use crate::CowStr;
use crate::Span;
use std::fmt;

use State::*;

pub(crate) fn parse(src: &str) -> Attributes {
    let mut a = Attributes::new();
    a.parse(src);
    a
}

pub fn valid<I: Iterator<Item = char>>(chars: I) -> (usize, bool) {
    let mut has_attr = false;
    let mut p = Parser::new();
    for c in chars {
        if p.step(c).is_some() {
            has_attr = true;
        }
        if matches!(p.state, Done | Invalid) {
            break;
        }
    }

    if matches!(p.state, Done) {
        (p.pos, has_attr)
    } else {
        (0, false)
    }
}

/// 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(&mut self, input: &'s str) -> bool {
        let mut p = Parser::new();
        for c in input.chars() {
            if let Some(elem) = p.step(c) {
                match elem {
                    Element::Class(c) => self.insert("class", c.of(input).into()),
                    Element::Identifier(i) => self.insert("id", i.of(input).into()),
                    Element::Attribute(a, v) => self.insert(a.of(input), v.of(input).into()),
                }
            }
            if matches!(p.state, Done | Invalid) {
                break;
            }
        }
        matches!(p.state, Done)
    }

    /// 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,
    Key,
    ValueFirst,
    Value,
    ValueQuoted,
    ValueEscape,
    Done,
    Invalid,
}

impl State {
    fn step(self, c: char) -> State {
        match self {
            Start if c == '{' => Whitespace,
            Start => Invalid,
            Whitespace => match c {
                '}' => Done,
                '.' => ClassFirst,
                '#' => IdentifierFirst,
                '%' => Comment,
                c if is_name(c) => Key,
                c if c.is_whitespace() => Whitespace,
                _ => Invalid,
            },
            Comment if c == '%' => Whitespace,
            Comment => Comment,
            ClassFirst if is_name(c) => Class,
            ClassFirst => Invalid,
            IdentifierFirst if is_name(c) => Identifier,
            IdentifierFirst => Invalid,
            s @ (Class | Identifier | Value) if is_name(c) => s,
            Class | Identifier | Value if c.is_whitespace() => Whitespace,
            Class | Identifier | Value if c == '}' => Done,
            Class | Identifier | Value => Invalid,
            Key if is_name(c) => Key,
            Key if c == '=' => ValueFirst,
            Key => Invalid,
            ValueFirst if is_name(c) => Value,
            ValueFirst if c == '"' => ValueQuoted,
            ValueFirst => Invalid,
            ValueQuoted if c == '"' => Whitespace,
            ValueQuoted if c == '\\' => ValueEscape,
            ValueQuoted | ValueEscape => ValueQuoted,
            Invalid | Done => panic!("{:?}", self),
        }
    }
}

struct Parser {
    pos: usize,
    pos_prev: usize,
    span1: Span,
    state: State,
}

impl Parser {
    fn new() -> Self {
        Parser {
            pos: 0,
            pos_prev: 0,
            span1: Span::new(0, 0),
            state: Start,
        }
    }

    fn step(&mut self, c: char) -> Option<Element> {
        let state_next = self.state.step(c);
        let st = std::mem::replace(&mut self.state, state_next);

        let elem = if st != self.state
            && !matches!((st, self.state), (ValueEscape, _) | (_, ValueEscape))
        {
            let span0 = Span::new(self.pos_prev, self.pos);
            self.pos_prev = self.pos;
            match st {
                Key => {
                    self.span1 = span0;
                    None
                }
                Class => Some(Element::Class(span0)),
                Identifier => Some(Element::Identifier(span0)),
                Value => Some(Element::Attribute(self.span1, span0)),
                ValueQuoted => Some(Element::Attribute(self.span1, span0.skip(1))),
                _ => None,
            }
        } else {
            None
        };

        self.pos += c.len_utf8();

        elem
    }
}

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

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

#[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));
    }
}