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+/* vi:set ts=8 sts=4 sw=4:
+ *
+ * VIM - Vi IMproved	by Bram Moolenaar
+ *
+ * Do ":help uganda"  in Vim to read copying and usage conditions.
+ * Do ":help credits" in Vim to see a list of people who contributed.
+ * See README.txt for an overview of the Vim source code.
+ */
+
+/*
+ * hashtab.c: Handling of a hashtable with Vim-specific properties.
+ *
+ * Each item in a hashtable has a NUL terminated string key.  A key can appear
+ * only once in the table.
+ *
+ * A hash number is computed from the key for quick lookup.  When the hashes
+ * of two different keys point to the same entry an algorithm is used to
+ * iterate over other entries in the table until the right one is found.
+ * To make the iteration work removed keys are different from entries where a
+ * key was never present.
+ *
+ * The mechanism has been partly based on how Python Dictionaries are
+ * implemented.  The algorithm is from Knuth Vol. 3, Sec. 6.4.
+ *
+ * The hashtable grows to accommodate more entries when needed.  At least 1/3
+ * of the entries is empty to keep the lookup efficient (at the cost of extra
+ * memory).
+ */
+
+#include "vim.h"
+
+#if defined(FEAT_EVAL) || defined(FEAT_SYN_HL) || defined(PROTO)
+
+#if 0
+# define HT_DEBUG	/* extra checks for table consistency  and statistics */
+
+static long hash_count_lookup = 0;	/* count number of hashtab lookups */
+static long hash_count_perturb = 0;	/* count number of "misses" */
+#endif
+
+/* Magic value for algorithm that walks through the array. */
+#define PERTURB_SHIFT 5
+
+static int hash_may_resize __ARGS((hashtab_T *ht, int minitems));
+
+#if 0 /* currently not used */
+/*
+ * Create an empty hash table.
+ * Returns NULL when out of memory.
+ */
+    hashtab_T *
+hash_create()
+{
+    hashtab_T *ht;
+
+    ht = (hashtab_T *)alloc(sizeof(hashtab_T));
+    if (ht != NULL)
+	hash_init(ht);
+    return ht;
+}
+#endif
+
+/*
+ * Initialize an empty hash table.
+ */
+    void
+hash_init(ht)
+    hashtab_T *ht;
+{
+    /* This zeroes all "ht_" entries and all the "hi_key" in "ht_smallarray". */
+    vim_memset(ht, 0, sizeof(hashtab_T));
+    ht->ht_array = ht->ht_smallarray;
+    ht->ht_mask = HT_INIT_SIZE - 1;
+}
+
+/*
+ * Free the array of a hash table.  Does not free the items it contains!
+ * If "ht" is not freed then you should call hash_init() next!
+ */
+    void
+hash_clear(ht)
+    hashtab_T *ht;
+{
+    if (ht->ht_array != ht->ht_smallarray)
+	vim_free(ht->ht_array);
+}
+
+/*
+ * Free the array of a hash table and all the keys it contains.  The keys must
+ * have been allocated.  "off" is the offset from the start of the allocate
+ * memory to the location of the key (it's always positive).
+ */
+    void
+hash_clear_all(ht, off)
+    hashtab_T	*ht;
+    int		off;
+{
+    int		todo;
+    hashitem_T	*hi;
+
+    todo = ht->ht_used;
+    for (hi = ht->ht_array; todo > 0; ++hi)
+    {
+	if (!HASHITEM_EMPTY(hi))
+	{
+	    vim_free(hi->hi_key - off);
+	    --todo;
+	}
+    }
+    hash_clear(ht);
+}
+
+/*
+ * Find "key" in hashtable "ht".  "key" must not be NULL.
+ * Always returns a pointer to a hashitem.  If the item was not found then
+ * HASHITEM_EMPTY() is TRUE.  The pointer is then the place where the key
+ * would be added.
+ * WARNING: The returned pointer becomes invalid when the hashtable is changed
+ * (adding, setting or removing an item)!
+ */
+    hashitem_T *
+hash_find(ht, key)
+    hashtab_T	*ht;
+    char_u	*key;
+{
+    return hash_lookup(ht, key, hash_hash(key));
+}
+
+/*
+ * Like hash_find(), but caller computes "hash".
+ */
+    hashitem_T *
+hash_lookup(ht, key, hash)
+    hashtab_T	*ht;
+    char_u	*key;
+    hash_T	hash;
+{
+    hash_T	perturb;
+    hashitem_T	*freeitem;
+    hashitem_T	*hi;
+    int		idx;
+
+#ifdef HT_DEBUG
+    ++hash_count_lookup;
+#endif
+
+    /*
+     * Quickly handle the most common situations:
+     * - return if there is no item at all
+     * - skip over a removed item
+     * - return if the item matches
+     */
+    idx = hash & ht->ht_mask;
+    hi = &ht->ht_array[idx];
+
+    if (hi->hi_key == NULL)
+	return hi;
+    if (hi->hi_key == HI_KEY_REMOVED)
+	freeitem = hi;
+    else if (hi->hi_hash == hash && STRCMP(hi->hi_key, key) == 0)
+	return hi;
+    else
+	freeitem = NULL;
+
+    /*
+     * Need to search through the table to find the key.  The algorithm
+     * to step through the table starts with large steps, gradually becoming
+     * smaller down to (1/4 table size + 1).  This means it goes through all
+     * table entries in the end.
+     * When we run into a NULL key it's clear that the key isn't there.
+     * Return the first available slot found (can be a slot of a removed
+     * item).
+     */
+    for (perturb = hash; ; perturb >>= PERTURB_SHIFT)
+    {
+#ifdef HT_DEBUG
+	++hash_count_perturb;	    /* count a "miss" for hashtab lookup */
+#endif
+	idx = (idx << 2) + idx + perturb + 1;
+	hi = &ht->ht_array[idx & ht->ht_mask];
+	if (hi->hi_key == NULL)
+	    return freeitem == NULL ? hi : freeitem;
+	if (hi->hi_hash == hash
+		&& hi->hi_key != HI_KEY_REMOVED
+		&& STRCMP(hi->hi_key, key) == 0)
+	    return hi;
+	if (hi->hi_key == HI_KEY_REMOVED && freeitem == NULL)
+	    freeitem = hi;
+    }
+}
+
+/*
+ * Print the efficiency of hashtable lookups.
+ * Useful when trying different hash algorithms.
+ * Called when exiting.
+ */
+    void
+hash_debug_results()
+{
+#ifdef HT_DEBUG
+    fprintf(stderr, "\r\n\r\n\r\n\r\n");
+    fprintf(stderr, "Number of hashtable lookups: %ld\r\n", hash_count_lookup);
+    fprintf(stderr, "Number of perturb loops: %ld\r\n", hash_count_perturb);
+    fprintf(stderr, "Percentage of perturb loops: %ld%%\r\n",
+				hash_count_perturb * 100 / hash_count_lookup);
+#endif
+}
+
+/*
+ * Add item with key "key" to hashtable "ht".
+ * Returns FAIL when out of memory or the key is already present.
+ */
+    int
+hash_add(ht, key)
+    hashtab_T	*ht;
+    char_u	*key;
+{
+    hash_T	hash = hash_hash(key);
+    hashitem_T	*hi;
+
+    hi = hash_lookup(ht, key, hash);
+    if (!HASHITEM_EMPTY(hi))
+    {
+	EMSG2(_(e_intern2), "hash_add()");
+	return FAIL;
+    }
+    return hash_add_item(ht, hi, key, hash);
+}
+
+/*
+ * Add item "hi" with "key" to hashtable "ht".  "key" must not be NULL and
+ * "hi" must have been obtained with hash_lookup() and point to an empty item.
+ * "hi" is invalid after this!
+ * Returns OK or FAIL (out of memory).
+ */
+    int
+hash_add_item(ht, hi, key, hash)
+    hashtab_T	*ht;
+    hashitem_T	*hi;
+    char_u	*key;
+    hash_T	hash;
+{
+    /* If resizing failed before and it fails again we can't add an item. */
+    if (ht->ht_error && hash_may_resize(ht, 0) == FAIL)
+	return FAIL;
+
+    ++ht->ht_used;
+    if (hi->hi_key == NULL)
+	++ht->ht_filled;
+    hi->hi_key = key;
+    hi->hi_hash = hash;
+
+    /* When the space gets low may resize the array. */
+    return hash_may_resize(ht, 0);
+}
+
+#if 0  /* not used */
+/*
+ * Overwrite hashtable item "hi" with "key".  "hi" must point to the item that
+ * is to be overwritten.  Thus the number of items in the hashtable doesn't
+ * change.
+ * Although the key must be identical, the pointer may be different, thus it's
+ * set anyway (the key is part of an item with that key).
+ * The caller must take care of freeing the old item.
+ * "hi" is invalid after this!
+ */
+    void
+hash_set(hi, key)
+    hashitem_T	*hi;
+    char_u	*key;
+{
+    hi->hi_key = key;
+}
+#endif
+
+/*
+ * Remove item "hi" from  hashtable "ht".  "hi" must have been obtained with
+ * hash_lookup().
+ * The caller must take care of freeing the item itself.
+ */
+    void
+hash_remove(ht, hi)
+    hashtab_T	*ht;
+    hashitem_T	*hi;
+{
+    --ht->ht_used;
+    hi->hi_key = HI_KEY_REMOVED;
+    hash_may_resize(ht, 0);
+}
+
+/*
+ * Lock a hashtable: prevent that ht_array changes.
+ * Don't use this when items are to be added!
+ * Must call hash_unlock() later.
+ */
+    void
+hash_lock(ht)
+    hashtab_T	*ht;
+{
+    ++ht->ht_locked;
+}
+
+#if 0	    /* currently not used */
+/*
+ * Lock a hashtable at the specified number of entries.
+ * Caller must make sure no more than "size" entries will be added.
+ * Must call hash_unlock() later.
+ */
+    void
+hash_lock_size(ht, size)
+    hashtab_T	*ht;
+    int		size;
+{
+    (void)hash_may_resize(ht, size);
+    ++ht->ht_locked;
+}
+#endif
+
+/*
+ * Unlock a hashtable: allow ht_array changes again.
+ * Table will be resized (shrink) when necessary.
+ * This must balance a call to hash_lock().
+ */
+    void
+hash_unlock(ht)
+    hashtab_T	*ht;
+{
+    --ht->ht_locked;
+    (void)hash_may_resize(ht, 0);
+}
+
+/*
+ * Shrink a hashtable when there is too much empty space.
+ * Grow a hashtable when there is not enough empty space.
+ * Returns OK or FAIL (out of memory).
+ */
+    static int
+hash_may_resize(ht, minitems)
+    hashtab_T	*ht;
+    int		minitems;		/* minimal number of items */
+{
+    hashitem_T	temparray[HT_INIT_SIZE];
+    hashitem_T	*oldarray, *newarray;
+    hashitem_T	*olditem, *newitem;
+    int		newi;
+    int		todo;
+    long_u	oldsize, newsize;
+    long_u	minsize;
+    long_u	newmask;
+    hash_T	perturb;
+
+    /* Don't resize a locked table. */
+    if (ht->ht_locked > 0)
+	return OK;
+
+#ifdef HT_DEBUG
+    if (ht->ht_used > ht->ht_filled)
+	EMSG("hash_may_resize(): more used than filled");
+    if (ht->ht_filled >= ht->ht_mask + 1)
+	EMSG("hash_may_resize(): table completely filled");
+#endif
+
+    if (minitems == 0)
+    {
+	/* Return quickly for small tables with at least two NULL items.  NULL
+	 * items are required for the lookup to decide a key isn't there. */
+	if (ht->ht_filled < HT_INIT_SIZE - 1
+					 && ht->ht_array == ht->ht_smallarray)
+	    return OK;
+
+	/*
+	 * Grow or refill the array when it's more than 2/3 full (including
+	 * removed items, so that they get cleaned up).
+	 * Shrink the array when it's less than 1/5 full.  When growing it is
+	 * at least 1/4 full (avoids repeated grow-shrink operations)
+	 */
+	oldsize = ht->ht_mask + 1;
+	if (ht->ht_filled * 3 < oldsize * 2 && ht->ht_used > oldsize / 5)
+	    return OK;
+
+	if (ht->ht_used > 1000)
+	    minsize = ht->ht_used * 2;  /* it's big, don't make too much room */
+	else
+	    minsize = ht->ht_used * 4;  /* make plenty of room */
+    }
+    else
+    {
+	/* Use specified size. */
+	if ((long_u)minitems < ht->ht_used)	/* just in case... */
+	    minitems = ht->ht_used;
+	minsize = minitems * 3 / 2;	/* array is up to 2/3 full */
+    }
+
+    newsize = HT_INIT_SIZE;
+    while (newsize < minsize)
+    {
+	newsize <<= 1;		/* make sure it's always a power of 2 */
+	if (newsize == 0)
+	    return FAIL;	/* overflow */
+    }
+
+    if (newsize == HT_INIT_SIZE)
+    {
+	/* Use the small array inside the hashdict structure. */
+	newarray = ht->ht_smallarray;
+	if (ht->ht_array == newarray)
+	{
+	    /* Moving from ht_smallarray to ht_smallarray!  Happens when there
+	     * are many removed items.  Copy the items to be able to clean up
+	     * removed items. */
+	    mch_memmove(temparray, newarray, sizeof(temparray));
+	    oldarray = temparray;
+	}
+	else
+	    oldarray = ht->ht_array;
+    }
+    else
+    {
+	/* Allocate an array. */
+	newarray = (hashitem_T *)alloc((unsigned)
+					      (sizeof(hashitem_T) * newsize));
+	if (newarray == NULL)
+	{
+	    /* Out of memory.  When there are NULL items still return OK.
+	     * Otherwise set ht_error, because lookup may result in a hang if
+	     * we add another item. */
+	    if (ht->ht_filled < ht->ht_mask)
+		return OK;
+	    ht->ht_error = TRUE;
+	    return FAIL;
+	}
+	oldarray = ht->ht_array;
+    }
+    vim_memset(newarray, 0, (size_t)(sizeof(hashitem_T) * newsize));
+
+    /*
+     * Move all the items from the old array to the new one, placing them in
+     * the right spot.  The new array won't have any removed items, thus this
+     * is also a cleanup action.
+     */
+    newmask = newsize - 1;
+    todo = ht->ht_used;
+    for (olditem = oldarray; todo > 0; ++olditem)
+	if (!HASHITEM_EMPTY(olditem))
+	{
+	    /*
+	     * The algorithm to find the spot to add the item is identical to
+	     * the algorithm to find an item in hash_lookup().  But we only
+	     * need to search for a NULL key, thus it's simpler.
+	     */
+	    newi = olditem->hi_hash & newmask;
+	    newitem = &newarray[newi];
+
+	    if (newitem->hi_key != NULL)
+		for (perturb = olditem->hi_hash; ; perturb >>= PERTURB_SHIFT)
+		{
+		    newi = (newi << 2) + newi + perturb + 1;
+		    newitem = &newarray[newi & newmask];
+		    if (newitem->hi_key == NULL)
+			break;
+		}
+	    *newitem = *olditem;
+	    --todo;
+	}
+
+    if (ht->ht_array != ht->ht_smallarray)
+	vim_free(ht->ht_array);
+    ht->ht_array = newarray;
+    ht->ht_mask = newmask;
+    ht->ht_filled = ht->ht_used;
+    ht->ht_error = FALSE;
+
+    return OK;
+}
+
+/*
+ * Get the hash number for a key.
+ * If you think you know a better hash function: Compile with HT_DEBUG set and
+ * run a script that uses hashtables a lot.  Vim will then print statistics
+ * when exiting.  Try that with the current hash algorithm and yours.  The
+ * lower the percentage the better.
+ */
+    hash_T
+hash_hash(key)
+    char_u	*key;
+{
+    hash_T	hash;
+    char_u	*p;
+
+    if ((hash = *key) == 0)
+	return (hash_T)0;	/* Empty keys are not allowed, but we don't
+				   want to crash if we get one. */
+    p = key + 1;
+
+#if 0
+    /* ElfHash algorithm, which is supposed to have an even distribution.
+     * Suggested by Charles Campbell. */
+    hash_T	g;
+
+    while (*p != NUL)
+    {
+	hash = (hash << 4) + *p++;	/* clear low 4 bits of hash, add char */
+	g = hash & 0xf0000000L;		/* g has high 4 bits of hash only */
+	if (g != 0)
+	    hash ^= g >> 24;		/* xor g's high 4 bits into hash */
+    }
+#else
+
+    /* A simplistic algorithm that appears to do very well.
+     * Suggested by George Reilly. */
+    while (*p != NUL)
+	hash = hash * 101 + *p++;
+#endif
+
+    return hash;
+}
+
+#endif