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//! A grid of styled cells, and the colours that go in it.
//!
//! Everything this crate paints goes here first, and only [`crate::term`] ever
//! turns it into escape sequences. That split is deliberate and is the same one
//! glimmer-tui makes on the jolt side: painting into a grid needs no terminal,
//! no raw mode and no TTY, so the whole widget layer is testable in a unit test
//! and in CI — `tui_headless` opens a screen and nothing else.

/// A terminal colour, in the three ways a caller can write one.
///
/// `Default` is not black: it is "whatever the terminal was using", which is
/// what a theme-respecting TUI wants for most of its surface.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum Color {
    #[default]
    Default,
    /// An index into the xterm 256-colour palette. 0–15 are the named ones.
    Indexed(u8),
    Rgb(u8, u8, u8),
}

impl Color {
    /// Parse a colour prop: a name (`red`, `bright-blue`, `default`), a palette
    /// index (`"33"`), a hex triple (`#ff6432` or `#f64`), or `r,g,b`.
    ///
    /// Answers `None` for anything else, which the caller reads as "leave the
    /// colour alone" rather than as an error — a typo'd colour should not take
    /// the widget away.
    pub fn parse(text: &str) -> Option<Self> {
        let text = text.trim();
        if text.is_empty() {
            return None;
        }
        if let Some(hex) = text.strip_prefix('#') {
            return Self::from_hex(hex);
        }
        if let Ok(index) = text.parse::<u8>() {
            return Some(Self::Indexed(index));
        }
        if text.contains(',') {
            let parts: Vec<&str> = text.split(',').map(str::trim).collect();
            if let [r, g, b] = parts[..] {
                return Some(Self::Rgb(r.parse().ok()?, g.parse().ok()?, b.parse().ok()?));
            }
            return None;
        }
        let (name, bright) = match text.strip_prefix("bright-") {
            Some(rest) => (rest, true),
            None => (text, false),
        };
        let base = match name {
            "black" => 0,
            "red" => 1,
            "green" => 2,
            "yellow" => 3,
            "blue" => 4,
            "magenta" => 5,
            "cyan" => 6,
            "white" => 7,
            "default" if !bright => return Some(Self::Default),
            _ => return None,
        };
        Some(Self::Indexed(base + if bright { 8 } else { 0 }))
    }

    fn from_hex(hex: &str) -> Option<Self> {
        let bytes = hex.as_bytes();
        let nib = |c: u8| (c as char).to_digit(16).map(|d| d as u8);
        match bytes.len() {
            3 => {
                let (r, g, b) = (nib(bytes[0])?, nib(bytes[1])?, nib(bytes[2])?);
                Some(Self::Rgb(r * 17, g * 17, b * 17))
            }
            6 => {
                let pair = |i: usize| Some(nib(bytes[i])? * 16 + nib(bytes[i + 1])?);
                Some(Self::Rgb(pair(0)?, pair(2)?, pair(4)?))
            }
            _ => None,
        }
    }
}

/// True for a character that is part of the glyph before it rather than one of
/// its own: a variation selector, a zero-width joiner, a skin tone.
fn continues_glyph(ch: char) -> bool {
    matches!(ch as u32, 0xFE00..=0xFE0F | 0x200D | 0x1F3FB..=0x1F3FF)
}

/// Split `text` into what a terminal draws as single glyphs.
///
/// One character is usually one glyph, but an emoji is often several: `↩️` is
/// an arrow and a variation selector saying to draw it as a picture, and a
/// family is three people and the joiners between them. Cutting between those
/// characters is what makes a reply arrow come out as the small mono arrow
/// rather than the emoji — the selector says which of the two a terminal
/// draws, and it has to travel with the character it is about.
pub fn glyphs(text: &str) -> Vec<String> {
    let mut out: Vec<String> = Vec::new();
    let mut joining = false;
    for ch in text.chars() {
        let continues = continues_glyph(ch);
        if (continues || joining) && !out.is_empty() {
            out.last_mut().expect("not empty").push(ch);
        } else {
            out.push(ch.to_string());
        }
        // A joiner welds what follows it onto what came before.
        joining = ch as u32 == 0x200D;
    }
    out
}

/// How many columns one glyph takes on screen.
///
/// Two for the emoji a terminal draws double width, one for everything else.
/// A character out of the emoji blocks is drawn wide on its own; an older
/// symbol or dingbat is drawn wide when it carries the selector that asks for
/// the picture, and narrow — a character among characters — when it does not.
/// This is the whole of what this backend knows about width, and it is enough
/// for what a chat client puts on a screen: text, and the emoji in it.
pub fn glyph_cols(glyph: &str) -> u16 {
    let mut chars = glyph.chars();
    let Some(base) = chars.next() else {
        return 0;
    };
    let emoji_presentation = glyph.chars().any(|c| c as u32 == 0xFE0F);
    match base as u32 {
        0x1F000.. => 2,
        0x2000..=0x2BFF if emoji_presentation => 2,
        _ => 1,
    }
}

/// The columns `text` takes, the same way [`Screen::text`] spends them.
pub fn text_cols(text: &str) -> u16 {
    glyphs(text).iter().map(|g| glyph_cols(g)).sum()
}

/// The attribute bits a cell can carry. A `u8` rather than a set of `bool`s
/// because a cell is copied a great many times a frame.
pub mod attr {
    pub const BOLD: u8 = 1 << 0;
    pub const DIM: u8 = 1 << 1;
    pub const UNDERLINE: u8 = 1 << 2;
    pub const REVERSE: u8 = 1 << 3;
    pub const BLINK: u8 = 1 << 4;
    pub const ITALIC: u8 = 1 << 5;
}

#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct Style {
    pub fg: Color,
    pub bg: Color,
    pub attrs: u8,
}

impl Style {
    pub fn with(mut self, bits: u8) -> Self {
        self.attrs |= bits;
        self
    }

    pub fn fg(mut self, color: Color) -> Self {
        self.fg = color;
        self
    }

    pub fn has(&self, bits: u8) -> bool {
        self.attrs & bits != 0
    }
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Cell {
    pub ch: char,
    /// The rest of the glyph, where it took more than one character: the
    /// selector that asks for a picture, the joiners in a family. Printed
    /// straight after `ch`, which is the only way a terminal draws them as
    /// the one glyph they are.
    pub tail: Option<Box<str>>,
    pub style: Style,
    /// The right half of a double-width character. It holds no character of
    /// its own: the glyph in the cell to its left is drawn across both, and
    /// writing anything here would print a second copy one column over.
    pub trail: bool,
}

impl Default for Cell {
    fn default() -> Self {
        Self {
            ch: ' ',
            tail: None,
            style: Style::default(),
            trail: false,
        }
    }
}

/// A rectangle in cells. Columns and rows, origin top left.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct Rect {
    pub x: u16,
    pub y: u16,
    pub w: u16,
    pub h: u16,
}

impl Rect {
    pub fn new(x: u16, y: u16, w: u16, h: u16) -> Self {
        Self { x, y, w, h }
    }

    pub fn is_empty(&self) -> bool {
        self.w == 0 || self.h == 0
    }

    pub fn contains(&self, x: u16, y: u16) -> bool {
        x >= self.x && x < self.x + self.w && y >= self.y && y < self.y + self.h
    }

    /// The rect left after taking `n` cells off every side. Saturating, so
    /// padding larger than the rect answers an empty one rather than wrapping.
    pub fn shrink(&self, n: u16) -> Self {
        let take = n.saturating_mul(2);
        Self {
            x: self.x.saturating_add(n),
            y: self.y.saturating_add(n),
            w: self.w.saturating_sub(take),
            h: self.h.saturating_sub(take),
        }
    }
}

#[derive(Clone, Debug)]
pub struct Screen {
    w: u16,
    h: u16,
    cells: Vec<Cell>,
}

impl Screen {
    pub fn new(w: u16, h: u16) -> Self {
        Self {
            w,
            h,
            cells: vec![Cell::default(); w as usize * h as usize],
        }
    }

    pub fn width(&self) -> u16 {
        self.w
    }

    pub fn height(&self) -> u16 {
        self.h
    }

    pub fn rect(&self) -> Rect {
        Rect::new(0, 0, self.w, self.h)
    }

    pub fn resize(&mut self, w: u16, h: u16) {
        if (w, h) != (self.w, self.h) {
            *self = Self::new(w, h);
        }
    }

    pub fn clear(&mut self) {
        self.cells.fill(Cell::default());
    }

    pub fn cell(&self, x: u16, y: u16) -> Option<&Cell> {
        if x < self.w && y < self.h {
            self.cells.get(y as usize * self.w as usize + x as usize)
        } else {
            None
        }
    }

    /// Make room at `i` for a cell that is, or is not, the right half of a
    /// wide glyph. Painting over half of a wide glyph takes the whole glyph
    /// off a terminal, so the grid has to lose the other half too: a lead
    /// left without its trail, or a trail without its lead, is a cell the
    /// flush believes is on screen and a terminal has already erased — and
    /// since it never changes again, it is never repainted. That is the
    /// stray pencil left behind once a row's chips are gone.
    fn displace(&mut self, i: usize, trail: bool) {
        let x = i % self.w as usize;
        if !trail && self.cells[i].trail && x > 0 {
            let lead = &mut self.cells[i - 1];
            lead.ch = ' ';
            lead.tail = None;
        }
        if x + 1 < self.w as usize && self.cells[i + 1].trail {
            let rest = &mut self.cells[i + 1];
            rest.ch = ' ';
            rest.tail = None;
            rest.trail = false;
        }
    }

    pub fn set(&mut self, x: u16, y: u16, ch: char, style: Style) {
        if x < self.w && y < self.h {
            let i = y as usize * self.w as usize + x as usize;
            self.displace(i, false);
            self.cells[i] = Cell {
                ch,
                tail: None,
                style,
                trail: false,
            };
        }
    }

    /// The cell a double-width character's right half sits in.
    fn set_trail(&mut self, x: u16, y: u16, style: Style) {
        if x < self.w && y < self.h {
            let i = y as usize * self.w as usize + x as usize;
            self.displace(i, true);
            self.cells[i] = Cell {
                ch: ' ',
                tail: None,
                style,
                trail: true,
            };
        }
    }

    /// Write `text` at `x, y`, clipped to `width` columns. Answers how many
    /// columns were used.
    pub fn text(&mut self, x: u16, y: u16, width: u16, text: &str, style: Style) -> u16 {
        let mut col = 0u16;
        for glyph in glyphs(text) {
            if col >= width {
                break;
            }
            let mut chars = glyph.chars();
            let ch = chars.next().unwrap_or(' ');
            // A control character in a label would move the cursor; show it as
            // a dot instead of letting it rearrange the screen.
            let ch = if (ch as u32) < 0x20 { '·' } else { ch };
            let cols = glyph_cols(&glyph);
            if col + cols > width {
                // Half of a wide glyph is a different character, so the last
                // column stays blank rather than showing one.
                break;
            }
            let at = x.saturating_add(col);
            self.set(at, y, ch, style);
            let rest: String = chars.collect();
            if !rest.is_empty() {
                self.set_tail(at, y, rest);
            }
            if cols == 2 {
                self.set_trail(x.saturating_add(col + 1), y, style);
            }
            col += cols;
        }
        col
    }

    /// Put a whole cell down as it is — what a scroll does when it copies the
    /// visible window of its content across. Cell by character would drop the
    /// rest of a glyph and the right half of a wide one, which is a reply
    /// arrow painted as the small mono arrow and a row a column out.
    pub fn put(&mut self, x: u16, y: u16, cell: Cell) {
        if x < self.w && y < self.h {
            let i = y as usize * self.w as usize + x as usize;
            self.displace(i, cell.trail);
            self.cells[i] = cell;
        }
    }

    /// The rest of a glyph, on the cell its first character went in.
    fn set_tail(&mut self, x: u16, y: u16, rest: String) {
        if x < self.w && y < self.h {
            let i = y as usize * self.w as usize + x as usize;
            self.cells[i].tail = Some(rest.into_boxed_str());
        }
    }

    /// Paint every cell of `rect` with `style`, keeping the characters — that
    /// is what a background is: a colour behind whatever is already there.
    pub fn fill(&mut self, rect: Rect, style: Style) {
        for y in rect.y..rect.y.saturating_add(rect.h) {
            for x in rect.x..rect.x.saturating_add(rect.w) {
                if x < self.w && y < self.h {
                    let i = y as usize * self.w as usize + x as usize;
                    // The character stays, and so does whether it is the half
                    // of one: a background is a colour, not a repaint.
                    self.cells[i].style = style;
                }
            }
        }
    }

    /// One row as text, trailing blanks trimmed. The whole reason the grid is
    /// addressable: a test asserts on lines, not on escape sequences.
    pub fn line(&self, y: u16) -> String {
        if y >= self.h {
            return String::new();
        }
        let start = y as usize * self.w as usize;
        // Without the trailing halves: they hold no character, and a reader —
        // a test, a bug report — wants the line as it looks.
        let mut row = String::new();
        for cell in self.cells[start..start + self.w as usize].iter() {
            if cell.trail {
                continue;
            }
            row.push(cell.ch);
            if let Some(tail) = &cell.tail {
                row.push_str(tail);
            }
        }
        row.trim_end().to_owned()
    }

    #[cfg(test)]
    pub fn lines(&self) -> Vec<String> {
        (0..self.h).map(|y| self.line(y)).collect()
    }

    pub(crate) fn cells(&self) -> &[Cell] {
        &self.cells
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn painting_over_half_of_a_wide_glyph_takes_the_other_half_with_it() {
        // Over the left half: the pencil's right half must not stay behind as
        // a trail with nothing to its left.
        let mut screen = Screen::new(6, 1);
        screen.text(0, 0, 6, "a✏️b", Style::default());
        screen.set(1, 0, 'x', Style::default());
        assert_eq!(screen.line(0), "ax b");
        assert!(!screen.cell(2, 0).unwrap().trail);

        // Over the right half: the left half is blank, as a terminal shows it.
        let mut screen = Screen::new(6, 1);
        screen.text(0, 0, 6, "a✏️b", Style::default());
        screen.set(2, 0, 'x', Style::default());
        assert_eq!(screen.line(0), "a xb");
        assert!(screen.cell(1, 0).unwrap().tail.is_none());

        // A wide glyph laid over the right half of another keeps only itself.
        let mut screen = Screen::new(6, 1);
        screen.text(0, 0, 6, "a✏️b", Style::default());
        screen.text(2, 0, 4, "🙂", Style::default());
        assert_eq!(screen.line(0), "a 🙂");
    }

    #[test]
    fn colours_parse_in_every_shape_a_caller_writes_them() {
        assert_eq!(Color::parse("red"), Some(Color::Indexed(1)));
        assert_eq!(Color::parse("bright-blue"), Some(Color::Indexed(12)));
        assert_eq!(Color::parse("default"), Some(Color::Default));
        assert_eq!(Color::parse("33"), Some(Color::Indexed(33)));
        assert_eq!(Color::parse("#ff6432"), Some(Color::Rgb(255, 100, 50)));
        assert_eq!(Color::parse("#f64"), Some(Color::Rgb(255, 102, 68)));
        assert_eq!(Color::parse("255,100,50"), Some(Color::Rgb(255, 100, 50)));
    }

    #[test]
    fn an_unreadable_colour_is_no_colour_rather_than_an_error() {
        assert_eq!(Color::parse("puce"), None);
        assert_eq!(Color::parse("#gg0000"), None);
        assert_eq!(Color::parse(""), None);
    }

    #[test]
    fn text_clips_to_the_width_it_was_given() {
        let mut screen = Screen::new(10, 2);
        screen.text(0, 0, 4, "abcdefg", Style::default());
        assert_eq!(screen.line(0), "abcd");
    }

    #[test]
    fn a_control_character_cannot_move_the_cursor() {
        let mut screen = Screen::new(6, 1);
        screen.text(0, 0, 6, "a\rb", Style::default());
        assert_eq!(screen.line(0), "a·b");
    }

    #[test]
    fn filling_a_rect_keeps_the_characters_under_it() {
        let mut screen = Screen::new(4, 1);
        screen.text(0, 0, 4, "hi", Style::default());
        screen.fill(screen.rect(), Style::default().with(attr::REVERSE));
        assert_eq!(screen.line(0), "hi");
        assert!(screen.cell(0, 0).unwrap().style.has(attr::REVERSE));
    }
}