Math-Evaluate OpenChirp Service

Overview

This is an OpenChirp service that evaluates a logical or mathematical expression upon receiving updates to transducers referenced in the expression.

Service Config

Key Name	Key Description	Key Example	Is Required?
Expressions	Comma separated list of mathematical or logical expressions	"(temp_c*1.8) + 32", temp_f > 100	Required
Output Topics	Comma separated list of corresponding output topics	temp_f, overtemp	Optional
Options	Comma separated list of optional behavior modifiers	boolasvalue	Optional

Options

- `boolasvalue` - Indicates that you want boolean values to be published
   as `0` or `1`.
   Without this modifier, booleans are published as `true` or `false`.
   This modifier currently does not allow the reverse direction, `0` or `1`
   being used as booleans.

Expression Syntax

Expressions are evaluated by the govaluate library. Please see the library's README.md or the library's MANUAL.md for help with expressions.

Developer Notes

• Transducer names with - must be used in the expression with the underscore, _, instead. This is because it is impossible to distinguish between the transducer name and the subtraction operator in the expression.

  Example: An expression of "door-relay && motion-activated" would be seen as (door - relay) && (motion - activated), where it depends on the transducers door, relay, motion, and activated. Using "door_relay && motion_activated" will cause the service to depend on the transducers door_relay, door-relay, motion_activated, and motion-activated.

• It should be noted that this service assumes that transducers are only one level deep. This is because we must use them as variable names in an expressions. Since "/" maps to divide, we cannot express hierarchical transducer names.

• We allow one type of immediate loop, where the output topic is one of the input variable names. This allows for things like incrementing transducers on events. This kind of immediate loop is detected and handled removing the input variable(which matches the output topic) from the list of transducers capable of triggering a reevaluation.