From b4f666118febfc95bf97e31aa1e39e3df9522dfa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Mon, 25 Apr 2022 19:57:18 +0200
Subject: [PATCH] grid: add abs-to-sb and sb-to-abs utility function
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These functions convert row numbers between absolute coordinates and
“scrollback relative” coordinates.

Absolute row numbers can be used to index into the grid->rows[] array.

Scrollback relative numbers are ordered with the *oldest* row first,
and the *newest* row last. That is, in these coordinates, row 0 is the
*first* (oldest) row in the scrollback history, and row N is the
*last* (newest) row.

Scrollback relative numbers are used when we need to sort things after
their age, when determining if something has scrolled out, or when
limiting an operation to ensure we don’t go past the scrollback
wrap-around.
---
 grid.c | 28 ++++++++++++++++++++++++++++
 grid.h |  5 +++++
 2 files changed, 33 insertions(+)

diff --git a/grid.c b/grid.c
index 5a0b5c7e..4a3995a0 100644
--- a/grid.c
+++ b/grid.c
@@ -15,6 +15,34 @@
 
 #define TIME_REFLOW 0
 
+/*
+ * “sb” (scrollback relative) coordinates
+ *
+ * The scrollback relative row number 0 is the *first*, and *oldest*
+ * row in the scrollback history (and thus the *first* row to be
+ * scrolled out). Thus, a higher number means further *down* in the
+ * scrollback, with the *highest* number being at the bottom of the
+ * screen, where new input appears.
+ */
+int
+grid_row_abs_to_sb(const struct grid *grid, int screen_rows, int abs_row)
+{
+    const int scrollback_start = grid->offset + screen_rows;
+    int rebased_row = abs_row - scrollback_start + grid->num_rows;
+
+    rebased_row &= grid->num_rows - 1;
+    return rebased_row;
+}
+
+int grid_row_sb_to_abs(const struct grid *grid, int screen_rows, int sb_rel_row)
+{
+    const int scrollback_start = grid->offset + screen_rows;
+    int abs_row = sb_rel_row + scrollback_start;
+
+    abs_row &= grid->num_rows - 1;
+    return abs_row;
+}
+
 static void
 ensure_row_has_extra_data(struct row *row)
 {
diff --git a/grid.h b/grid.h
index 7819db4d..22bd76bb 100644
--- a/grid.h
+++ b/grid.h
@@ -21,6 +21,10 @@ void grid_resize_and_reflow(
     size_t tracking_points_count,
     struct coord *const _tracking_points[static tracking_points_count]);
 
+/* Convert row numbers between scrollback-relative and absolute coordinates */
+int grid_row_abs_to_sb(const struct grid *grid, int screen_rows, int abs_row);
+int grid_row_sb_to_abs(const struct grid *grid, int screen_rows, int sb_rel_row);
+
 static inline int
 grid_row_absolute(const struct grid *grid, int row_no)
 {
@@ -33,6 +37,7 @@ grid_row_absolute_in_view(const struct grid *grid, int row_no)
     return (grid->view + row_no) & (grid->num_rows - 1);
 }
 
+
 static inline struct row *
 _grid_row_maybe_alloc(struct grid *grid, int row_no, bool alloc_if_null)
 {