use crate::CowStr;
use crate::DiscontinuousString;
use crate::Span;
use std::borrow::Cow;
use std::fmt;

use State::*;

pub(crate) fn parse<'s, S: DiscontinuousString<'s>>(chars: S) -> Attributes<'s> {
    let mut a = Attributes::new();
    a.parse(chars);
    a
}

pub fn valid<I: Iterator<Item = char>>(chars: I) -> (usize, bool) {
    let mut has_attr = false;
    let mut p = Parser::new(chars);
    for e in &mut p {
        match e {
            Element::Class(..) | Element::Identifier(..) | Element::Attribute(..) => {
                has_attr = true;
            }
            Element::Invalid => return (0, false),
        }
    }
    (p.pos, has_attr)
}

/// Stores an attribute value that supports backslash escapes of ASCII punctuation upon displaying,
/// without allocating.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct AttributeValue<'s> {
    raw: CowStr<'s>,
}

impl<'s> AttributeValue<'s> {
    /// Processes the attribute value escapes and returns an iterator of the parts of the value
    /// that should be displayed.
    pub fn parts(&'s self) -> AttributeValueParts<'s> {
        AttributeValueParts { ahead: &self.raw }
    }
}

impl<'s> From<&'s str> for AttributeValue<'s> {
    fn from(value: &'s str) -> Self {
        Self { raw: value.into() }
    }
}

impl<'s> From<CowStr<'s>> for AttributeValue<'s> {
    fn from(value: CowStr<'s>) -> Self {
        Self { raw: value }
    }
}

impl<'s> From<String> for AttributeValue<'s> {
    fn from(value: String) -> Self {
        Self { raw: value.into() }
    }
}

impl<'s> fmt::Display for AttributeValue<'s> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.parts().try_for_each(|part| f.write_str(part))
    }
}

/// An iterator over the parts of an [`AttributeValue`] that should be displayed.
pub struct AttributeValueParts<'s> {
    ahead: &'s str,
}

impl<'s> Iterator for AttributeValueParts<'s> {
    type Item = &'s str;

    fn next(&mut self) -> Option<Self::Item> {
        for (i, _) in self.ahead.match_indices('\\') {
            match self.ahead.as_bytes().get(i + 1) {
                Some(b'\\') => {
                    let next = &self.ahead[..i + 1];
                    self.ahead = &self.ahead[i + 2..];
                    return Some(next);
                }
                Some(c) if c.is_ascii_punctuation() => {
                    let next = &self.ahead[..i];
                    self.ahead = &self.ahead[i + 1..];
                    return Some(next);
                }
                _ => {}
            }
        }

        (!self.ahead.is_empty()).then(|| std::mem::take(&mut self.ahead))
    }
}

/// A collection of attributes, i.e. a key-value map.
// Attributes are relatively rare, we choose to pay 8 bytes always and sometimes an extra
// indirection instead of always 24 bytes.
#[allow(clippy::box_vec)]
#[derive(Clone, PartialEq, Eq, Default)]
pub struct Attributes<'s>(Option<Box<Vec<(&'s str, AttributeValue<'s>)>>>);

impl<'s> Attributes<'s> {
    /// Create an empty collection.
    #[must_use]
    pub fn new() -> Self {
        Self::default()
    }

    #[must_use]
    pub(crate) fn take(&mut self) -> Self {
        Self(self.0.take())
    }

    pub(crate) fn parse<S: DiscontinuousString<'s>>(&mut self, input: S) -> bool {
        #[inline]
        fn borrow(cow: CowStr) -> &str {
            match cow {
                Cow::Owned(_) => panic!(),
                Cow::Borrowed(s) => s,
            }
        }

        for elem in Parser::new(input.chars()) {
            match elem {
                Element::Class(c) => self.insert("class", input.src(c).into()),
                Element::Identifier(i) => self.insert("id", input.src(i).into()),
                Element::Attribute(a, v) => self.insert(borrow(input.src(a)), input.src(v).into()),
                Element::Invalid => return false,
            }
        }
        true
    }

    /// Combine all attributes from both objects, prioritizing self on conflicts.
    pub(crate) fn union(&mut self, other: Self) {
        if let Some(attrs0) = &mut self.0 {
            if let Some(mut attrs1) = other.0 {
                for (key, val) in attrs1.drain(..) {
                    if !attrs0.iter().any(|(k, _)| *k == key) {
                        attrs0.push((key, val));
                    }
                }
            }
        } else {
            self.0 = other.0;
        }
    }

    /// Insert an attribute. If the attribute already exists, the previous value will be
    /// overwritten, unless it is a "class" attribute. In that case the provided value will be
    /// appended to the existing value.
    pub fn insert(&mut self, key: &'s str, val: AttributeValue<'s>) {
        if self.0.is_none() {
            self.0 = Some(Vec::new().into());
        };

        let attrs = self.0.as_mut().unwrap();
        if let Some(i) = attrs.iter().position(|(k, _)| *k == key) {
            let prev = &mut attrs[i].1;
            if key == "class" {
                *prev = format!("{} {}", prev, val).into();
            } else {
                *prev = val;
            }
        } else {
            attrs.push((key, val));
        }
    }

    /// Returns true if the collection contains no attributes.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.0.as_ref().map_or(true, |v| v.is_empty())
    }

    /// Returns a reference to the value corresponding to the attribute key.
    #[must_use]
    pub fn get(&self, key: &str) -> Option<&AttributeValue<'s>> {
        self.iter().find(|(k, _)| *k == key).map(|(_, v)| v)
    }

    /// Returns an iterator over the attributes in undefined order.
    pub fn iter(&self) -> impl Iterator<Item = (&'s str, &AttributeValue<'s>)> + '_ {
        self.0.iter().flat_map(|v| v.iter().map(|(a, b)| (*a, b)))
    }
}

#[cfg(test)]
impl<'s> FromIterator<(&'s str, &'s str)> for Attributes<'s> {
    fn from_iter<I: IntoIterator<Item = (&'s str, &'s str)>>(iter: I) -> Self {
        let attrs = iter
            .into_iter()
            .map(|(a, v)| (a, v.into()))
            .collect::<Vec<_>>();
        if attrs.is_empty() {
            Attributes::new()
        } else {
            Attributes(Some(attrs.into()))
        }
    }
}

impl<'s> std::fmt::Debug for Attributes<'s> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{{")?;
        let mut first = true;
        for (k, v) in self.iter() {
            if !first {
                write!(f, ", ")?;
            }
            first = false;
            write!(f, "{}=\"{}\"", k, v.raw)?;
        }
        write!(f, "}}")
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum State {
    Start,
    Whitespace,
    Comment,
    ClassFirst,
    Class,
    IdentifierFirst,
    Identifier,
    Attribute,
    ValueFirst,
    Value,
    ValueQuoted,
    Done,
    Invalid,
}

struct Parser<I> {
    chars: I,
    pos: usize,
    pos_prev: usize,
    state: State,
}

impl<I: Iterator<Item = char>> Parser<I> {
    fn new(chars: I) -> Self {
        Parser {
            chars,
            pos: 0,
            pos_prev: 0,
            state: Start,
        }
    }

    fn step_char(&mut self) -> Option<State> {
        self.chars.next().map(|c| {
            self.pos_prev = self.pos;
            self.pos += c.len_utf8();
            match self.state {
                Start => match c {
                    '{' => Whitespace,
                    _ => Invalid,
                },
                Whitespace => match c {
                    '}' => Done,
                    '.' => ClassFirst,
                    '#' => IdentifierFirst,
                    '%' => Comment,
                    c if c.is_ascii_alphanumeric() || matches!(c, '_' | ':' | '-') => Attribute,
                    c if c.is_whitespace() => Whitespace,
                    _ => Invalid,
                },
                Comment => {
                    if c == '%' {
                        Whitespace
                    } else {
                        Comment
                    }
                }
                s @ (ClassFirst | IdentifierFirst) => {
                    if is_name(c) {
                        match s {
                            ClassFirst => Class,
                            IdentifierFirst => Identifier,
                            _ => panic!(),
                        }
                    } else {
                        Invalid
                    }
                }
                s @ (Class | Identifier | Value) => {
                    if is_name(c) {
                        s
                    } else if c.is_whitespace() {
                        Whitespace
                    } else if c == '}' {
                        Done
                    } else {
                        Invalid
                    }
                }
                Attribute => {
                    if is_name(c) {
                        Attribute
                    } else if c == '=' {
                        ValueFirst
                    } else {
                        Invalid
                    }
                }
                ValueFirst => {
                    if is_name(c) {
                        Value
                    } else if c == '"' {
                        ValueQuoted
                    } else {
                        Invalid
                    }
                }
                ValueQuoted => match c {
                    '\\' => {
                        if let Some(c) = self.chars.next() {
                            self.pos_prev = self.pos;
                            self.pos += c.len_utf8();
                        }
                        ValueQuoted
                    }
                    '"' => Whitespace,
                    _ => ValueQuoted,
                },
                Invalid | Done => panic!("{:?}", self.state),
            }
        })
    }

    fn step(&mut self) -> (State, Span) {
        let start = self.pos_prev;

        if self.state == Done {
            return (Done, Span::empty_at(start));
        }

        if self.state == Invalid {
            return (Invalid, Span::empty_at(start));
        }

        while let Some(state_next) = self.step_char() {
            if self.state != state_next {
                return (
                    std::mem::replace(&mut self.state, state_next),
                    Span::new(start, self.pos_prev),
                );
            }
        }

        (
            if self.state == Done { Done } else { Invalid },
            Span::new(start, self.pos_prev),
        )
    }
}

pub fn is_name(c: char) -> bool {
    c.is_ascii_alphanumeric() || matches!(c, ':' | '_' | '-')
}

enum Element {
    Class(Span),
    Identifier(Span),
    Attribute(Span, Span),
    Invalid,
}

impl<I: Iterator<Item = char>> Iterator for Parser<I> {
    type Item = Element;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            let (st, span0) = self.step();
            return match st {
                ClassFirst | IdentifierFirst => {
                    let (st, span1) = self.step();
                    Some(match st {
                        Class => Element::Class(span1),
                        Identifier => Element::Identifier(span1),
                        _ => return Some(Element::Invalid),
                    })
                }
                Attribute => {
                    let (st, _span1) = self.step();
                    match st {
                        ValueFirst => {
                            let (st, span2) = self.step();
                            match st {
                                Value => Some(Element::Attribute(span0, span2)),
                                ValueQuoted => Some(Element::Attribute(span0, span2.skip(1))),
                                Invalid => Some(Element::Invalid),
                                _ => panic!("{:?}", st),
                            }
                        }
                        Invalid => Some(Element::Invalid),
                        _ => panic!("{:?}", st),
                    }
                }
                Comment | Start | Whitespace => continue,
                Done => None,
                Invalid => Some(Element::Invalid),
                _ => panic!("{:?}", st),
            };
        }
    }
}

#[cfg(test)]
mod test {
    macro_rules! test_attr {
        ($src:expr $(,$($av:expr),* $(,)?)?) => {
            #[allow(unused)]
            let mut attr = super::Attributes::new();
            attr.parse($src);
            let actual = attr.iter().collect::<Vec<_>>();
            let expected = &[$($($av),*,)?];
            for i in 0..actual.len() {
                let actual_val = format!("{}", actual[i].1);
                assert_eq!((actual[i].0, actual_val.as_str()), expected[i], "\n\n{}\n\n", $src);
            }
        };
    }

    #[test]
    fn empty() {
        test_attr!("{}");
    }

    #[test]
    fn class_id() {
        test_attr!(
            "{.some_class #some_id}",
            ("class", "some_class"),
            ("id", "some_id"),
        );
        test_attr!("{.a .b}", ("class", "a b"));
        test_attr!("{#a #b}", ("id", "b"));
    }

    #[test]
    fn unicode_whitespace() {
        test_attr!("{.a .b}", ("class", "a b"));
    }

    #[test]
    fn value_unquoted() {
        test_attr!(
            "{attr0=val0 attr1=val1}",
            ("attr0", "val0"),
            ("attr1", "val1"),
        );
    }

    #[test]
    fn value_quoted() {
        test_attr!(
            r#"{attr0="val0" attr1="val1"}"#,
            ("attr0", "val0"),
            ("attr1", "val1"),
        );
        test_attr!(
            r#"{#id .class style="color:red"}"#,
            ("id", "id"),
            ("class", "class"),
            ("style", "color:red")
        );
    }

    #[test]
    fn comment() {
        test_attr!("{%%}");
        test_attr!("{ % abc % }");
        test_attr!(
            "{ .some_class % abc % #some_id}",
            ("class", "some_class"),
            ("id", "some_id"),
        );
    }

    #[test]
    fn escape() {
        test_attr!(
            r#"{attr="with escaped \~ char"}"#,
            ("attr", "with escaped ~ char")
        );
        test_attr!(
            r#"{key="quotes \" should be escaped"}"#,
            ("key", r#"quotes " should be escaped"#)
        );
    }

    #[test]
    fn escape_backslash() {
        test_attr!(r#"{attr="with\\backslash"}"#, ("attr", r"with\backslash"));
        test_attr!(
            r#"{attr="with many backslashes\\\\"}"#,
            ("attr", r"with many backslashes\\")
        );
        test_attr!(
            r#"{attr="\\escaped backslash at start"}"#,
            ("attr", r"\escaped backslash at start")
        );
    }

    #[test]
    fn only_escape_punctuation() {
        test_attr!(r#"{attr="do not \escape"}"#, ("attr", r"do not \escape"));
        test_attr!(
            r#"{attr="\backslash at the beginning"}"#,
            ("attr", r"\backslash at the beginning")
        );
    }

    #[test]
    fn valid_full() {
        let src = "{.class %comment%}";
        assert_eq!(super::valid(src.chars()), (src.len(), true));
    }

    #[test]
    fn valid_empty() {
        let src = "{}";
        assert_eq!(super::valid(src.chars()), (src.len(), false));
    }

    #[test]
    fn valid_whitespace() {
        let src = "{ \n }";
        assert_eq!(super::valid(src.chars()), (src.len(), false));
    }

    #[test]
    fn valid_comment() {
        let src = "{%comment%}";
        assert_eq!(super::valid(src.chars()), (src.len(), false));
    }

    #[test]
    fn valid_trailing() {
        let src = "{.class}";
        assert_eq!(
            super::valid(src.chars().chain("{.ignore}".chars())),
            (src.len(), true),
        );
    }

    #[test]
    fn valid_invalid() {
        assert_eq!(super::valid(" {.valid}".chars()), (0, false));
        assert_eq!(super::valid("{.class invalid}".chars()), (0, false));
        assert_eq!(super::valid("abc".chars()), (0, false));
        assert_eq!(super::valid("{.abc.}".chars()), (0, false));
    }
}