use std::borrow::Cow;
use std::io::Write;
use std::sync::mpsc::Sender;
use std::sync::{Arc, Mutex};
use std::{cmp, fmt, fs, io};

#[cfg(feature = "syslog-4")]
use std::collections::HashMap;

use log::{self, Log};

use {log_impl, Filter, FormatCallback, Formatter};

#[cfg(feature = "syslog-3")]
use syslog_3;
#[cfg(feature = "syslog-4")]
use {Syslog4Rfc3164Logger, Syslog4Rfc5424Logger};

/// The base dispatch logger.
///
/// This allows for formatting log records, limiting what records can be passed
/// through, and then dispatching records to other dispatch loggers or output
/// loggers.
///
/// Note that all methods are position-insensitive.
/// `Dispatch::new().format(a).chain(b)` produces the exact same result
/// as `Dispatch::new().chain(b).format(a)`. Given this, it is preferred to put
/// 'format' and other modifiers before 'chain' for the sake of clarity.
///
/// Example usage demonstrating all features:
///
/// ```no_run
/// # // no_run because this creates log files.
/// #[macro_use]
/// extern crate log;
/// extern crate fern;
///
/// use std::{fs, io};
///
/// # fn setup_logger() -> Result<(), fern::InitError> {
/// fern::Dispatch::new()
///     .format(|out, message, record| {
///         out.finish(format_args!(
///             "[{}][{}] {}",
///             record.level(),
///             record.target(),
///             message,
///         ))
///     })
///     .chain(
///         fern::Dispatch::new()
///             // by default only accept warn messages
///             .level(log::LevelFilter::Warn)
///             // accept info messages from the current crate too
///             .level_for("my_crate", log::LevelFilter::Info)
///             // `io::Stdout`, `io::Stderr` and `io::File` can be directly passed in.
///             .chain(io::stdout()),
///     )
///     .chain(
///         fern::Dispatch::new()
///             // output all messages
///             .level(log::LevelFilter::Trace)
///             // except for hyper, in that case only show info messages
///             .level_for("hyper", log::LevelFilter::Info)
///             // `log_file(x)` equates to
///             // `OpenOptions::new().write(true).append(true).create(true).open(x)`
///             .chain(fern::log_file("persistent-log.log")?)
///             .chain(fs::OpenOptions::new()
///                 .write(true)
///                 .create(true)
///                 .truncate(true)
///                 .create(true)
///                 .open("/tmp/temp.log")?),
///     )
///     .chain(
///         fern::Dispatch::new()
///             .level(log::LevelFilter::Error)
///             .filter(|_meta_data| {
///                 // as an example, randomly reject half of the messages
///                 # /*
///                 rand::random()
///                 # */
///                 # true
///             })
///             .chain(io::stderr()),
///     )
///     // and finally, set as the global logger!
///     .apply()?;
/// # Ok(())
/// # }
/// #
/// # fn main() { setup_logger().expect("failed to set up logger") }
/// ```
#[must_use = "this is only a logger configuration and must be consumed with into_log() or apply()"]
pub struct Dispatch {
    format: Option<Box<Formatter>>,
    children: Vec<OutputInner>,
    default_level: log::LevelFilter,
    levels: Vec<(Cow<'static, str>, log::LevelFilter)>,
    filters: Vec<Box<Filter>>,
}

/// Logger which is usable as an output for multiple other loggers.
///
/// This struct contains a built logger stored in an [`Arc`], and can be
/// safely cloned.
///
/// See [`Dispatch::into_shared`].
///
/// [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
/// [`Dispatch::into_shared`]: struct.Dispatch.html#method.into_shared
#[derive(Clone)]
pub struct SharedDispatch {
    inner: Arc<log_impl::Dispatch>,
    min_level: log::LevelFilter,
}

impl Dispatch {
    /// Creates a dispatch, which will initially do nothing.
    #[inline]
    pub fn new() -> Self {
        Dispatch {
            format: None,
            children: Vec::new(),
            default_level: log::LevelFilter::Trace,
            levels: Vec::new(),
            filters: Vec::new(),
        }
    }

    /// Sets the formatter of this dispatch. The closure should accept a
    /// callback, a message and a log record, and write the resulting
    /// format to the writer.
    ///
    /// The log record is passed for completeness, but the `args()` method of
    /// the record should be ignored, and the [`fmt::Arguments`] given
    /// should be used instead. `record.args()` may be used to retrieve the
    /// _original_ log message, but in order to allow for true log
    /// chaining, formatters should use the given message instead whenever
    /// including the message in the output.
    ///
    /// To avoid all allocation of intermediate results, the formatter is
    /// "completed" by calling a callback, which then calls the rest of the
    /// logging chain with the new formatted message. The callback object keeps
    /// track of if it was called or not via a stack boolean as well, so if
    /// you don't use `out.finish` the log message will continue down
    /// the logger chain unformatted.
    ///
    /// [`fmt::Arguments`]: https://doc.rust-lang.org/std/fmt/struct.Arguments.html
    ///
    /// Example usage:
    ///
    /// ```
    /// fern::Dispatch::new().format(|out, message, record| {
    ///     out.finish(format_args!(
    ///         "[{}][{}] {}",
    ///         record.level(),
    ///         record.target(),
    ///         message
    ///     ))
    /// })
    ///     # .into_log();
    /// ```
    #[inline]
    pub fn format<F>(mut self, formatter: F) -> Self
    where
        F: Fn(FormatCallback, &fmt::Arguments, &log::Record) + Sync + Send + 'static,
    {
        self.format = Some(Box::new(formatter));
        self
    }

    /// Adds a child to this dispatch.
    ///
    /// All log records which pass all filters will be formatted and then sent
    /// to all child loggers in sequence.
    ///
    /// Note: If the child logger is also a Dispatch, and cannot accept any log
    /// records, it will be dropped. This only happens if the child either
    /// has no children itself, or has a minimum log level of
    /// [`LevelFilter::Off`].
    ///
    /// [`LevelFilter::Off`]: https://docs.rs/log/0.4/log/enum.LevelFilter.html#variant.Off
    ///
    /// Example usage:
    ///
    /// ```
    /// fern::Dispatch::new()
    ///     .chain(fern::Dispatch::new().chain(std::io::stdout()))
    ///     # .into_log();
    /// ```
    #[inline]
    pub fn chain<T: Into<Output>>(mut self, logger: T) -> Self {
        self.children.push(logger.into().0);
        self
    }

    /// Sets the overarching level filter for this logger. All messages not
    /// already filtered by something set by [`Dispatch::level_for`] will
    /// be affected.
    ///
    /// All messages filtered will be discarded if less severe than the given
    /// level.
    ///
    /// Default level is [`LevelFilter::Trace`].
    ///
    /// [`Dispatch::level_for`]: #method.level_for
    /// [`LevelFilter::Trace`]: https://docs.rs/log/0.4/log/enum.LevelFilter.html#variant.Trace
    ///
    /// Example usage:
    ///
    /// ```
    /// # extern crate log;
    /// # extern crate fern;
    /// #
    /// # fn main() {
    /// fern::Dispatch::new()
    ///     .level(log::LevelFilter::Info)
    ///     # .into_log();
    /// # }
    /// ```
    #[inline]
    pub fn level(mut self, level: log::LevelFilter) -> Self {
        self.default_level = level;
        self
    }

    /// Sets a per-target log level filter. Default target for log messages is
    /// `crate_name::module_name` or
    /// `crate_name` for logs in the crate root. Targets can also be set with
    /// `info!(target: "target-name", ...)`.
    ///
    /// For each log record fern will first try to match the most specific
    /// level_for, and then progressively more general ones until either a
    /// matching level is found, or the default level is used.
    ///
    /// For example, a log for the target `hyper::http::h1` will first test a
    /// level_for for `hyper::http::h1`, then for `hyper::http`, then for
    /// `hyper`, then use the default level.
    ///
    /// Examples:
    ///
    /// A program wants to include a lot of debugging output, but the library
    /// "hyper" is known to work well, so debug output from it should be
    /// excluded:
    ///
    /// ```
    /// # extern crate log;
    /// # extern crate fern;
    /// #
    /// # fn main() {
    /// fern::Dispatch::new()
    ///     .level(log::LevelFilter::Trace)
    ///     .level_for("hyper", log::LevelFilter::Info)
    ///     # .into_log();
    /// # }
    /// ```
    ///
    /// A program has a ton of debug output per-module, but there is so much
    /// that debugging more than one module at a time is not very useful.
    /// The command line accepts a list of modules to debug, while keeping the
    /// rest of the program at info level:
    ///
    /// ```
    /// # extern crate log;
    /// # extern crate fern;
    /// #
    /// fn setup_logging<T, I>(verbose_modules: T) -> Result<(), fern::InitError>
    /// where
    ///     I: AsRef<str>,
    ///     T: IntoIterator<Item = I>,
    /// {
    ///     let mut config = fern::Dispatch::new()
    ///         .level(log::LevelFilter::Info);
    ///
    ///     for module_name in verbose_modules {
    ///         config = config.level_for(
    ///             format!("my_crate_name::{}", module_name.as_ref()),
    ///             log::LevelFilter::Debug,
    ///         );
    ///     }
    ///
    ///     config.chain(std::io::stdout()).apply()?;
    ///
    ///     Ok(())
    /// }
    /// #
    /// # // we're ok with apply() failing.
    /// # fn main() { let _ = setup_logging(&["hi"]); }
    /// ```
    #[inline]
    pub fn level_for<T: Into<Cow<'static, str>>>(
        mut self,
        module: T,
        level: log::LevelFilter,
    ) -> Self {
        let module = module.into();

        if let Some((index, _)) = self
            .levels
            .iter()
            .enumerate()
            .find(|&(_, &(ref name, _))| name == &module)
        {
            self.levels.remove(index);
        }

        self.levels.push((module, level));
        self
    }

    /// Adds a custom filter which can reject messages passing through this
    /// logger.
    ///
    /// The logger will continue to process log records only if all filters
    /// return `true`.
    ///
    /// [`Dispatch::level`] and [`Dispatch::level_for`] are preferred if
    /// applicable.
    ///
    /// [`Dispatch::level`]: #method.level
    /// [`Dispatch::level_for`]: #method.level_for
    ///
    /// Example usage:
    ///
    /// This sends error level messages to stderr and others to stdout.
    ///
    /// ```
    /// # extern crate log;
    /// # extern crate fern;
    /// #
    /// # fn main() {
    /// fern::Dispatch::new()
    ///     .level(log::LevelFilter::Info)
    ///     .chain(
    ///         fern::Dispatch::new()
    ///             .filter(|metadata| {
    ///                 // Reject messages with the `Error` log level.
    ///                 metadata.level() != log::LevelFilter::Error
    ///             })
    ///             .chain(std::io::stderr()),
    ///     )
    ///     .chain(
    ///         fern::Dispatch::new()
    ///             .level(log::LevelFilter::Error)
    ///             .chain(std::io::stdout()),
    ///     )
    ///     # .into_log();
    /// # }
    #[inline]
    pub fn filter<F>(mut self, filter: F) -> Self
    where
        F: Fn(&log::Metadata) -> bool + Send + Sync + 'static,
    {
        self.filters.push(Box::new(filter));
        self
    }

    /// Builds this dispatch and stores it in a clonable structure containing
    /// an [`Arc`].
    ///
    /// Once "shared", the dispatch can be used as an output for multiple other
    /// dispatch loggers.
    ///
    /// Example usage:
    ///
    /// This separates info and warn messages, sending info to stdout + a log
    /// file, and warn to stderr + the same log file. Shared is used so the
    /// program only opens "file.log" once.
    ///
    /// ```no_run
    /// # extern crate log;
    /// # extern crate fern;
    /// #
    /// # fn setup_logger() -> Result<(), fern::InitError> {
    ///
    /// let file_out = fern::Dispatch::new()
    ///     .chain(fern::log_file("file.log")?)
    ///     .into_shared();
    ///
    /// let info_out = fern::Dispatch::new()
    ///     .level(log::LevelFilter::Debug)
    ///     .filter(|metadata|
    ///         // keep only info and debug (reject warn and error)
    ///         metadata.level() <= log::Level::Info)
    ///     .chain(std::io::stdout())
    ///     .chain(file_out.clone());
    ///
    /// let warn_out = fern::Dispatch::new()
    ///     .level(log::LevelFilter::Warn)
    ///     .chain(std::io::stderr())
    ///     .chain(file_out);
    ///
    /// fern::Dispatch::new()
    ///     .chain(info_out)
    ///     .chain(warn_out)
    ///     .apply();
    ///
    /// # Ok(())
    /// # }
    /// #
    /// # fn main() { setup_logger().expect("failed to set up logger"); }
    /// ```
    ///
    /// [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
    pub fn into_shared(self) -> SharedDispatch {
        let (min_level, dispatch) = self.into_dispatch();

        SharedDispatch {
            inner: Arc::new(dispatch),
            min_level: min_level,
        }
    }

    /// Builds this into the actual logger implementation.
    ///
    /// This could probably be refactored, but having everything in one place
    /// is also nice.
    fn into_dispatch(self) -> (log::LevelFilter, log_impl::Dispatch) {
        let Dispatch {
            format,
            children,
            default_level,
            levels,
            mut filters,
        } = self;

        let mut max_child_level = log::LevelFilter::Off;

        let output = children
            .into_iter()
            .flat_map(|child| match child {
                OutputInner::Stdout { stream, line_sep } => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Stdout(log_impl::Stdout {
                        stream: stream,
                        line_sep: line_sep,
                    }))
                }
                OutputInner::Stderr { stream, line_sep } => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Stderr(log_impl::Stderr {
                        stream: stream,
                        line_sep: line_sep,
                    }))
                }
                OutputInner::File { stream, line_sep } => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::File(log_impl::File {
                        stream: Mutex::new(io::BufWriter::new(stream)),
                        line_sep: line_sep,
                    }))
                }
                OutputInner::Writer { stream, line_sep } => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Writer(log_impl::Writer {
                        stream: Mutex::new(stream),
                        line_sep: line_sep,
                    }))
                }
                OutputInner::Sender { stream, line_sep } => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Sender(log_impl::Sender {
                        stream: Mutex::new(stream),
                        line_sep: line_sep,
                    }))
                }
                #[cfg(feature = "syslog-3")]
                OutputInner::Syslog3(log) => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Syslog3(log_impl::Syslog3 { inner: log }))
                }
                #[cfg(feature = "syslog-4")]
                OutputInner::Syslog4Rfc3164(logger) => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Syslog4Rfc3164(log_impl::Syslog4Rfc3164 {
                        inner: Mutex::new(logger),
                    }))
                }
                #[cfg(feature = "syslog-4")]
                OutputInner::Syslog4Rfc5424 { logger, transform } => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Syslog4Rfc5424(log_impl::Syslog4Rfc5424 {
                        inner: Mutex::new(logger),
                        transform,
                    }))
                }
                OutputInner::Panic => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::Panic(log_impl::Panic))
                }
                OutputInner::Dispatch(child_dispatch) => {
                    let (child_level, child) = child_dispatch.into_dispatch();
                    if child_level > log::LevelFilter::Off {
                        max_child_level = cmp::max(max_child_level, child_level);
                        Some(log_impl::Output::Dispatch(child))
                    } else {
                        None
                    }
                }
                OutputInner::SharedDispatch(child_dispatch) => {
                    let SharedDispatch {
                        inner: child,
                        min_level: child_level,
                    } = child_dispatch;

                    if child_level > log::LevelFilter::Off {
                        max_child_level = cmp::max(max_child_level, child_level);
                        Some(log_impl::Output::SharedDispatch(child))
                    } else {
                        None
                    }
                }
                OutputInner::OtherBoxed(child_log) => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::OtherBoxed(child_log))
                }
                OutputInner::OtherStatic(child_log) => {
                    max_child_level = log::LevelFilter::Trace;
                    Some(log_impl::Output::OtherStatic(child_log))
                }
            })
            .collect();

        let min_level = levels
            .iter()
            .map(|t| t.1)
            .max()
            .map_or(default_level, |lvl| cmp::max(lvl, default_level));
        let real_min = cmp::min(min_level, max_child_level);

        filters.shrink_to_fit();

        let dispatch = log_impl::Dispatch {
            output: output,
            default_level: default_level,
            levels: levels.into(),
            format: format,
            filters: filters,
        };

        (real_min, dispatch)
    }

    /// Builds this logger into a `Box<log::Log>` and calculates the minimum
    /// log level needed to have any effect.
    ///
    /// While this method is exposed publicly, [`Dispatch::apply`] is typically
    /// used instead.
    ///
    /// The returned LevelFilter is a calculation for all level filters of this
    /// logger and child loggers, and is the minimum log level needed to
    /// for a record to have any chance of passing through this logger.
    ///
    /// [`Dispatch::apply`]: #method.apply
    ///
    /// Example usage:
    ///
    /// ```
    /// # extern crate log;
    /// # extern crate fern;
    /// #
    /// # fn main() {
    /// let (min_level, log) = fern::Dispatch::new()
    ///     .level(log::LevelFilter::Info)
    ///     .chain(std::io::stdout())
    ///     .into_log();
    ///
    /// assert_eq!(min_level, log::LevelFilter::Info);
    /// # }
    /// ```
    pub fn into_log(self) -> (log::LevelFilter, Box<log::Log>) {
        let (level, logger) = self.into_dispatch();
        if level == log::LevelFilter::Off {
            (level, Box::new(log_impl::Null))
        } else {
            (level, Box::new(logger))
        }
    }

    /// Builds this logger and instantiates it as the global [`log`] logger.
    ///
    /// # Errors:
    ///
    /// This function will return an error if a global logger has already been
    /// set to a previous logger.
    ///
    /// [`log`]: https://github.com/rust-lang-nursery/log
    pub fn apply(self) -> Result<(), log::SetLoggerError> {
        let (max_level, log) = self.into_log();

        log::set_boxed_logger(log)?;
        log::set_max_level(max_level);

        Ok(())
    }
}

/// This enum contains various outputs that you can send messages to.
enum OutputInner {
    /// Prints all messages to stdout with `line_sep` separator.
    Stdout {
        stream: io::Stdout,
        line_sep: Cow<'static, str>,
    },
    /// Prints all messages to stderr with `line_sep` separator.
    Stderr {
        stream: io::Stderr,
        line_sep: Cow<'static, str>,
    },
    /// Writes all messages to file with `line_sep` separator.
    File {
        stream: fs::File,
        line_sep: Cow<'static, str>,
    },
    /// Writes all messages to the writer with `line_sep` separator.
    Writer {
        stream: Box<Write + Send>,
        line_sep: Cow<'static, str>,
    },
    /// Writes all messages to mpst::Sender with `line_sep` separator.
    Sender {
        stream: Sender<String>,
        line_sep: Cow<'static, str>,
    },
    /// Passes all messages to other dispatch.
    Dispatch(Dispatch),
    /// Passes all messages to other dispatch that's shared.
    SharedDispatch(SharedDispatch),
    /// Passes all messages to other logger.
    OtherBoxed(Box<Log>),
    /// Passes all messages to other logger.
    OtherStatic(&'static Log),
    /// Passes all messages to the syslog.
    #[cfg(feature = "syslog-3")]
    Syslog3(syslog_3::Logger),
    /// Passes all messages to the syslog.
    #[cfg(feature = "syslog-4")]
    Syslog4Rfc3164(Syslog4Rfc3164Logger),
    /// Sends all messages through the transform then passes to the syslog.
    #[cfg(feature = "syslog-4")]
    Syslog4Rfc5424 {
        logger: Syslog4Rfc5424Logger,
        transform: Box<
            Fn(&log::Record) -> (i32, HashMap<String, HashMap<String, String>>, String)
                + Sync
                + Send,
        >,
    },
    /// Panics with messages text for all messages.
    Panic,
}

/// Logger which will panic whenever anything is logged. The panic
/// will be exactly the message of the log.
///
/// `Panic` is useful primarily as a secondary logger, filtered by warning or
/// error.
///
/// # Examples
///
/// This configuration will output all messages to stdout and panic if an Error
/// message is sent.
///
/// ```
/// # extern crate fern;
/// # extern crate log;
/// fern::Dispatch::new()
///     // format, etc.
///     .chain(std::io::stdout())
///     .chain(
///         fern::Dispatch::new()
///             .level(log::LevelFilter::Error)
///             .chain(fern::Panic)
///     )
///     # /*
///     .apply()?;
///     # */ .into_log();
/// ```
///
/// This sets up a "panic on warn+" logger, and ignores errors so it can be
/// called multiple times.
///
/// This might be useful in test setup, for example, to disallow warn-level
/// messages.
///
/// ```no_run
/// # extern crate fern;
/// # extern crate log;
/// fn setup_panic_logging() {
///     fern::Dispatch::new()
///         .level(log::LevelFilter::Warn)
///         .chain(fern::Panic)
///         .apply()
///         // ignore errors from setting up logging twice
///         .ok();
/// }
/// ```
pub struct Panic;

/// Configuration for a logger output.
pub struct Output(OutputInner);

impl From<Dispatch> for Output {
    /// Creates an output logger forwarding all messages to the dispatch.
    fn from(log: Dispatch) -> Self {
        Output(OutputInner::Dispatch(log))
    }
}

impl From<SharedDispatch> for Output {
    /// Creates an output logger forwarding all messages to the dispatch.
    fn from(log: SharedDispatch) -> Self {
        Output(OutputInner::SharedDispatch(log))
    }
}

impl From<Box<Log>> for Output {
    /// Creates an output logger forwarding all messages to the custom logger.
    fn from(log: Box<Log>) -> Self {
        Output(OutputInner::OtherBoxed(log))
    }
}

impl From<&'static Log> for Output {
    /// Creates an output logger forwarding all messages to the custom logger.
    fn from(log: &'static Log) -> Self {
        Output(OutputInner::OtherStatic(log))
    }
}

impl From<fs::File> for Output {
    /// Creates an output logger which writes all messages to the file with
    /// `\n` as the separator.
    ///
    /// File writes are buffered and flushed once per log record.
    fn from(file: fs::File) -> Self {
        Output(OutputInner::File {
            stream: file,
            line_sep: "\n".into(),
        })
    }
}

impl From<Box<Write + Send>> for Output {
    /// Creates an output logger which writes all messages to the writer with
    /// `\n` as the separator.
    ///
    /// This does no buffering and it is up to the writer to do buffering as
    /// needed (eg. wrap it in `BufWriter`). However, flush is called after
    /// each log record.
    fn from(writer: Box<Write + Send>) -> Self {
        Output(OutputInner::Writer {
            stream: writer,
            line_sep: "\n".into(),
        })
    }
}

impl From<io::Stdout> for Output {
    /// Creates an output logger which writes all messages to stdout with the
    /// given handle and `\n` as the separator.
    fn from(stream: io::Stdout) -> Self {
        Output(OutputInner::Stdout {
            stream: stream,
            line_sep: "\n".into(),
        })
    }
}

impl From<io::Stderr> for Output {
    /// Creates an output logger which writes all messages to stderr with the
    /// given handle and `\n` as the separator.
    fn from(stream: io::Stderr) -> Self {
        Output(OutputInner::Stderr {
            stream: stream,
            line_sep: "\n".into(),
        })
    }
}

impl From<Sender<String>> for Output {
    /// Creates an output logger which writes all messages to the given
    /// mpsc::Sender with  '\n' as the separator.
    ///
    /// All messages sent to the mpsc channel are suffixed with '\n'.
    fn from(stream: Sender<String>) -> Self {
        Output(OutputInner::Sender {
            stream: stream,
            line_sep: "\n".into(),
        })
    }
}

#[cfg(feature = "syslog-3")]
impl From<syslog_3::Logger> for Output {
    /// Creates an output logger which writes all messages to the given syslog
    /// output.
    ///
    /// Log levels are translated trace => debug, debug => debug, info =>
    /// informational, warn => warning, and error => error.
    ///
    /// This requires the `"syslog-3"` feature.
    fn from(log: syslog_3::Logger) -> Self {
        Output(OutputInner::Syslog3(log))
    }
}

#[cfg(feature = "syslog-3")]
impl From<Box<syslog_3::Logger>> for Output {
    /// Creates an output logger which writes all messages to the given syslog
    /// output.
    ///
    /// Log levels are translated trace => debug, debug => debug, info =>
    /// informational, warn => warning, and error => error.
    ///
    /// Note that while this takes a Box<Logger> for convenience (syslog
    /// methods return Boxes), it will be immediately unboxed upon storage
    /// in the configuration structure. This will create a configuration
    /// identical to that created by passing a raw `syslog::Logger`.
    ///
    /// This requires the `"syslog-3"` feature.
    fn from(log: Box<syslog_3::Logger>) -> Self {
        Output(OutputInner::Syslog3(*log))
    }
}

#[cfg(feature = "syslog-4")]
impl From<Syslog4Rfc3164Logger> for Output {
    /// Creates an output logger which writes all messages to the given syslog.
    ///
    /// Log levels are translated trace => debug, debug => debug, info =>
    /// informational, warn => warning, and error => error.
    ///
    /// Note that due to https://github.com/Geal/rust-syslog/issues/41,
    /// logging to this backend requires one allocation per log call.
    ///
    /// This is for RFC 3164 loggers. To use an RFC 5424 logger, use the
    /// [`Output::syslog_5424`] helper method.
    ///
    /// This requires the `"syslog-4"` feature.
    fn from(log: Syslog4Rfc3164Logger) -> Self {
        Output(OutputInner::Syslog4Rfc3164(log))
    }
}

impl From<Panic> for Output {
    /// Creates an output logger which will panic with message text for all
    /// messages.
    fn from(_: Panic) -> Self {
        Output(OutputInner::Panic)
    }
}

impl Output {
    /// Returns a file logger using a custom separator.
    ///
    /// If the default separator of `\n` is acceptable, an [`fs::File`]
    /// instance can be passed into [`Dispatch::chain`] directly.
    ///
    /// ```no_run
    /// # fn setup_logger() -> Result<(), fern::InitError> {
    /// fern::Dispatch::new()
    ///     .chain(std::fs::File::create("log")?)
    ///     # .into_log();
    /// # Ok(())
    /// # }
    /// #
    /// # fn main() { setup_logger().expect("failed to set up logger"); }
    /// ```
    ///
    /// ```no_run
    /// # fn setup_logger() -> Result<(), fern::InitError> {
    /// fern::Dispatch::new()
    ///     .chain(fern::log_file("log")?)
    ///     # .into_log();
    /// # Ok(())
    /// # }
    /// #
    /// # fn main() { setup_logger().expect("failed to set up logger"); }
    /// ```
    ///
    /// Example usage (using [`fern::log_file`]):
    ///
    /// ```no_run
    /// # fn setup_logger() -> Result<(), fern::InitError> {
    /// fern::Dispatch::new()
    ///     .chain(fern::Output::file(fern::log_file("log")?, "\r\n"))
    ///     # .into_log();
    /// # Ok(())
    /// # }
    /// #
    /// # fn main() { setup_logger().expect("failed to set up logger"); }
    /// ```
    ///
    /// [`fs::File`]: https://doc.rust-lang.org/std/fs/struct.File.html
    /// [`Dispatch::chain`]: struct.Dispatch.html#method.chain
    /// [`fern::log_file`]: fn.log_file.html
    pub fn file<T: Into<Cow<'static, str>>>(file: fs::File, line_sep: T) -> Self {
        Output(OutputInner::File {
            stream: file,
            line_sep: line_sep.into(),
        })
    }

    /// Returns a logger using arbitrary write object and custom separator.
    ///
    /// If the default separator of `\n` is acceptable, an `Box<Write + Send>`
    /// instance can be passed into [`Dispatch::chain`] directly.
    ///
    /// ```no_run
    /// # fn setup_logger() -> Result<(), fern::InitError> {
    /// // Anything implementing 'Write' works.
    /// let mut writer = std::io::Cursor::new(Vec::<u8>::new());
    ///
    /// fern::Dispatch::new()
    ///     // as long as we explicitly cast into a type-erased Box
    ///     .chain(Box::new(writer) as Box<std::io::Write + Send>)
    ///     # .into_log();
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() { setup_logger().expect("failed to set up logger"); }
    /// ```
    ///
    /// Example usage:
    ///
    /// ```no_run
    /// # fn setup_logger() -> Result<(), fern::InitError> {
    /// let writer = Box::new(std::io::Cursor::new(Vec::<u8>::new()));
    ///
    /// fern::Dispatch::new()
    ///     .chain(fern::Output::writer(writer, "\r\n"))
    ///     # .into_log();
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() { setup_logger().expect("failed to set up logger"); }
    /// ```
    ///
    /// [`Dispatch::chain`]: struct.Dispatch.html#method.chain
    pub fn writer<T: Into<Cow<'static, str>>>(writer: Box<Write + Send>, line_sep: T) -> Self {
        Output(OutputInner::Writer {
            stream: writer,
            line_sep: line_sep.into(),
        })
    }

    /// Returns an stdout logger using a custom separator.
    ///
    /// If the default separator of `\n` is acceptable, an `io::Stdout`
    /// instance can be passed into `Dispatch::chain()` directly.
    ///
    /// ```
    /// fern::Dispatch::new()
    ///     .chain(std::io::stdout())
    ///     # .into_log();
    /// ```
    ///
    /// Example usage:
    ///
    /// ```
    /// fern::Dispatch::new()
    ///     // some unix tools use null bytes as message terminators so
    ///     // newlines in messages can be treated differently.
    ///     .chain(fern::Output::stdout("\0"))
    ///     # .into_log();
    /// ```
    pub fn stdout<T: Into<Cow<'static, str>>>(line_sep: T) -> Self {
        Output(OutputInner::Stdout {
            stream: io::stdout(),
            line_sep: line_sep.into(),
        })
    }

    /// Returns an stderr logger using a custom separator.
    ///
    /// If the default separator of `\n` is acceptable, an `io::Stderr`
    /// instance can be passed into `Dispatch::chain()` directly.
    ///
    /// ```
    /// fern::Dispatch::new()
    ///     .chain(std::io::stderr())
    ///     # .into_log();
    /// ```
    ///
    /// Example usage:
    ///
    /// ```
    /// fern::Dispatch::new()
    ///     .chain(fern::Output::stderr("\n\n\n"))
    ///     # .into_log();
    /// ```
    pub fn stderr<T: Into<Cow<'static, str>>>(line_sep: T) -> Self {
        Output(OutputInner::Stderr {
            stream: io::stderr(),
            line_sep: line_sep.into(),
        })
    }

    /// Returns a mpsc::Sender logger using a custom separator.
    ///
    /// If the default separator of `\n` is acceptable, an
    /// `mpsc::Sender<String>` instance can be passed into `Dispatch::
    /// chain()` directly.
    ///
    /// Each log message will be suffixed with the separator, then sent as a
    /// single String to the given sender.
    ///
    /// ```
    /// use std::sync::mpsc::channel;
    ///
    /// let (tx, rx) = channel();
    /// fern::Dispatch::new()
    ///     .chain(tx)
    ///     # .into_log();
    /// ```
    pub fn sender<T: Into<Cow<'static, str>>>(sender: Sender<String>, line_sep: T) -> Self {
        Output(OutputInner::Sender {
            stream: sender,
            line_sep: line_sep.into(),
        })
    }

    /// Returns a logger which logs into an RFC5424 syslog.
    ///
    /// This method takes an additional transform method to turn the log data
    /// into RFC5424 data.
    ///
    /// I've honestly got no clue what the expected keys and values are for
    /// this kind of logging, so I'm just going to link [the rfc] instead.
    ///
    /// If you're an expert on syslog logging and would like to contribute
    /// an example to put here, it would be gladly accepted!
    ///
    /// This requires the `"syslog-4"` feature.
    ///
    /// [the rfc]: https://tools.ietf.org/html/rfc5424
    #[cfg(feature = "syslog-4")]
    pub fn syslog_5424<F>(logger: Syslog4Rfc5424Logger, transform: F) -> Self
    where
        F: Fn(&log::Record) -> (i32, HashMap<String, HashMap<String, String>>, String)
            + Sync
            + Send
            + 'static,
    {
        Output(OutputInner::Syslog4Rfc5424 {
            logger,
            transform: Box::new(transform),
        })
    }

    /// Returns a logger which simply calls the given function with each message.
    ///
    /// The function will be called inline in the thread the log occurs on.
    ///
    /// Example usage:
    ///
    /// ```
    /// fern::Dispatch::new()
    ///     .chain(fern::Output::call(|record| {
    ///         // this is mundane, but you can do anything here.
    ///         println!("{}", record.args());
    ///     }))
    ///     # .into_log();
    /// ```
    pub fn call<F>(func: F) -> Self
    where
        F: Fn(&log::Record) + Sync + Send + 'static,
    {
        struct CallShim<F>(F);
        impl<F> log::Log for CallShim<F>
        where
            F: Fn(&log::Record) + Sync + Send + 'static,
        {
            fn enabled(&self, _: &log::Metadata) -> bool {
                true
            }
            fn log(&self, record: &log::Record) {
                (self.0)(record)
            }
            fn flush(&self) {}
        }

        Self::from(Box::new(CallShim(func)) as Box<log::Log>)
    }
}

impl Default for Dispatch {
    /// Returns a logger configuration that does nothing with log records.
    ///
    /// Equivalent to [`Dispatch::new`].
    ///
    /// [`Dispatch::new`]: #method.new
    fn default() -> Self {
        Self::new()
    }
}

impl fmt::Debug for Dispatch {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        struct LevelsDebug<'a>(&'a [(Cow<'static, str>, log::LevelFilter)]);
        impl<'a> fmt::Debug for LevelsDebug<'a> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                f.debug_map()
                    .entries(self.0.iter().map(|t| (t.0.as_ref(), t.1)))
                    .finish()
            }
        }
        struct FiltersDebug<'a>(&'a [Box<Filter>]);
        impl<'a> fmt::Debug for FiltersDebug<'a> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                f.debug_list()
                    .entries(self.0.iter().map(|_| "<filter closure>"))
                    .finish()
            }
        }
        f.debug_struct("Dispatch")
            .field(
                "format",
                &self.format.as_ref().map(|_| "<formatter closure>"),
            )
            .field("children", &self.children)
            .field("default_level", &self.default_level)
            .field("levels", &LevelsDebug(&self.levels))
            .field("filters", &FiltersDebug(&self.filters))
            .finish()
    }
}

impl fmt::Debug for OutputInner {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            OutputInner::Stdout {
                ref stream,
                ref line_sep,
            } => f
                .debug_struct("Output::Stdout")
                .field("stream", stream)
                .field("line_sep", line_sep)
                .finish(),
            OutputInner::Stderr {
                ref stream,
                ref line_sep,
            } => f
                .debug_struct("Output::Stderr")
                .field("stream", stream)
                .field("line_sep", line_sep)
                .finish(),
            OutputInner::File {
                ref stream,
                ref line_sep,
            } => f
                .debug_struct("Output::File")
                .field("stream", stream)
                .field("line_sep", line_sep)
                .finish(),
            OutputInner::Writer { ref line_sep, .. } => f
                .debug_struct("Output::Writer")
                .field("stream", &"<unknown writer>")
                .field("line_sep", line_sep)
                .finish(),
            OutputInner::Sender {
                ref stream,
                ref line_sep,
            } => f
                .debug_struct("Output::Sender")
                .field("stream", stream)
                .field("line_sep", line_sep)
                .finish(),
            #[cfg(feature = "syslog-3")]
            OutputInner::Syslog3(_) => f
                .debug_tuple("Output::Syslog3")
                .field(&"<unprintable syslog::Logger>")
                .finish(),
            #[cfg(feature = "syslog-4")]
            OutputInner::Syslog4Rfc3164 { .. } => f
                .debug_tuple("Output::Syslog4Rfc3164")
                .field(&"<unprintable syslog::Logger>")
                .finish(),
            #[cfg(feature = "syslog-4")]
            OutputInner::Syslog4Rfc5424 { .. } => f
                .debug_tuple("Output::Syslog4Rfc5424")
                .field(&"<unprintable syslog::Logger>")
                .finish(),
            OutputInner::Dispatch(ref dispatch) => {
                f.debug_tuple("Output::Dispatch").field(dispatch).finish()
            }
            OutputInner::SharedDispatch(_) => f
                .debug_tuple("Output::SharedDispatch")
                .field(&"<built Dispatch logger>")
                .finish(),
            OutputInner::OtherBoxed { .. } => f
                .debug_tuple("Output::OtherBoxed")
                .field(&"<boxed logger>")
                .finish(),
            OutputInner::OtherStatic { .. } => f
                .debug_tuple("Output::OtherStatic")
                .field(&"<boxed logger>")
                .finish(),
            OutputInner::Panic => f.debug_tuple("Output::Panic").finish(),
        }
    }
}

impl fmt::Debug for Output {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.0.fmt(f)
    }
}