fresh-editor 0.2.25

//! Pure visual-row-aware scrollbar math.
//!
//! Two functions:
//!
//! - [`scrollbar_jump_visual`] — convert a click ratio on the scrollbar
//!   track into a `(line_byte, view_line_offset)` scroll target.
//! - [`scrollbar_drag_relative_visual`] — follow the thumb under the mouse
//!   as the user drags, preserving the click offset on the thumb so the
//!   cursor stays pinned to the same spot on the thumb.
//!
//! Both take a `&mut Buffer` only because `Buffer::line_iterator` requires
//! mutable access for lazy-load internals; neither touches `Editor`.

use crate::model::buffer::Buffer;
use crate::primitives::line_wrapping::{wrap_line, WrapConfig};

/// Width estimate of the gutter, used to build the wrap config. Kept in
/// sync with the real gutter sizing in the render path (indicator + digits
/// + separator).
fn estimated_gutter_width(buffer: &Buffer) -> usize {
    let line_count = buffer.line_count().unwrap_or(1);
    let digits = (line_count as f64).log10().floor() as usize + 1;
    1 + digits.max(4) + 3
}

/// Build a map of `(line_start_byte, visual_row_offset_within_line)` for
/// every visual row in the buffer, and the total row count.
fn build_visual_row_map(
    buffer: &mut Buffer,
    viewport_width: usize,
) -> (Vec<(usize, usize)>, usize) {
    let gutter_width = estimated_gutter_width(buffer);
    let wrap_config = WrapConfig::new(viewport_width, gutter_width, true, true);

    let mut total_visual_rows = 0;
    let mut visual_row_positions: Vec<(usize, usize)> = Vec::new();

    let mut iter = buffer.line_iterator(0, 80);
    while let Some((line_start, content)) = iter.next_line() {
        let line_content = content.trim_end_matches(['\n', '\r']).to_string();
        let segments = wrap_line(&line_content, &wrap_config);
        let visual_rows_in_line = segments.len().max(1);

        for offset in 0..visual_rows_in_line {
            visual_row_positions.push((line_start, offset));
        }
        total_visual_rows += visual_rows_in_line;
    }

    (visual_row_positions, total_visual_rows)
}

/// Pick the `(byte, offset)` at `target_row`, falling back to the last
/// valid row if the index is out of range.
fn position_at(visual_row_positions: &[(usize, usize)], target_row: usize) -> (usize, usize) {
    if target_row < visual_row_positions.len() {
        visual_row_positions[target_row]
    } else {
        visual_row_positions.last().copied().unwrap_or((0, 0))
    }
}

/// Calculate scroll position for a visual-row-aware scrollbar *jump*.
///
/// Returns `(byte_position, view_line_offset)` — the start of the line
/// and the wrap-segment offset inside that line.
pub(crate) fn scrollbar_jump_visual(
    buffer: &mut Buffer,
    ratio: f64,
    viewport_height: usize,
    viewport_width: usize,
) -> (usize, usize) {
    if buffer.len() == 0 || viewport_height == 0 {
        return (0, 0);
    }

    let (visual_row_positions, total_visual_rows) = build_visual_row_map(buffer, viewport_width);
    if total_visual_rows == 0 {
        return (0, 0);
    }

    let max_scroll_row = total_visual_rows.saturating_sub(viewport_height);
    if max_scroll_row == 0 {
        // Content fits in viewport, no scrolling needed
        return (0, 0);
    }

    let target_row = (ratio * max_scroll_row as f64).round() as usize;
    let target_row = target_row.min(max_scroll_row);

    position_at(&visual_row_positions, target_row)
}

/// Calculate scroll position for a visual-row-aware scrollbar *drag*.
///
/// The thumb follows the mouse position, accounting for where on the thumb
/// the user originally clicked.
#[allow(clippy::too_many_arguments)]
pub(crate) fn scrollbar_drag_relative_visual(
    buffer: &mut Buffer,
    current_row: u16,
    scrollbar_y: u16,
    scrollbar_height: usize,
    drag_start_row: u16,
    drag_start_top_byte: usize,
    drag_start_view_line_offset: usize,
    viewport_height: usize,
    viewport_width: usize,
) -> (usize, usize) {
    if buffer.len() == 0 || viewport_height == 0 || scrollbar_height <= 1 {
        return (0, 0);
    }

    let (visual_row_positions, total_visual_rows) = build_visual_row_map(buffer, viewport_width);
    if total_visual_rows == 0 {
        return (0, 0);
    }

    let max_scroll_row = total_visual_rows.saturating_sub(viewport_height);
    if max_scroll_row == 0 {
        return (0, 0);
    }

    // Find the visual row corresponding to drag_start_top_byte + view_line_offset.
    // First find the line start, then add the offset for wrapped lines.
    let line_start_visual_row = visual_row_positions
        .iter()
        .position(|(byte, _)| *byte >= drag_start_top_byte)
        .unwrap_or(0);
    let start_visual_row =
        (line_start_visual_row + drag_start_view_line_offset).min(max_scroll_row);

    // Thumb size — same formula as the scrollbar renderer.
    let thumb_size_raw = (viewport_height as f64 / total_visual_rows as f64
        * scrollbar_height as f64)
        .ceil() as usize;
    let max_thumb_size = (scrollbar_height as f64 * 0.8).floor() as usize;
    let thumb_size = thumb_size_raw
        .max(1)
        .min(max_thumb_size)
        .min(scrollbar_height);

    let max_thumb_start = scrollbar_height.saturating_sub(thumb_size);

    // Where the thumb was at drag start, in scrollbar coordinates.
    let start_scroll_ratio = start_visual_row as f64 / max_scroll_row as f64;
    let thumb_row_at_start = scrollbar_y as f64 + start_scroll_ratio * max_thumb_start as f64;

    // Offset within the thumb where the user clicked.
    let click_offset = drag_start_row as f64 - thumb_row_at_start;

    // Target thumb position from current mouse position.
    let target_thumb_row = current_row as f64 - click_offset;

    // Inverse of the thumb_start formula.
    let target_scroll_ratio = if max_thumb_start > 0 {
        ((target_thumb_row - scrollbar_y as f64) / max_thumb_start as f64).clamp(0.0, 1.0)
    } else {
        0.0
    };

    let target_row = (target_scroll_ratio * max_scroll_row as f64).round() as usize;
    let target_row = target_row.min(max_scroll_row);

    position_at(&visual_row_positions, target_row)
}