Modules/_sqlite/cache.c

/* cache .c - a LRU cache
 *
 * Copyright (C) 2004-2010 Gerhard Häring <gh@ghaering.de>
 *
 * This file is part of pysqlite.
 *
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will the authors be held liable for any damages
 * arising from the use of this software.
 *
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 *
 * 1. The origin of this software must not be misrepresented; you must not
 *    claim that you wrote the original software. If you use this software
 *    in a product, an acknowledgment in the product documentation would be
 *    appreciated but is not required.
 * 2. Altered source versions must be plainly marked as such, and must not be
 *    misrepresented as being the original software.
 * 3. This notice may not be removed or altered from any source distribution.
 */

#include "cache.h"
#include <limits.h>

/* only used internally */
static pysqlite_Node *
pysqlite_new_node(PyObject *key, PyObject *data)
{
    pysqlite_Node* node;

    node = (pysqlite_Node*) (pysqlite_NodeType->tp_alloc(pysqlite_NodeType, 0));
    if (!node) {
        return NULL;
    }

    node->key = Py_NewRef(key);
    node->data = Py_NewRef(data);

    node->prev = NULL;
    node->next = NULL;

    return node;
}

static int
node_traverse(pysqlite_Node *self, visitproc visit, void *arg)
{
    Py_VISIT(Py_TYPE(self));
    Py_VISIT(self->key);
    Py_VISIT(self->data);
    return 0;
}

static int
node_clear(pysqlite_Node *self)
{
    Py_CLEAR(self->key);
    Py_CLEAR(self->data);
    return 0;
}

static void
pysqlite_node_dealloc(pysqlite_Node *self)
{
    PyTypeObject *tp = Py_TYPE(self);
    PyObject_GC_UnTrack(self);
    tp->tp_clear((PyObject *)self);
    tp->tp_free(self);
    Py_DECREF(tp);
}

static int
pysqlite_cache_init(pysqlite_Cache *self, PyObject *args, PyObject *kwargs)
{
    PyObject* factory;
    int size = 10;

    self->factory = NULL;

    if (!PyArg_ParseTuple(args, "O|i", &factory, &size)) {
        return -1;
    }

    /* minimum cache size is 5 entries */
    if (size < 5) {
        size = 5;
    }
    self->size = size;
    self->first = NULL;
    self->last = NULL;

    self->mapping = PyDict_New();
    if (!self->mapping) {
        return -1;
    }

    self->factory = Py_NewRef(factory);

    self->decref_factory = 1;

    return 0;
}

static int
cache_traverse(pysqlite_Cache *self, visitproc visit, void *arg)
{
    Py_VISIT(Py_TYPE(self));
    Py_VISIT(self->mapping);
    if (self->decref_factory) {
        Py_VISIT(self->factory);
    }

    pysqlite_Node *node = self->first;
    while (node) {
        Py_VISIT(node);
        node = node->next;
    }
    return 0;
}

static int
cache_clear(pysqlite_Cache *self)
{
    Py_CLEAR(self->mapping);
    if (self->decref_factory) {
        Py_CLEAR(self->factory);
    }

    /* iterate over all nodes and deallocate them */
    pysqlite_Node *node = self->first;
    self->first = NULL;
    while (node) {
        pysqlite_Node *delete_node = node;
        node = node->next;
        Py_CLEAR(delete_node);
    }
    return 0;
}

static void
pysqlite_cache_dealloc(pysqlite_Cache *self)
{
    if (!self->factory) {
        /* constructor failed, just get out of here */
        return;
    }

    PyObject_GC_UnTrack(self);
    PyTypeObject *tp = Py_TYPE(self);
    tp->tp_clear((PyObject *)self);
    tp->tp_free(self);
    Py_DECREF(tp);
}

PyObject* pysqlite_cache_get(pysqlite_Cache* self, PyObject* key)
{
    pysqlite_Node* node;
    pysqlite_Node* ptr;
    PyObject* data;

    node = (pysqlite_Node*)PyDict_GetItemWithError(self->mapping, key);
    if (node) {
        /* an entry for this key already exists in the cache */

        /* increase usage counter of the node found */
        if (node->count < LONG_MAX) {
            node->count++;
        }

        /* if necessary, reorder entries in the cache by swapping positions */
        if (node->prev && node->count > node->prev->count) {
            ptr = node->prev;

            while (ptr->prev && node->count > ptr->prev->count) {
                ptr = ptr->prev;
            }

            if (node->next) {
                node->next->prev = node->prev;
            } else {
                self->last = node->prev;
            }
            if (node->prev) {
                node->prev->next = node->next;
            }
            if (ptr->prev) {
                ptr->prev->next = node;
            } else {
                self->first = node;
            }

            node->next = ptr;
            node->prev = ptr->prev;
            if (!node->prev) {
                self->first = node;
            }
            ptr->prev = node;
        }
    }
    else if (PyErr_Occurred()) {
        return NULL;
    }
    else {
        /* There is no entry for this key in the cache, yet. We'll insert a new
         * entry in the cache, and make space if necessary by throwing the
         * least used item out of the cache. */

        if (PyDict_GET_SIZE(self->mapping) == self->size) {
            if (self->last) {
                node = self->last;

                if (PyDict_DelItem(self->mapping, self->last->key) != 0) {
                    return NULL;
                }

                if (node->prev) {
                    node->prev->next = NULL;
                }
                self->last = node->prev;
                node->prev = NULL;

                Py_DECREF(node);
            }
        }

        /* We cannot replace this by PyObject_CallOneArg() since
         * PyObject_CallFunction() has a special case when using a
         * single tuple as argument. */
        data = PyObject_CallFunction(self->factory, "O", key);

        if (!data) {
            return NULL;
        }

        node = pysqlite_new_node(key, data);
        if (!node) {
            return NULL;
        }
        node->prev = self->last;

        Py_DECREF(data);

        if (PyDict_SetItem(self->mapping, key, (PyObject*)node) != 0) {
            Py_DECREF(node);
            return NULL;
        }

        if (self->last) {
            self->last->next = node;
        } else {
            self->first = node;
        }
        self->last = node;
    }

    return Py_NewRef(node->data);
}

static PyObject *
pysqlite_cache_display(pysqlite_Cache *self, PyObject *args)
{
    pysqlite_Node* ptr;
    PyObject* prevkey;
    PyObject* nextkey;
    PyObject* display_str;

    ptr = self->first;

    while (ptr) {
        if (ptr->prev) {
            prevkey = ptr->prev->key;
        } else {
            prevkey = Py_None;
        }

        if (ptr->next) {
            nextkey = ptr->next->key;
        } else {
            nextkey = Py_None;
        }

        display_str = PyUnicode_FromFormat("%S <- %S -> %S\n",
                                           prevkey, ptr->key, nextkey);
        if (!display_str) {
            return NULL;
        }
        PyObject_Print(display_str, stdout, Py_PRINT_RAW);
        Py_DECREF(display_str);

        ptr = ptr->next;
    }

    Py_RETURN_NONE;
}

static PyType_Slot node_slots[] = {
    {Py_tp_dealloc, pysqlite_node_dealloc},
    {Py_tp_traverse, node_traverse},
    {Py_tp_clear, node_clear},
    {0, NULL},
};

static PyType_Spec node_spec = {
    .name = MODULE_NAME ".Node",
    .basicsize = sizeof(pysqlite_Node),
    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
    .slots = node_slots,
};
PyTypeObject *pysqlite_NodeType = NULL;

static PyMethodDef cache_methods[] = {
    {"get", (PyCFunction)pysqlite_cache_get, METH_O,
        PyDoc_STR("Gets an entry from the cache or calls the factory function to produce one.")},
    {"display", (PyCFunction)pysqlite_cache_display, METH_NOARGS,
        PyDoc_STR("For debugging only.")},
    {NULL, NULL}
};

static PyType_Slot cache_slots[] = {
    {Py_tp_dealloc, pysqlite_cache_dealloc},
    {Py_tp_methods, cache_methods},
    {Py_tp_init, pysqlite_cache_init},
    {Py_tp_traverse, cache_traverse},
    {Py_tp_clear, cache_clear},
    {0, NULL},
};

static PyType_Spec cache_spec = {
    .name = MODULE_NAME ".Cache",
    .basicsize = sizeof(pysqlite_Cache),
    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
    .slots = cache_slots,
};
PyTypeObject *pysqlite_CacheType = NULL;

int
pysqlite_cache_setup_types(PyObject *mod)
{
    pysqlite_NodeType = (PyTypeObject *)PyType_FromModuleAndSpec(mod, &node_spec, NULL);
    if (pysqlite_NodeType == NULL) {
        return -1;
    }

    pysqlite_CacheType = (PyTypeObject *)PyType_FromModuleAndSpec(mod, &cache_spec, NULL);
    if (pysqlite_CacheType == NULL) {
        return -1;
    }
    return 0;
}