DoubleSpscRingBuffer

High-performance non-blocking SPSC ring buffer specialized for Double.

Layout: a single Array[Long] storing raw doubleToRawLongBits. The FastFlow signal uses two reserved NaN bit patterns: EMPTY_BITS (0xFFF8_0000_0000_0001L) is the empty marker, DONE_BITS (0xFFF8_0000_0000_0002L) is the in-band end-of-stream marker. If the user offers a Double whose raw bits happen to equal either reserved pattern, it is canonicalized to Double.NaN's standard bit pattern (0x7FF8_0000_0000_0000L) on insertion.

This canonicalization is unobservable: IEEE 754 guarantees NaN != NaN, all NaNs compare equal under reference NaN semantics, and there is no portable Scala/Java way to round-trip a specific NaN payload through arithmetic. The remapping only affects two specific raw bit patterns, which the user could not have meaningfully produced through normal floating-point operations.

In-band "done" sentinel: producers can call offerDone() to insert the end-of-stream marker after the last data value. Consumers reading via pollPacked() receive EMPTY_BITS for empty, DONE_BITS for the sentinel, or the raw doubleToRawLongBits value for data. This removes the need for a separate control queue alongside the data queue.

One memory-ordered op per insert/remove, one cache line per slot.

SPSC: one producer thread, one consumer thread.