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parent: Developers
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Profile developers guide
========================
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Technical reference for BRouter profile scripts

1. TOC
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The tag-value lookup table
--------------------------

Within the routing data files (rd5), tag information is encoded in a binary
bitstream for the way tags and the node tags each.

To encode and decode to/from this bitstream, a lookup table is used that
contains all the tags and values that are considered for encoding.

For each tag there are 2 special values:

- `<empty>` if the tag is not set or the value is empty
- `unknown` if the value is not contained in the table

Each value can have optional *aliases*, these alias values are encoded into the
same binary value as the associated primary value.

A profile must use the primary value in expressions, as aliases trigger a parse
error. E.g. if there is a line in `lookups.dat` file:

`bicycle;0001245560 yes allowed`

then a profile must use `bicycle=yes`, as `bicycle=allowed` gives an error.

The numbers in the lookup table are statistical information on the frequency of
the values in the map of Germany - these are just informational and are not
processed by BRouter.

Context-Separation
------------------

Way-tags and Node-Tags are treated independently, so there are different
sections in the lookup table as well as in the profile scripts for each context.
The special tags: `---context:way` and `---context:node` mark the beginning of
each section.

An exception from context separation is the node-context, where variables from
the way-context of the originating way can be accessed using the `way:` prefix.
For the variable nodeaccessgranted there's an additional legacy-hack to access
it as a lookup value without prefix:

`if nodeaccessgranted=yes then ...`

while in the general case the prefixed expressions are variables:

`if greater way:costfactor 5 then ...`

In the profile scripts there is a third context `global` which contains global
configuration which is shared for all contexts and is accessible by the routing
engine.

The variables from the `global` section in the profile scripts are read-only
visible in the `way` and `node` sections of the scripts.

Predefined variables in the profile scripts
-------------------------------------------

Some variable names are pre-defined and accessed by the routing engine:

- for the global section these are:

  - 7 elevation configuration parameters:

    - `downhillcost`
    - `downhillcutoff`
    - `uphillcost`
    - `uphillcutoff`
    - `elevationpenaltybuffer`
    - `elevationmaxbuffer`
    - `elevationbufferreduce`

  - 3 boolean mode-hint flags

    - `validForBikes`
    - `validForFoot`
    - `validForCars`

  - 2 variables to change the heuristic coefficients for the 2 routing passes (
    <0 disables a routing pass )

    - `pass1coefficient`
    - `pass2coefficient`

  - 3 variables to influence the generation of turn-instructions

    - `turnInstructionMode`          0=none, 1=auto-choose, 2=locus-style,
      3=osmand-style
    - `turnInstructionCatchingRange` default=40m
    - `turnInstructionRoundabouts`   default=true generate explicit roundabout
      hints

  - variables to modify BRouter behaviour

    - `processUnusedTags` default=false

       If an OSM tag is unused within the profile, BRouter totally ignores the
       tag existence. Skipping unused tags improves BRouter speed. As a side
       effect, the tag is not even listed in the route segment table nor the
       table exported as CSV. Setting it to true/1, Brouter-web Data page will
       list all tags present in the RD5 file.

- for the way section these are

  - `turncost`
  - `initialcost`
  - `costfactor`
  - `uphillcostfactor`
  - `downhillcostfactor`
  - `nodeaccessgranted`
  - `initialclassifier`
  - `priorityclassifier`

- for the node section this is just

  - `initialcost`

Operators of the profile scripts
--------------------------------

The profile scripts use polish notation (operator first).

The `assign` operator is special: it can be used only on the top level of the
expression hierarchy and has 2 operands:

`assign <variable-name> <expression>`

It just assigns the expression value to this variable (which can be a predefined
variable or any other variable, which in this case is defined implicitly). The
expression can be a complex expression using other operators.

All other operators can be used recursively to an unlimited complexity, which
means that each operand can be a composed expression starting with an operator
and so on.

All expressions have one of the following basic forms:

- `<numeric value>`
- `<numeric variable>`
- `<lookup-match>`
- `<1-op-operator> <operand>`
- `<2-op-operator> <operand> <operand>`
- `<3-op-operator> <operand> <operand> <operand>`

- A numeric value is just a number, floating point, with `.` as decimal
  separator. Boolean values are treated as numbers as well, with `0` = `false`
  and every nonzero value = `true`.

- A lookup match has the form `<tag-name>=<value>`, e.g. `highway=primary` Only
  the primary values can be used in lookup-matches, not aliases. The `<empty>`
  value is referred to as an empty string, e.g. `access=`

- 1 Operand operators are:

  - `not <boolean expression>`

- 2 Operand operators are:

  - `or       <boolean expression 1> <boolean expression 2>`
  - `and      <boolean expression 1> <boolean expression 2>`
  - `xor      <boolean expression 1> <boolean expression 2>`
  - `multiply <numeric expression 1> <numeric expression 2>`
  - `add      <numeric expression 1> <numeric expression 2>`
  - `sub      <numeric expression 1> <numeric expression 2>`
  - `max      <numeric expression 1> <numeric expression 2>`
  - `min      <numeric expression 1> <numeric expression 2>`
  - `equal    <numeric expression 1> <numeric expression 2>`
  - `greater  <numeric expression 1> <numeric expression 2>`
  - `lesser   <numeric expression 1> <numeric expression 2>`

- 3 Operand operators are:

  - `switch <boolean-expression> <true-expression> <false-expression>`

    So the switch expression has a numeric value which is the true-expression if
    the boolean expression is true, the false-expression otherwise.

Syntactic Sugar
---------------

To improve the readablity of the profile scripts, some syntactic variations are
possible:

- if then else: `if` can be used instead of the `switch` operator, if the
  additional keywords `then` and `else` are placed between the operators:

  `if <boolean-expression> then <true-expression> else <false-expression>`

- Parentheses: each expression can be surrounded by parentheses:
  `(<expression>)`

  Please note that the profile syntax, due to the polish notation, does not need
  parentheses, they are always optional. However, if there are parentheses, the
  parser checks if they really match the expression boundaries.

- or-ing lookup-matches: the pipe-symbol can be used as a short syntax for
  lookup matches where more than one value is accepted for a key:
  `highway=primary|secondary|tertiary`

- additional `=` symbol for `assign` operations:

  `assign <variable-name> = <expression>`

- boolean constants: `true` and `false` can be used instead of 1 and 0

Please note that the tokenizer always expects blank space to separate symbols
and expressions so it is not allowed to place parentheses or the `=` symbol
without separating blank space!

The initial cost classifier
---------------------------

To trigger the addition of the `initialcost`, another variable is used:
`initialclassifier` - any change in the value of that variable leads to adding
the value of `initialcost`.

Initial cost is used typically for a ferry, where you want to apply a penalty
independent of the length of the ferry line.

Another useful case may be an initial cost for bicycle mounting/dismounting,
having set an initialclassifier for ways without bicycle access, with high
initialcost. For backward compatibility, if `initialclassifier` = 0, it is
replaced by the costfactor.

The priority classifier
-----------------------

`priorityclassifier` is a BRouter numerical parameter calculated for ways and
used for generation of pictogram/voice navigation instructions.

Higher values means the more significant (noticeable) way, as far as it can be
predicted from OSM data.

To avoid a navigation instruction flood, it was decided that the instructions
are provided only if:

1. You are supposed to turn at a crossroad/junction and some other ways having
   the same or higher `priorityclassifier` value.

2. You are supposed to go straight ahead and some other ways having the higher
   `priorityclassifier` value.

The elevation buffer ( From Poutnik's glossary )
------------------------------------------------

With related 3 internal BRouter variables:

- `elevationpenaltybuffer`
- `elevationmaxbuffer`
- `elevationbufferreduce`

the Elevation Buffer is BRouter feature to filter elevation noise along the
route. It may be real, or caused by the artefacts of used SRTM elevation data.

From every elevation change is at the first place cut out amount
`10*up/downhillcutoff` per every km of the way length. What remains, starts to
accumulate in the buffer. IF cutoff demand of elevation per length is not
saturated from incoming elevation, it is applied on elevation remaining in the
buffer as well.

E.g. if the way climbs 20 m along 500 m, and `uphillcutoff=3.0`, then
`10*3.0*0.5 km = 15 m` is taken away and only remaining 5 m accumulates. But if
it climbed only 10 m on those 500 m, all 10 m would be *swallowed* by cutoff,
together with up to 5 m from the buffer, if there were any.

When elevation does not fit the buffer of size `elevationmaxbuffer`, it is
converted by up/downhillcost ratio to Elevationcost portion of Equivalentlength.
Up/downhillcostfactors are used, if defined, otherwise costfactor is used.

- `elevationpenaltybuffer` - default 5(m).

  The variable value is used for 2 purposes

  - with `buffer content > elevationpenaltybuffer`, it starts partially convert
    the buffered elevation to ElevationCost by Up/downhillcost

  - with `elevation taken = MIN (buffer content - elevationpenaltybuffer, WayLength[km] * elevationbufferreduce*10`
    Up/downhillcost factor takes place instead of costfactor at the percentage
    of how much is `WayLength[km] * elevationbufferreduce*10` is saturated by
    the buffer content above elevationpenaltybuffer.

- `elevationmaxbuffer` - default 10(m)

  is the size of the buffer, above which all elevation is converted to
  Elevationcost by Up/Downhillcost ratio, and - if defined -
  Up/downhillcostfactor fully replaces costfactor in way cost calculation.

- `elevationbufferreduce` - default 0(slope%)

  is rate of conversion of the buffer content above elevationpenaltybuffer to
  ElevationCost. For a way of length L, the amount of converted elevation is
  L[km] * elevationbufferreduce[%] * 10. The elevation to Elevationcost
  conversion ratio is given by Up/downhillcost.

Example:   Let's examine steady slopes with `elevationmaxbuffer=10`,
`elevationpenaltybuffer=5`, `elevationbufferreduce=0.5`, `cutoffs=1.5`,
`up/downhillcosts=60`.

All slopes within 0 .. 1.5% are swallowed by the cutoff.

- For slope 1.75%, there will remain 0.25%.

  That saturates the elevationbufferreduce 0.5% by 50%. That gives Way cost to
  be calculated 50% from costfactor and 50% from Up/downhillcostfactor.
  Additionally, 0.25% gives 2.5m per 1km, converted to 2.5*60 = 150m of
  Elevationcost.

- For slope 2.0%, there will remain 0.5%.

  That saturates the elevationbufferreduce 0.5% by 100%. That gives Way cost to
  be calculated fully from `up/downhillcostfactor`. Additionally, 0.5% gives 5m
  per 1km, converted to 5*60 = 300m of Elevationcost. Up to slope 2.0% the
  buffer value stays at 5m = `elevationpenaltybuffer`.

- For slope 2.5%, there will remain 1.0% after cutoff subtract, and 0.5% after
  the buffer reduce subtract.

  The remaining 0.5% accumulates in the buffer by rate 5 m/km. When the buffer
  is full (elevationmaxbuffer), the elevation transforms to elevationcost by
  full rate of 1.0%, i.e. 10 m/km, giving elevationcost 10*60=600 m/km.


Technical constraints
---------------------

- The costfactor is required to be >= 1, otherwise the cost-cutoff logic of the
  routing algorithm does not work and you get wrong results.

- The profile should be able to find a route with an average costfactor not very
  much larger than one, because otherwise the routing algorithm will not find a
  reasonable cost-cutoff, leading to a very large search area and thus to long
  processing times.

- Forbidden ways or nodes must be treated as very high cost, because there is no
  *forbidden* value. Technically, values >= 10000 for a (way-)costfactor, and >=
  1000000 for a nodes `initalcost` are treated as infinity, so please use these
  as the *forbidden* values.

- Ways with costfactor >= 10000 are considered as if they did not exist at all.

- Ways with costfactor = 9999 are considered as if they did not exist during
  route calculation, but the navigation hint generator takes them into account.


Developing and debugging scripts
--------------------------------

For developing scripts, the *brouter-web* web-application is your friend. You
can use that either online at https://brouter.de/brouter-web or set up a local
installation.

*brouter-web* has a window at the lower left corner with a *Profile* and a
*Data* tab. Here, you can upload profile scripts and see the individual cost
calculations per way-section in the *Data*-tab.

For profile debugging activate `assign processUnusedTags = true` to see all
present OSM tags on the Data tab, not just those used in the tested profile.


Lookup-Table evolution and the the *major* and *minor* versions
---------------------------------------------------------------

The lookup-table is allowed to grow over time, to include more tags and values
as needed. To support that evolution, it carries a major and a minor version
number. These numbers are also encoded into the routing data files, taken from
the lookups.dat that is used to pre-process the routing data files.

A major version change is considered to always break compatibility between the
routing datafiles and the lookup table.

A minor version change keeps the routing data files and the lookup-table
compatible in both directions, using the following rules:

- if the data contains a key that is not contained in the lookup tables, it is
  ignored

- if the data contains a value that is not contained in the lookup tables (but
  its key is known) that value is treated as `unknown`

- if a profile uses a key that is not present in the data, it sees empty
  (=unset) values for that key

- if a profile uses a value that is not present in the data, lookup matches for
  that value are always false.

For a minor version change it is required that tags are only appended at the end
of the table (or replace one of the dummy tags located between the way-tags and
the relation pseudo-tags), and that values are only appended at the end of the
value lists. This is because the routing data files address tags and values by
their sequence numbers, so changing sequences would produce garbage data.

Other resources
---------------

See [Poutnik's glossary](https://github.com/poutnikl/Brouter-profiles/wiki/Glossary)
as a complementary source about various profile internals.
