Turbo NSS
---------

glibc nss library for passwd and group.

Steps
-----

A known implementation runs id(1) at ~250 rps sequentially. Our goal is 10k
ID/s.

id(1) works as follows:
- lookup user by name.
- get all additional gids (an array attached to a member).
- for each additional gid, return the group name.

Assuming a member is in ~100 groups on average, that's 1M group lookups per
second. We need to convert gid to a group index quickly.

Data structures
---------------

Basic data structures that allow efficient storage:

```lang=c
// reminder:
typedef uid_t uint32;
typedef gid_t uint32;

// 6*32b = 6*4B = 24B/user
typedef struct {
  uid_t uid;
  gid_t gid;
  name_offset uint32; // offset into *usernames
  gecos_offset uint32; // offset into *gecos
  shell_offset uint32; // offset into *shells
  additional_groups_offset uint32; // offset into additional_groups
} user;

const char* usernames; // all concatenated usernames, fsst-compressed
const char* gecoss; // all concatenated gecos, fsst-compressed
const char* shells; // all concatenated home directories, fsst-compressed
const uint8_t additional_groups; // all additional_groups, turbo compressed

typedef struct {
  gid_t gid;
  name_offset uint32; // offset into *groupnames
  members_offset uint32; // offset into members
}

const char* groupnames; // all concatenated group names, fsst-compressed
const uint8_8 members; // all concatenated members, turbo compressed
```

"turbo compression" encodes a list of uids/gids with this algorithm:
1. sort ascending.
2. extract deltas and subtract 1: `awk '{diff=$0-prev; prev=$0; print
   diff-1}'`.
3. varint-encode these deltas into an uint32, like protobuf or utf8.

With typical group memberships (as of writing) this requires ~1.3-1.5 byte per
entry.

Indexes
-------

The following operations need to be fast, in order of importance:

1. lookup gid -> group (this is on hot path in id).
2. lookup uid -> user.
3. lookup username -> user.
4. lookup groupname -> group.
5. (optional) iterate users using a defined order (`getent passwd`).
6. (optional) iterate groups using a defined order (`getent group`).