NAME
    Data::SortedSet::Shared - shared-memory sorted set (ZSET) for Linux

SYNOPSIS
        use Data::SortedSet::Shared;

        # up to 1M members, anonymous shared mapping
        my $z = Data::SortedSet::Shared->new(undef, 1_000_000);

        $z->add(42, 1500);              # member 42 with score 1500
        $z->add(7,  1500);              # ties broken by member id
        $z->incr(42, 50);               # 42 -> 1550 (returns the new score)

        my @top   = $z->rev_range_by_rank(0, 9);    # top 10 members (highest score)
        my $rank  = $z->rank(42);                    # 0-based rank (lowest score = 0)
        my $score = $z->score(42);
        my @near  = $z->range_by_score(1400, 1600);  # members scored in [1400, 1600]

        my ($m, $s) = $z->pop_min;       # remove + return the lowest

        $z->each(sub { my ($member, $score) = @_; ... });   # in score order

DESCRIPTION
    An ordered set in shared memory, in the spirit of a Redis sorted set:
    each member is a 64-bit integer carrying a double score, and members are
    kept in score order. It is backed by an order-statistics B+tree -- so
    "rank", range, and "pop" are O(log n) and ranges scan sequentially
    through doubly-linked leaves -- paired with a member-to-score hash
    index, so "score" and "exists" are O(1).

    The total order is (score, member): members with equal scores are
    ordered by member id, which gives a well-defined rank and a
    deterministic "pop_min"/"pop_max".

    Multiple processes can map the same set and read and write it
    concurrently; access is serialized by a write-preferring futex rwlock
    that recovers automatically if a lock holder dies (see "CRASH SAFETY").

    Members are 64-bit integers. For string-keyed sets, see "String-keyed
    sets" and Data::SortedSet::Shared::Strings (bundled). Scores must not be
    NaN. Linux-only. Requires 64-bit Perl.

METHODS
  Constructors
        my $z = Data::SortedSet::Shared->new($path, $max [, $mode]);
        my $z = Data::SortedSet::Shared->new(undef, $max);        # anonymous
        my $z = Data::SortedSet::Shared->new_memfd($name, $max);
        my $z = Data::SortedSet::Shared->new_from_fd($fd);

    $path is the backing file ("undef" for an anonymous mapping); $max is
    the maximum number of members. When reopening an existing file or memfd,
    the stored header wins and the caller's $max is ignored. Backing files
    are created with mode 0600 by default; pass an octal $mode (e.g. 0660)
    to opt into cross-user sharing. The mode applies only when the file is
    created (it is ignored when attaching an existing file); the exact mode
    is applied via "fchmod", so umask does not narrow it. "new_memfd"
    creates a Linux memfd (transferable via its "memfd" descriptor);
    "new_from_fd" reopens one in another process.

  String-keyed sets
        my $z = Data::SortedSet::Shared->new_strings(max => 1_000_000);
        $z->add("alice", 1500);
        my @top = $z->rev_range_by_rank(0, 9);    # ("alice", ...)

    "new_strings" returns a Data::SortedSet::Shared::Strings -- the same API
    as this class but with string members. Keys are interned to dense ids
    via Data::Intern::Shared (a prerequisite of this distribution), so the
    set is still shared across processes by id. Ties among equal scores
    break by interning id, not lexicographically. See
    Data::SortedSet::Shared::Strings for the full options ("set"/"keys"
    backing paths, "max_keys", "arena", "mode"), and the separate "wrap"
    constructor that adopts two existing objects.

  Mutators
        $z->add($member, $score);     # 1 new, 0 existing (score updated), undef if full
        $z->incr($member, $delta);    # add to the score (creating at $delta); returns new score
        $z->remove($member);          # true if removed, false if absent
        my $n = $z->add_many([ [$m1,$s1], [$m2,$s2], ... ]);   # bulk; returns count of new
        $z->clear;

    "add" inserts a new member or updates an existing member's score,
    returning 1 or 0 respectively, or "undef" if the pool is full and the
    member is new. $score may be any finite or infinite value but not NaN
    (croaks). "incr" creates an absent member at $delta (like Redis ZINCRBY)
    and croaks if the result would be NaN, or if the pool is full and the
    member is new.

    "add_many" applies a whole batch under a single lock; each row is an
    "[member, score]" arrayref, malformed or NaN-scored rows are skipped,
    and it stops at $max. It returns the number of members newly inserted,
    which can be fewer than the number of new rows if the pool fills
    mid-batch.

  Lookup and count
        $z->score($member);           # the score, or undef if absent
        $z->exists($member);
        $z->count;                    # number of members
        $z->rank($member);            # 0-based rank (lowest score = 0), or undef
        $z->rev_rank($member);        # rank from the top, or undef
        $z->count_in_score($min, $max);   # members with score in [min, max] (inclusive)

  Rank and range
        $z->at_rank($r);              # member at rank $r (negative counts from the end), or undef
        my @m = $z->range_by_rank($start, $stop);       # members in [start .. stop] by rank
        my @m = $z->rev_range_by_rank(0, 9);            # top 10 (highest scores first)
        my @m = $z->range_by_score($min, $max, %opts);  # members scored in [min, max], ascending
        my @m = $z->rev_range_by_score($max, $min, %opts);

    Rank indices are 0-based and may be negative (counting from the end,
    like Perl slices); "range_by_*" bounds are inclusive. "range_by_score" /
    "rev_range_by_score" accept "limit => $n" and "offset => $k" (a negative
    "offset" is treated as 0). All range and rank methods return members;
    pass "withscores => 1" for a flat "(member, score, ...)" list instead.

  Pop and peek
        my ($member, $score) = $z->pop_min;    # remove + return the lowest, or () if empty
        my ($member, $score) = $z->pop_max;
        my ($member, $score) = $z->peek_min;   # without removing
        my ($member, $score) = $z->peek_max;

  Iteration
        $z->each(sub { my ($member, $score) = @_; ... });

    "each" snapshots all members under the read lock, then invokes the
    callback once per member in score order after the lock is released, so
    the callback may safely call back into the set.

  Introspection and lifecycle
        $z->count; $z->max_entries; $z->stats;     # see STATS
        $z->path; $z->memfd; $z->sync; $z->unlink;     # or Class->unlink($path)
        $z->eventfd; $z->fileno; $z->notify; $z->eventfd_consume;

    "sync" flushes the mapping to its backing store and "unlink" removes the
    backing file (also callable as "Class->unlink($path)"). "path" returns
    the backing file path ("undef" for an anonymous or memfd-backed set) and
    "memfd" returns the descriptor of a "new_memfd" set (-1 otherwise). The
    eventfd methods let another process wait for updates: "eventfd" lazily
    creates an eventfd and returns its descriptor (croaks on failure;
    calling it again returns the same fd), "fileno" returns the current
    eventfd descriptor or -1, "notify" writes a wakeup (returning false if
    no eventfd is attached), and "eventfd_consume" reads and resets the
    counter, returning it as an integer or "undef" when nothing is pending.

SHARING ACROSS PROCESSES
    The set lives in a shared mapping, so several processes operate on the
    same data with no serialization layer in between. There are three ways
    to share it:

    *   A backing file -- every process calls "new($path, $max)" on the same
        path. The first to arrive creates and sizes the file (serialized by
        an exclusive lock); the rest map it.

    *   An anonymous mapping inherited across "fork" -- create with
        "new(undef, $max)" before forking; the parent and its children then
        share the one mapping.

    *   A memfd -- create with "new_memfd($name, $max)" and hand its "memfd"
        descriptor to an unrelated process (over a UNIX socket with
        "SCM_RIGHTS", or while the creator is alive via "/proc/$pid/fd/$n"),
        which reopens it with new_from_fd($fd).

        # children populate a fork-shared set; the parent reads the result
        my $z = Data::SortedSet::Shared->new(undef, 1_000_000);
        for my $k (1 .. 4) {
            unless (fork) {                                  # child
                $z->add($k * 1_000_000 + $_, rand) for 1 .. 1000;
                exit;
            }
        }
        1 while wait != -1;                                  # reap children
        print $z->count, "\n";                               # 4000

    Every operation is serialized by the rwlock, so concurrent writers do
    not corrupt the tree. A writer can wake readers blocked in other
    processes through the eventfd interface: it calls "notify" after a
    batch, and a reader selects on "fileno" then drains the count with
    "eventfd_consume".

COMPLEXITY
    "score"/"exists"/"peek_*" are O(1); "add"/"remove"/"incr"/"rank"/
    "at_rank"/"pop_*" and locating a range bound are O(log n); a range or
    iteration of "k" members is O(log n + k), scanning sequentially through
    the linked leaves.

STATS
    stats() returns a hashref with keys: "count", "max_entries", "height"
    (B+tree height), "node_capacity", "nodes_used", "index_slots",
    "index_load" (occupied fraction of the member index), "ops" (running
    count of write-path calls, whether or not they changed the set), and
    "mmap_size" (bytes).

SECURITY
    Backing files are created with mode 0600 (owner-only) by default, so
    only the creating user can open and attach them. To share a backing file
    across users, pass an explicit octal file mode such as 0660 as the last
    argument to "new"; the mode is applied only when the file is created (an
    existing file keeps its own permissions). The file is opened with
    "O_NOFOLLOW", so a symlink planted at the path is refused, and created
    with "O_EXCL"; the on-disk header is validated when the file is
    attached. Any process you grant write access to a shared mapping is
    trusted not to corrupt its contents while other processes are using it.

CRASH SAFETY
    The write lock is a futex-based rwlock with PID-encoded ownership; if a
    writer dies while holding it, the next writer detects the dead owner and
    recovers. Reader slots are reclaimed similarly. Recovery restores
    locking only, never tree consistency: a writer killed mid-mutation (a
    node split, underflow, or insert) can leave the B+tree structurally
    corrupt. Limitation: PID reuse is not detected, which is very unlikely
    in practice but cannot be ruled out.

    Reader-slot exhaustion (slotless readers): dead-process recovery
    attributes a crashed lock holder's contribution through its reader-slot.
    The slot table holds 1024 entries (one per concurrent reader process).
    If more than that many reader processes share one mapping at once, a
    reader that cannot claim a slot proceeds "slotless" -- it still takes
    the read lock but leaves no per-process record. If such a slotless
    reader is then killed while holding the read lock, its share of the lock
    cannot be attributed to a dead process, so writer recovery cannot
    reclaim it and writers may block until the mapping is recreated.
    Reaching this needs more than 1024 concurrent reader processes on one
    mapping plus a crash in the brief read-lock window; the dead-process
    slot reclaim keeps the table from filling with stale entries, so in
    practice it is very unlikely.

SEE ALSO
    Data::SortedSet::Shared::Strings (string-keyed variant, bundled with
    this distribution), Data::Intern::Shared, Data::SpatialHash::Shared,
    Data::HashMap::Shared, Data::Heap::Shared, Data::Graph::Shared, and the
    rest of the "Data::*::Shared" family.

AUTHOR
    vividsnow

LICENSE
    This is free software; you can redistribute it and/or modify it under
    the same terms as Perl itself.

