From 528e91aece2c073b38c0fd85e4efb16a5cc62555 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Sat, 15 May 2021 12:54:59 +0200
Subject: [PATCH] grid: take scrollback start into account when sorting the
 tracking points array
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The row numbers in the tracking points are in absolute
numbers. However, when we walk the old grid, we do so starting in the
beginning of the scrollback history.

We must ensure the tracking points are sorted such that the *first*
one we see is the “oldest” one. I.e. the one furthest back in the
scrollback history.
---
 grid.c | 38 +++++++++++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/grid.c b/grid.c
index bb8bc577..fbc9820d 100644
--- a/grid.c
+++ b/grid.c
@@ -384,18 +384,32 @@ _line_wrap(struct grid *old_grid, struct row **new_grid, struct row *row,
     return new_row;
 }
 
+struct tp_cmp_ctx {
+    int scrollback_start;
+    int rows;
+};
+
 static int
-tp_cmp(const void *_a, const void *_b)
+tp_cmp(const void *_a, const void *_b, void *_arg)
 {
     const struct coord *a = *(const struct coord **)_a;
     const struct coord *b = *(const struct coord **)_b;
+    const struct tp_cmp_ctx *ctx = _arg;
 
-    if (a->row < b->row)
+    int a_row = (a->row - ctx->scrollback_start + ctx->rows) & (ctx->rows - 1);
+    int b_row = (b->row - ctx->scrollback_start + ctx->rows) & (ctx->rows - 1);
+
+    xassert(a_row >= 0);
+    xassert(a_row < ctx->rows);
+    xassert(b_row >= 0);
+    xassert(b_row < ctx->rows);
+
+    if (a_row < b_row)
         return -1;
-    if (a->row > b->row)
+    if (a_row > b_row)
         return 1;
 
-    xassert(a->row == b->row);
+    xassert(a_row == b_row);
 
     if (a->col < b->col)
         return -1;
@@ -471,13 +485,23 @@ grid_resize_and_reflow(
     if (!view_follows)
         tracking_points[tracking_points_count + 2] = &viewport;
 
-    qsort(tracking_points, tp_count - 1, sizeof(tracking_points[0]), &tp_cmp);
+    if (old_rows > 0) {
+        qsort_r(
+            tracking_points, tp_count - 1, sizeof(tracking_points[0]), &tp_cmp,
+            &(struct tp_cmp_ctx){.scrollback_start = offset, .rows = old_rows});
+    }
 
     /* NULL terminate */
     struct coord terminator = {-1, -1};
     tracking_points[tp_count - 1] = &terminator;
     struct coord **next_tp = &tracking_points[0];
 
+    LOG_DBG("scrollback-start=%d", offset);
+    for (size_t i = 0; i < tp_count - 1; i++) {
+        LOG_DBG("TP #%zu: row=%d, col=%d",
+                i, tracking_points[i]->row, tracking_points[i]->col);
+    }
+
     /*
      * Walk the old grid
      */
@@ -613,10 +637,6 @@ grid_resize_and_reflow(
                     next_tp++;
                     tp = *next_tp;
                 } while (tp->row == old_row_idx && tp->col == c);
-
-                xassert((tp->row < 0 && tp->col < 0) ||
-                        tp->row > old_row_idx ||
-                        (tp->row == old_row_idx && tp->col > c));
             }
 
             if (unlikely(on_uri))