xpcom/glue/pldhash.h

   1 /* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
   2 /* ***** BEGIN LICENSE BLOCK *****
   3  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
   4  *
   5  * The contents of this file are subject to the Mozilla Public License Version
   6  * 1.1 (the "License"); you may not use this file except in compliance with
   7  * the License. You may obtain a copy of the License at
   8  * http://www.mozilla.org/MPL/
   9  *
  10  * Software distributed under the License is distributed on an "AS IS" basis,
  11  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
  12  * for the specific language governing rights and limitations under the
  13  * License.
  14  *
  15  * The Original Code is Mozilla JavaScript code.
  16  *
  17  * The Initial Developer of the Original Code is
  18  * Netscape Communications Corporation.
  19  * Portions created by the Initial Developer are Copyright (C) 1999-2001
  20  * the Initial Developer. All Rights Reserved.
  21  *
  22  * Contributor(s):
  23  *   Brendan Eich <brendan@mozilla.org> (Original Author)
  24  *
  25  * Alternatively, the contents of this file may be used under the terms of
  26  * either of the GNU General Public License Version 2 or later (the "GPL"),
  27  * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
  28  * in which case the provisions of the GPL or the LGPL are applicable instead
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  36  *
  37  * ***** END LICENSE BLOCK ***** */
  38
  39 #ifndef pldhash_h___
  40 #define pldhash_h___
  41 /*
  42  * Double hashing, a la Knuth 6.
  43  * GENERATED BY js/src/plify_jsdhash.sed -- DO NOT EDIT!!!
  44  */
  45 #include "nscore.h"
  46
  47 PR_BEGIN_EXTERN_C
  48
  49 #if defined(__GNUC__) && defined(__i386__) && (__GNUC__ >= 3) && !defined(XP_OS2)
  50 #define PL_DHASH_FASTCALL __attribute__ ((regparm (3),stdcall))
  51 #elif defined(XP_WIN)
  52 #define PL_DHASH_FASTCALL __fastcall
  53 #else
  54 #define PL_DHASH_FASTCALL
  55 #endif
  56
  57 #ifdef DEBUG_XXXbrendan
  58 #define PL_DHASHMETER 1
  59 #endif
  60
  61 /* Table size limit, do not equal or exceed (see min&maxAlphaFrac, below). */
  62 #undef PL_DHASH_SIZE_LIMIT
  63 #define PL_DHASH_SIZE_LIMIT     PR_BIT(24)
  64
  65 /* Minimum table size, or gross entry count (net is at most .75 loaded). */
  66 #ifndef PL_DHASH_MIN_SIZE
  67 #define PL_DHASH_MIN_SIZE 16
  68 #elif (PL_DHASH_MIN_SIZE & (PL_DHASH_MIN_SIZE - 1)) != 0
  69 #error "PL_DHASH_MIN_SIZE must be a power of two!"
  70 #endif
  71
  72 /*
  73  * Multiplicative hash uses an unsigned 32 bit integer and the golden ratio,
  74  * expressed as a fixed-point 32-bit fraction.
  75  */
  76 #define PL_DHASH_BITS           32
  77 #define PL_DHASH_GOLDEN_RATIO   0x9E3779B9U
  78
  79 /* Primitive and forward-struct typedefs. */
  80 typedef PRUint32                PLDHashNumber;
  81 typedef struct PLDHashEntryHdr  PLDHashEntryHdr;
  82 typedef struct PLDHashEntryStub PLDHashEntryStub;
  83 typedef struct PLDHashTable     PLDHashTable;
  84 typedef struct PLDHashTableOps  PLDHashTableOps;
  85
  86 /*
  87  * Table entry header structure.
  88  *
  89  * In order to allow in-line allocation of key and value, we do not declare
  90  * either here.  Instead, the API uses const void *key as a formal parameter.
  91  * The key need not be stored in the entry; it may be part of the value, but
  92  * need not be stored at all.
  93  *
  94  * Callback types are defined below and grouped into the PLDHashTableOps
  95  * structure, for single static initialization per hash table sub-type.
  96  *
  97  * Each hash table sub-type should nest the PLDHashEntryHdr structure at the
  98  * front of its particular entry type.  The keyHash member contains the result
  99  * of multiplying the hash code returned from the hashKey callback (see below)
 100  * by PL_DHASH_GOLDEN_RATIO, then constraining the result to avoid the magic 0
 101  * and 1 values.  The stored keyHash value is table size invariant, and it is
 102  * maintained automatically by PL_DHashTableOperate -- users should never set
 103  * it, and its only uses should be via the entry macros below.
 104  *
 105  * The PL_DHASH_ENTRY_IS_LIVE macro tests whether entry is neither free nor
 106  * removed.  An entry may be either busy or free; if busy, it may be live or
 107  * removed.  Consumers of this API should not access members of entries that
 108  * are not live.
 109  *
 110  * However, use PL_DHASH_ENTRY_IS_BUSY for faster liveness testing of entries
 111  * returned by PL_DHashTableOperate, as PL_DHashTableOperate never returns a
 112  * non-live, busy (i.e., removed) entry pointer to its caller.  See below for
 113  * more details on PL_DHashTableOperate's calling rules.
 114  */
 115 struct PLDHashEntryHdr {
 116     PLDHashNumber       keyHash;        /* every entry must begin like this */
 117 };
 118
 119 #define PL_DHASH_ENTRY_IS_FREE(entry)   ((entry)->keyHash == 0)
 120 #define PL_DHASH_ENTRY_IS_BUSY(entry)   (!PL_DHASH_ENTRY_IS_FREE(entry))
 121 #define PL_DHASH_ENTRY_IS_LIVE(entry)   ((entry)->keyHash >= 2)
 122
 123 /*
 124  * A PLDHashTable is currently 8 words (without the PL_DHASHMETER overhead)
 125  * on most architectures, and may be allocated on the stack or within another
 126  * structure or class (see below for the Init and Finish functions to use).
 127  *
 128  * To decide whether to use double hashing vs. chaining, we need to develop a
 129  * trade-off relation, as follows:
 130  *
 131  * Let alpha be the load factor, esize the entry size in words, count the
 132  * entry count, and pow2 the power-of-two table size in entries.
 133  *
 134  *   (PLDHashTable overhead)    > (PLHashTable overhead)
 135  *   (unused table entry space) > (malloc and .next overhead per entry) +
 136  *                                (buckets overhead)
 137  *   (1 - alpha) * esize * pow2 > 2 * count + pow2
 138  *
 139  * Notice that alpha is by definition (count / pow2):
 140  *
 141  *   (1 - alpha) * esize * pow2 > 2 * alpha * pow2 + pow2
 142  *   (1 - alpha) * esize        > 2 * alpha + 1
 143  *
 144  *   esize > (1 + 2 * alpha) / (1 - alpha)
 145  *
 146  * This assumes both tables must keep keyHash, key, and value for each entry,
 147  * where key and value point to separately allocated strings or structures.
 148  * If key and value can be combined into one pointer, then the trade-off is:
 149  *
 150  *   esize > (1 + 3 * alpha) / (1 - alpha)
 151  *
 152  * If the entry value can be a subtype of PLDHashEntryHdr, rather than a type
 153  * that must be allocated separately and referenced by an entry.value pointer
 154  * member, and provided key's allocation can be fused with its entry's, then
 155  * k (the words wasted per entry with chaining) is 4.
 156  *
 157  * To see these curves, feed gnuplot input like so:
 158  *
 159  *   gnuplot> f(x,k) = (1 + k * x) / (1 - x)
 160  *   gnuplot> plot [0:.75] f(x,2), f(x,3), f(x,4)
 161  *
 162  * For k of 2 and a well-loaded table (alpha > .5), esize must be more than 4
 163  * words for chaining to be more space-efficient than double hashing.
 164  *
 165  * Solving for alpha helps us decide when to shrink an underloaded table:
 166  *
 167  *   esize                     > (1 + k * alpha) / (1 - alpha)
 168  *   esize - alpha * esize     > 1 + k * alpha
 169  *   esize - 1                 > (k + esize) * alpha
 170  *   (esize - 1) / (k + esize) > alpha
 171  *
 172  *   alpha < (esize - 1) / (esize + k)
 173  *
 174  * Therefore double hashing should keep alpha >= (esize - 1) / (esize + k),
 175  * assuming esize is not too large (in which case, chaining should probably be
 176  * used for any alpha).  For esize=2 and k=3, we want alpha >= .2; for esize=3
 177  * and k=2, we want alpha >= .4.  For k=4, esize could be 6, and alpha >= .5
 178  * would still obtain.  See the PL_DHASH_MIN_ALPHA macro further below.
 179  *
 180  * The current implementation uses a configurable lower bound on alpha, which
 181  * defaults to .25, when deciding to shrink the table (while still respecting
 182  * PL_DHASH_MIN_SIZE).
 183  *
 184  * Note a qualitative difference between chaining and double hashing: under
 185  * chaining, entry addresses are stable across table shrinks and grows.  With
 186  * double hashing, you can't safely hold an entry pointer and use it after an
 187  * ADD or REMOVE operation, unless you sample table->generation before adding
 188  * or removing, and compare the sample after, dereferencing the entry pointer
 189  * only if table->generation has not changed.
 190  *
 191  * The moral of this story: there is no one-size-fits-all hash table scheme,
 192  * but for small table entry size, and assuming entry address stability is not
 193  * required, double hashing wins.
 194  */
 195 struct PLDHashTable {
 196     const PLDHashTableOps *ops;         /* virtual operations, see below */
 197     void                *data;          /* ops- and instance-specific data */
 198     PRInt16             hashShift;      /* multiplicative hash shift */
 199     uint8               maxAlphaFrac;   /* 8-bit fixed point max alpha */
 200     uint8               minAlphaFrac;   /* 8-bit fixed point min alpha */
 201     PRUint32            entrySize;      /* number of bytes in an entry */
 202     PRUint32            entryCount;     /* number of entries in table */
 203     PRUint32            removedCount;   /* removed entry sentinels in table */
 204     PRUint32            generation;     /* entry storage generation number */
 205     char                *entryStore;    /* entry storage */
 206 #ifdef PL_DHASHMETER
 207     struct PLDHashStats {
 208         PRUint32        searches;       /* total number of table searches */
 209         PRUint32        steps;          /* hash chain links traversed */
 210         PRUint32        hits;           /* searches that found key */
 211         PRUint32        misses;         /* searches that didn't find key */
 212         PRUint32        lookups;        /* number of PL_DHASH_LOOKUPs */
 213         PRUint32        addMisses;      /* adds that miss, and do work */
 214         PRUint32        addOverRemoved; /* adds that recycled a removed entry */
 215         PRUint32        addHits;        /* adds that hit an existing entry */
 216         PRUint32        addFailures;    /* out-of-memory during add growth */
 217         PRUint32        removeHits;     /* removes that hit, and do work */
 218         PRUint32        removeMisses;   /* useless removes that miss */
 219         PRUint32        removeFrees;    /* removes that freed entry directly */
 220         PRUint32        removeEnums;    /* removes done by Enumerate */
 221         PRUint32        grows;          /* table expansions */
 222         PRUint32        shrinks;        /* table contractions */
 223         PRUint32        compresses;     /* table compressions */
 224         PRUint32        enumShrinks;    /* contractions after Enumerate */
 225     } stats;
 226 #endif
 227 };
 228
 229 /*
 230  * Size in entries (gross, not net of free and removed sentinels) for table.
 231  * We store hashShift rather than sizeLog2 to optimize the collision-free case
 232  * in SearchTable.
 233  */
 234 #define PL_DHASH_TABLE_SIZE(table)  PR_BIT(PL_DHASH_BITS - (table)->hashShift)
 235
 236 /*
 237  * Table space at entryStore is allocated and freed using these callbacks.
 238  * The allocator should return null on error only (not if called with nbytes
 239  * equal to 0; but note that pldhash.c code will never call with 0 nbytes).
 240  */
 241 typedef void *
 242 (* PR_CALLBACK PLDHashAllocTable)(PLDHashTable *table, PRUint32 nbytes);
 243
 244 typedef void
 245 (* PR_CALLBACK PLDHashFreeTable) (PLDHashTable *table, void *ptr);
 246
 247 /*
 248  * Compute the hash code for a given key to be looked up, added, or removed
 249  * from table.  A hash code may have any PLDHashNumber value.
 250  */
 251 typedef PLDHashNumber
 252 (* PR_CALLBACK PLDHashHashKey)   (PLDHashTable *table, const void *key);
 253
 254 /*
 255  * Compare the key identifying entry in table with the provided key parameter.
 256  * Return PR_TRUE if keys match, PR_FALSE otherwise.
 257  */
 258 typedef PRBool
 259 (* PR_CALLBACK PLDHashMatchEntry)(PLDHashTable *table,
 260                                       const PLDHashEntryHdr *entry,
 261                                       const void *key);
 262
 263 /*
 264  * Copy the data starting at from to the new entry storage at to.  Do not add
 265  * reference counts for any strong references in the entry, however, as this
 266  * is a "move" operation: the old entry storage at from will be freed without
 267  * any reference-decrementing callback shortly.
 268  */
 269 typedef void
 270 (* PR_CALLBACK PLDHashMoveEntry)(PLDHashTable *table,
 271                                      const PLDHashEntryHdr *from,
 272                                      PLDHashEntryHdr *to);
 273
 274 /*
 275  * Clear the entry and drop any strong references it holds.  This callback is
 276  * invoked during a PL_DHASH_REMOVE operation (see below for operation codes),
 277  * but only if the given key is found in the table.
 278  */
 279 typedef void
 280 (* PR_CALLBACK PLDHashClearEntry)(PLDHashTable *table,
 281                                       PLDHashEntryHdr *entry);
 282
 283 /*
 284  * Called when a table (whether allocated dynamically by itself, or nested in
 285  * a larger structure, or allocated on the stack) is finished.  This callback
 286  * allows table->ops-specific code to finalize table->data.
 287  */
 288 typedef void
 289 (* PR_CALLBACK PLDHashFinalize)  (PLDHashTable *table);
 290
 291 /*
 292  * Initialize a new entry, apart from keyHash.  This function is called when
 293  * PL_DHashTableOperate's PL_DHASH_ADD case finds no existing entry for the
 294  * given key, and must add a new one.  At that point, entry->keyHash is not
 295  * set yet, to avoid claiming the last free entry in a severely overloaded
 296  * table.
 297  */
 298 typedef PRBool
 299 (* PR_CALLBACK PLDHashInitEntry)(PLDHashTable *table,
 300                                      PLDHashEntryHdr *entry,
 301                                      const void *key);
 302
 303 /*
 304  * Finally, the "vtable" structure for PLDHashTable.  The first eight hooks
 305  * must be provided by implementations; they're called unconditionally by the
 306  * generic pldhash.c code.  Hooks after these may be null.
 307  *
 308  * Summary of allocation-related hook usage with C++ placement new emphasis:
 309  *  allocTable          Allocate raw bytes with malloc, no ctors run.
 310  *  freeTable           Free raw bytes with free, no dtors run.
 311  *  initEntry           Call placement new using default key-based ctor.
 312  *                      Return PR_TRUE on success, PR_FALSE on error.
 313  *  moveEntry           Call placement new using copy ctor, run dtor on old
 314  *                      entry storage.
 315  *  clearEntry          Run dtor on entry.
 316  *  finalize            Stub unless table->data was initialized and needs to
 317  *                      be finalized.
 318  *
 319  * Note the reason why initEntry is optional: the default hooks (stubs) clear
 320  * entry storage:  On successful PL_DHashTableOperate(tbl, key, PL_DHASH_ADD),
 321  * the returned entry pointer addresses an entry struct whose keyHash member
 322  * has been set non-zero, but all other entry members are still clear (null).
 323  * PL_DHASH_ADD callers can test such members to see whether the entry was
 324  * newly created by the PL_DHASH_ADD call that just succeeded.  If placement
 325  * new or similar initialization is required, define an initEntry hook.  Of
 326  * course, the clearEntry hook must zero or null appropriately.
 327  *
 328  * XXX assumes 0 is null for pointer types.
 329  */
 330 struct PLDHashTableOps {
 331     /* Mandatory hooks.  All implementations must provide these. */
 332     PLDHashAllocTable   allocTable;
 333     PLDHashFreeTable    freeTable;
 334     PLDHashHashKey      hashKey;
 335     PLDHashMatchEntry   matchEntry;
 336     PLDHashMoveEntry    moveEntry;
 337     PLDHashClearEntry   clearEntry;
 338     PLDHashFinalize     finalize;
 339
 340     /* Optional hooks start here.  If null, these are not called. */
 341     PLDHashInitEntry    initEntry;
 342 };
 343
 344 /*
 345  * Default implementations for the above ops.
 346  */
 347 NS_COM_GLUE void *
 348 PL_DHashAllocTable(PLDHashTable *table, PRUint32 nbytes);
 349
 350 NS_COM_GLUE void
 351 PL_DHashFreeTable(PLDHashTable *table, void *ptr);
 352
 353 NS_COM_GLUE PLDHashNumber
 354 PL_DHashStringKey(PLDHashTable *table, const void *key);
 355
 356 /* A minimal entry contains a keyHash header and a void key pointer. */
 357 struct PLDHashEntryStub {
 358     PLDHashEntryHdr hdr;
 359     const void      *key;
 360 };
 361
 362 NS_COM_GLUE PLDHashNumber
 363 PL_DHashVoidPtrKeyStub(PLDHashTable *table, const void *key);
 364
 365 NS_COM_GLUE PRBool
 366 PL_DHashMatchEntryStub(PLDHashTable *table,
 367                        const PLDHashEntryHdr *entry,
 368                        const void *key);
 369
 370 NS_COM_GLUE PRBool
 371 PL_DHashMatchStringKey(PLDHashTable *table,
 372                        const PLDHashEntryHdr *entry,
 373                        const void *key);
 374
 375 NS_COM_GLUE void
 376 PL_DHashMoveEntryStub(PLDHashTable *table,
 377                       const PLDHashEntryHdr *from,
 378                       PLDHashEntryHdr *to);
 379
 380 NS_COM_GLUE void
 381 PL_DHashClearEntryStub(PLDHashTable *table, PLDHashEntryHdr *entry);
 382
 383 NS_COM_GLUE void
 384 PL_DHashFreeStringKey(PLDHashTable *table, PLDHashEntryHdr *entry);
 385
 386 NS_COM_GLUE void
 387 PL_DHashFinalizeStub(PLDHashTable *table);
 388
 389 /*
 390  * If you use PLDHashEntryStub or a subclass of it as your entry struct, and
 391  * if your entries move via memcpy and clear via memset(0), you can use these
 392  * stub operations.
 393  */
 394 NS_COM_GLUE const PLDHashTableOps *
 395 PL_DHashGetStubOps(void);
 396
 397 /*
 398  * Dynamically allocate a new PLDHashTable using malloc, initialize it using
 399  * PL_DHashTableInit, and return its address.  Return null on malloc failure.
 400  * Note that the entry storage at table->entryStore will be allocated using
 401  * the ops->allocTable callback.
 402  */
 403 NS_COM_GLUE PLDHashTable *
 404 PL_NewDHashTable(const PLDHashTableOps *ops, void *data, PRUint32 entrySize,
 405                  PRUint32 capacity);
 406
 407 /*
 408  * Finalize table's data, free its entry storage (via table->ops->freeTable),
 409  * and return the memory starting at table to the malloc heap.
 410  */
 411 NS_COM_GLUE void
 412 PL_DHashTableDestroy(PLDHashTable *table);
 413
 414 /*
 415  * Initialize table with ops, data, entrySize, and capacity.  Capacity is a
 416  * guess for the smallest table size at which the table will usually be less
 417  * than 75% loaded (the table will grow or shrink as needed; capacity serves
 418  * only to avoid inevitable early growth from PL_DHASH_MIN_SIZE).
 419  */
 420 NS_COM_GLUE PRBool
 421 PL_DHashTableInit(PLDHashTable *table, const PLDHashTableOps *ops, void *data,
 422                   PRUint32 entrySize, PRUint32 capacity);
 423
 424 /*
 425  * Set maximum and minimum alpha for table.  The defaults are 0.75 and .25.
 426  * maxAlpha must be in [0.5, 0.9375] for the default PL_DHASH_MIN_SIZE; or if
 427  * MinSize=PL_DHASH_MIN_SIZE <= 256, in [0.5, (float)(MinSize-1)/MinSize]; or
 428  * else in [0.5, 255.0/256].  minAlpha must be in [0, maxAlpha / 2), so that
 429  * we don't shrink on the very next remove after growing a table upon adding
 430  * an entry that brings entryCount past maxAlpha * tableSize.
 431  */
 432 NS_COM_GLUE void
 433 PL_DHashTableSetAlphaBounds(PLDHashTable *table,
 434                             float maxAlpha,
 435                             float minAlpha);
 436
 437 /*
 438  * Call this macro with k, the number of pointer-sized words wasted per entry
 439  * under chaining, to compute the minimum alpha at which double hashing still
 440  * beats chaining.
 441  */
 442 #define PL_DHASH_MIN_ALPHA(table, k)                                          \
 443     ((float)((table)->entrySize / sizeof(void *) - 1)                         \
 444      / ((table)->entrySize / sizeof(void *) + (k)))
 445
 446 /*
 447  * Default max/min alpha, and macros to compute the value for the |capacity|
 448  * parameter to PL_NewDHashTable and PL_DHashTableInit, given default or any
 449  * max alpha, such that adding entryCount entries right after initializing the
 450  * table will not require a reallocation (so PL_DHASH_ADD can't fail for those
 451  * PL_DHashTableOperate calls).
 452  *
 453  * NB: PL_DHASH_CAP is a helper macro meant for use only in PL_DHASH_CAPACITY.
 454  * Don't use it directly!
 455  */
 456 #define PL_DHASH_DEFAULT_MAX_ALPHA 0.75
 457 #define PL_DHASH_DEFAULT_MIN_ALPHA 0.25
 458
 459 #define PL_DHASH_CAP(entryCount, maxAlpha)                                    \
 460     ((PRUint32)((double)(entryCount) / (maxAlpha)))
 461
 462 #define PL_DHASH_CAPACITY(entryCount, maxAlpha)                               \
 463     (PL_DHASH_CAP(entryCount, maxAlpha) +                                     \
 464      (((PL_DHASH_CAP(entryCount, maxAlpha) * (uint8)(0x100 * (maxAlpha)))     \
 465        >> 8) < (entryCount)))
 466
 467 #define PL_DHASH_DEFAULT_CAPACITY(entryCount)                                 \
 468     PL_DHASH_CAPACITY(entryCount, PL_DHASH_DEFAULT_MAX_ALPHA)
 469
 470 /*
 471  * Finalize table's data, free its entry storage using table->ops->freeTable,
 472  * and leave its members unchanged from their last live values (which leaves
 473  * pointers dangling).  If you want to burn cycles clearing table, it's up to
 474  * your code to call memset.
 475  */
 476 NS_COM_GLUE void
 477 PL_DHashTableFinish(PLDHashTable *table);
 478
 479 /*
 480  * To consolidate keyHash computation and table grow/shrink code, we use a
 481  * single entry point for lookup, add, and remove operations.  The operation
 482  * codes are declared here, along with codes returned by PLDHashEnumerator
 483  * functions, which control PL_DHashTableEnumerate's behavior.
 484  */
 485 typedef enum PLDHashOperator {
 486     PL_DHASH_LOOKUP = 0,        /* lookup entry */
 487     PL_DHASH_ADD = 1,           /* add entry */
 488     PL_DHASH_REMOVE = 2,        /* remove entry, or enumerator says remove */
 489     PL_DHASH_NEXT = 0,          /* enumerator says continue */
 490     PL_DHASH_STOP = 1           /* enumerator says stop */
 491 } PLDHashOperator;
 492
 493 /*
 494  * To lookup a key in table, call:
 495  *
 496  *  entry = PL_DHashTableOperate(table, key, PL_DHASH_LOOKUP);
 497  *
 498  * If PL_DHASH_ENTRY_IS_BUSY(entry) is true, key was found and it identifies
 499  * entry.  If PL_DHASH_ENTRY_IS_FREE(entry) is true, key was not found.
 500  *
 501  * To add an entry identified by key to table, call:
 502  *
 503  *  entry = PL_DHashTableOperate(table, key, PL_DHASH_ADD);
 504  *
 505  * If entry is null upon return, then either the table is severely overloaded,
 506  * and memory can't be allocated for entry storage via table->ops->allocTable;
 507  * Or if table->ops->initEntry is non-null, the table->ops->initEntry op may
 508  * have returned false.
 509  *
 510  * Otherwise, entry->keyHash has been set so that PL_DHASH_ENTRY_IS_BUSY(entry)
 511  * is true, and it is up to the caller to initialize the key and value parts
 512  * of the entry sub-type, if they have not been set already (i.e. if entry was
 513  * not already in the table, and if the optional initEntry hook was not used).
 514  *
 515  * To remove an entry identified by key from table, call:
 516  *
 517  *  (void) PL_DHashTableOperate(table, key, PL_DHASH_REMOVE);
 518  *
 519  * If key's entry is found, it is cleared (via table->ops->clearEntry) and
 520  * the entry is marked so that PL_DHASH_ENTRY_IS_FREE(entry).  This operation
 521  * returns null unconditionally; you should ignore its return value.
 522  */
 523 NS_COM_GLUE PLDHashEntryHdr * PL_DHASH_FASTCALL
 524 PL_DHashTableOperate(PLDHashTable *table, const void *key, PLDHashOperator op);
 525
 526 /*
 527  * Remove an entry already accessed via LOOKUP or ADD.
 528  *
 529  * NB: this is a "raw" or low-level routine, intended to be used only where
 530  * the inefficiency of a full PL_DHashTableOperate (which rehashes in order
 531  * to find the entry given its key) is not tolerable.  This function does not
 532  * shrink the table if it is underloaded.  It does not update stats #ifdef
 533  * PL_DHASHMETER, either.
 534  */
 535 NS_COM_GLUE void
 536 PL_DHashTableRawRemove(PLDHashTable *table, PLDHashEntryHdr *entry);
 537
 538 /*
 539  * Enumerate entries in table using etor:
 540  *
 541  *   count = PL_DHashTableEnumerate(table, etor, arg);
 542  *
 543  * PL_DHashTableEnumerate calls etor like so:
 544  *
 545  *   op = etor(table, entry, number, arg);
 546  *
 547  * where number is a zero-based ordinal assigned to live entries according to
 548  * their order in table->entryStore.
 549  *
 550  * The return value, op, is treated as a set of flags.  If op is PL_DHASH_NEXT,
 551  * then continue enumerating.  If op contains PL_DHASH_REMOVE, then clear (via
 552  * table->ops->clearEntry) and free entry.  Then we check whether op contains
 553  * PL_DHASH_STOP; if so, stop enumerating and return the number of live entries
 554  * that were enumerated so far.  Return the total number of live entries when
 555  * enumeration completes normally.
 556  *
 557  * If etor calls PL_DHashTableOperate on table with op != PL_DHASH_LOOKUP, it
 558  * must return PL_DHASH_STOP; otherwise undefined behavior results.
 559  *
 560  * If any enumerator returns PL_DHASH_REMOVE, table->entryStore may be shrunk
 561  * or compressed after enumeration, but before PL_DHashTableEnumerate returns.
 562  * Such an enumerator therefore can't safely set aside entry pointers, but an
 563  * enumerator that never returns PL_DHASH_REMOVE can set pointers to entries
 564  * aside, e.g., to avoid copying live entries into an array of the entry type.
 565  * Copying entry pointers is cheaper, and safe so long as the caller of such a
 566  * "stable" Enumerate doesn't use the set-aside pointers after any call either
 567  * to PL_DHashTableOperate, or to an "unstable" form of Enumerate, which might
 568  * grow or shrink entryStore.
 569  *
 570  * If your enumerator wants to remove certain entries, but set aside pointers
 571  * to other entries that it retains, it can use PL_DHashTableRawRemove on the
 572  * entries to be removed, returning PL_DHASH_NEXT to skip them.  Likewise, if
 573  * you want to remove entries, but for some reason you do not want entryStore
 574  * to be shrunk or compressed, you can call PL_DHashTableRawRemove safely on
 575  * the entry being enumerated, rather than returning PL_DHASH_REMOVE.
 576  */
 577 typedef PLDHashOperator
 578 (* PR_CALLBACK PLDHashEnumerator)(PLDHashTable *table, PLDHashEntryHdr *hdr,
 579                                       PRUint32 number, void *arg);
 580
 581 NS_COM_GLUE PRUint32
 582 PL_DHashTableEnumerate(PLDHashTable *table, PLDHashEnumerator etor, void *arg);
 583
 584 #ifdef PL_DHASHMETER
 585 #include <stdio.h>
 586
 587 NS_COM_GLUE void
 588 PL_DHashTableDumpMeter(PLDHashTable *table, PLDHashEnumerator dump, FILE *fp);
 589 #endif
 590
 591 PR_END_EXTERN_C
 592
 593 #endif /* pldhash_h___ */