Discarding

pub const Discarding = struct {
    count: u64,
    writer: Writer,

    pub fn init(buffer: []u8) Discarding {
        return .{
            .count = 0,
            .writer = .{
                .vtable = &.{
                    .drain = Discarding.drain,
                    .sendFile = Discarding.sendFile,
                },
                .buffer = buffer,
            },
        };
    }

    /// Includes buffered data (no need to flush).
    pub fn fullCount(d: *const Discarding) u64 {
        return d.count + d.writer.end;
    }

    pub fn drain(w: *Writer, data: []const []const u8, splat: usize) Error!usize {
        const d: *Discarding = @alignCast(@fieldParentPtr("writer", w));
        const slice = data[0 .. data.len - 1];
        const pattern = data[slice.len];
        var written: usize = pattern.len * splat;
        for (slice) |bytes| written += bytes.len;
        d.count += w.end + written;
        w.end = 0;
        return written;
    }

    pub fn sendFile(w: *Writer, file_reader: *File.Reader, limit: Limit) FileError!usize {
        if (File.Handle == void) return error.Unimplemented;
        const d: *Discarding = @alignCast(@fieldParentPtr("writer", w));
        d.count += w.end;
        w.end = 0;
        if (limit == .nothing) return 0;
        if (file_reader.getSize()) |size| {
            const n = limit.minInt64(size - file_reader.pos);
            if (n == 0) return error.EndOfStream;
            file_reader.seekBy(@intCast(n)) catch return error.Unimplemented;
            w.end = 0;
            d.count += n;
            return n;
        } else |_| {
            // Error is observable on `file_reader` instance, and it is better to
            // treat the file as a pipe.
            return error.Unimplemented;
        }
    }
}