From 95a8048282609c534cb1237b62415c2f01a2af11 Mon Sep 17 00:00:00 2001
From: Greg Depoire--Ferrer <greg.depoire@gmail.com>
Date: Sat, 13 Jun 2020 20:11:22 +0200
Subject: [PATCH 1/3] Add fast path to til::bitmap::translate using bit shifts

---
 src/inc/til/bitmap.h | 56 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/src/inc/til/bitmap.h b/src/inc/til/bitmap.h
index 3cb88830a54..ddc072d0361 100644
--- a/src/inc/til/bitmap.h
+++ b/src/inc/til/bitmap.h
@@ -194,6 +194,13 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
         // optional fill the uncovered area with bits.
         void translate(const til::point delta, bool fill = false)
         {
+            if (delta.x() == 0)
+            {
+                // fast path by using bit shifting
+                translate_y(delta.y(), fill);
+                return;
+            }
+
             // FUTURE: PERF: GH #4015: This could use in-place walk semantics instead of a temporary.
             til::bitmap other{ _sz };
 
@@ -382,6 +389,55 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
         }
 
     private:
+        void translate_y(ptrdiff_t delta_y, bool fill)
+        {
+            if (delta_y == 0)
+            {
+                return;
+            }
+
+            const auto bitShift = delta_y * _sz.width();
+
+            const auto newBits = static_cast<size_t>(std::abs(bitShift));
+            const bool isLeftShift = bitShift > 0;
+
+            if (newBits >= _bits.size())
+            {
+                if (fill)
+                {
+                    set_all();
+                }
+                else
+                {
+                    reset_all();
+                }
+                return;
+            }
+
+            if (isLeftShift)
+            {
+                // This operator doesn't modify the size of `_bits`: the
+                // new bits are set to 0.
+                _bits <<= newBits;
+            }
+            else
+            {
+                _bits >>= newBits;
+            }
+
+            if (fill)
+            {
+                if (isLeftShift)
+                {
+                    _bits.set(0, newBits, true);
+                }
+                else
+                {
+                    _bits.set(_bits.size() - newBits, newBits, true);
+                }
+            }
+        }
+
         til::size _sz;
         til::rectangle _rc;
         dynamic_bitset<> _bits;

From 37f33f66ccdea54948f15701825faf9576472cca Mon Sep 17 00:00:00 2001
From: Greg Depoire--Ferrer <greg.depoire@gmail.com>
Date: Sat, 13 Jun 2020 20:45:29 +0200
Subject: [PATCH 2/3] fix Static Analysis

---
 src/inc/til/bitmap.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/inc/til/bitmap.h b/src/inc/til/bitmap.h
index ddc072d0361..752c998c463 100644
--- a/src/inc/til/bitmap.h
+++ b/src/inc/til/bitmap.h
@@ -398,7 +398,11 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
 
             const auto bitShift = delta_y * _sz.width();
 
+#pragma warning(push)
+            // we can't depend on GSL here (some libraries use BLOCK_GSL), so we use static_cast for explicit narrowing
+#pragma warning(disable : 26472)
             const auto newBits = static_cast<size_t>(std::abs(bitShift));
+#pragma warning(pop)
             const bool isLeftShift = bitShift > 0;
 
             if (newBits >= _bits.size())

From 1e1491c921c3fe4680ca51f4ebefad0422b85226 Mon Sep 17 00:00:00 2001
From: Greg Depoire--Ferrer <greg.depoire@gmail.com>
Date: Sun, 14 Jun 2020 09:03:47 +0200
Subject: [PATCH 3/3] Add _runs.reset() in translate_y

---
 src/inc/til/bitmap.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/inc/til/bitmap.h b/src/inc/til/bitmap.h
index 752c998c463..d1f5dff7fd1 100644
--- a/src/inc/til/bitmap.h
+++ b/src/inc/til/bitmap.h
@@ -440,6 +440,8 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
                     _bits.set(_bits.size() - newBits, newBits, true);
                 }
             }
+
+            _runs.reset(); // reset cached runs on any non-const method
         }
 
         til::size _sz;