Do not use a hash to determine if a window must be redrawn

Collision happens
Fixes #569

2 files changed, 39 insertions, 17 deletions

diff --git a/src/window.cc b/src/window.cc
index d4429407..4ebd9e2c 100644
--- a/src/window.cc
+++ b/src/window.cc
@@ -72,22 +72,38 @@ void Window::center_column(CharCount buffer_column)
     display_column_at(buffer_column, m_dimensions.column/2_char);
 }
 
-size_t Window::compute_hash(const Context& context) const
+Window::Setup Window::build_setup(const Context& context) const
 {
-    size_t res = hash_values(m_position, context.ui().dimensions(), context.buffer().timestamp());
-
-    auto& selections = context.selections();
-    res = combine_hash(res, hash_value(selections.main_index()));
-    for (auto& sel : selections)
-        res = combine_hash(res, hash_values((const ByteCoord&)sel.cursor(), sel.anchor()));
-
-    return res;
+    Vector<BufferRange> selections;
+    for (auto& sel : context.selections())
+        selections.push_back({sel.cursor(), sel.anchor()});
+
+    return { m_position,
+             context.ui().dimensions(),
+             context.buffer().timestamp(),
+             context.selections().main_index(),
+             std::move(selections) };
 }
 
 bool Window::needs_redraw(const Context& context) const
 {
-    size_t hash = compute_hash(context);
-    return hash != m_hash;
+    auto& selections = context.selections();
+
+    if (m_position != m_last_setup.position or
+        context.ui().dimensions() != m_last_setup.dimensions or
+        context.buffer().timestamp() != m_last_setup.timestamp or
+        selections.main_index() != m_last_setup.main_selection or
+        selections.size() != m_last_setup.selections.size())
+        return true;
+
+    for (int i = 0; i < selections.size(); ++i)
+    {
+        if (selections[i].cursor() != m_last_setup.selections[i].begin or
+            selections[i].anchor() != m_last_setup.selections[i].end)
+            return true;
+    }
+
+    return false;
 }
 
 const DisplayBuffer& Window::update_display_buffer(const Context& context)
@@ -120,7 +136,7 @@ const DisplayBuffer& Window::update_display_buffer(const Context& context)
         line.trim(m_position.column, m_dimensions.column, true);
     m_display_buffer.optimize();
 
-    m_hash = compute_hash(context);
+    m_last_setup = build_setup(context);
 
     return m_display_buffer;
 }
diff --git a/src/window.hh b/src/window.hh
index fd347c79..40e0fef8 100644
--- a/src/window.hh
+++ b/src/window.hh
@@ -41,7 +41,7 @@ public:
     Buffer& buffer() const { return *m_buffer; }
 
     bool needs_redraw(const Context& context) const;
-    void force_redraw() { m_hash = -1; }
+    void force_redraw() { m_last_setup = Setup{}; }
 
     ByteCoord offset_coord(ByteCoord coord, CharCount offset);
     ByteCoordAndTarget offset_coord(ByteCoordAndTarget coord, LineCount offset);
@@ -55,8 +55,6 @@ private:
 
     void run_hook_in_own_context(StringView hook_name, StringView param);
 
-    size_t compute_hash(const Context& context) const;
-
     SafePtr<Buffer> m_buffer;
 
     CharCoord m_position;
@@ -66,8 +64,16 @@ private:
     HighlighterGroup m_highlighters;
     HighlighterGroup m_builtin_highlighters;
 
-    // hash used to determine if a redraw is necessary
-    size_t m_hash = -1;
+    struct Setup
+    {
+        CharCoord position;
+        CharCoord dimensions;
+        size_t timestamp;
+        size_t main_selection;
+        Vector<BufferRange> selections;
+    };
+    Setup build_setup(const Context& context) const;
+    Setup m_last_setup;
 };
 
 }