//! Audis is an implementation of a multi-index audit log,
//! built atop the Redis key-value store solution.
//!
//! An audit log consists of zero or more Event objects,
//! indexed against one or more subjects, each.
//! The correspondence between Redis databases and audit
//! logs is 1:1 -- each audit log inhabits precisely one
//! Redis instance, and an instance can only house a single
//! audit log.
//!
//! ## Example: Logging Events
//!
//! The simplest way to use audis is to point it at a Redis
//! instance and start logging to it:
//!
//! ```rust,no_run
//! extern crate audis;
//!
//! fn main() {
//!     let client = audis::Client::connect("redis://127.0.0.1:6379").unwrap();
//!
//!     client.log(&audis::Event{
//!         id: "foo1".to_string(),
//!         data: "{\"some\":\"data\"}".to_string(),
//!         subjects: vec![
//!             "system".to_string(),
//!             "user:42".to_string(),
//!         ],
//!     }).unwrap();
//!
//!     // ... etc ...
//! }
//! ```
//!
//! ## Retrieving The Audit Log
//!
//! What good is an audit log if you can't ever review it?
//!
//! ```rust,no_run
//! extern crate audis;
//!
//! fn main() {
//!     let client = audis::Client::connect("redis://127.0.0.1:6379").unwrap();
//!
//!     for subject in &client.subjects().unwrap() {
//!         println!("## {} ######################", subject);
//!         for event in &client.retrieve(subject).unwrap() {
//!             println!("  {}", event.data);
//!         }
//!         println!("");
//!     }
//! }
//! ```
//!
//! ## Distributed Audit Logging via Threads
//!
//! A common pattern with audis is to delegate a single thread
//! to the task of shunting event logs into Redis, allowing other
//! threads to focus on their work, without being slowed down by
//! momentary hiccups in the auditing layer.
//!
//! You can do this via the `background()` function, which returns
//! a buffered channel you can send events to, and the join handle
//! of the executing background thread:
//!
//! ```rust,no_run
//! extern crate audis;
//!
//! fn main() {
//!     let client = audis::Client::connect("redis://127.0.0.1:6379").unwrap();
//!
//!     // buffer 50 events
//!     let (tx, thread) = client.background(50).unwrap();
//!
//!     tx.send(audis::Event{
//!         id: "foo1".to_string(),
//!         data: "{\"some\":\"data\"}".to_string(),
//!         subjects: vec![
//!             "system".to_string(),
//!             "user:42".to_string(),
//!         ],
//!     }).unwrap();
//!
//!     // ... etc ...
//!
//!     thread.join().unwrap();
//! }
//! ```
//!
//! ## Implementation Details
//!
//! Audis uses four (4) types of objects A) the events
//! themselves, B) reference counts for inserted events,
//! C) per-subject lists of events, sorted in insertion-order,
//! and D) a master list of all known subjects.
//!
//! Each audit event is stored as an opaque blob, usually
//! JSON -- for the purposes of this library, the exact contents
//! of events is immaterial.  Each event gets its own Redis key,
//! derived from its globally unique ID, in the form `audit:$id`.
//! This makes retrieving events an _O(1)_ operation, using the
//! Redis `GET` command.
//!
//! Accompanying each event object is a reference count, kept
//! under a parallel keying structure that appends `:ref` to the
//! main event object key.  These reference counts are integers
//! that track how many different subjects are currently still
//! referencing the given event.  For example, `audit:ae2:ref`
//! is the reference count key for `audit:ae2`.
//!
//! Each subject in the audit log maintains its own list of
//! event IDs that are relevant to it.  These lists are stored
//! under keys derived from the subject itself.  Callers are
//! strongly urged to ensure that subject names are as unique
//! as they need to be for analysis.
//!
//! Finally, a single Redis Set, called `subjects`, exists to
//! track the complete set of known subject strings.  This
//! facilitates discovery of the different subsets of the audit
//! log.
//!
//! Here is some pseudocode for the insertion logic of the
//! `LOG(e)` operation, where `e` is an object:
//!
//! ```redis-pseudo-code
//! LOG(e):
//!     var id = $e[id]
//!     SETNX "audit:$id" $e[data]
//!     for s in $e[subjects]:
//!         SADD "subjects" "$s"
//!         RPUSH "$s" "$id"
//!         INCR "audit:$id:ref"
//! ```
//!
//! Technically speaking, `LOG(e)` runs in _O(n)_, linearly
//! to the number of subjects that the audit log event applies
//! to.  However, given that this `n` is usually very small
//! (almost always < 100), `LOG(e)` performs well.
//!
//! `RETR(s)` is straightforward: iterate over the subject list
//! in Redis via `LRANGE` and then `GET` the referenced event
//! objects:
//!
//! ```redis-pseudo-code
//! RETR(s):
//!     var log = []
//!     for id in LRANGE "$s" 0 -1:
//!         $log.append( GET "audit:$id" )
//!     return $log
//! ```
//!
//! Since `LOG(e)` only ever adds to our audit log dataset,
//! and `RETR(s)` is a read-only operation, our Redis footprint
//! will forever grow, unless we define operations to clear out
//! old log entries.  Deleting parts of our audit log seems
//! wrong and counter-productive -- the whole point is to know
//! what happened!  Storage (especially memory), however, isn't
//! unlimited, and for debugging purposes (at least), audit log
//! events become less relevant over time.
//!
//! For these reasons, we define two pruning operations:
//! `TRUNC(s,n)`, for truncating a subject eventset to the most
//! recent `n` audit events, and `PURGE(s,last)`, for deleting
//! a events from a subject eventset, until a given ID is found.
//! (That ID will also be removed, as well).
//!
//! These two operations allow us to define a cleanup routine,
//! outside of the audis library, which can do things like
//! render and persist audit events to a log file in a filesystem
//! or external blobstore (i.e. S3), before ultimately deleting
//! them from Redis.
//!
//! Here is the pseudo-code for `TRUNC(s,n)`:
//!
//! ```redis-pseudo-code
//! TRUNC(s,n):
//!     var end = 0 - n - 1
//!     for id in LRANGE "$s" 0 $end:
//!         LPOP "$s"
//!         DECR "audit:$id:ref"
//!         if GET "audit:$id:ref" <= 0:
//!             DEL "audit:$id:ref" "audit:$id"
//! ```
//!
//! As events are truncated from the subject's index, the
//! associated reference counts are checked to determine if any
//! larger cleanup (via `DEL`) needs to be performed.
//!
//! `PURGE(s,last)` is similar:
//!
//! ```redis-pseudo-code
//! PURGE(s,last):
//!     for id in LRANGE "$s" 0 -1:
//!         LPOP "$s"
//!         DECR "audit:$id:ref"
//!         if GET "audit:$id:ref" <= 0:
//!             DEL "audit:$id:ref" "audit:$id"
//!         if $id == $last
//!             break
//! ```
//!
//! Both of these operations suffer from massive problems
//! when run concurrently with each other, or with other
//! calls to themselves.  A future version of this library
//! will correct this, by the judicious use of `LOCK()`/`UNLOCK()`
//! primitives implemented inside of the same Redis database.
//!

use redis;
use std::sync::mpsc::{sync_channel, SyncSender};
use std::thread::{spawn, JoinHandle};

macro_rules! id {
    ($x:expr) => {
        format!("audit:{}", $x)
    };
}

macro_rules! idref {
    ($x:expr) => {
        format!("audit:{}:ref", $x)
    };
}

pub type AudisResult<T> = redis::RedisResult<T>;

/// A single Redis endpoint housing an audit log.
pub struct Client {
    url: String,
    redis: redis::Client,
}

/// An event, suitable for logging in the audit log.
pub struct Event {
    pub id: String,
    pub data: String,
    pub subjects: Vec<String>,
}

impl Client {
    /// Connect to a Redis instance, by URL.
    ///
    /// This implementation understands the same URL formats
    /// that the underlying `redis` crate understands.
    /// Primarily, this means the following should work:
    ///
    ///  - redis://127.0.0.1:6379
    ///  - redis://localhost
    ///  - unix:/path/to/redis.sock
    ///
    pub fn connect(url: &str) -> AudisResult<Client> {
        let c = Client {
            url: url.to_string(),
            redis: redis::Client::open(url)?,
        };
        match c.ping() {
            Ok(_) => Ok(c),
            Err(e) => Err(e),
        }
    }

    /// Delegate event logging to a background thread.
    ///
    /// This function spins up a new thread, with a copy of the
    /// audis Client object, and returns a channel for sending
    /// new audis::Event objects to be logged, and the thread
    /// JoinHandle for waiting on the thread to finish.
    ///
    /// The sending channel is buffered, and will have enough
    /// space to keep `n` Event objects in memory.  If `n` is
    /// passed as zero, a suitable default will be used instead.
    ///
    /// If the background thread encounters an error while trying
    /// to log an Event to the Redis backend, it will print out
    /// the error and attempt to recover.
    ///
    /// To shut down the background thread, drop the returned
    /// SyncSender<Event> object and then join the thread's
    /// JoinHandle.
    ///
    pub fn background(&self, n: usize) -> AudisResult<(SyncSender<Event>, JoinHandle<()>)> {
        let c = Client {
            url: self.url.to_string(),
            redis: redis::Client::open(self.url.as_str())?,
        };
        let (tx, rx) = sync_channel(if n == 0 { 100 } else { n });

        let t = spawn(move || {
            for e in rx {
                match c.log(&e) {
                    Err(err) => println!("audis failed to log event {}: {}", e.id, err),
                    Ok(_) => (),
                };
            }
        });

        Ok((tx, t))
    }

    /// Return the list of all known subjects.
    pub fn subjects(&self) -> AudisResult<Vec<String>> {
        self.smembers("subjects")
    }

    /// Log an event to the audit log.
    pub fn log(&self, e: &Event) -> AudisResult<&Client> {
        self.setnx(&id!(e.id), &e.data)?;
        for s in &e.subjects {
            self.sadd("subjects", s)?.rpush(s, &e.id)?.incr(&e.id)?;
        }
        Ok(self)
    }

    /// Retrieve the full list of events for the given subject.
    pub fn retrieve(&self, log: &str) -> AudisResult<Vec<Event>> {
        let mut events: Vec<Event> = vec![];
        for id in self.lrange(&log, "0", "-1")? {
            events.push(Event {
                id: String::from(&id),
                data: self.get(&id!(id))?,
                subjects: vec![],
            })
        }

        Ok(events)
    }

    /// Truncate a subject so that it only contains `n` Events.
    pub fn truncate(&self, log: &str, n: u32) -> AudisResult<&Client> {
        for id in self.lrange(&log, "0", &format!("-{}", n + 1))? {
            self.lpop(&log)?.deref(&id)?;
        }
        Ok(self)
    }

    /// Delete the Event `last` and all prior events from a given subject.
    pub fn purge(&self, log: &str, last: &str) -> AudisResult<&Client> {
        for id in self.lrange(&log, "0", "-1")? {
            self.lpop(&log)?.deref(&id)?;
            if id == last {
                break;
            }
        }

        Ok(self)
    }

    fn query<T: redis::FromRedisValue>(&self, cmd: &mut redis::Cmd) -> AudisResult<T> {
        cmd.query(&mut self.redis.get_connection()?)
    }

    fn ping(&self) -> AudisResult<&Client> {
        self.query(&mut redis::cmd("PING"))?;
        Ok(self)
    }

    fn lrange(&self, key: &str, a: &str, b: &str) -> AudisResult<Vec<String>> {
        self.query(redis::cmd("LRANGE").arg(key).arg(a).arg(b))
    }

    fn smembers(&self, key: &str) -> AudisResult<Vec<String>> {
        self.query(redis::cmd("SMEMBERS").arg(key))
    }

    fn rpush(&self, log: &str, id: &str) -> AudisResult<&Client> {
        self.query(redis::cmd("RPUSH").arg(log).arg(id))?;
        Ok(self)
    }

    fn lpop(&self, log: &str) -> AudisResult<&Client> {
        self.query(redis::cmd("LPOP").arg(log))?;
        Ok(self)
    }

    fn decr(&self, key: &str) -> AudisResult<&Client> {
        self.query(redis::cmd("DECR").arg(key))?;
        Ok(self)
    }

    fn incr(&self, key: &str) -> AudisResult<&Client> {
        self.query(redis::cmd("INCR").arg(key))?;
        Ok(self)
    }

    fn setnx(&self, key: &str, data: &str) -> AudisResult<&Client> {
        let s: i32 = self.query(redis::cmd("SETNX").arg(key).arg(data))?;
        if s == 1 {
            Ok(self)
        } else {
            Err(redis::RedisError::from((
                redis::ErrorKind::IoError,
                "duplicate key detected",
            )))
        }
    }

    fn sadd(&self, key: &str, data: &str) -> AudisResult<&Client> {
        self.query(redis::cmd("SADD").arg(key).arg(data))?;
        Ok(self)
    }

    fn get(&self, key: &str) -> AudisResult<String> {
        self.query(redis::cmd("GET").arg(key))
    }

    fn del(&self, id: &str) -> AudisResult<&Client> {
        self.query(redis::cmd("DEL").arg(id!(id)).arg(idref!(id)))?;
        Ok(self)
    }

    // Dereference (and possibly delete) an audit event.
    fn deref(&self, id: &str) -> AudisResult<&Client> {
        if self.decr(&idref!(id))?.get(&idref!(id))? == "0" {
            self.del(id)?;
        }
        Ok(self)
    }
}