use egui::text::{CCursorRange, LayoutJob};

use crate::{
    easy_mark::easy_mark_parser,
    markdown::{
        span::Span,
        tokenizer::{Token, TokenKind, tokenize},
    },
};

/// Highlight easymark, memoizing previous output to save CPU.
///
/// In practice, the highlighter is fast enough not to need any caching.
#[derive(Default)]
pub struct MemoizedHighlighter {
    style: egui::Style,
    code: String,
    output: LayoutJob,
}

impl MemoizedHighlighter {
    pub fn highlight(
        &mut self,
        egui_style: &egui::Style,
        code: &str,
        cursor: Option<CCursorRange>,
    ) -> LayoutJob {
        if (&self.style, self.code.as_str()) != (egui_style, code) {
            self.style = egui_style.clone();
            code.clone_into(&mut self.code);
            self.output = highlight_easymark(egui_style, code, cursor);
        }
        self.output.clone()
    }
}

pub fn highlight_easymark(
    egui_style: &egui::Style,
    text: &str,

    // TODO: hide special characters where cursor isn't
    _cursor: Option<CCursorRange>,
) -> LayoutJob {
    let mut job = LayoutJob::default();
    let mut style = easy_mark_parser::Style::default();

    let mut prev = TokenKind::Newline;

    let tokens: Vec<_> = tokenize(text).collect();
    let mut tokens = &tokens[..];

    const CODE_INDENT: f32 = 10.0;

    while !tokens.is_empty() {
        let token = tokens.first().unwrap();
        tokens = &tokens[1..];

        let start_of_line = prev == TokenKind::Newline;
        prev = token.kind;

        match token.kind {
            TokenKind::CodeBlock if start_of_line => {
                let astyle = format_from_style(
                    egui_style,
                    &easy_mark_parser::Style {
                        code: true,
                        ..Default::default()
                    },
                );

                let span = collect_until(
                    token,
                    &mut tokens,
                    series([TokenKind::Newline, TokenKind::CodeBlock]),
                );

                job.append(&*span, CODE_INDENT, astyle.clone());
                style = Default::default();
                continue;
            }

            TokenKind::Newline => style = easy_mark_parser::Style::default(),
            TokenKind::Strong => style.strong ^= true,
            TokenKind::Italic => style.italics ^= true,
            TokenKind::Strikethrough => style.strikethrough ^= true,
            TokenKind::Heading(_h) if start_of_line => style.heading = true,
            TokenKind::Quote if start_of_line => style.quoted = true,

            TokenKind::CodeBlock | TokenKind::Mono => {
                style.code = true;
                let span = collect_until(
                    token,
                    &mut tokens,
                    any_of([TokenKind::Mono, TokenKind::CodeBlock, TokenKind::Newline]),
                );
                job.append(&*span, 0.0, format_from_style(egui_style, &style));
                style.code = false;
                continue;
            }

            TokenKind::Heading(..) | TokenKind::Quote | TokenKind::Text => {}
        }

        job.append(&token.span, 0.0, format_from_style(egui_style, &style));
    }

    job
}

fn series<'a, const N: usize>(of: [TokenKind; N]) -> impl FnMut(&[Token<'a>; N]) -> bool {
    move |token| {
        of.iter()
            .zip(token)
            .all(|(kind, token)| kind == &token.kind)
    }
}

fn any_of<'a, const N: usize>(these: [TokenKind; N]) -> impl FnMut(&[Token<'a>; 1]) -> bool {
    move |[token]| these.contains(&token.kind)
}

/// Collect all tokens up to and including `pattern`, and merge them into a signle span.
///
/// `N` determines how many specific and consecutive tokens we are looking for.
/// i.e. if we were looking for a [TokenKind::Newline] followed by a [TokenKind::Quote], `N`
/// would equal `2`.
///
/// `pattern` is a function that accepts an array of `N` tokens and returns `true` if they match,
/// i.e. if we should stop collecting. [any_of] and [series] can help to construct this function.
///
/// The collected tokens will be split off the head of the slice referred to by `tokens`.
///
/// # Panic
/// Panics if `tokens` does not contain only consecutive adjacent spans.
fn collect_until<'a, const N: usize>(
    token: &Token<'a>,
    tokens: &mut &[Token<'a>],
    pattern: impl FnMut(&[Token<'a>; N]) -> bool,
) -> Span<'a>
where
    for<'b> &'b [Token<'a>; N]: TryFrom<&'b [Token<'a>]>,
{
    let mut windows = tokens
        .windows(N)
        .map(|slice| <&[Token<'a>; N]>::try_from(slice).ok().unwrap());

    let split_at = match windows.position(pattern) {
        Some(i) => i + N,
        None => tokens.len(), // consume everything
    };

    let (consume, keep) = tokens.split_at(split_at);
    *tokens = keep;

    consume
        .iter()
        .fold(token.span.clone(), |span: Span<'_>, token| {
            span.try_merge(&token.span).unwrap()
        })
}

pub fn highlight_easymark_old(
    egui_style: &egui::Style,
    mut text: &str,

    // TODO: hide special characters where cursor isn't
    _cursor: Option<CCursorRange>,
) -> LayoutJob {
    let mut job = LayoutJob::default();
    let mut style = easy_mark_parser::Style::default();
    let mut start_of_line = true;

    const CODE_INDENT: f32 = 10.0;

    while !text.is_empty() {
        if start_of_line && text.starts_with("```") {
            let astyle = format_from_style(
                egui_style,
                &easy_mark_parser::Style {
                    code: true,
                    ..Default::default()
                },
            );

            // Render the initial backticks as spaces
            text = &text[3..];
            job.append("   ", CODE_INDENT, astyle.clone());

            match text.find("\n```") {
                Some(n) => {
                    for line in text[..n + 1].lines() {
                        job.append(line, CODE_INDENT, astyle.clone());
                        job.append("\n", 0.0, astyle.clone());
                    }
                    // Render the final backticks as spaces
                    job.append("   ", CODE_INDENT, astyle);
                    text = &text[n + 4..];
                }
                None => {
                    job.append(text, 0.0, astyle.clone());
                    text = "";
                }
            };
            style = Default::default();
            continue;
        }

        if text.starts_with('`') {
            style.code = true;
            let end = text[1..]
                .find(&['`', '\n'][..])
                .map_or_else(|| text.len(), |i| i + 2);
            job.append(&text[..end], 0.0, format_from_style(egui_style, &style));
            text = &text[end..];
            style.code = false;
            continue;
        }

        let skip;

        // zero-width space
        let _zws = "\u{200b}";

        let mut apply_basic_style =
            |text: &mut &str,
             style: &mut easy_mark_parser::Style,
             access: fn(&mut easy_mark_parser::Style) -> &mut bool| {
                let skip = if *access(style) {
                    // Include the character that is ending this style:
                    job.append(&text[..1], 0.0, format_from_style(egui_style, style));
                    *text = &text[1..];
                    0
                } else {
                    1
                };
                *access(style) ^= true;
                skip
            };

        if text.starts_with('*') {
            skip = apply_basic_style(&mut text, &mut style, |style| &mut style.strong);
        } else if text.starts_with('/') {
            skip = apply_basic_style(&mut text, &mut style, |style| &mut style.italics);
        } else if text.starts_with('_') {
            skip = apply_basic_style(&mut text, &mut style, |style| &mut style.underline);
        } else if text.starts_with('$') {
            skip = apply_basic_style(&mut text, &mut style, |style| &mut style.small);
        } else if text.starts_with('~') {
            skip = apply_basic_style(&mut text, &mut style, |style| &mut style.strikethrough);
        } else if text.starts_with('^') {
            skip = apply_basic_style(&mut text, &mut style, |style| &mut style.raised);
        } else if text.starts_with('\\') && text.len() >= 2 {
            skip = 2;
        } else if start_of_line && text.starts_with(' ') {
            // we don't preview indentation, because it is confusing
            skip = 1;
        } else if start_of_line && text.starts_with("###### ") {
            style.heading = true;
            skip = 7;
        } else if start_of_line && text.starts_with("##### ") {
            style.heading = true;
            skip = 6;
        } else if start_of_line && text.starts_with("#### ") {
            style.heading = true;
            skip = 5;
        } else if start_of_line && text.starts_with("### ") {
            style.heading = true;
            skip = 4;
        } else if start_of_line && text.starts_with("## ") {
            style.heading = true;
            skip = 3;
        } else if start_of_line && text.starts_with("# ") {
            style.heading = true;
            skip = 2;
        } else if start_of_line && text.starts_with("> ") {
            style.quoted = true;
            skip = 2;
            // we don't preview indentation, because it is confusing
        } else if start_of_line && text.trim_start().starts_with("- ") {
            job.append("• ", 0.0, format_from_style(egui_style, &style));
            text = &text[2..];
            skip = 0;
            // we don't preview indentation, because it is confusing
        } else {
            skip = 0;
        }
        // Note: we don't preview underline, strikethrough and italics because it confuses things.

        // Swallow everything up to the next special character:
        let line_end = text[skip..]
            .find('\n')
            .map_or_else(|| text.len(), |i| (skip + i + 1));
        let end = text[skip..]
            .find(&['*', '`', '~', '_', '/', '$', '^', '\\', '<', '['][..])
            .map_or_else(|| text.len(), |i| (skip + i).max(1));

        if line_end <= end {
            job.append(
                &text[..line_end],
                0.0,
                format_from_style(egui_style, &style),
            );
            text = &text[line_end..];
            start_of_line = true;
            style = Default::default();
        } else {
            job.append(&text[..end], 0.0, format_from_style(egui_style, &style));
            text = &text[end..];
            start_of_line = false;
        }
    }

    job
}

fn format_from_style(
    egui_style: &egui::Style,
    emark_style: &easy_mark_parser::Style,
) -> egui::text::TextFormat {
    use egui::{Align, Color32, Stroke, TextStyle};

    let color = if emark_style.strong || emark_style.heading {
        egui_style.visuals.strong_text_color()
    } else if emark_style.quoted {
        egui_style.visuals.weak_text_color()
    } else {
        egui_style.visuals.text_color()
    };

    let text_style = if emark_style.heading {
        TextStyle::Heading
    } else if emark_style.code {
        TextStyle::Monospace
    } else if emark_style.small | emark_style.raised {
        TextStyle::Small
    } else {
        TextStyle::Body
    };

    let background = if emark_style.code {
        egui_style.visuals.code_bg_color
    } else {
        Color32::TRANSPARENT
    };

    let underline = if emark_style.underline {
        Stroke::new(1.0, color)
    } else {
        Stroke::NONE
    };

    let strikethrough = if emark_style.strikethrough {
        Stroke::new(1.0, color)
    } else {
        Stroke::NONE
    };

    let valign = if emark_style.raised {
        Align::TOP
    } else {
        Align::BOTTOM
    };

    egui::text::TextFormat {
        font_id: text_style.resolve(egui_style),
        color,
        background,
        italics: emark_style.italics,
        underline,
        strikethrough,
        valign,
        ..Default::default()
    }
}