tiny.acp.ReaderCapacity

1/// A caller learns the buffer size before any buffer exists, for example to set aside the reader's2/// share of one larger region of memory. The structure holds the line limit and the buffer size it3/// needs. `derive` builds it. Another program in the repository adds the buffer size into the size4/// of one region it allocates up front. The package exports it as `acp.ReaderCapacity`.5pub const Capacity = struct {6    /// The line limit, copied from `Limits`.7    message_bytes: usize,8    /// The buffer size: the line limit plus one byte, for the newline after a longest line. The9    /// value is always above the line limit and above zero.10    storage_bytes: usize,1112    /// A caller works out the buffer size for a line limit and learns early when the limit is too13    /// large. The function returns the line limit with its buffer size. The call fails with14    /// `error.CapacityOverflow` when the limit plus one does not fit in a `usize`. The call15    /// allocates nothing.16    pub fn derive(limits: Limits) DeriveError!Capacity {17        const storage_bytes = std.math.add(usize, limits.message_bytes, 1) catch18            return error.CapacityOverflow;19        std.debug.assert(storage_bytes > limits.message_bytes);20        std.debug.assert(storage_bytes != 0);21        return .{22            .message_bytes = limits.message_bytes,23            .storage_bytes = storage_bytes,24        };25    }26};

1pub const ReaderCapacity = reader.Capacity;