use smallvec::SmallVec;

use crate::{

    parse,

    parse::{Event, Section},

};

/// A type store without allocation all events that are typically preceding the first section.

pub type FrontMatterEvents<'a> = SmallVec<[Event<'a>; 8]>;

/// A zero-copy `git-config` file parser.

///

/// This is parser exposes low-level syntactic events from a `git-config` file.

/// Generally speaking, you'll want to use [`File`] as it wraps

/// around the parser to provide a higher-level abstraction to a `git-config`

/// file, including querying, modifying, and updating values.

///

/// This parser guarantees that the events emitted are sufficient to

/// reconstruct a `git-config` file identical to the source `git-config`

/// when writing it.

///

/// # Differences between a `.ini` parser

///

/// While the `git-config` format closely resembles the [`.ini` file format],

/// there are subtle differences that make them incompatible. For one, the file

/// format is not well defined, and there exists no formal specification to

/// adhere to.

///

/// For concrete examples, some notable differences are:

/// - `git-config` sections permit subsections via either a quoted string

///   (`[some-section "subsection"]`) or via the deprecated dot notation

///   (`[some-section.subsection]`). Successful parsing these section names is not

///   well defined in typical `.ini` parsers. This parser will handle these cases

///   perfectly.

/// - Comment markers are not strictly defined either. This parser will always

///   and only handle a semicolon or octothorpe (also known as a hash or number

///   sign).

/// - Global properties may be allowed in `.ini` parsers, but is strictly

///   disallowed by this parser.

/// - Only `\t`, `\n`, `\b` `\\` are valid escape characters.

/// - Quoted and semi-quoted values will be parsed (but quotes will be included

///   in event outputs). An example of a semi-quoted value is `5"hello world"`,

///   which should be interpreted as `5hello world` after

///   [normalization][crate::value::normalize()].

/// - Line continuations via a `\` character is supported (inside or outside of quotes)

/// - Whitespace handling similarly follows the `git-config` specification as

///   closely as possible, where excess whitespace after a non-quoted value are

///   trimmed, and line continuations onto a new line with excess spaces are kept.

/// - Only equal signs (optionally padded by spaces) are valid name/value

///   delimiters.

///

/// Note that things such as case-sensitivity or duplicate sections are

/// _not_ handled. This parser is a low level _syntactic_ interpreter

/// and higher level wrappers around this parser, which may

/// or may not be zero-copy, should handle _semantic_ values. This also means

/// that string-like values are not interpreted. For example, `hello"world"`

/// would be read at a high level as `helloworld` but this parser will return

/// the former instead, with the extra quotes. This is because it is not the

/// responsibility of the parser to interpret these values, and doing so would

/// necessarily require a copy, which this parser avoids.

///

/// # Trait Implementations

///

/// - This struct does _not_ implement [`FromStr`] due to lifetime

///   constraints implied on the required `from_str` method. Instead, it provides

///   [`From<&'_ str>`].

///

/// # Idioms

///

/// If you do want to use this parser, there are some idioms that may help you

/// with interpreting sequences of events.

///

/// ## `Value` events do not immediately follow `Key` events

///

/// Consider the following `git-config` example:

///

/// ```text

/// [core]

///   autocrlf = input

/// ```

///

/// Because this parser guarantees perfect reconstruction, there are many

/// non-significant events that occur in addition to the ones you may expect:

///

/// ```

/// # use gix_config::parse::{Event, Events, section};

/// # use std::borrow::Cow;

/// # use std::convert::TryFrom;

/// # let section_header = section::Header::new("core", None).unwrap();

/// # let section_data = "[core]\n  autocrlf = input";

/// # assert_eq!(Events::from_str(section_data).unwrap().into_vec(), vec![

/// Event::SectionHeader(section_header),

/// Event::Newline(Cow::Borrowed("\n".into())),

/// Event::Whitespace(Cow::Borrowed("  ".into())),

/// Event::SectionValueName(section::ValueName::try_from("autocrlf")?),

/// Event::Whitespace(Cow::Borrowed(" ".into())),

/// Event::KeyValueSeparator,

/// Event::Whitespace(Cow::Borrowed(" ".into())),

/// Event::Value(Cow::Borrowed("input".into())),

/// # ]);

/// # Ok::<_, Box<dyn std::error::Error>>(())

/// ```

///

/// Note the two whitespace events between the key and value pair! Those two

/// events actually refer to the whitespace between the name and value and the

/// equal sign. So if the config instead had `autocrlf=input`, those whitespace

/// events would no longer be present.

///

/// ## `KeyValueSeparator` event is not guaranteed to emit

///

/// Consider the following `git-config` example:

///

/// ```text

/// [core]

///   autocrlf

/// ```

///

/// This is a valid config with a `autocrlf` key having an implicit `true`

/// value. This means that there is not a `=` separating the key and value,

/// which means that the corresponding event won't appear either:

///

/// ```

/// # use gix_config::parse::{Event, Events, section};

/// # use std::borrow::Cow;

/// # use std::convert::TryFrom;

/// # let section_header = section::Header::new("core", None).unwrap();

/// # let section_data = "[core]\n  autocrlf";

/// # assert_eq!(Events::from_str(section_data).unwrap().into_vec(), vec![

/// Event::SectionHeader(section_header),

/// Event::Newline(Cow::Borrowed("\n".into())),

/// Event::Whitespace(Cow::Borrowed("  ".into())),

/// Event::SectionValueName(section::ValueName::try_from("autocrlf")?),

/// Event::Value(Cow::Borrowed("".into())),

/// # ]);

/// # Ok::<_, Box<dyn std::error::Error>>(())

/// ```

///

/// ## Quoted values are not unquoted

///

/// Consider the following `git-config` example:

///

/// ```text

/// [core]

/// autocrlf=true""

/// filemode=fa"lse"

/// ```

///

/// Both these events, when fully processed, should normally be `true` and

/// `false`. However, because this parser is zero-copy, we cannot process

/// partially quoted values, such as the `false` example. As a result, to

/// maintain consistency, the parser will just take all values as literals. The

/// relevant event stream emitted is thus emitted as:

///

/// ```

/// # use gix_config::parse::{Event, Events, section};

/// # use std::borrow::Cow;

/// # use std::convert::TryFrom;

/// # let section_header = section::Header::new("core", None).unwrap();

/// # let section_data = "[core]\nautocrlf=true\"\"\nfilemode=fa\"lse\"";

/// # assert_eq!(Events::from_str(section_data).unwrap().into_vec(), vec![

/// Event::SectionHeader(section_header),

/// Event::Newline(Cow::Borrowed("\n".into())),

/// Event::SectionValueName(section::ValueName::try_from("autocrlf")?),

/// Event::KeyValueSeparator,

/// Event::Value(Cow::Borrowed(r#"true"""#.into())),

/// Event::Newline(Cow::Borrowed("\n".into())),

/// Event::SectionValueName(section::ValueName::try_from("filemode")?),

/// Event::KeyValueSeparator,

/// Event::Value(Cow::Borrowed(r#"fa"lse""#.into())),

/// # ]);

/// # Ok::<_, Box<dyn std::error::Error>>(())

/// ```

///

/// ## Whitespace after line continuations are part of the value

///

/// Consider the following `git-config` example:

///

/// ```text

/// [some-section]

/// file=a\

///     c

/// ```

///

/// Because how `git-config` treats continuations, the whitespace preceding `c`

/// are in fact part of the value of `file`. The fully interpreted key/value

/// pair is actually `file=a    c`. As a result, the parser will provide this

/// split value accordingly:

///

/// ```

/// # use gix_config::parse::{Event, Events, section};

/// # use std::borrow::Cow;

/// # use std::convert::TryFrom;

/// # let section_header = section::Header::new("some-section", None).unwrap();

/// # let section_data = "[some-section]\nfile=a\\\n    c";

/// # assert_eq!(Events::from_str(section_data).unwrap().into_vec(), vec![

/// Event::SectionHeader(section_header),

/// Event::Newline(Cow::Borrowed("\n".into())),

/// Event::SectionValueName(section::ValueName::try_from("file")?),

/// Event::KeyValueSeparator,

/// Event::ValueNotDone(Cow::Borrowed("a".into())),

/// Event::Newline(Cow::Borrowed("\n".into())),

/// Event::ValueDone(Cow::Borrowed("    c".into())),

/// # ]);

/// # Ok::<_, Box<dyn std::error::Error>>(())

/// ```

///

/// [`File`]: crate::File

/// [`.ini` file format]: https://en.wikipedia.org/wiki/INI_file

/// [`git`'s documentation]: https://git-scm.com/docs/git-config#_configuration_file

/// [`FromStr`]: std::str::FromStr

/// [`From<&'_ str>`]: std::convert::From

#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Default)]

pub struct Events<'a> {

    /// Events seen before the first section.

    pub frontmatter: FrontMatterEvents<'a>,

    /// All parsed sections.

    pub sections: Vec<Section<'a>>,

}

impl Events<'static> {

    /// Parses the provided bytes, returning an [`Events`] that contains allocated

    /// and owned events. This is similar to [`Events::from_bytes()`], but performance

    /// is degraded as it requires allocation for every event.

    ///

    /// Use `filter` to only include those events for which it returns true.

    pub fn from_bytes_owned<'a>(

        input: &'a [u8],

        filter: Option<fn(&Event<'a>) -> bool>,

    ) -> Result<Events<'static>, parse::Error> {

        from_bytes(input, &|e| e.to_owned(), filter)

    }

}

impl<'a> Events<'a> {

    /// Attempt to zero-copy parse the provided bytes. On success, returns a

    /// [`Events`] that provides methods to accessing leading comments and sections

    /// of a `git-config` file and can be converted into an iterator of [`Event`]

    /// for higher level processing.

    ///

    /// Use `filter` to only include those events for which it returns true.

    pub fn from_bytes(input: &'a [u8], filter: Option<fn(&Event<'a>) -> bool>) -> Result<Events<'a>, parse::Error> {

        from_bytes(input, &std::convert::identity, filter)

    }

    /// Attempt to zero-copy parse the provided `input` string.

    ///

    /// Prefer the [`from_bytes()`][Self::from_bytes()] method if UTF8 encoding

    /// isn't guaranteed.

    #[allow(clippy::should_implement_trait)]

    pub fn from_str(input: &'a str) -> Result<Events<'a>, parse::Error> {

        Self::from_bytes(input.as_bytes(), None)

    }

    /// Consumes the parser to produce an iterator of all contained events.

    #[must_use = "iterators are lazy and do nothing unless consumed"]

    #[allow(clippy::should_implement_trait)]

    pub fn into_iter(self) -> impl std::iter::FusedIterator<Item = parse::Event<'a>> {

        self.frontmatter.into_iter().chain(

            self.sections

                .into_iter()

                .flat_map(|section| std::iter::once(parse::Event::SectionHeader(section.header)).chain(section.events)),

        )

    }

    /// Place all contained events into a single `Vec`.

    pub fn into_vec(self) -> Vec<parse::Event<'a>> {

        self.into_iter().collect()

    }

}

impl<'a> TryFrom<&'a str> for Events<'a> {

    type Error = parse::Error;

    fn try_from(value: &'a str) -> Result<Self, Self::Error> {

        Self::from_str(value)

    }

}

impl<'a> TryFrom<&'a [u8]> for Events<'a> {

    type Error = parse::Error;

    fn try_from(value: &'a [u8]) -> Result<Self, Self::Error> {

        Events::from_bytes(value, None)

    }

}

fn from_bytes<'a, 'b>(

    input: &'a [u8],

    convert: &dyn Fn(Event<'a>) -> Event<'b>,

    filter: Option<fn(&Event<'a>) -> bool>,

) -> Result<Events<'b>, parse::Error> {

    let mut header = None;

    let mut events = Vec::with_capacity(256);

    let mut frontmatter = FrontMatterEvents::default();

    let mut sections = Vec::new();

    parse::from_bytes(input, &mut |e: Event<'_>| match e {

        Event::SectionHeader(next_header) => {

            match header.take() {

                None => {

                    frontmatter = std::mem::take(&mut events).into_iter().collect();

                }

                Some(prev_header) => {

                    sections.push(parse::Section {

                        header: prev_header,

                        events: std::mem::take(&mut events),

                    });

                }

            }

            header = match convert(Event::SectionHeader(next_header)) {

                Event::SectionHeader(h) => h,

                _ => unreachable!("BUG: convert must not change the event type, just the lifetime"),

            }

            .into();

        }

        event => {

            if filter.map_or(true, |f| f(&event)) {

                events.push(convert(event));

            }

        }

    })?;

    match header {

        None => {

            frontmatter = events.into_iter().collect();

        }

        Some(prev_header) => {

            sections.push(parse::Section {

                header: prev_header,

                events: std::mem::take(&mut events),

            });

        }

    }

    Ok(Events { frontmatter, sections })

}