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+2007/03/30 - Header storage in trees
+
+This documentation describes how to store headers in radix trees, providing
+fast access to any known position, while retaining the ability to grow/reduce
+any arbitrary header without having to recompute all positions.
+
+Principle :
+ We have a radix tree represented in an integer array, which represents the
+ total number of bytes used by all headers whose position is below it. This
+ ensures that we can compute any header's position in O(log(N)) where N is
+ the number of headers.
+
+Example with N=16 :
+
+ +-----------------------+
+ | |
+ +-----------+ +-----------+
+ | | | |
+ +-----+ +-----+ +-----+ +-----+
+ | | | | | | | |
+ +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
+ | | | | | | | | | | | | | | | |
+
+ 0 1 2 3 4 5 6 7 8 9 A B C D E F
+
+ To reach header 6, we have to compute hdr[0]+hdr[4]+hdr[6]
+
+ With this method, it becomes easy to grow any header and update the array.
+ To achieve this, we have to replace one after the other all bits on the
+ right with one 1 followed by zeroes, and update the position if it's higher
+ than current position, and stop when it's above number of stored headers.
+
+ For instance, if we want to grow hdr[6], we proceed like this :
+
+ 6 = 0110 (BIN)
+
+ Let's consider the values to update :
+
+ (bit 0) : (0110 & ~0001) | 0001 = 0111 = 7 > 6 => update
+ (bit 1) : (0110 & ~0011) | 0010 = 0110 = 6 <= 6 => leave it
+ (bit 2) : (0110 & ~0111) | 0100 = 0100 = 4 <= 6 => leave it
+ (bit 4) : (0110 & ~1111) | 1000 = 1000 = 8 > 6 => update
+ (bit 5) : larger than array size, stop.
+
+
+It's easy to walk through the tree too. We only have one iteration per bit
+changing from X to the ancestor, and one per bit from the ancestor to Y.
+The ancestor is found while walking. To go from X to Y :
+
+ pos = pos(X)
+
+ while (Y != X) {
+ if (Y > X) {
+ // walk from Y to ancestor
+ pos += hdr[Y]
+ Y &= (Y - 1)
+ } else {
+ // walk from X to ancestor
+ pos -= hdr[X]
+ X &= (X - 1)
+ }
+ }
+
+However, it is not trivial anymore to linearly walk the tree. We have to move
+from a known place to another known place, but a jump to next entry costs the
+same as a jump to a random place.
+
+Other caveats :
+ - it is not possible to remove a header, it is only possible to empty it.
+ - it is not possible to insert a header, as that would imply a renumbering.
+ => this means that a "defrag" function is required. Headers should preferably
+ be added, then should be stuffed on top of destroyed ones, then only
+ inserted if absolutely required.
+
+
+When we have this, we can then focus on a 32-bit header descriptor which would
+look like this :
+
+{
+ unsigned line_len :13; /* total line length, including CRLF */
+ unsigned name_len :6; /* header name length, max 63 chars */
+ unsigned sp1 :5; /* max spaces before value : 31 */
+ unsigned sp2 :8; /* max spaces after value : 255 */
+}
+
+Example :
+
+ Connection: close \r\n
+ <---------+-----+-----+-------------> line_len
+ <-------->| | | name_len
+ <-----> | sp1
+ <-------------> sp2
+Rem:
+ - if there are more than 31 spaces before the value, the buffer will have to
+ be moved before being registered
+
+ - if there are more than 255 spaces after the value, the buffer will have to
+ be moved before being registered
+
+ - we can use the empty header name as an indicator for a deleted header
+
+ - it would be wise to format a new request before sending lots of random
+ spaces to the servers.
+
+ - normal clients do not send such crap, so those operations *may* reasonably
+ be more expensive than the rest provided that other ones are very fast.
+
+It would be handy to have the following macros :
+
+ hdr_eon(hdr) => end of name
+ hdr_sov(hdr) => start of value
+ hdr_eof(hdr) => end of value
+ hdr_vlen(hdr) => length of value
+ hdr_hlen(hdr) => total header length
+
+
+A 48-bit encoding would look like this :
+
+ Connection: close \r\n
+ <---------+------+---+--------------> eoh = 16 bits
+ <-------->| | | eon = 8 bits
+ <--------------->| | sov = 8 bits
+ <---> vlen = 16 bits
+