gitui 0.22.1

blazing fast terminal-ui for git
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use std::cell::Cell;

use tui::{backend::Backend, layout::Rect, Frame};

use crate::{
	components::ScrollType,
	ui::{draw_scrollbar, style::SharedTheme},
};

pub struct VerticalScroll {
	top: Cell<usize>,
	max_top: Cell<usize>,
}

impl VerticalScroll {
	pub const fn new() -> Self {
		Self {
			top: Cell::new(0),
			max_top: Cell::new(0),
		}
	}

	pub fn get_top(&self) -> usize {
		self.top.get()
	}

	pub fn reset(&self) {
		self.top.set(0);
	}

	pub fn move_top(&self, move_type: ScrollType) -> bool {
		let old = self.top.get();
		let max = self.max_top.get();

		let new_scroll_top = match move_type {
			ScrollType::Down => old.saturating_add(1),
			ScrollType::Up => old.saturating_sub(1),
			ScrollType::Home => 0,
			ScrollType::End => max,
			_ => old,
		};

		let new_scroll_top = new_scroll_top.clamp(0, max);

		if new_scroll_top == old {
			return false;
		}

		self.top.set(new_scroll_top);

		true
	}

	pub fn update(
		&self,
		selection: usize,
		selection_max: usize,
		visual_height: usize,
	) -> usize {
		let new_top = calc_scroll_top(
			self.get_top(),
			visual_height,
			selection,
			selection_max,
		);
		self.top.set(new_top);

		if visual_height == 0 {
			self.max_top.set(0);
		} else {
			let new_max = selection_max.saturating_sub(visual_height);
			self.max_top.set(new_max);
		}

		new_top
	}

	pub fn update_no_selection(
		&self,
		line_count: usize,
		visual_height: usize,
	) -> usize {
		self.update(self.get_top(), line_count, visual_height)
	}

	pub fn draw<B: Backend>(
		&self,
		f: &mut Frame<B>,
		r: Rect,
		theme: &SharedTheme,
	) {
		draw_scrollbar(
			f,
			r,
			theme,
			self.max_top.get(),
			self.top.get(),
		);
	}
}

const fn calc_scroll_top(
	current_top: usize,
	height_in_lines: usize,
	selection: usize,
	selection_max: usize,
) -> usize {
	if height_in_lines == 0 {
		return 0;
	}
	if selection_max <= height_in_lines {
		return 0;
	}

	if current_top + height_in_lines <= selection {
		selection.saturating_sub(height_in_lines) + 1
	} else if current_top > selection {
		selection
	} else {
		current_top
	}
}

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

	#[test]
	fn test_scroll_no_scroll_to_top() {
		assert_eq!(calc_scroll_top(1, 10, 4, 4), 0);
	}

	#[test]
	fn test_scroll_zero_height() {
		assert_eq!(calc_scroll_top(4, 0, 4, 3), 0);
	}
}