//! Contains `XmlEvent` datatype, instances of which are emitted by the parser.

use std::borrow::Cow;
use std::fmt;

use crate::attribute::OwnedAttribute;
use crate::common::XmlVersion;
use crate::name::OwnedName;
use crate::namespace::Namespace;

/// An element of an XML input stream.
///
/// Items of this enum are emitted by `reader::EventReader`. They correspond to different
/// elements of an XML document.
#[derive(PartialEq, Clone)]
pub enum XmlEvent {
    /// Corresponds to XML document declaration.
    ///
    /// This event is always emitted before any other event. It is emitted
    /// even if the actual declaration is not present in the document.
    StartDocument {
        /// XML version.
        ///
        /// If XML declaration is not present, defaults to `Version10`.
        version: XmlVersion,

        /// XML document encoding.
        ///
        /// If XML declaration is not present or does not contain `encoding` attribute,
        /// defaults to `"UTF-8"`. This field is currently used for no other purpose than
        /// informational.
        encoding: String,

        /// XML standalone declaration.
        ///
        /// If XML document is not present or does not contain `standalone` attribute,
        /// defaults to `None`. This field is currently used for no other purpose than
        /// informational.
        standalone: Option<bool>,
    },

    /// Denotes to the end of the document stream.
    ///
    /// This event is always emitted after any other event (except `Error`). After it
    /// is emitted for the first time, it will always be emitted on next event pull attempts.
    EndDocument,

    /// Denotes an XML processing instruction.
    ///
    /// This event contains a processing instruction target (`name`) and opaque `data`. It
    /// is up to the application to process them.
    ProcessingInstruction {
        /// Processing instruction target.
        name: String,

        /// Processing instruction content.
        data: Option<String>,
    },

    /// Denotes a beginning of an XML element.
    ///
    /// This event is emitted after parsing opening tags or after parsing bodiless tags. In the
    /// latter case `EndElement` event immediately follows.
    StartElement {
        /// Qualified name of the element.
        name: OwnedName,

        /// A list of attributes associated with the element.
        ///
        /// Currently attributes are not checked for duplicates (TODO)
        attributes: Vec<OwnedAttribute>,

        /// Contents of the namespace mapping at this point of the document.
        namespace: Namespace,
    },

    /// Denotes an end of an XML element.
    ///
    /// This event is emitted after parsing closing tags or after parsing bodiless tags. In the
    /// latter case it is emitted immediately after corresponding `StartElement` event.
    EndElement {
        /// Qualified name of the element.
        name: OwnedName,
    },

    /// Denotes CDATA content.
    ///
    /// This event contains unparsed data. No unescaping will be performed.
    ///
    /// It is possible to configure a parser to emit `Characters` event instead of `CData`. See
    /// `pull::ParserConfiguration` structure for more information.
    CData(String),

    /// Denotes a comment.
    ///
    /// It is possible to configure a parser to ignore comments, so this event will never be emitted.
    /// See `pull::ParserConfiguration` structure for more information.
    Comment(String),

    /// Denotes character data outside of tags.
    ///
    /// Contents of this event will always be unescaped, so no entities like `&lt;` or `&amp;` or `&#123;`
    /// will appear in it.
    ///
    /// It is possible to configure a parser to trim leading and trailing whitespace for this event.
    /// See `pull::ParserConfiguration` structure for more information.
    Characters(String),

    /// Denotes a chunk of whitespace outside of tags.
    ///
    /// It is possible to configure a parser to emit `Characters` event instead of `Whitespace`.
    /// See `pull::ParserConfiguration` structure for more information. When combined with whitespace
    /// trimming, it will eliminate standalone whitespace from the event stream completely.
    Whitespace(String),
}

impl fmt::Debug for XmlEvent {
    #[cold]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            XmlEvent::StartDocument { ref version, ref encoding, standalone } =>
                write!(f, "StartDocument({}, {}, {:?})", version, *encoding, standalone),
            XmlEvent::EndDocument =>
                write!(f, "EndDocument"),
            XmlEvent::ProcessingInstruction { ref name, ref data } =>
                write!(f, "ProcessingInstruction({}{})", *name, match *data {
                    Some(ref data) => format!(", {data}"),
                    None       => String::new()
                }),
            XmlEvent::StartElement { ref name, ref attributes, namespace: Namespace(ref namespace) } =>
                write!(f, "StartElement({}, {:?}{})", name, namespace, if attributes.is_empty() {
                    String::new()
                } else {
                    let attributes: Vec<String> = attributes.iter().map(
                        |a| format!("{} -> {}", a.name, a.value)
                    ).collect();
                    format!(", [{}]", attributes.join(", "))
                }),
            XmlEvent::EndElement { ref name } =>
                write!(f, "EndElement({name})"),
            XmlEvent::Comment(ref data) =>
                write!(f, "Comment({data})"),
            XmlEvent::CData(ref data) =>
                write!(f, "CData({data})"),
            XmlEvent::Characters(ref data) =>
                write!(f, "Characters({data})"),
            XmlEvent::Whitespace(ref data) =>
                write!(f, "Whitespace({data})")
        }
    }
}

impl XmlEvent {
    /// Obtains a writer event from this reader event.
    ///
    /// This method is useful for streaming processing of XML documents where the output
    /// is also an XML document. With this method it is possible to process some events
    /// while passing other events through to the writer unchanged:
    ///
    /// ```rust
    /// use std::str;
    ///
    /// use xml::{EventReader, EventWriter};
    /// use xml::reader::XmlEvent as ReaderEvent;
    /// use xml::writer::XmlEvent as WriterEvent;
    ///
    /// let mut input: &[u8] = b"<hello>world</hello>";
    /// let mut output: Vec<u8> = Vec::new();
    ///
    /// {
    ///     let mut reader = EventReader::new(&mut input);
    ///     let mut writer = EventWriter::new(&mut output);
    ///
    ///     for e in reader {
    ///         match e.unwrap() {
    ///             ReaderEvent::Characters(s) =>
    ///                 writer.write(WriterEvent::characters(&s.to_uppercase())).unwrap(),
    ///             e => if let Some(e) = e.as_writer_event() {
    ///                 writer.write(e).unwrap()
    ///             }
    ///         }
    ///     }
    /// }
    ///
    /// assert_eq!(
    ///     str::from_utf8(&output).unwrap(),
    ///     r#"<?xml version="1.0" encoding="UTF-8"?><hello>WORLD</hello>"#
    /// );
    /// ```
    ///
    /// Note that this API may change or get additions in future to improve its ergonomics.
    #[must_use]
    pub fn as_writer_event(&self) -> Option<crate::writer::events::XmlEvent<'_>> {
        match *self {
            XmlEvent::StartDocument { version, ref encoding, standalone } =>
                Some(crate::writer::events::XmlEvent::StartDocument {
                    version,
                    encoding: Some(encoding),
                    standalone
                }),
            XmlEvent::ProcessingInstruction { ref name, ref data } =>
                Some(crate::writer::events::XmlEvent::ProcessingInstruction {
                    name,
                    data: data.as_ref().map(|s| &**s)
                }),
            XmlEvent::StartElement { ref name, ref attributes, ref namespace } =>
                Some(crate::writer::events::XmlEvent::StartElement {
                    name: name.borrow(),
                    attributes: attributes.iter().map(|a| a.borrow()).collect(),
                    namespace: Cow::Borrowed(namespace)
                }),
            XmlEvent::EndElement { ref name } =>
                Some(crate::writer::events::XmlEvent::EndElement { name: Some(name.borrow()) }),
            XmlEvent::Comment(ref data) => Some(crate::writer::events::XmlEvent::Comment(data)),
            XmlEvent::CData(ref data) => Some(crate::writer::events::XmlEvent::CData(data)),
            XmlEvent::Characters(ref data) |
            XmlEvent::Whitespace(ref data) => Some(crate::writer::events::XmlEvent::Characters(data)),
            XmlEvent::EndDocument => None,
        }
    }
}