//! ScrollPane — a generic scrollable widget with gutter support.
//!
//! Renders only the visible portion of a list of items, caching
//! wrapped heights for performance. Handles scroll offset,
//! pin-to-bottom, scrollbar, and an optional gutter column.

use ratatui::prelude::*;
use ratatui::widgets::{Block, Paragraph, Scrollbar, ScrollbarOrientation, ScrollbarState, Wrap};

// ── Trait for scrollable items ─────────────────────────────────

/// Anything that can appear in a ScrollPane.
pub trait ScrollItem {
    /// The content lines for this item.
    fn content(&self) -> Text<'_>;

    /// Optional gutter annotation (rendered at the first visual line).
    fn gutter(&self) -> Option<Span<'_>> {
        None
    }
}

// Blanket impls for common types

impl ScrollItem for Line<'static> {
    fn content(&self) -> Text<'_> {
        Text::from(self.clone())
    }
}

impl ScrollItem for Vec<Line<'static>> {
    fn content(&self) -> Text<'_> {
        Text::from(self.clone())
    }
}

impl ScrollItem for Text<'static> {
    fn content(&self) -> Text<'_> {
        self.clone()
    }
}

// ── State ──────────────────────────────────────────────────────

pub struct ScrollPaneState {
    /// Scroll offset in visual (wrapped) lines.
    pub offset: u16,
    /// When true, auto-scroll to bottom on new content.
    /// Set to false when user scrolls up.
    pub pinned: bool,
    /// Cached wrapped height per item.
    heights: Vec<u16>,
    /// Width these heights were computed at.
    cached_width: u16,
    /// Total visual lines (sum of heights).
    pub total_visual: u16,
    /// Last rendered viewport height.
    pub viewport_height: u16,
}

impl Default for ScrollPaneState {
    fn default() -> Self {
        Self {
            offset: 0,
            pinned: false,
            heights: Vec::new(),
            cached_width: 0,
            total_visual: 0,
            viewport_height: 0,
        }
    }
}

impl ScrollPaneState {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn scroll_up(&mut self, n: u16) {
        self.offset = self.offset.saturating_sub(n);
        self.pinned = true; // user is scrolling, pin position
    }

    pub fn scroll_down(&mut self, n: u16) {
        let max = self.max_offset();
        self.offset = (self.offset + n).min(max);
        if self.offset >= max {
            self.pinned = false; // back at bottom, unpin
        }
    }

    fn max_offset(&self) -> u16 {
        self.total_visual.saturating_sub(self.viewport_height)
    }

    /// Invalidate height cache (e.g. when items change).
    pub fn invalidate(&mut self) {
        self.heights.clear();
        self.cached_width = 0;
    }

    /// Invalidate heights from index onwards (for append-only patterns).
    pub fn invalidate_from(&mut self, index: usize) {
        self.heights.truncate(index);
    }

    /// Convert a screen row (relative to viewport) to an item index and
    /// column, given the content items for text extraction.
    pub fn screen_to_item(&self, mouse_x: u16, mouse_y: u16, lines: &[Line<'_>]) -> Option<(usize, usize)> {
        if lines.is_empty() || self.cached_width == 0 {
            return None;
        }
        let (first, sub_scroll, _) = visible_range(&self.heights, self.offset, self.viewport_height);

        let mut row = -(sub_scroll as i32);
        for line_idx in first..lines.len() {
            let h = self.heights.get(line_idx).copied().unwrap_or(1) as i32;
            if (mouse_y as i32) < row + h {
                let line_text: String = lines[line_idx].spans.iter().map(|s| s.content.as_ref()).collect();
                let col = (mouse_x as usize).min(line_text.len());
                return Some((line_idx, col));
            }
            row += h;
        }
        Some((lines.len().saturating_sub(1), 0))
    }

    /// Compute or update cached heights for the given items and width.
    fn ensure_heights<T: ScrollItem>(&mut self, items: &[T], width: u16, gutter_width: u16) {
        let text_width = width.saturating_sub(gutter_width);
        self.ensure_heights_inner(items.len(), text_width, |i| {
            Paragraph::new(items[i].content())
                .wrap(Wrap { trim: false })
                .line_count(text_width) as u16
        });
    }

    fn ensure_heights_inner(&mut self, count: usize, text_width: u16, height_fn: impl Fn(usize) -> u16) {
        if text_width == 0 {
            return;
        }

        if self.cached_width != text_width {
            self.heights.clear();
            self.cached_width = text_width;
        }

        while self.heights.len() < count {
            let h = height_fn(self.heights.len());
            self.heights.push(h.max(1));
        }

        self.heights.truncate(count);
        self.total_visual = self.heights.iter().sum();
    }
}

// ── Widget ─────────────────────────────────────────────────────

pub struct ScrollPane<'a, T> {
    items: &'a [T],
    block: Option<Block<'a>>,
    gutter_width: u16,
    pin_to_bottom: bool,
}

impl<'a, T: ScrollItem> ScrollPane<'a, T> {
    pub fn new(items: &'a [T]) -> Self {
        Self {
            items,
            block: None,
            gutter_width: 0,
            pin_to_bottom: false,
        }
    }

    pub fn block(mut self, block: Block<'a>) -> Self {
        self.block = Some(block);
        self
    }

    pub fn gutter_width(mut self, width: u16) -> Self {
        self.gutter_width = width;
        self
    }

    pub fn pin_to_bottom(mut self, pin: bool) -> Self {
        self.pin_to_bottom = pin;
        self
    }
}

impl<T: ScrollItem> StatefulWidget for ScrollPane<'_, T> {
    type State = ScrollPaneState;

    fn render(self, area: Rect, buf: &mut Buffer, state: &mut Self::State) {
        // Render block and get inner area
        let inner = if let Some(ref block) = self.block {
            let inner = block.inner(area);
            block.clone().render(area, buf);
            inner
        } else {
            area
        };

        if inner.width < 2 || inner.height == 0 {
            return;
        }

        state.viewport_height = inner.height;

        // Compute heights
        state.ensure_heights(self.items, inner.width, self.gutter_width);

        // Auto-scroll to bottom
        if self.pin_to_bottom && !state.pinned {
            state.offset = state.max_offset();
        }

        // Clamp offset
        state.offset = state.offset.min(state.max_offset());

        // Find visible range
        let (first, sub_scroll, last) =
            visible_range(&state.heights, state.offset, inner.height);

        // Split into gutter + text areas
        let (gutter_area, text_area) = if self.gutter_width > 0 {
            let cols = Layout::horizontal([
                Constraint::Length(self.gutter_width),
                Constraint::Min(1),
            ])
            .split(inner);
            (Some(cols[0]), cols[1])
        } else {
            (None, inner)
        };

        // Render visible items
        let mut content_lines: Vec<Line<'_>> = Vec::new();
        let mut gutter_lines: Vec<Line<'_>> = Vec::new();

        for i in first..last {
            let item = &self.items[i];

            for line in item.content().lines {
                content_lines.push(line);
            }

            // Gutter: annotation at the first visual line of each
            // item, blank lines for the rest (including wrapped lines).
            if self.gutter_width > 0 {
                let item_height = state.heights[i] as usize;
                if let Some(g) = item.gutter() {
                    gutter_lines.push(Line::from(g));
                } else {
                    gutter_lines.push(Line::raw(""));
                }
                for _ in 1..item_height {
                    gutter_lines.push(Line::raw(""));
                }
            }
        }

        // Render text
        let text_para = Paragraph::new(content_lines)
            .wrap(Wrap { trim: false })
            .scroll((sub_scroll, 0));
        text_para.render(text_area, buf);

        // Render gutter
        if let Some(gutter_area) = gutter_area {
            let gutter_para = Paragraph::new(gutter_lines)
                .scroll((sub_scroll, 0));
            gutter_para.render(gutter_area, buf);
        }

        // Render scrollbar
        let content_len = state.total_visual as usize;
        let visible = inner.height as usize;
        if content_len > visible {
            let mut sb_state = ScrollbarState::new(content_len)
                .position(state.offset as usize);
            Scrollbar::new(ScrollbarOrientation::VerticalRight).render(
                inner.inner(Margin { vertical: 0, horizontal: 0 }),
                buf,
                &mut sb_state,
            );
        }
    }
}

// ── Visible range computation ──────────────────────────────────

/// Given per-item wrapped heights, a scroll offset in visual lines,
/// and the viewport height, return:
///   (first_item, sub_scroll_within_first_item, last_item_exclusive)
pub fn visible_range(heights: &[u16], scroll: u16, viewport: u16) -> (usize, u16, usize) {
    if heights.is_empty() {
        return (0, 0, 0);
    }

    // Find first visible item
    let mut row = 0u16;
    let mut first = 0;
    let mut row_at_first = 0u16;
    for (i, &h) in heights.iter().enumerate() {
        if row + h > scroll {
            first = i;
            row_at_first = row;
            break;
        }
        row += h;
        if i == heights.len() - 1 {
            first = heights.len();
            row_at_first = row;
        }
    }

    let sub_scroll = scroll.saturating_sub(row_at_first);

    // Find last visible item
    let mut last = first;
    let mut visible = 0u16;
    for i in first..heights.len() {
        visible += heights[i];
        last = i + 1;
        if visible >= viewport + sub_scroll {
            break;
        }
    }

    (first, sub_scroll, last)
}

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

    #[test]
    fn visible_range_basic() {
        let heights = vec![1, 1, 1, 1, 1];
        assert_eq!(visible_range(&heights, 0, 3), (0, 0, 3));
        assert_eq!(visible_range(&heights, 2, 3), (2, 0, 5));
    }

    #[test]
    fn visible_range_wrapped() {
        // Item 1 wraps to 3 lines, others are 1 line
        let heights = vec![1, 3, 1, 1];
        assert_eq!(visible_range(&heights, 0, 3), (0, 0, 2));
        assert_eq!(visible_range(&heights, 1, 3), (1, 0, 2));
        assert_eq!(visible_range(&heights, 2, 3), (1, 1, 3));
    }

    #[test]
    fn visible_range_empty() {
        let heights: Vec<u16> = vec![];
        assert_eq!(visible_range(&heights, 0, 10), (0, 0, 0));
    }
}