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urbit/outside/bpt/bitmapped_patricia_tree.c
~hatteb-mitlyd 3e72845fb3 fix BPT constant, closes #140
2014-05-24 13:17:33 -07:00

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// -*- mode: c; coding: utf-8 -*- */
//
// Copyright 2010, 2011, Matthias Andreas Benkard.
//
//-----------------------------------------------------------------------------
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
//-----------------------------------------------------------------------------
//
// An implementation of a bitmapped Patricia tree.
//// Purpose ////
//
// The idea is to use a locally mutable, bitmapped Patricia tree as a
// variable binding store (i.e. environment) in compiled code. In this
// way, there is no need for excessive copying when an independent
// environment must be set up (such as when initiating the processing of
// a new node in the search space). Instead, significant amounts of
// structure can be shared between child and parent environments.
//// Motivation ////
//
// 1. Patricia trees are very amenable to structure sharing.
//
// 2. Furthermore, big-endian Patricia trees are especially efficient
// when indices are allocated sequentially, as is the case for
// variables in code emitted by our compiler.
//
// 3. Finally, bitmapping improves the performance of copying because
// copying an array is much cheaper than copying an equivalent branch
// in a tree. As we need to shallow-copy the tree at potentially
// each choice point, copying needs to be fast.
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "bitmapped_patricia_tree.h"
#ifndef BPT_EXPLICIT_CONFIGURATION
#define CHUNK_LENGTH 5
#define KEY_LENGTH 32
#define OFFSET_MASK 0x1f //((1 << chunk_length) - 1)
#define MAX_CHUNKS 7 //key_length / chunk_length + ((key_length % chunk_length == 0) ? 0 : 1)
#define LAST_CHUNK_LENGTH 2 //key_length - ((max_chunks - 1) * chunk_length)
#endif //!BPT_EXPLICIT_CONFIGURATION
typedef struct bpt_nonempty *bpt_nonempty_t;
typedef struct bpt_node *bpt_node_t;
typedef struct bpt_leaf *bpt_leaf_t;
struct bpt {
enum bpt_tag tag;
int refcount;
bool mutable;
bpt_key_t prefix;
};
struct bpt_leaf {
struct bpt bpt; // poor man's inheritance
void *value;
#ifdef BPT_ENABLE_DEALLOC_HOOKS
void (*dealloc_hook)(bpt_key_t, void *); // not actually used anywhere in client code
#endif
};
struct bpt_node {
struct bpt bpt; // poor man's inheritance
unsigned int branching_chunk;
bpt_key_bitmask_t bitmask;
bpt_t *children;
};
// Forward declarations.
void init_bpt_leaf(bpt_t leaf, bpt_key_t key, void *value);
bpt_t bpt_make_leaf(bpt_key_t key, void *value);
// Boilerplate definitions.
void bpt_retain0(bpt_t bpt, void *user_data) {
bpt_retain(bpt);
}
void bpt_seal0(bpt_t bpt, void *user_data) {
bpt_seal(bpt);
}
void bpt_release0(bpt_t bpt, void *user_data) {
bpt_release(bpt);
}
// Implementation.
void init_bpt_leaf(bpt_t a_leaf, bpt_key_t key, void *value) {
bpt_leaf_t leaf = (bpt_leaf_t)a_leaf;
leaf->bpt.tag = BPT_LEAF;
leaf->bpt.mutable = true;
leaf->bpt.prefix = key;
leaf->value = value;
#ifdef BPT_ENABLE_DEALLOC_HOOKS
leaf->dealloc_hook = NULL;
#endif
leaf->bpt.refcount = 1;
}
void init_bpt_node(bpt_node_t node, bpt_key_t prefix, unsigned int branching_chunk) {
node->bpt.tag = BPT_INNER_NODE;
node->bpt.mutable = true;
node->bpt.prefix = prefix;
node->branching_chunk = branching_chunk;
node->bitmask = 0;
node->children = NULL;
node->bpt.refcount = 1;
}
bpt_t bpt_make_leaf(bpt_key_t key, void *value) {
bpt_leaf_t leaf = malloc(sizeof *leaf);
init_bpt_leaf((bpt_t)leaf, key, value);
return (bpt_t)leaf;
}
bpt_node_t bpt_make_node(bpt_key_t prefix, unsigned int branching_chunk) {
bpt_node_t node = malloc(sizeof *node);
init_bpt_node(node, prefix, branching_chunk);
return node;
}
static inline unsigned int bpt_number_of_leading_zeros(bpt_key_t x);
static inline unsigned int bpt_number_of_trailing_zeros(bpt_key_t x);
static inline unsigned int bpt_popcount(bpt_key_bitmask_t key);
static unsigned int bpt_compute_child_index(bpt_key_bitmask_t bitmask, unsigned int child_number);
static inline uint_fast8_t bpt_offset_of_key(bpt_key_t key, unsigned int branching_chunk);
static bpt_key_t bpt_prefix_of_key(bpt_key_t key, unsigned int branching_chunk);
static inline unsigned int bpt_branching_chunk(bpt_t bpt);
static unsigned int bpt_find_diverging_chunk(bpt_key_t key1, bpt_key_t key2);
static void bpt_for_children(bpt_t bpt, void (*thunk)(bpt_t, void*), void *user_data);
static void bpt_for_children(bpt_t bpt, void (*thunk)(bpt_t, void*), void *user_data) {
if (bpt && bpt->tag == BPT_INNER_NODE) {
bpt_node_t b = (bpt_node_t)bpt;
bpt_t *iter = b->children;
bpt_t *children_end = b->children + bpt_popcount(b->bitmask);
while (iter < children_end) {
thunk(*iter, user_data);
iter++;
}
}
}
void *bpt_get(bpt_t bpt, bpt_key_t key) {
void **pointer = bpt_get_pointer(bpt, key);
if (pointer) {
return *pointer;
} else {
return NULL;
}
}
bpt_leaf_t bpt_get_leaf(bpt_t bpt, bpt_key_t key)
{
if (!bpt) {
return NULL;
} else if (bpt->tag == BPT_LEAF) {
bpt_leaf_t b = (bpt_leaf_t)bpt;
if (bpt->prefix == key) {
return b;
} else {
return NULL;
}
} else {
bpt_node_t b = (bpt_node_t)bpt;
int child_number = bpt_offset_of_key(key, b->branching_chunk);
if ((1 << child_number) & b->bitmask) {
int child_index = bpt_compute_child_index(b->bitmask, child_number);
return bpt_get_leaf(b->children[child_index], key);
} else {
return NULL;
}
}
}
void **bpt_get_pointer(bpt_t bpt, bpt_key_t key)
{
bpt_leaf_t leaf = bpt_get_leaf(bpt, key);
if (!leaf) {
return NULL;
} else {
return &leaf->value;
}
}
bool bpt_has_key(bpt_t bpt, bpt_key_t key) {
return (bpt_get_leaf(bpt, key) != NULL);
}
bpt_t bpt_assoc(bpt_t bpt, bpt_key_t key, void *value) {
if (!bpt) {
return (bpt_t)bpt_make_leaf(key, value);
} else {
bpt_key_t prefix = bpt->prefix;
if (bpt_prefix_of_key(key, bpt_branching_chunk(bpt)) != prefix) {
unsigned int diverging_chunk = bpt_find_diverging_chunk(key, prefix);
bpt_key_t my_number_in_parent = bpt_offset_of_key(prefix, diverging_chunk);
bpt_key_t their_number_in_parent = bpt_offset_of_key(key, diverging_chunk);
bpt_node_t new_node = bpt_make_node(bpt_prefix_of_key(prefix, diverging_chunk), diverging_chunk);
new_node->bitmask = (1 << my_number_in_parent) | (1 << their_number_in_parent);
new_node->children = malloc(sizeof (*new_node->children) * 2);
if (my_number_in_parent < their_number_in_parent) {
new_node->children[0] = bpt;
new_node->children[1] = bpt_make_leaf(key, value);
} else {
new_node->children[0] = bpt_make_leaf(key, value);
new_node->children[1] = bpt;
}
bpt_retain(bpt);
return (bpt_t)new_node;
} else {
if (bpt->tag == BPT_LEAF) {
bpt_leaf_t b = (bpt_leaf_t)bpt;
if (bpt->mutable) {
b->value = value;
bpt_retain(bpt);
return bpt;
} else {
return (bpt_t)bpt_make_leaf(key, value);
}
} else {
bpt_node_t b = (bpt_node_t)bpt;
uint_fast8_t child_number = bpt_offset_of_key(key, b->branching_chunk);
unsigned int child_index = bpt_compute_child_index(b->bitmask, child_number);
if ((1 << child_number) & b->bitmask) {
// We already have a child to pass the value to. Do that.
bpt_t child = b->children[child_index];
bpt_t new_child = bpt_assoc(child, key, value);
if (new_child == child) {
bpt_release(child);
bpt_retain(bpt);
return bpt;
} else {
if (bpt->mutable) {
bpt_release(child);
b->children[child_index] = new_child;
bpt_retain(bpt);
return bpt;
} else {
bpt_node_t new_node = malloc(sizeof *new_node);
*new_node = *b;
new_node->bpt.refcount = 1;
new_node->bpt.mutable = true;
unsigned int number_of_children = bpt_popcount(b->bitmask);
size_t size_of_child_array = sizeof (*new_node->children) * number_of_children;
new_node->children = malloc(size_of_child_array);
memcpy(new_node->children, b->children, size_of_child_array);
new_node->children[child_index] = new_child;
// Retain the children copied into the new node.
bpt_for_children((bpt_t)new_node, bpt_retain0, NULL);
bpt_release(new_child);
return (bpt_t)new_node;
}
}
} else {
// Create a new child.
unsigned int number_of_children = bpt_popcount(b->bitmask);
size_t new_size_of_child_array = sizeof (*b->children) * (number_of_children + 1);
if (bpt->mutable) {
b->children = realloc(b->children, new_size_of_child_array);
memmove(b->children + child_index + 1, b->children + child_index, sizeof (*b->children) * (number_of_children - child_index));
b->children[child_index] = bpt_make_leaf(key, value);
b->bitmask |= 1 << child_number;
bpt_retain(bpt);
return bpt;
} else {
bpt_t *new_children = malloc(new_size_of_child_array);
memcpy(new_children, b->children, sizeof (*b->children) * child_index);
memcpy(new_children + child_index + 1,
b->children + child_index,
sizeof (*b->children) * (number_of_children - child_index));
new_children[child_index] = bpt_make_leaf(key, value);
bpt_node_t new_node = bpt_make_node(b->bpt.prefix, b->branching_chunk);
new_node->children = new_children;
new_node->bitmask = b->bitmask | (1 << child_number);
// Retain the children copied into the new node.
bpt_for_children(bpt, bpt_retain0, NULL);
return (bpt_t)new_node;
}
}
}
}
}
}
bpt_t bpt_dissoc(bpt_t bpt, bpt_key_t key) {
if (!bpt || (bpt_prefix_of_key(key, bpt_branching_chunk(bpt)) != bpt->prefix)) {
bpt_retain(bpt);
return bpt;
} else if (bpt->tag == BPT_LEAF) {
// Key matches.
return NULL;
} else {
// Prefix matches.
bpt_node_t b = (bpt_node_t)bpt;
uint_fast8_t child_number = bpt_offset_of_key(key, b->branching_chunk);
if ((1 << child_number) & b->bitmask) {
unsigned int child_index = bpt_compute_child_index(b->bitmask, child_number);
bpt_t child = b->children[child_index];
bpt_t new_child = bpt_dissoc(child, key);
if (new_child == child) {
bpt_release(child);
bpt_retain(bpt);
return bpt;
} else {
unsigned int number_of_children = bpt_popcount(b->bitmask);
if (!new_child && number_of_children == 2) {
// When there is only a single child left, we replace ourselves
// with that child.
bpt_t remaining_child = b->children[1-child_index];
bpt_retain(remaining_child);
return remaining_child;
} else if (bpt->mutable) {
bpt_release(child);
if (!new_child) {
// We don't reallocate the array because it wouldn't really
// gain us anything (except maybe non-confusion of a
// conservative GC).
memmove(b->children + child_index, b->children + child_index + 1, sizeof(*b->children) * (number_of_children - child_index - 1));
b->bitmask &= ~(1 << child_number);
bpt_retain(bpt);
return bpt;
} else {
b->children[child_index] = new_child;
bpt_retain(bpt);
return bpt;
}
} else {
// If all else fails, allocate a new node.
bpt_t *new_children;
bpt_key_bitmask_t bitmask;
if (!new_child) {
new_children = malloc((sizeof *new_children) * (number_of_children - 1));
memcpy(new_children, b->children, sizeof (*b->children) * child_index);
memcpy(new_children + child_index,
b->children + child_index + 1,
sizeof (*b->children) * (number_of_children - child_index - 1));
bitmask = b->bitmask & ~(1 << child_number);
} else {
new_children = malloc((sizeof *new_children) * number_of_children);
memcpy(new_children, b->children, sizeof (*b->children) * number_of_children);
new_children[child_index] = new_child;
bitmask = b->bitmask;
}
bpt_node_t new_node = bpt_make_node(b->bpt.prefix, b->branching_chunk);
new_node->children = new_children;
new_node->bitmask = bitmask;
// Retain the children copied into the new node.
bpt_for_children((bpt_t)new_node, bpt_retain0, NULL);
bpt_release(new_child);
return (bpt_t)new_node;
}
}
} else {
bpt_retain(bpt);
return bpt;
}
}
}
void bpt_seal(bpt_t bpt) {
if (bpt) {
if (bpt->mutable) {
bpt->mutable = false;
if (bpt->tag == BPT_INNER_NODE) {
bpt_for_children(bpt, bpt_seal0, NULL);
}
}
}
}
/////////////// Helper functions ///////////////
static unsigned int bpt_compute_child_index(bpt_key_bitmask_t bitmask, unsigned int child_number) {
// Compute the sparse array index given a flat array index.
return bpt_popcount(bitmask & ((1 << child_number) - 1));
}
static inline uint_fast8_t bpt_offset_of_key(bpt_key_t key, unsigned int chunk_number) {
// Little-enidan:
//return (key >> (chunk_number * CHUNK_LENGTH)) & OFFSET_MASK;
// Big-endian:
int shift = 0;
if (chunk_number <= MAX_CHUNKS - 2) {
shift += LAST_CHUNK_LENGTH;
}
if (chunk_number <= MAX_CHUNKS - 3) {
shift += ((MAX_CHUNKS - 2 - chunk_number) * CHUNK_LENGTH);
}
return (key >> shift) & (chunk_number == MAX_CHUNKS - 1 ? ((1 << LAST_CHUNK_LENGTH) - 1) : OFFSET_MASK);
}
static bpt_key_t bpt_prefix_of_key(bpt_key_t key, unsigned int chunk_number) {
if (chunk_number == MAX_CHUNKS) {
return key;
} else {
// Little-endian:
//return key & ((1 << (chunk_number * CHUNK_LENGTH)) - 1)
// Big-endian:
return key & (((1 << (chunk_number * CHUNK_LENGTH)) - 1) << (KEY_LENGTH - (chunk_number * CHUNK_LENGTH)));
}
}
static inline unsigned int bpt_branching_chunk(bpt_t bpt) {
assert(bpt);
if (bpt->tag == BPT_LEAF) {
return MAX_CHUNKS;
} else {
return ((bpt_node_t)bpt)->branching_chunk;
}
}
static inline unsigned int bpt_popcount(bpt_key_bitmask_t x) {
return __builtin_popcount(x);
}
static inline unsigned int bpt_number_of_leading_zeros(bpt_key_t x) {
return __builtin_clz(x);
}
static inline unsigned int bpt_number_of_trailing_zeros(bpt_key_t x) {
return __builtin_ctz(x);
}
static unsigned int bpt_find_diverging_chunk(bpt_key_t a, bpt_key_t b) {
// Little-endian:
//return bpt_number_of_trailing_zeros(a ^ b) / CHUNK_LENGTH;
// Big-endian:
return bpt_number_of_leading_zeros(a ^ b) / CHUNK_LENGTH;
}
void bpt_retain(bpt_t bpt) {
if (bpt) {
__sync_fetch_and_add(&bpt->refcount, 1);
}
}
void bpt_release(bpt_t bpt) {
if (bpt) {
if (__sync_sub_and_fetch(&bpt->refcount, 1) == 0) {
bpt_dealloc(bpt);
}
}
}
void bpt_dealloc(bpt_t bpt) {
if (bpt) {
if (bpt->tag == BPT_LEAF) {
bpt_leaf_t b = (bpt_leaf_t)bpt;
#ifdef BPT_ENABLE_DEALLOC_HOOKS
if (b->dealloc_hook) {
b->dealloc_hook(b->bpt.prefix, b->value);
}
#endif
free(b);
} else {
bpt_node_t b = (bpt_node_t)bpt;
bpt_for_children(bpt, bpt_release0, NULL);
free(b->children);
free(b);
}
}
}
#ifdef BPT_ENABLE_DEALLOC_HOOKS
void bpt_leaf_set_dealloc_hook(bpt_leaf_t bpt, void (*hook)(bpt_key_t, void*)) {
if (bpt) {
bpt->dealloc_hook = hook;
}
}
void bpt_set_dealloc_hook(bpt_t bpt, bpt_key_t key, void (*hook)(bpt_key_t, void*)) {
bpt_leaf_set_dealloc_hook(bpt_get_leaf(bpt, key), hook);
}
#endif
/* Utilities */
struct bpt_for_mappings_closure_data {
void (*thunk)(bpt_key_t, void*, void*);
void *user_data;
};
static void bpt_for_mappings_iter(bpt_t bpt, void *closure_data_) {
struct bpt_for_mappings_closure_data *closure_data = closure_data_;
if (bpt->tag == BPT_LEAF) {
bpt_leaf_t leaf = (bpt_leaf_t)bpt;
closure_data->thunk(bpt->prefix, leaf->value, closure_data->user_data);
} else {
bpt_for_children(bpt, bpt_for_mappings_iter, closure_data);
}
}
void bpt_for_mappings(bpt_t bpt, void (*thunk)(bpt_key_t, void*, void*), void *user_data) {
struct bpt_for_mappings_closure_data closure_data =
{ .user_data = user_data, .thunk = thunk };
bpt_for_mappings_iter(bpt, &closure_data);
}
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