1 files changed, 119 insertions, 0 deletions
diff --git a/src/modes/display/content_window.rs b/src/modes/display/content_window.rs
new file mode 100644
index 0000000..42adb9c
--- /dev/null
+++ b/src/modes/display/content_window.rs
@@ -0,0 +1,119 @@
+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 element.
+    pub top: usize,
+    /// The index of the last displayed element + 1.
+    pub bottom: usize,
+    /// The number of displayble elements.
+    pub len: usize,
+    /// The height of the terminal.
+    pub height: usize,
+}
+
+impl ContentWindow {
+    /// The padding around the last displayed filename
+    const WINDOW_PADDING: usize = 4;
+    /// The space of the top element
+    pub const WINDOW_MARGIN_TOP: usize = 2;
+    /// How many rows are reserved for the header ?
+    pub const HEADER_ROWS: usize = 3;
+    /// How many rows are reserved for the footer ?
+    const FOOTER_ROWS: usize = 1;
+    /// Footer and bottom padding
+    const BOTTOM_ROWS: usize = 2;
+
+    /// How many rows could be displayed with given height ?
+    /// It's not the number of rows displayed since the content may
+    /// not be long enough to fill the window.
+    fn nb_displayed_rows(height: usize) -> usize {
+        height - Self::HEADER_ROWS - Self::FOOTER_ROWS
+    }
+
+    /// Default value for the bottom index.
+    /// minimum of terminal height minus reserved rows and the length of the content.
+    fn default_bottom(len: usize, height: usize) -> usize {
+        min(height - Self::BOTTOM_ROWS, len)
+    }
+
+    /// Returns a new `ContentWindow` instance with values depending of
+    /// number of displayable elements and height of the terminal screen.
+    pub fn new(len: usize, terminal_height: usize) -> Self {
+        let height = Self::nb_displayed_rows(terminal_height);
+        let top = 0;
+        let bottom = Self::default_bottom(len, height);
+        ContentWindow {
+            top,
+            bottom,
+            len,
+            height,
+        }
+    }
+
+    /// Set the height of file window.
+    pub fn set_height(&mut self, terminal_height: usize) {
+        self.height = Self::nb_displayed_rows(terminal_height);
+        self.bottom = Self::default_bottom(self.len, self.height);
+    }
+
+    /// Move the window one line up if possible.
+    /// Does nothing if the index can't be reached.
+    pub fn scroll_up_one(&mut self, index: usize) {
+        if (index < self.top + Self::WINDOW_PADDING || index > self.bottom) && self.top > 0 {
+            self.top -= 1;
+            self.bottom -= 1;
+        }
+        self.scroll_to(index)
+    }
+
+    /// Move the window one line down if possible.
+    /// Does nothing if the index can't be reached.
+    pub fn scroll_down_one(&mut self, index: usize) {
+        if self.len < self.height {
+            return;
+        }
+        if index < self.top || index > self.bottom - Self::WINDOW_PADDING {
+            self.top += 1;
+            self.bottom += 1;
+        }
+        self.scroll_to(index)
+    }
+
+    /// Scroll the window to this index if possible.
+    /// Does nothing if the index can't be reached.
+    pub fn scroll_to(&mut self, index: usize) {
+        if self.len < self.height {
+            return;
+        }
+        if self.is_index_outside_window(index) {
+            self.top = max(index, Self::WINDOW_PADDING) - Self::WINDOW_PADDING;
+            self.bottom = (self.top + Self::default_bottom(self.height, self.len))
+                .checked_sub(Self::BOTTOM_ROWS)
+                .unwrap_or(2);
+        }
+    }
+
+    /// Reset the window to the first item of the content.
+    pub fn reset(&mut self, len: usize) {
+        self.len = len;
+        self.top = 0;
+        self.bottom = Self::default_bottom(self.len, self.height);
+    }
+
+    /// True iff the index is outside the displayed window or
+    /// too close from the border.
+    /// User shouldn't be able to reach the last elements
+    fn is_index_outside_window(&self, index: usize) -> bool {
+        index < self.top || index >= self.bottom
+    }
+
+    pub fn is_row_in_header(row: u16) -> bool {
+        row < Self::HEADER_ROWS as u16
+    }
+}