Add StableRowIndex concept

This will make it easier to manage both the viewport and the
selection in the gui layer.

refs: https://github.com/wez/wezterm/issues/106

term/src/lib.rs

@@ -53,6 +53,20 @@ pub type VisibleRowIndex = i64;
 /// to want to scroll back or select more than ~2billion lines of scrollback.
 pub type ScrollbackOrVisibleRowIndex = i32;
 
+/// Allows referencing a logical line in the scrollback, allowing for scrolling.
+/// The StableRowIndex counts from the top of the scrollback, growing larger
+/// as you move down through the display rows.
+/// Initially the very first line as StableRowIndex==0.  If the scrollback
+/// is filled and lines are purged (say we need to purge 5 lines), then whichever
+/// line is first in the scrollback (PhysRowIndex==0) will now have StableRowIndex==5
+/// which is the same value that that logical line had prior to data being purged
+/// out of the scrollback.
+///
+/// As per ScrollbackOrVisibleRowIndex above, a StableRowIndex can never
+/// legally be a negative number.  We're just using a differently sized type
+/// to have the compiler assist us in detecting improper usage.
+pub type StableRowIndex = isize;
+
 /// Returns true if r1 intersects r2
 pub fn intersects_range<T: Ord + Copy>(r1: Range<T>, r2: Range<T>) -> bool {
     use std::cmp::{max, min};

term/src/screen.rs

@@ -19,6 +19,11 @@ pub struct Screen {
     /// would otherwise have exceeded the line capacity
     pub lines: VecDeque<Line>,
 
+    /// Whenever we scroll a line off the top of the scrollback, we
+    /// increment this.  We use this offset to translate between
+    /// PhysRowIndex and StableRowIndex.
+    stable_row_index_offset: usize,
+
     /// config so we can access Maximum number of lines of scrollback
     config: Arc<dyn TerminalConfiguration>,
     allow_scrollback: bool,
@@ -62,6 +67,7 @@ impl Screen {
             allow_scrollback,
             physical_rows,
             physical_cols,
+            stable_row_index_offset: 0,
         }
     }
 
@@ -197,6 +203,27 @@ impl Screen {
         self.phys_row(range.start)..self.phys_row(range.end)
     }
 
+    #[inline]
+    pub fn phys_to_stable_row_index(&self, phys: PhysRowIndex) -> StableRowIndex {
+        (phys + self.stable_row_index_offset) as StableRowIndex
+    }
+
+    #[inline]
+    pub fn stable_row_to_phys(&self, stable: StableRowIndex) -> Option<PhysRowIndex> {
+        let idx = stable - self.stable_row_index_offset as isize;
+        if idx < 0 || idx >= self.lines.len() as isize {
+            // Index is no longer valid
+            None
+        } else {
+            Some(idx as PhysRowIndex)
+        }
+    }
+
+    #[inline]
+    pub fn visible_row_to_stable_row(&self, vis: VisibleRowIndex) -> StableRowIndex {
+        self.phys_to_stable_row_index(self.phys_row(vis))
+    }
+
     /// ---------
     /// |
     /// |--- top
@@ -268,6 +295,10 @@ impl Screen {
             self.lines.remove(remove_idx);
         }
 
+        if remove_idx == 0 {
+            self.stable_row_index_offset += lines_removed;
+        }
+
         if scroll_region.end as usize == self.physical_rows {
             // It's cheaper to push() than it is insert() at the end
             for _ in 0..to_add {

term/src/test/mod.rs

@@ -529,20 +529,35 @@ fn test_scrollup() {
     let mut term = TestTerm::new(2, 1, 4);
     term.print("1\n");
     assert_all_contents(&term, &["1", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 0);
+
     term.print("2\n");
     assert_all_contents(&term, &["1", "2", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 1);
+
     term.print("3\n");
     assert_all_contents(&term, &["1", "2", "3", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 2);
+
     term.print("4\n");
     assert_all_contents(&term, &["1", "2", "3", "4", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 3);
+
     term.print("5\n");
     assert_all_contents(&term, &["1", "2", "3", "4", "5", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 4);
+
     term.print("6\n");
     assert_all_contents(&term, &["2", "3", "4", "5", "6", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 5);
+
     term.print("7\n");
     assert_all_contents(&term, &["3", "4", "5", "6", "7", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 6);
+
     term.print("8\n");
     assert_all_contents(&term, &["4", "5", "6", "7", "8", " "]);
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 7);
 }
 
 #[test]