Gh-523: GafferPop Custom Features (#524)

* document types handling

* move options to reference guide

* set up custom functions and predicates pages

* document the custom cypher functions

* Apply suggestions from code review

Co-authored-by: cn337131 <141730190+cn337131@users.noreply.github.com>
Co-authored-by: wb36499 <166839644+wb36499@users.noreply.github.com>

---------

Co-authored-by: cn337131 <141730190+cn337131@users.noreply.github.com>
Co-authored-by: wb36499 <166839644+wb36499@users.noreply.github.com>

docs/reference/gremlin-guide/custom-functions.md

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+# GafferPop Custom Functions
+
+This page details each additional function that is available in
+the Gaffer Gremlin API (GafferPop). These functions may be exclusive
+to GafferPop so are likely not available in other databases that
+support the Gremlin query language.
+
+All examples featured on this page assume the [Tinkerpop modern dataset](https://tinkerpop.apache.org/docs/current/images/tinkerpop-modern.png).
+
+## OpenCypher Extensions
+
+The following are additional functions provided by the [OpenCypher extensions](https://github.com/opencypher/cypher-for-gremlin/tree/master/tinkerpop/cypher-gremlin-extensions)
+for Gremlin.
+
+## cypherToString
+
+Can be used to cast the input value to a `String` type.
+
+!!! example
+
+    ```groovy
+    g.V().values('age').map(cypherToString())
+    ```
+
+    Result:
+
+    ```text
+    ['27', '29', '35', '32']
+    ```
+
+## cypherToBoolean
+
+Can be used to cast the input value to a `Boolean` type. Will convert `String`
+representations e.g. "true" or "false", otherwise return a Cypher null value.
+
+!!! example
+
+    ```groovy
+    g.V().values('name').map(cypherToBoolean())
+    ```
+
+    Result:
+
+    ```text
+    ['  cypher.null',
+     '  cypher.null',
+     '  cypher.null',
+     '  cypher.null',
+     '  cypher.null',
+     '  cypher.null']
+    ```
+
+## cypherToInteger
+
+Can be used to cast the input value to an `Integer` type. Note if value is
+a `Float` or `Double` rounding may be applied.
+
+!!! example
+
+    ```groovy
+    g.E().values('weight').map(cypherToInteger())
+    ```
+
+    Result:
+
+    ```text
+    [0, 0, 0, 1, 1, 0]
+    ```
+
+## cypherToFloat
+
+Can be used to cast the input value to a `Float` type.
+
+!!! example
+
+    ```groovy
+    g.V().values('age').map(cypherToFloat())
+    ```
+
+    Result:
+
+    ```text
+    [27.0, 29.0, 35.0, 32.0]
+    ```
+
+## cypherRound
+
+Can be used to round numerical input values.
+
+!!! example
+
+    ```groovy
+    g.E().values('weight').map(cypherRound())
+    ```
+
+    Result:
+
+    ```text
+    [0, 0, 0, 1, 1, 1]
+    ```
+
+## cypherProperties
+
+Used to extract only the properties from the input `Vertex` or `Edge`.
+Similar to running an `elementMap()` or `propertyMap()` step.
+
+!!! example
+
+    ```groovy
+    g.V().map(cypherProperties())
+    ```
+
+    Result:
+
+    ```json
+    [{'name': 'vadas', 'age': 27},
+     {'name': 'ripple', 'lang': 'java'},
+     {'name': 'marko', 'age': 29},
+     {'name': 'peter', 'age': 35},
+     {'name': 'josh', 'age': 32},
+     {'name': 'lop', 'lang': 'java'}]
+    ```
+
+## cypherSize
+
+Can be used to return the size of a `Collection` or `String`.
+
+!!! example
+
+    ```groovy
+    g.E().values('weight').fold().map(cypherSize())
+    ```
+
+    Result:
+
+    ```text
+    [6]
+    ```
+
+## cypherPlus
+
+Can be used to add two inputs together to act like the plus operator
+in Cypher e.g. `RETURN 1 + 2`.
+
+Note this requires a slightly more complex input of pair list e.g. `[1, 2]` to
+use in a Gremlin query. The example below uses a `project()` step to pair each
+'age' with the constant `1` so that each age is incremented by 1 in the result.
+
+!!! example
+
+    ```groovy
+    g.V().hasLabel('person')
+        .project('result')
+            .by(__.project('a', 'b')
+                .by(__.values('age'))
+                .by(__.constant(1))
+                .select(values)
+                .map(cypherPlus()))
+    ```
+
+    Result:
+
+    ```text
+    [{'result': 28}, {'result': 30}, {'result': 36}, {'result': 33}]
+    ```
+
+## cypherReverse
+
+Can be used to reverse a `Collection` or `String`.
+
+!!! example
+
+    ```groovy
+    g.V().values('name').map(cypherReverse())
+    ```
+
+    Result:
+
+    ```text
+    ['sadav', 'elppir', 'okram', 'retep', 'hsoj', 'pol']
+    ```
+
+## cypherSubstring
+
+Can be used to extract a substring from the result by using indexes for start
+and/or end point of the string.
+
+Note requires complex input pairs by using the project step for the input string
+and index. The example below cuts the first two chars off the start of the
+`name`.
+
+!!! example
+
+    ```groovy
+    g.V().hasLabel('person')
+        .project('result')
+            .by(__.project('a', 'b')
+                .by(__.values('name'))
+                .by(__.constant(2))
+                .select(values)
+                .map(cypherSubstring()))
+    ```
+
+    ```text
+    [{'result': 'das'}, {'result': 'rko'}, {'result': 'ter'}, {'result': 'sh'}]
+    ```
+
+## cypherTrim
+
+Can be used to run a [`trim()`](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#trim())
+on a given string input, basically all leading and trailing whitespace is
+removed.
+
+!!! example
+
+    ```groovy
+    g.V().values('name').map(cypherTrim())
+    ```
+
+    Result:
+
+    ```text
+    ['vadas', 'ripple', 'marko', 'peter', 'josh', 'lop']
+    ```
+
+## cypherToUpper
+
+Can be used to uppercase the input string.
+
+!!! example
+
+    ```groovy
+    g.V().values('name').map(cypherToUpper())
+    ```
+
+    Result:
+
+    ```text
+    ['VADAS', 'RIPPLE', 'MARKO', 'PETER', 'JOSH', 'LOP']
+    ```
+
+## cypherToLower
+
+Can be used to lowercase the input string.
+
+!!! example
+
+    ```groovy
+    g.V().values('name').map(cypherToLower())
+    ```
+
+    Result:
+
+    ```text
+    ['vadas', 'ripple', 'marko', 'peter', 'josh', 'lop']
+    ```
+
+## cypherSplit
+
+Can be used to split an input string based on a given character.
+
+Note requires complex input pairs by using the project step for the input string
+and split character. The example below splits on an `e`.
+
+!!! example
+
+    ```groovy
+    g.V().hasLabel('person')
+        .project('result')
+            .by(__.project('a', 'b')
+                .by(__.values('name'))
+                .by(__.constant('e'))
+                .select(values)
+                .map(cypherSplit()))
+    ```
+
+    ```text
+    [{'result': ['vadas']},
+     {'result': ['marko']},
+     {'result': ['p', 't', 'r']},
+     {'result': ['josh']}]
+    ```

docs/reference/gremlin-guide/custom-predicates.md

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+# GafferPop Custom Predicates
+
+This page details each additional predicate that is available in
+the Gaffer Gremlin API (GafferPop). These predicates may be exclusive
+to GafferPop so are likely not available in other databases that
+support the Gremlin query language.
+
+!!! note
+    Currently no custom predicates in GafferPop.

docs/reference/gremlin-guide/gaffer-options.md

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+# Custom Options for Gremlin
+
+This page details the available options you can pass in a Gremlin
+`with()` step when using the GafferPop API for querying.
+
+## Operation Options
+
+Key: `operationOptions`
+
+Allows passing options to the underlying Gaffer Operations, this is the same as
+the `options` field on a standard JSON query. This will be applied to all
+operations in the query.
+
+!!! example
+
+    ```groovy
+    g.with("operationOptions", "gaffer.federatedstore.operation.graphIds:graphA").V().toList()
+    ```
+
+## GetAllElements Limit
+
+Key `getAllElementsLimit`
+
+Limits the amount of elements returned if performing an unseeded query e.g. a
+`GetAllElements` operation. This will override the default for the current
+query, see the [admin guide](../../administration-guide/gaffer-deployment/gremlin.md#configuring-the-gafferpop-library)
+for more detail on setting up defaults.
+
+!!! example
+
+    ```groovy
+    g.with("getAllElementsLimit", 100).V().toList()
+    ```
+
+## Has Step Filter Stage
+
+Key: `hasStepFilterStage`
+
+Controls which phase the filtering from a Gremlin `has()` stage is applied to
+the results. This will apply to all `has()` steps in the query and override the
+default, see the [admin guide](../../administration-guide/gaffer-deployment/gremlin.md#configuring-the-gafferpop-library)
+for more detail on setting up defaults.
+
+!!! example
+
+    ```groovy
+    g.with("hasStepFilterStage", "PRE_AGGREGATION").V().has("count" P.gt(1)).toList()
+    ```
+
+## Cypher Query
+
+Key: `cypher`
+
+Translates the given Cypher query to Gremlin and executes it on the Graph. This
+can be used in combination with other `with()` steps as they will be protected
+from translation.
+
+!!! example
+
+    ```groovy
+    g.with("cypher", "MATCH (p:person) RETURN p").call().toList()
+    ```