Add :test:function, :test:select, and :test:format function definitions (#769)

* Add test:number and test:plural function definitions

* Rename as test:func & test:select, add test:format

* Apply suggestions from code review

* Add "Operands" and "Options" sections to look like registry.md function definitions

* Apply suggestions from code review

Co-authored-by: Addison Phillips <[email protected]>

* Linkify all the things

* Drop note about not rounding

* Rename FD as DecimalPlaces

---------

Co-authored-by: Addison Phillips <[email protected]>

Author: eemeli

test/README.md

@@ -23,3 +23,103 @@ Some examples of test harnesses using these tests, from the source repository:
 A [JSON schema](./schemas/) is included for the test files in this repository.
 
 For users of Visual Studio Code, a [settings file](./.vscode/settings.json) is included that enables schema validation while editing the test files.
+
+## Test Functions
+
+As the behaviour of some of the default registry _functions_
+such as `:number` and `:datetime`
+is dependent on locale-specific data and may vary between implementations,
+the following _functions_ are defined for **test use only**:
+
+### `:test:function`
+
+This function is valid both as a _selector_ and as a _formatter_.
+
+#### Operands
+
+The function `:test:function` requires a [Number Operand](/spec/registry.md#number-operands) as its _operand_.
+
+#### Options
+
+The only _option_ `:test:function` recognizes is `decimalPlaces`,
+a _digit size option_ for which only `0` and `1` are valid values.
+
+All other _options_ and their values are ignored.
+
+#### Behavior
+
+When resolving a `:test:function` expression,
+its `Input` and `DecimalPlaces` values are determined as follows:
+
+1. Let `DecimalPlaces` be 0.
+1. Let `arg` be the resolved value of the _expression_ _operand_.
+1. If `arg` is the resolved value of an _expression_
+   with a `:test:function`, `:test:select`, or `:test:format` _annotation_
+   for which resolution has succeeded, then
+   1. Let `Input` be the `Input` value of `arg`.
+   1. Set `DecimalPlaces` to be `DecimalPlaces` value of `arg`.
+1. Else if `arg` is a numerical value
+   or a string matching the `number-literal` production, then
+   1. Let `Input` be the numerical value of `arg`.
+1. Else,
+   1. Emit "bad-input" _Resolution Error_.
+   1. Use a _fallback value_ as the resolved value of the _expression_.
+      Further steps of this algorithm are not followed.
+1. If the `decimalPlaces` _option_ is set, then
+   1. If its value resolves to a numerical integer value 0 or 1
+      or their corresponding string representations `'0'` or `'1'`, then
+      1. Set `DecimalPlaces` to be the numerical value of the _option_.
+   1. Else if its value is not an unresolved value set by _option resolution_,
+      1. Emit "bad-option" _Resolution Error_.
+      1. Use a _fallback value_ as the resolved value of the _expression_.
+
+When `:test:function` is used as a _selector_,
+the behaviour of calling it as the `rv` value of MatchSelectorKeys(`rv`, `keys`)
+(see [Resolve Preferences](/spec/formatting.md#resolve-preferences) for more information)
+depends on its `Input` and `DecimalPlaces` values.
+
+- If the `Input` is 1 and `DecimalPlaces` is 1,
+  the method will return some slice of the list « `'1.0'`, `'1'` »,
+  depending on whether those values are included in `keys`.
+- If the `Input` is 1 and `DecimalPlaces` is 0,
+  the method will return the list « `'1'` » if `keys` includes `'1'`, or an empty list otherwise.
+- If the `Input` is any other value, the method will return an empty list.
+
+When an _expression_ with a `:test:function` _annotation_ is assigned to a _variable_ by a _declaration_
+and that _variable_ is used as an _option_ value,
+its resolved value is the `Input` value.
+
+When `:test:function` is used as a _formatter_,
+a _placeholder_ resolving to a value with a `:test:function` _expression_
+is formatted as a concatenation of the following parts:
+
+1. If `Input` is less than 0, the character `-` U+002D Hyphen-Minus.
+1. The truncated absolute integer value of `Input`, i.e. floor(abs(`Input`)),
+   formatted as a sequence of decimal digit characters (U+0030...U+0039).
+1. If `DecimalPlaces` is 1, then
+   1. The character `.` U+002E Full Stop.
+   1. The single decimal digit character representing the value floor((abs(`Input`) - floor(abs(`Input`))) \* 10)
+
+If the formatting target is a sequence of parts,
+each of the above parts will be emitted separately
+rather than being concatenated into a single string.
+
+
+### `:test:select`
+
+This _function_ accepts the same _operands_ and _options_,
+and behaves exactly the same as `:test:function`,
+except that it cannot be used for formatting.
+
+When `:test:select` is used as a _formatter_,
+a "not-formattable" error is emitted and the _placeholder_ is formatted with
+a _fallback value_.
+
+### `:test:format`
+
+This _function_ accepts the same _operands_ and _options_,
+and behaves exactly the same as `:test:function`,
+except that it cannot be used for selection.
+
+When `:test:format` is used as a _selector_,
+the steps under 2.iii. of [Resolve Selectors](/spec/formatting.md#resolve-selectors) are followed.