Faded ink on blank cells is not sent to the terminal: a focus change sent every blank of the other panes again

1 files changed, 116 insertions, 9 deletions

diff --git a/src/tty/tty.zig b/src/tty/tty.zig
index 84552ddc..97c720ea 100644
--- a/src/tty/tty.zig
+++ b/src/tty/tty.zig
@@ -1442,7 +1442,7 @@ const Shell = struct {
         const shadowed = chipShadows(s.frame.allocator(), surface, s.core, truecolor) catch surface;
         const flashed = s.flash.apply(s.frame.allocator(), shadowed, s.core, truecolor) catch shadowed;
         const trailed = s.trail.apply(s.frame.allocator(), flashed, s.core, truecolor) catch flashed;
-        paintCells(win, trailed.cells, trailed.cols, trailed.rows);
+        paintCells(vx, win, trailed.cells, trailed.cols, trailed.rows);
         tz_cells.end();
         for (surface.images[0..surface.nimages]) |maybe| {
             const place = maybe orelse continue;
@@ -2159,22 +2159,129 @@ fn mouseEvent(m: vaxis.Mouse) ?pardes.Event {
     } };
 }
 
-fn paintCells(win: vaxis.Window, cells: []const pardes.Cell, cols: u16, rows: u16) void {
+fn paintCells(vx: *const vaxis.Vaxis, win: vaxis.Window, cells: []const pardes.Cell, cols: u16, rows: u16) void {
     win.clear();
     var y: u16 = 0;
     while (y < rows) : (y += 1) {
+        const row = cells[@as(usize, y) * cols ..][0..cols];
         var x: u16 = 0;
+        // The cell to the left as written, for a blank that needs it.
+        var left: ?Written = null;
         while (x < cols) : (x += 1) {
-            const cell = &cells[@as(usize, y) * cols + x];
-            if (cell.default) continue;
-            win.writeCell(x, y, .{
-                .char = .{ .grapheme = cell.grapheme() },
-                .style = vaxisStyle(cell.style),
-            });
+            const cell = &row[x];
+            if (cell.default) {
+                left = null;
+                continue;
+            }
+            const grapheme = cell.grapheme();
+            var style = vaxisStyle(cell.style);
+            if (inkless(grapheme, style)) blankInk(vx, row, x, y, &style, left);
+            left = .{ .grapheme = grapheme, .style = style };
+            win.writeCell(x, y, .{ .char = .{ .grapheme = grapheme }, .style = style });
         }
     }
 }
 
+const Written = struct { grapheme: []const u8, style: vaxis.Style };
+
+/// A blank's ink shows nowhere (no underline, strike or reverse to show it).
+fn inkless(grapheme: []const u8, style: vaxis.Style) bool {
+    return std.mem.eql(u8, grapheme, " ") and !style.reverse and !style.strikethrough and style.ul_style == .off;
+}
+
+/// A blank whose ink alone changed (a fade: InactiveDim on a focus change)
+/// keeps the ink the terminal has there and so is not sent at all:
+/// otherwise every blank of every pane a fade touched went, 7 KB a click
+/// for three shells. Only a short gap between two cells that are sent
+/// anyway is cheaper sent than jumped (a cursor move): it takes the ink of
+/// the cell before it, so the run needs no colour change either.
+fn blankInk(vx: *const vaxis.Vaxis, row: []const pardes.Cell, x: u16, y: u16, style: *vaxis.Style, left: ?Written) void {
+    const last = lastCell(vx, x, y) orelse return;
+    if (vx.refresh or last.default or last.skipped or !std.mem.eql(u8, last.char.items, " ")) return;
+    if (sameColor(last.style.fg, style.fg) or !sameButInk(last.style, style.*)) return;
+    var kept = style.*;
+    kept.fg = last.style.fg;
+    if (left) |l| if (sends(vx, x - 1, y, l.grapheme, l.style)) {
+        var end = x + 1;
+        while (end < row.len and end - x < 8) : (end += 1) {
+            const next = &row[end];
+            if (next.default) break;
+            const next_style = vaxisStyle(next.style);
+            if (inkless(next.grapheme(), next_style)) continue;
+            if (sends(vx, end, y, next.grapheme(), next_style)) {
+                style.fg = l.style.fg;
+                return;
+            }
+            break;
+        }
+    };
+    style.* = kept;
+}
+
+/// `Style.eql` but for the ink, and cheap: this runs for every blank of
+/// every frame.
+fn sameButInk(a: vaxis.Style, b: vaxis.Style) bool {
+    return sameColor(a.bg, b.bg) and sameColor(a.ul, b.ul) and a.ul_style == b.ul_style and
+        a.bold == b.bold and a.dim == b.dim and a.italic == b.italic and a.blink == b.blink and
+        a.reverse == b.reverse and a.invisible == b.invisible and a.strikethrough == b.strikethrough;
+}
+
+fn sameColor(a: vaxis.Color, b: vaxis.Color) bool {
+    return switch (a) {
+        .default => b == .default,
+        .index => |i| b == .index and b.index == i,
+        .rgb => |rgb| b == .rgb and std.mem.eql(u8, &rgb, &b.rgb),
+    };
+}
+
+/// Whether vaxis sends this cell: it differs from what the terminal shows.
+fn sends(vx: *const vaxis.Vaxis, x: u16, y: u16, grapheme: []const u8, style: vaxis.Style) bool {
+    const last = lastCell(vx, x, y) orelse return true;
+    return vx.refresh or last.default or last.skipped or !std.mem.eql(u8, last.char.items, grapheme) or
+        !sameColor(last.style.fg, style.fg) or !sameButInk(last.style, style);
+}
+
+fn lastCell(vx: *const vaxis.Vaxis, x: u16, y: u16) ?*const vaxis.AllocatingScreen.InternalCell {
+    if (x >= vx.screen.width) return null;
+    const i = @as(usize, y) * vx.screen.width + x;
+    return if (i < vx.screen_last.buf.len) &vx.screen_last.buf[i] else null;
+}
+
+test "a fade sends the text it dims, not the blanks beside it" {
+    const gpa = std.testing.allocator;
+    var env = try std.testing.environ.createMap(gpa);
+    defer env.deinit();
+    var vx = try vaxis.init(std.testing.io, gpa, &env, .{});
+    var out: std.Io.Writer.Allocating = .init(gpa);
+    defer out.deinit();
+    defer vx.deinit(gpa, &out.writer);
+    try vx.resize(gpa, &out.writer, .{ .rows = 1, .cols = 32, .x_pixel = 0, .y_pixel = 0 });
+    // `a`, a long gap, `b`, a two-cell gap, `c`, a long gap, an underlined
+    // blank (which shows its ink) and one more blank.
+    var cells: [32]pardes.Cell = @splat(.{ .default = false });
+    cells[0].text[0] = 'a';
+    cells[12].text[0] = 'b';
+    cells[15].text[0] = 'c';
+    cells[30].style.ul = .single;
+    var sent: [2][]u8 = .{ &.{}, &.{} };
+    defer for (sent) |bytes| gpa.free(bytes);
+    for ([_][3]u8{ .{ 200, 200, 200 }, .{ 90, 90, 90 } }, 0..) |ink, frame| {
+        for (&cells) |*cell| cell.style.fg = .{ .rgb = ink };
+        out.clearRetainingCapacity();
+        paintCells(&vx, vx.window(), &cells, 32, 1);
+        try vx.render(&out.writer);
+        sent[frame] = try out.toOwnedSlice();
+    }
+    // Against a cleared screen every cell goes. After the fade the letters
+    // go, the short gap between two of them (cheaper than a cursor move),
+    // and the underlined blank; the long gaps are jumped.
+    try std.testing.expectEqual(@as(usize, 29), std.mem.count(u8, sent[0], " "));
+    try std.testing.expectEqual(@as(usize, 3), std.mem.count(u8, sent[1], " "));
+    try std.testing.expect(std.mem.indexOf(u8, sent[1], "b  c") != null);
+    try std.testing.expect(std.mem.indexOf(u8, sent[1], "90:90:90") != null);
+    try std.testing.expect(std.mem.indexOfScalar(u8, sent[1], 'a') != null);
+}
+
 /// T1 (§9.2): each notice chip casts a shadow one cell right and one down,
 /// inside its own pane's body: its ground 35% toward black in OKLab on a
 /// cell with text; on a blank one a half block (▌ on the right, ▀ below,
@@ -2570,7 +2677,7 @@ const Attach = struct {
     fn paint(a: *Attach) void {
         a.dirty = false;
         const win = a.vx.window();
-        paintCells(win, a.client.grid.items, a.client.cols, a.client.rows);
+        paintCells(a.vx, win, a.client.grid.items, a.client.cols, a.client.rows);
         if (a.client.cursor) |cur| paintCursor(win, cur.x, cur.y, cur.bar);
         a.vx.render(a.tty.writer()) catch {};
     }