/* Memory layout of a zipmap, for the map "foo" => "bar", "hello" => "world":
*
- * <status><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
+ * <zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
*
- * <status> is 1 byte status. Currently only 1 bit is used: if the least
- * significant bit is set, it means the zipmap needs to be defragmented.
+ * <zmlen> is 1 byte length that holds the current size of the zipmap.
+ * When the zipmap length is greater than or equal to 254, this value
+ * is not used and the zipmap needs to be traversed to find out the length.
*
* <len> is the length of the following string (key or value).
* <len> lengths are encoded in a single value or in a 5 bytes value.
* or even in order to add a key/value pair if it fits.
*
* <free> is always an unsigned 8 bit number, because if after an
- * update operation there are more than a few free bytes, they'll be converted
- * into empty space prefixed by the special value 254.
+ * update operation there are more than a few free bytes, the zipmap will be
+ * reallocated to make sure it is as small as possible.
*
* The most compact representation of the above two elements hash is actually:
*
- * "\x00\x03foo\x03\x00bar\x05hello\x05\x00world\xff"
+ * "\x02\x03foo\x03\x00bar\x05hello\x05\x00world\xff"
*
- * Empty space is marked using a 254 bytes + a <len> (coded as already
- * specified). The length includes the 254 bytes in the count and the
- * space taken by the <len> field. So for instance removing the "foo" key
- * from the zipmap above will lead to the following representation:
- *
- * "\x00\xfd\x10........\x05hello\x05\x00world\xff"
- *
- * Note that because empty space, keys, values, are all prefixed length
- * "objects", the lookup will take O(N) where N is the numeber of elements
+ * Note that because keys and values are prefixed length "objects",
+ * the lookup will take O(N) where N is the number of elements
* in the zipmap and *not* the number of bytes needed to represent the zipmap.
* This lowers the constant times considerably.
*/
/* The following defines the max value for the <free> field described in the
* comments above, that is, the max number of trailing bytes in a value. */
-#define ZIPMAP_VALUE_MAX_FREE 5
+#define ZIPMAP_VALUE_MAX_FREE 4
/* The following macro returns the number of bytes needed to encode the length
* for the integer value _l, that is, 1 byte for lengths < ZIPMAP_BIGLEN and
* free space if any). */
static unsigned int zipmapRawEntryLength(unsigned char *p) {
unsigned int l = zipmapRawKeyLength(p);
-
return l + zipmapRawValueLength(p+l);
}
* If 'update' is not NULL, *update is set to 1 if the key was
* already preset, otherwise to 0. */
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {
- unsigned int zmlen;
+ unsigned int zmlen, offset;
unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);
unsigned int empty, vempty;
unsigned char *p;
/* Increase zipmap length (this is an insert) */
if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;
} else {
- unsigned char *b = p;
-
/* Key found. Is there enough space for the new value? */
/* Compute the total length: */
if (update) *update = 1;
- freelen = zipmapRawKeyLength(b);
- b += freelen;
- freelen += zipmapRawValueLength(b);
+ freelen = zipmapRawEntryLength(p);
if (freelen < reqlen) {
- /* Move remaining entries to the current position, so this
- * pair can be appended. Note: the +1 in memmove is caused
- * by the end-of-zipmap byte. */
- memmove(p, p+freelen, zmlen-((p-zm)+freelen+1));
+ /* Store the offset of this key within the current zipmap, so
+ * it can be resized. Then, move the tail backwards so this
+ * pair fits at the current position. */
+ offset = p-zm;
zm = zipmapResize(zm, zmlen-freelen+reqlen);
- p = zm+zmlen-1-freelen;
- zmlen = zmlen-1-freelen+reqlen;
+ p = zm+offset;
+
+ /* The +1 in the number of bytes to be moved is caused by the
+ * end-of-zipmap byte. Note: the *original* zmlen is used. */
+ memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
+ zmlen = zmlen-freelen+reqlen;
freelen = reqlen;
}
}
- /* Ok we have a suitable block where to write the new key/value
- * entry. */
+ /* We now have a suitable block where the key/value entry can
+ * be written. If there is too much free space, move the tail
+ * of the zipmap a few bytes to the front and shrink the zipmap,
+ * as we want zipmaps to be very space efficient. */
empty = freelen-reqlen;
- /* If there is too much free space mark it as a free block instead
- * of adding it as trailing empty space for the value, as we want
- * zipmaps to be very space efficient. */
if (empty >= ZIPMAP_VALUE_MAX_FREE) {
- memmove(p+reqlen, p+freelen, zmlen-((p-zm)+freelen+1));
+ /* First, move the tail <empty> bytes to the front, then resize
+ * the zipmap to be <empty> bytes smaller. */
+ offset = p-zm;
+ memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen -= empty;
zm = zipmapResize(zm, zmlen);
+ p = zm+offset;
vempty = 0;
} else {
vempty = empty;
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