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const std = @import("std");

const Writer = struct {
    ptr: *Base,
    vtable: *const struct { flush: *const fn (ptr: *Base) void },
    pub const Base = struct {
        buffer: []u8,
        tail: usize = 0,
    };
    pub fn write_byte(self: Writer, byte: u8) void {
        if (self.ptr.tail == self.ptr.buffer.len) {
            self.vtable.flush(self.ptr);
            self.ptr.tail = 0;
        }

self.ptr.buffer[self.ptr.tail] = byte;
        self.ptr.tail += 1;
    }
    pub fn flush(self: Writer) void {
        self.vtable.flush(self.ptr);
    }
};

const File = struct { handle: std.posix.fd_t, base: Writer.Base };
fn flush(ptr: *Writer.Base) void {
    const self: *File = @fieldParentPtr("base", ptr);
    _ = std.posix.write(self.handle, ptr.buffer[0..ptr.tail]) catch unreachable;
}
pub fn main() !void {
    var buffer: [5]u8 = undefined;
    var stdout: File = .{
        .handle = 1,
        .base = .{ .buffer = buffer[0..] },
    };
    const writer: Writer = .{ .ptr = &stdout.base, .vtable = &.{ .flush = flush } };
    writer.write_byte('H');
    writer.write_byte('e');
    writer.write_byte('l');
    writer.write_byte('l');
    writer.write_byte('o');
    writer.write_byte('w');
    writer.write_byte('o');
    writer.write_byte('r');
    writer.write_byte('l');
    writer.write_byte('d');
    writer.write_byte('\n');
    writer.write_byte('w');
    writer.write_byte('o');
    writer.write_byte('r');
    writer.write_byte('w');
    writer.write_byte('w');
    writer.write_byte('w');
    writer.write_byte('o');
    writer.write_byte('r');
    writer.write_byte('o');
    writer.write_byte('r');
    writer.write_byte('o');
    writer.write_byte('r');
    writer.flush();
}