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updated zipmap documentation to match the implementation

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Pieter Noordhuis 2010-04-01 13:24:18 +02:00
parent 8c6700720d
commit bfded2aa51
1 changed files with 9 additions and 15 deletions
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24
zipmap.c
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@ -42,10 +42,11 @@
/* 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.
@ -62,22 +63,15 @@
* 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.
*/