use std::cmp::{max, min};

/// Holds the information about the displayed window of lines.
/// When there's too much lines to display in one screen, we can scroll
/// and this struct is responsible for that.
/// Scrolling is done with `scroll_to`, `scroll_up_one`, `scroll_down_one`
/// methods.
#[derive(Debug, Clone)]
pub struct ContentWindow {
    /// The index of the first displayed file.
    pub top: usize,
    /// The index of the last displayed file.
    pub bottom: usize,
    /// The number of displayble file in the current folder.
    pub len: usize,
    /// The height of the terminal.
    pub height: usize,
}

impl ContentWindow {
    /// Returns a new `ContentWindow` instance with values depending of
    /// number of files and height of the terminal screen.
    pub fn new(len: usize, height: usize) -> Self {
        ContentWindow {
            top: 0,
            bottom: min(len, height - Self::RESERVED_ROWS),
            len,
            height: height - Self::RESERVED_ROWS,
        }
    }

    /// The padding around the last displayed file
    const WINDOW_PADDING: usize = 4;
    /// The space of the top element
    pub const WINDOW_MARGIN_TOP: usize = 2;
    /// How many rows are reserved at bottom
    const RESERVED_ROWS: usize = 3;

    /// Set the height of file window.
    pub fn set_height(&mut self, height: usize) {
        self.height = height - Self::RESERVED_ROWS;
    }

    /// Move the window one line up if possible.
    pub fn scroll_up_one(&mut self, index: usize) {
        if index < self.top + Self::WINDOW_PADDING && self.top > 0 {
            self.top -= 1;
            self.bottom -= 1;
        }
    }

    /// Move the window one line down if possible.
    pub fn scroll_down_one(&mut self, index: usize) {
        if self.len < self.height {
            return;
        }
        if index > self.bottom - Self::WINDOW_PADDING
            && self.bottom < self.len - Self::WINDOW_MARGIN_TOP
        {
            self.top += 1;
            self.bottom += 1;
        }
    }

    /// Reset the window to the first files of the current directory.
    pub fn reset(&mut self, len: usize) {
        self.len = len;
        self.top = 0;
        self.bottom = min(len, self.height);
    }

    /// Scroll the window to this index if possible.
    pub fn scroll_to(&mut self, index: usize) {
        if index < self.top || index > self.bottom {
            self.top = max(index, Self::WINDOW_PADDING) - Self::WINDOW_PADDING;
            self.bottom = self.top + min(self.len, self.height - 3);
        }
    }
}