What is the test for a jury to determine whether there is a deliberate and premeditated purpose to kill?

Premeditation and deliberation can occur in a brief interval. "The test is not time, but reflection. 'Thoughts may follow each other with great rapidity and cold, calculated judgment may be arrived at quickly.' " (People v. Bloyd (1987) 43 Cal.3d 333, 348.)

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"[D]irect evidence of a deliberate and premeditated purpose to kill is not required. The necessary elements of deliberation and premeditation may be inferred from proof of such facts and circumstances in the case as will furnish a reasonable foundation for such an inference, and where the evidence is not in law insufficient, the matter is exclusively within the province of the jury for determination." (People v. Isby (1947) 30 Cal.2d 879, 888.)