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diff --git a/bitmapped_patricia_tree.c b/bitmapped_patricia_tree.c
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+++ b/bitmapped_patricia_tree.c
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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"
+
+#define CHUNK_LENGTH 5
+#define KEY_LENGTH 32
+#define OFFSET_MASK 0x1ffff  //((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)
+
+
+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_RELEASE_HOOKS
+  void (*release_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;
+};
+
+
+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_RELEASE_HOOKS
+  leaf->release_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));
+
+
+static void bpt_for_children(bpt_t bpt, void (*thunk)(bpt_t)) {
+  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);
+      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(new_node->children[0]);
+      bpt_retain(new_node->children[1]);
+      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, bpt_retain);
+              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_retain);
+            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[0] ? b->children[0]
+                                                  : b->children[1]);
+          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_retain);
+          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_seal);
+      }
+    }
+  }
+}
+
+
+/////////////// 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) {
+    bpt->refcount++;
+  }
+}
+
+void bpt_release(bpt_t bpt) {
+  if (bpt) {
+    if (--(bpt->refcount) == 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_RELEASE_HOOKS
+      if (b->release_hook) {
+        b->release_hook(b->bpt.prefix, b->value);
+      }
+#endif
+      free(b);
+    } else {
+      bpt_node_t b = (bpt_node_t)bpt;
+      bpt_for_children(bpt, bpt_release);
+      free(b->children);
+      free(b);
+    }
+  }
+}
+
+#ifdef BPT_ENABLE_RELEASE_HOOKS
+void bpt_leaf_set_dealloc_hook(bpt_leaf_t bpt, void (*hook)(bpt_key_t, void*)) {
+  if (bpt) {
+    bpt->release_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