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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.

SPDX-License-Identifier: curl
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# `uint32_t` Sets

The multi handle tracks added easy handles via an `uint32_t` it calls an
`mid`. There are four data structures for `uint32_t` optimized for the multi
use case.

## `uint32_tbl`

`uint32_table`, implemented in `uint-table.[ch]` manages an array of `void *`.
The `uint32_t` is the index into this array. It is created with a *capacity*
which can be *resized*. The table assigns the index when a `void *` is
*added*. It keeps track of the last assigned index and uses the next available
larger index for a subsequent add. Reaching *capacity* it wraps around.

The table *can not* store `NULL` values. The largest possible index
is `UINT32_MAX - 1`.

The table is iterated over by asking for the *first* existing index, meaning
the smallest number that has an entry, if the table is not empty. To get
the *next* entry, one passes the index of the previous iteration step. It does
not matter if the previous index is still in the table. Sample code for a table
iteration would look like this:

```c
uint32_t int mid;
void *entry;

if(Curl_uint32_tbl_first(tbl, &mid, &entry)) {
  do {
     /* operate on entry with index mid */
  }
  while(Curl_uint_tbl_next(tbl, mid, &mid, &entry));
}

```

This iteration has the following properties:

* entries in the table can be added/removed safely.
* all entries that are not removed during the iteration are visited.
* the table may be resized to a larger capacity without affecting visited
  entries.
* entries added with a larger index than the current are visited.

### Memory

For storing 1000 entries, the table would allocate one block of 8KB on
a 64-bit system, plus the 2 pointers and 3 `uint32_t` in its base `struct
uint32_tbl`. A resize allocates a completely new pointer array, copy
the existing entries and free the previous one.

### Performance

Lookups of entries are only an index into the array, O(1) with a tiny 1.
Adding entries and iterations are more work:

1. adding an entry means "find the first free index larger than the previous
   assigned one". Worst case for this is a table with only a single free index
   where `capacity - 1` checks on `NULL` values would be performed, O(N). If
   the single free index is randomly distributed, this would be O(N/2).
2. iterating a table scans for the first not `NULL` entry after the start
   index. This makes a complete iteration O(N) work.

In the multi use case, point 1 is remedied by growing the table so that a good
chunk of free entries always exists.

Point 2 is less of an issue for a multi, since it does not really matter when
the number of transfer is relatively small. A multi managing a larger set
needs to operate event based anyway and table iterations rarely are needed.

For these reasons, the simple implementation was preferred. Should this become
a concern, there are options like "free index lists" or, alternatively, an
internal bitset that scans better.

## `uint32_bset`

A bitset for `uint32_t` values, allowing fast add/remove operations. It is
initialized with a *capacity*, meaning it can store only the numbers in the
range `[0, capacity-1]`. It can be *resized* and safely *iterated*.
`uint32_bset` is designed to operate in combination with `uint_tbl`.

The bitset keeps an array of `uint64_t`. The first array entry keeps the
numbers 0 to 63, the second 64 to 127 and so on. A bitset with capacity 1024
would therefore allocate an array of 16 64-bit values (128 bytes). Operations
for an unsigned int divide it by 64 for the array index and then
check/set/clear the bit of the remainder.

Iterator works the same as with `uint32_tbl`: ask the bitset for the *first*
number present and then use that to get the *next* higher number present. Like
the table, this safe for adds/removes and growing the set while iterating.

### Memory

The set only needs 1 bit for each possible number.
A bitset for 40000 transfers occupies 5KB of memory.

## Performance

Operations for add/remove/check are O(1). Iteration needs to scan for the next
bit set. The number of scans is small (see memory footprint) and, for checking
bits, many compilers offer primitives for special CPU instructions.

## `uint32_spbset`

While the memory footprint of `uint32_bset` is good, it still needs 5KB to
store the single number 40000. This is not optimal when many are needed. For
example, in event based processing, each socket needs to keep track of the
transfers involved. There are many sockets potentially, but each one mostly
tracks a single transfer or few (on HTTP/2 connection borderline up to 100).

For such uses cases, the `uint32_spbset` is intended: track a small number of
unsigned int, potentially rather "close" together. It keeps "chunks" with an
offset and has no capacity limit.

Example: adding the number 40000 to an empty sparse bitset would have one
chunk with offset 39936, keeping track of the numbers 39936 to 40192 (a chunk
has 4 64-bit values). The numbers in that range can be handled without further
allocations.

The worst case is then storing 100 numbers that lie in separate intervals.
Then 100 chunks would need to be allocated and linked, resulting in overall 4
KB of memory used.

Iterating a sparse bitset works the same as for bitset and table.

## `uint32_hash`

At last, there are places in libcurl such as the HTTP/2 and HTTP/3 protocol
implementations that need to store their own data related to a transfer.
`uint32_hash` allows then to associate an unsigned int, e.g. the transfer's
`mid`, to their own data.
