The fastest stretch of growth

A greenhouse records the height of a plant in millimetres, always the same number of days apart. You are asked which stretch between two consecutive readings had the fastest growth, and how fast that was.

Write fastest_growth(heights, gap), which returns the largest average rate of change between two consecutive readings, in millimetres per day. gap is the number of days between readings.

Worked example, with heights = [40, 110, 180, 240] and gap = 7. The three stretches give $\frac{110-40}{7} = 10$, $\frac{180-110}{7} = 10$ and $\frac{240-180}{7} \approx 8.571$. The largest is 10, so the answer is 10.

• Constraints: heights has at most 50 entries, and gap is between 1 and 30.
• Heights may fall as well as rise, so a rate can be negative. Return the largest one, which may still be negative.
• If there are fewer than two readings there is no stretch at all, so return 0.
• Answers are compared with a small tolerance, so an ordinary division is fine. Take care in a language where dividing two whole numbers throws the fraction away.