RPN Rules

RPN Rules

The "RPN Rules" module is an advanced feature that let's you define pretty flexible custom rules to execute actions (mostly changing relay and light statuses) based on different inputs. Rules are defined as a series of commands using Reverse Polish Notation, also called "postfix" notation, where operators come after the operands.

This might look somewhat complicated but it actually has major benefits over the usual "infix" notation:

• Commands (rules) are faster to process
• Rules are simple to understand
• It's easier to implement

Table of Contents

• RPN Rules
  • Table of Contents
  • Main concepts
    • Postfix notation
    • Tokens
    • Stack
    • Operators
    • Variables
    • MQTT variables
    • Rule execution
  • Examples
    • Toggle a heater depending on a temperature sensor
    • Open light at night if there is presence
    • Emulate the schedule
    • Light color following power consumption
    • Individually synchronize relays
  • Terminal commands

Main concepts

Postfix notation

First you should familiarize yourself with RPN calculation. Reverse Polish notation (RPN) is a mathematical notation in which operators follow their operands. It does not need any parentheses as long as each operator has a fixed number of operands.

A simple calculation in infix notation might look like this:

( 4 - 2 ) * 5 + 1 =

The same calculation in RPN (postfix) will look like this:

4 2 - 5 * 1 +

It results in a shorter expression since parenthesis are unnecessary. Also the equals sign is not needed since all results are stored in the stack. From the computer point of view is much simpler to evaluate since it doesn't have to look forward for the operands.

Tokens

Each element in an expression is called "token" (not a standard name). Tokens can be:

• Numbers, always considered as real numbers (floats)
• Operators (+, -, ...)
• Variables (starting with a $ sign)

Stack

As I already mentioned, operands (numbers) are pushed into a stack and later operators pop the number of operands from the stack. Let's take a closer look to a simple RPN expression: 4 2 - 5 * 1 +. The first column shows the next token to process and the third column the status of the stack after processing the token. At the beginning the stack is (usually) empty.

token	action	stack
4	pushes '4' into the stack	4
2	pushes '2' into the stack	4 2
-	pops 2 values from the stack performs the subtraction and pushes the result to the stack	2
5	pushes '5' into the stack	2 5
*	pops 2 values from the stack performs the multiplication and pushes the result to the stack	10
1	pushes '1' into the stack	10 1
+	pops 2 values from the stack performs the sum and pushes the result to the stack	11

Operators

Of course you need to know the vocabulary, i.e. the available operators. There are different types of operators:

• Mathematical (+, -, *, ...)
• Stack manipulation (dup, swap, ...)
• Comparison (eq, gt, cmp, ...)
• Logical (and, or, ...)
• Time management (now, hour, minute,...)
• ESPurna related (relay, channel)

See RPNLib README for the full list.

Time / Date

Operator	Description	Notes
now	Pushes the current epoch time into the stack	Warning: Core 2.3.0 this returns the time in local time zone offset, >= 2.3.0 this returns UTC timestamp
utc	Pushes the current epoch time into the stack	Warning: Core >= 2.3.0 this is an alias for now
hour	Gets a timestamp from the stack and returns the hour
minute	Gets a timestamp from the stack and returns the minute
dow	Gets a timestamp from the stack and returns the day of week	Sunday is 1, Monday is 2, etc.
day	Gets a timestamp from the stack and returns the day of month
month	Gets a timestamp from the stack and returns the month	January is 1, February is 2, etc.

Notice: Depending on Arduino Core version, semantics of now and utc change. With older Core versions, now returns local timestamp and utc returns UTC. With newer Core versions, underlying time library changes made it so now and utc now return the same timestamp. To reference UTC time instead of the one in local timezone, please append utc_ prefix to any of the conversion functions. e.g., instead of minute it becomes utc_minute

Relays

Operator	Description
relay	Gets a value and a relay number from the stack and changes that relay to the given status, status can be 0, 1 or 2 (toggle)
relay_reset	Similar to relay, but re-schedules any timers attached to the relay (Flood, ON / OFF delays, Pulse etc.)

Lights

Operator	Description
black	Turns off all channels (only lights)
update	Updates channels
channel	Gets a value and a channel number from the stack and changes that chanel to the given value (from 0 to 255). Does not update the light, you must call "update" at the end.

Debugging

Operator	Description
showstack	Shows current stack contents in the global debug log
dbgmsg	Prints the specified string to the global debug log

Execution control

Operator	Description
end	Ends rule execution when top stack value resolves to boolean false WARNING! Before 1.15.0, operator used to end rule execution when stack value resolved to boolean true
ifn	Will choose between 1st (top) and 2nd stack elements, depending if 3rd stack element resolves to true (choose 2nd) or false (choose 1st)

Note that ifn, unlike generic programming language's if, does not allow different 'branches' and rpnlib does not allow operator references (yet) to be stored on the stack. Any token encountered while parsing the expression and matching with the operator name will immediately call the associated function.

Variables

Variables are stored in the module context so they can be used from your expressions. Variables are fed from different sources:

• Relay status ($relay0, $relay1,...)
• Lights channel values ($channel0, $channel1,...)
• Sensors ($temperature0, $humidity0,...)
• MQTT (custom names)

By default they are persistent (meaning that once defined they are available for any rule execution in the future) but they do not persist across reboots. This behavior is called "sticky variables" and can be changed from the UI. Non sticky variables get deleted after the next rule execution. This can be useful to avoid dead locks when used with other features like relay pulses.

MQTT variables

MQTT variables are a special type of variable that gets its content from MQTT topics. You can define an MQTT and a variable name and every time the topic receives a message the variables will get updated and the rules will be executed. Remember than variable names in the RPN expressions begin with the $ sign.

Rule execution

Rules get executed X milliseconds after the last variable update. The execution delay is 100ms by default but it can be changed from the web UI. This allows for a buffer time so all variables coming from sensors get updated before rules are executed.

All variables changes trigger the rule execution after the delay time. So every time a relay is toggled, every time a sensor reports values or every time one of the MQTT topic gets updated the rules will be executed.

Examples

Toggle a heater depending on a temperature sensor

Imagine you have a device with a relay controlling a heater and a temperature sensor. You can implement a simple thermostat with hysteresis using the input variables of the sensor ($temperature0) and the relay ($relay0):

$temperature0 18 24 cmp3 1 + [ 1 $relay0 0 ] index 0 relay

Let's split the expression in parts:

sub-expression	description	output
$temperature0 18 24 cmp3	Compares the temperature to the 18-24 range, it will output -1 if temperature is below 18, +1 if it is above 24 and 0 in the middle	-1 to 1
1 +	We need to shift the cmp3 result to use the index operator	0 to 2
[ 1 $relay0 0 ] index	We define an index of 3 different values (number of items between [ and ]) that will be: 1, the current status of the relay (0 or 1) and 0. The sub-expression will return one of them depending on the value in the stack preceding the [	0 or 1
0 relay	It will set the relay number 0 status according to the value on the stack	(empty)

This expression will ensure the relay in ON if the temperature is below 18C and OFF if it's above 24C. If the reported temperature is in between 18 and 24 the relay status will not change.

Open light at night if there is presence

In this case image we have a simple WiFi relay device and a PIR in the same room that reports presence via MQTT to the /livingroom/motion topic. We first define an MQTT topic for it using motion as the variable name. Then we can trigger the light if there is motion between 22h and 8h in the morning.

now hour 8 23 cmp3 abs $motion and 0 relay

Again, let's analyze it step by step

sub-expression	description	output
now hour	Will return the current hour	0 to 23
8 23 cmp3 abs	Compare the hour in the stack to the 8-23 range and get the absolute value, will output 1 if it's below 8 or above 23, 0 otherwise	0 or 1
$motion and	We do an AND with the motion value, so we will now have a 1 if it's nighttime AND there is motion	0 or 1
0 relay	We use the result value to turn on or off the relay #0	(empty)

Since our device will only have a relay, this rule will be executed every minute (triggered by the NTP module) and when there is a new value in the motion topic. Alternatively, if you disable the "stickiness" of the variables, the expression will fail except when the $motion is defined which will only happen after we receive a message to the motion topic.

Keep in mind that if the result changed the relay status, the relay change will trigger the rule execution again! Try to avoid loops in the rules like, for instance: 1 $relay0 - 0 relay. This simple expression will turn the relay ON and OFF and ON again and OFF again forever!!

Emulate the schedule

The schedule is different to the previous examples since it will only perform an action at a given time, not every time there is a time or status update like the RPN Rules module. But we can easily emulate this behaviour by using $tick1m variable that is set every minute:

$tick1m dup minute 0 eq end hour 8 eq end null &tick1m = 1 0 relay 

The $tick1m variable contains the timestamp, while the 2nd expression using &tick1m resets it to null, removing it from variables list after the expression is done. The end operator takes and argument from the stack and ends the execution if the argument resolves to false. Hence:

sub-expression	description	output
$tick1m dup	Check if $tick1m is set and duplicate it's value on the stack
minute 0 eq end	Will end the execution if the current minute is not 0	(empty)
hour 8 eq end	Will end the execution if the current hour is not 8	(empty)
null &tick1m =	Set tick1m variable's value to null, after which the variable will be disallowed to be used by the $tick1m expression
1 0 relay	Turn relay 0 to ON, this will only happen at 8:00!

It is also possible to schedule execution to happen every hour by checking for $tick1h variable.

Emulate periodic tasks

RPN allows to start a periodic or one-shot timers (aka RPN runners), that will trigger rule execution when expired.

For example, execute the right part of the expression after timer is done:

5000u every_ms 2 0 relay

Execute the right part of the expression when variable is set to true, but wait 5 seconds first and then set variable to false:

$test end 5000u once_ms false &test = "topic" "message" mqtt_send

Note that 5000u every_ms defined in multiple rules will create a single timer.

Light color following power consumption

Some devices, like the Smartlife Mini Smart Socket, have an LED light (sometime RGB) and a power consumption monitor. Wouldn't it be cool that the light would turn more and more red as the power consumption rises?

black $power0 0 1000 0 255 map 0 channel update

This expression maps a power value between 0 and 1000W to a number between 0 and 255 and then it feeds it to channel 0 (usually red). Initially, it sets all channels to 0 (black) and at the end forces the light driver to update the color. Nice.

Individually synchronize relays

ESPurna has a relay synchronization feature that offers different ways to synchronize relays when there is more than one available, but all of these options perform actions of all the relays, turning them ON or OFF to match the required pattern. What if you have 4 different relays and you want to synchronize relays 0 and 1 together but leave 2 and 3 untied? Easy.

$relay0 1 relay

What if you want them to have opposite value?

1 $relay0 - 1 relay

Now you can control relays 0 and 1 by just changing relay 0. If you want to be able to perform an action on either 0 or 1 and toggle the other as well, you can create one rule for each but remember to disable "Sticky variables" to avoid getting into a loop:

1 $relay0 - 1 relay
1 $relay1 - 0 relay

Terminal commands

The module exposes different terminal commands to test and evaluate expressions and variables.

• RPN.OPS will output the list of available operators and the number of required arguments for each of them
• RPN.TEST "<expression>" will execute the expression and output the stack at the end, useful to do partial tests of sub-expressions
• RPN.VARS will output the current defined variables and their values
• RPN.RUNNERS will show the currently running timers