add: Direct Renderer implementation rendering the fixed size of the root Container

A corresponding example has been added, which is very minimalistic as of now,
but will add further functionality to test the corresponding workflow that is
usual in `zterm` such that it in the best case only a swap in the renderer to
switch from alternate mode drawing to direct drawing.

build.zig

@@ -24,6 +24,8 @@ pub fn build(b: *std.Build) void {
         palette,
         // error handling
         errors,
+        // non alternate screen applications
+        direct,
     };
 
     const example = b.option(Examples, "example", "Example to build and/or run. (default: all)") orelse .all;
@@ -73,6 +75,8 @@ pub fn build(b: *std.Build) void {
                     .palette => "examples/styles/palette.zig",
                     // error handling
                     .errors => "examples/errors.zig",
+                    // non-alternate screen
+                    .direct => "examples/direct.zig",
                     .all => unreachable, // should never happen
                 }),
                 .target = target,

examples/direct.zig

@@ -0,0 +1,46 @@
+pub fn main() !void {
+    errdefer |err| log.err("Application Error: {any}", .{err});
+
+    var allocator: std.heap.DebugAllocator(.{}) = .init;
+    defer if (allocator.deinit() == .leak) log.err("memory leak", .{});
+
+    const gpa = allocator.allocator();
+
+    var app: App = .init(.{}, .{});
+    var renderer = zterm.Renderer.Direct.init(gpa);
+    defer renderer.deinit();
+
+    var container: App.Container = try .init(gpa, .{
+        .layout = .{
+            .direction = .horizontal,
+            .separator = .{ .enabled = true },
+        },
+        .size = .{
+            .dim = .{ .y = 30 },
+            .grow = .horizontal_only,
+        },
+    }, .{});
+    defer container.deinit();
+
+    try container.append(try .init(gpa, .{
+        .rectangle = .{ .fill = .blue },
+    }, .{}));
+
+    try container.append(try .init(gpa, .{
+        .rectangle = .{ .fill = .green },
+    }, .{}));
+
+    container.resize(&app.model, try renderer.resize());
+    container.reposition(&app.model, .{});
+    try renderer.render(@TypeOf(container), &container, App.Model, &app.model);
+    try renderer.flush();
+}
+
+pub const panic = App.panic_handler;
+const log = std.log.scoped(.default);
+
+const std = @import("std");
+const assert = std.debug.assert;
+const zterm = @import("zterm");
+const input = zterm.input;
+const App = zterm.App(struct {}, union(enum) {});

src/render.zig

@@ -91,6 +91,7 @@ pub const Buffered = struct {
         var writer = terminal.writer();
         const s = this.screen;
         const vs = this.virtual_screen;
+
         for (0..this.size.y) |row| {
             for (0..this.size.x) |col| {
                 const idx = (row * this.size.x) + col;
@@ -123,6 +124,87 @@ pub const Buffered = struct {
     }
 };
 
+pub const Direct = struct {
+    gpa: Allocator,
+    size: Point,
+    resized: bool,
+    screen: []Cell,
+
+    pub fn init(gpa: Allocator) @This() {
+        return .{
+            .gpa = gpa,
+            .size = .{},
+            .resized = true,
+            .screen = undefined,
+        };
+    }
+
+    pub fn deinit(this: *@This()) void {
+        this.gpa.free(this.screen);
+    }
+
+    pub fn resize(this: *@This()) !Point {
+        // TODO the size in the y-axis is *not fixed* and *not limited*
+        // how can I achive this?
+        this.size = .{};
+        if (!this.resized) {
+            this.gpa.free(this.screen);
+            this.screen = undefined;
+        }
+        this.resized = true;
+        return terminal.getTerminalSize();
+    }
+
+    pub fn clear(this: *@This()) !void {
+        _ = this;
+        try terminal.clearScreen();
+    }
+
+    /// Render provided cells at size (anchor and dimension) into the *virtual screen*.
+    pub fn render(this: *@This(), comptime Container: type, container: *Container, comptime Model: type, model: *const Model) !void {
+        const size: Point = container.size;
+        const origin: Point = container.origin;
+
+        if (this.resized) {
+        this.size = size;
+        const n = @as(usize, this.size.x) * @as(usize, this.size.y);
+        this.screen = try this.gpa.alloc(Cell, n);
+        @memset(this.screen, .{});
+        this.resized = false;
+        }
+
+        const cells: []const Cell = try container.content(model);
+
+        var idx: usize = 0;
+        var vs = this.screen;
+        const anchor: usize = (@as(usize, origin.y) * @as(usize, this.size.x)) + @as(usize, origin.x);
+
+
+        blk: for (0..size.y) |row| {
+            for (0..size.x) |col| {
+                vs[anchor + (row * this.size.x) + col] = cells[idx];
+                idx += 1;
+
+                if (cells.len == idx) break :blk;
+            }
+        }
+        // free immediately
+        container.allocator.free(cells);
+        for (container.elements.items) |*element| try this.render(Container, element, Model, model);
+    }
+
+    pub fn flush(this: *@This()) !void {
+        var writer = terminal.writer();
+        for (0..this.size.y) |row| {
+            for (0..this.size.x) |col| {
+                const idx = (row * this.size.x) + col;
+                const cvs = this.screen[idx];
+                try cvs.value(&writer);
+            }
+        }
+    }
+};
+
 const std = @import("std");
 const meta = std.meta;
 const assert = std.debug.assert;