fish-shell/src/lru.h

/** \file lru.h

    Least-recently-used cache implementation
*/

#ifndef FISH_LRU_H
#define FISH_LRU_H

#include <assert.h>
#include <wchar.h>
#include <map>
#include <set>
#include <list>
#include "common.h"

/** A predicate to compare dereferenced pointers */
struct dereference_less_t
{
    template <typename ptr_t>
    bool operator()(ptr_t p1, ptr_t p2) const
    {
        return *p1 < *p2;
    }
};

class lru_node_t
{
    template<class T> friend class lru_cache_t;

    /** Our linked list pointer */
    lru_node_t *prev, *next;

public:
    /** The key used to look up in the cache */
    const wcstring key;

    /** Constructor */
    explicit lru_node_t(const wcstring &pkey) : prev(NULL), next(NULL), key(pkey) { }

    /** Virtual destructor that does nothing for classes that inherit lru_node_t */
    virtual ~lru_node_t() {}

    /** operator< for std::set */
    bool operator<(const lru_node_t &other) const
    {
        return key < other.key;
    }
};

template<class node_type_t>
class lru_cache_t
{
private:

    /** Max node count. This may be (transiently) exceeded by add_node_without_eviction, which is used from background threads. */
    const size_t max_node_count;

    /** Count of nodes */
    size_t node_count;

    /** The set of nodes */
    typedef std::set<lru_node_t *, dereference_less_t> node_set_t;
    node_set_t node_set;

    void promote_node(node_type_t *node)
    {
        /* We should never promote the mouth */
        assert(node != &mouth);

        /* First unhook us */
        node->prev->next = node->next;
        node->next->prev = node->prev;

        /* Put us after the mouth */
        node->next = mouth.next;
        node->next->prev = node;
        node->prev = &mouth;
        mouth.next = node;
    }

    void evict_node(node_type_t *condemned_node)
    {
        /* We should never evict the mouth */
        assert(condemned_node != NULL && condemned_node != &mouth);

        /* Remove it from the linked list */
        condemned_node->prev->next = condemned_node->next;
        condemned_node->next->prev = condemned_node->prev;

        /* Remove us from the set */
        node_set.erase(condemned_node);
        node_count--;

        /* Tell ourselves */
        this->node_was_evicted(condemned_node);
    }

    void evict_last_node(void)
    {
        /* Simple */
        evict_node((node_type_t *)mouth.prev);
    }

    static lru_node_t *get_previous(lru_node_t *node)
    {
        return node->prev;
    }

protected:

    /** Head of the linked list */
    lru_node_t mouth;

    /** Overridable callback for when a node is evicted */
    virtual void node_was_evicted(node_type_t *node) { }

public:

    /** Constructor */
    explicit lru_cache_t(size_t max_size = 1024) : max_node_count(max_size), node_count(0), mouth(wcstring())
    {
        /* Hook up the mouth to itself: a one node circularly linked list! */
        mouth.prev = mouth.next = &mouth;
    }

    /** Note that we do not evict nodes in our destructor (even though they typically need to be deleted by their creator). */
    virtual ~lru_cache_t() { }


    /** Returns the node for a given key, or NULL */
    node_type_t *get_node(const wcstring &key)
    {
        node_type_t *result = NULL;

        /* Construct a fake node as our key */
        lru_node_t node_key(key);

        /* Look for it in the set */
        node_set_t::iterator iter = node_set.find(&node_key);

        /* If we found a node, promote and return it */
        if (iter != node_set.end())
        {
            result = static_cast<node_type_t*>(*iter);
            promote_node(result);
        }
        return result;
    }

    /** Evicts the node for a given key, returning true if a node was evicted. */
    bool evict_node(const wcstring &key)
    {
        /* Construct a fake node as our key */
        lru_node_t node_key(key);

        /* Look for it in the set */
        node_set_t::iterator iter = node_set.find(&node_key);
        if (iter == node_set.end())
            return false;

        /* Evict the given node */
        evict_node(static_cast<node_type_t*>(*iter));
        return true;
    }

    /** Adds a node under the given key. Returns true if the node was added, false if the node was not because a node with that key is already in the set. */
    bool add_node(node_type_t *node)
    {
        /* Add our node without eviction */
        if (! this->add_node_without_eviction(node))
            return false;

        /* Evict */
        while (node_count > max_node_count)
            evict_last_node();

        /* Success */
        return true;
    }

    /** Adds a node under the given key without triggering eviction. Returns true if the node was added, false if the node was not because a node with that key is already in the set. */
    bool add_node_without_eviction(node_type_t *node)
    {
        assert(node != NULL && node != &mouth);

        /* Try inserting; return false if it was already in the set */
        if (! node_set.insert(node).second)
            return false;

        /* Add the node after the mouth */
        node->next = mouth.next;
        node->next->prev = node;
        node->prev = &mouth;
        mouth.next = node;

        /* Update the count. This may push us over the maximum node count. */
        node_count++;
        
        /* Success */
        return true;
    }

    /** Counts nodes */
    size_t size(void)
    {
        return node_count;
    }

    /** Evicts all nodes */
    void evict_all_nodes(void)
    {
        while (node_count > 0)
        {
            evict_last_node();
        }
    }

    /** Iterator for walking nodes, from least recently used to most */
    class iterator
    {
        lru_node_t *node;
    public:
        explicit iterator(lru_node_t *val) : node(val) { }
        void operator++()
        {
            node = lru_cache_t::get_previous(node);
        }
        void operator++(int x)
        {
            node = lru_cache_t::get_previous(node);
        }
        bool operator==(const iterator &other)
        {
            return node == other.node;
        }
        bool operator!=(const iterator &other)
        {
            return !(*this == other);
        }
        node_type_t *operator*()
        {
            return static_cast<node_type_t *>(node);
        }
    };

    iterator begin()
    {
        return iterator(mouth.prev);
    }
    iterator end()
    {
        return iterator(&mouth);
    }
};


#endif