Lookup tables

The workshop's exhaust should run harder as the temperature rises – but not linearly. Up to 22 °C hardly anything, from 30 °C strongly, from 34 °C fully. A formula for that would be long and hard to change. A table is short and everybody sees at once what happens.

Precomputing values#

A lookup table holds finished results. Instead of calculating, the chip looks them up. That pays off in three cases:

• The curve has no simple formula (kinks, steps, empirical values).
• The formula is expensive, for example with pow, exp or a loop.
• Somebody should adjust the values later without understanding the calculation.

Table in the stack#

You set the table up once before start: with poke. Every slot is a step:

Index from the reading#

Four steps turn a temperature into a table slot: subtract the offset, divide by the step width, round down, limit.

sub idx temp CELSIUSOFFSET
sub idx idx T_START          # offset
div idx idx STEP          # step width
floor idx idx
clamp idx idx 0 LAST      # never outside the table
get value db idx

27 °C gives (27 − 20) / 2 = 3.5, rounded down 3 – the pump runs at 20. Below 20 °C the index becomes negative, clamp turns it into 0. Above 34 °C it stays at 7.