assert!(cache.get(&~"foo1").is_none());
+        cache.put(~"foo2", ~"bar2update");
+        cache.put(~"foo4", ~"bar4");
+        assert!(cache.get(&~"foo3").is_none());
+    }
+
+    #[test]
+    fn test_pop() {
+        let mut cache: LruCache<int, int> = LruCache::new(2);
+        cache.put(1, 10);
+        cache.put(2, 20);
+        assert_eq!(cache.len(), 2);
+        let opt1 = cache.pop(&1);
+        assert!(opt1.is_some());
+        assert_eq!(opt1.unwrap(), 10);
+        assert!(cache.get(&1).is_none());
+        assert_eq!(cache.len(), 1);
+    }
+
+    #[test]
+    fn test_change_capacity() {
+        let mut cache: LruCache<int, int> = LruCache::new(2);
+        assert_eq!(cache.capacity(), 2);
+        cache.put(1, 10);
+        cache.put(2, 20);
+        cache.change_capacity(1);
+        assert!(cache.get(&1).is_none());
+        assert_eq!(cache.capacity(), 1);
+    }
+
+    #[test]
+    fn test_to_str() {
+        let mut cache: LruCache<int, int> = LruCache::new(3);
+        cache.put(1, 10);
+        cache.put(2, 20);
+        cache.put(3, 30);
+        assert_eq!(cache.to_str(), ~"{3: 30, 2: 20, 1: 10}");
+        cache.put(2, 22);
+        assert_eq!(cache.to_str(), ~"{2: 22, 3: 30, 1: 10}");
+        cache.put(6, 60);
+        assert_eq!(cache.to_str(), ~"{6: 60, 2: 22, 3: 30}");
+        cache.get(&3);
+        assert_eq!(cache.to_str(), ~"{3: 30, 6: 60, 2: 22}");
+        cache.change_capacity(2);
+        assert_eq!(cache.to_str(), ~"{3: 30, 6: 60}");
+    }
+
+    #[test]
+    fn test_clear() {
+        let mut cache: LruCache<int, int> = LruCache::new(2);
+        cache.put(1, 10);
+        cache.put(2, 20);
+        cache.clear();
+        assert!(cache.get(&1).is_none());
+        assert!(cache.get(&2).is_none());
+        assert_eq!(cache.to_str(), ~"{}");
+    }
+}