1. A data stream processing unit for performing a computation upon a stream of data that is organized according to a plurality of contexts, the data stream processing unit comprising: a plurality of processing elements arranged in a physical sequence, wherein each given processing element comprises: a first memory circuit; a data input channel, coupled to the first memory circuit, configured to receive into the first memory circuit an input datum in the stream of data from a coupled data output channel of a processing element preceding the given processing element in the sequence, the input datum being associated with a context in the plurality of contexts; a metadata input channel, coupled to the first memory circuit, configured to receive into the first memory circuit an input metadata signal from a coupled metadata output channel of the preceding processing element, the input metadata signal representing a first state of the computation; a computing circuit configured as a programmable finite state machine having a plurality of states that operates, as a function of a position of the given processing element in the sequence and the context, on the input datum and the input metadata signal to produce an output datum and an output metadata signal, the output metadata signal representing a second state of the computation that is different from the first state; a data output channel, coupled to the first memory circuit, configured to transmit the output datum to a processing element succeeding the given processing element in the sequence; a metadata output channel, coupled to the first memory circuit, configured to transmit the output metadata signal to the succeeding processing element; and a second memory circuit, coupled to the computing circuit; wherein the computing circuit is further configured so that, when a first input datum is associated with a first context but a next input datum is associated with a different, second context, the computing circuit (a) stores, in the second memory circuit, an active state of the programmable finite state machine in association with data identifying the first context, and (b) retrieves, from the second memory circuit, a new state of the programmable finite state machine according to data identifying the second context.