Clarify some types for data in problem.yaml

Fixes C1, C2, C3, and C5 from #372.

Author: mpsijm

spec/2023-07-draft.md

@@ -144,7 +144,7 @@ Any unknown keys should be treated as an error.
 Key                                               | Type                                          | Required  | Default
 ------------------------------------------------- | --------------------------------------------- | --------- | -------
 [problem_format_version](#problem-format-version) | String                                        | Yes       |
-[type](#type)                                     | String or sequence of strings                 | No        | `pass-fail`
+[type](#type)                                     | String or non-empty sequence of strings       | No        | `pass-fail`
 [name](#name)                                     | String or map of strings                      | Yes       |
 [uuid](#uuid-and-version)                         | String                                        | Yes       |
 [version](#uuid-and-version)                      | String                                        | No        |
@@ -155,7 +155,7 @@ Key                                               | Type
 [embargo-until](#problem-publication-embargo)     | Date                                          | No        |
 [limits](#limits)                                 | Map with keys as defined below                | No        | See below
 [keywords](#keywords)                             | Sequence of strings                           | No        |
-[languages](#languages)                           | String or sequence of strings                 | No        | `all`
+[languages](#languages)                           | String or non-empty sequence of strings       | No        | `all`
 [allow_file_writing](#allow-file-writing)         | Boolean                                       | No        | `false`
 [constants](#constants)                           | Map of strings to int, float, or string       | No        |
 
@@ -172,7 +172,7 @@ Documentation for version `<version>` is available at `https://www.kattis.com/pr
 ### Type
 
 Type of problem.
-Must be either a single string or a sequence of strings, from the table below, with no repetition.
+Must be either a single string or a non-empty sequence of strings, from the table below, with no repetition.
 Two values listed as incompatible must not both be in the sequence.
 
 Value           | Incompatible with           | Comments
@@ -187,7 +187,7 @@ Value           | Incompatible with           | Comments
 
 The name of the problem in each language for which a problem statement exists.
 If there are statements in more than one language, the `name` field must be a map with the language codes as keys and the problem names as values.
-The set of languages for which `name` is given must **exactly** match the set of languages for which a problem statement exists.
+The set of languages for which `name` is given must **exactly** match the set of languages for which a [problem statement](#problem-statements) exists.
 
 A deliberately complex and construed example:
 ```yaml
@@ -220,15 +220,16 @@ This specification currently does not imply any more semantic meaning to these f
 
 Map specifying who should get credits for creating this problem.
 A person is specified as a string with the full name, optionally followed by an email wrapped in `<>`, (e.g.: `Full Name` or `Full Name <[email protected]>`).
+Each of the keys in this section is optional.
 
-Key                | Type                                   | Comments
------------------- | ---------------------------------------| --------
-`authors`          | Person or sequence of persons          | The people who conceptualized the problem.
-`contributors`     | Person or sequence of persons          | The people who developed the problem package, such as the statement, validators, and test data.
-`testers`          | Person or sequence of persons          | The people who tested the problem package, for example, by providing a solution and reviewing the statement.
-`translators`      | Map of strings to sequences of persons | The people who translated the statement to other languages. Each key must be a language code as described in [General Requirements](#general-requirements).
-`packagers`        | Person or sequence of persons          | The people who created the problem package out of an existing problem.
-`acknowledgements` | Person or sequence of persons          | Extra acknowledgements or special thanks in addition to the previously mentioned.
+Key                | Type                                                        | Comments
+------------------ | ----------------------------------------------------------- | --------
+`authors`          | Person or non-empty sequence of persons                     | The people who conceptualized the problem.
+`contributors`     | Person or non-empty sequence of persons                     | The people who developed the problem package, such as the statement, validators, and test data.
+`testers`          | Person or non-empty sequence of persons                     | The people who tested the problem package, for example, by providing a solution and reviewing the statement.
+`translators`      | Map of strings to persons or non-empty sequences of persons | The people who translated the statement to other languages. Each key must be a language code as described in [General Requirements](#general-requirements).
+`packagers`        | Person or non-empty sequence of persons                     | The people who created the problem package out of an existing problem.
+`acknowledgements` | Person or non-empty sequence of persons                     | Extra acknowledgements or special thanks in addition to the previously mentioned.
 
 A full example would be
 ```yaml
@@ -316,28 +317,28 @@ Values other than `unknown` or `public domain` require `rights_owner` to have a
 ### Problem Publication Embargo
 
 The `embargo-until` key, if present, declares that the problem package should not be made publicly available (in problem archives, online judges, etc.) until a certain date and time.
-The value of this key must be a calendar date, or date and time of day in UTC, in ISO-8601 extended format (`YYYY-MM-DD` or `YYYY-MM-DDThh:mm:ssZ`). 
+The value of this key must be a calendar date, or date and time of day in UTC, in ISO-8601 extended format (`YYYY-MM-DD` or `YYYY-MM-DDThh:mm:ssZ`).
 The time defaults to the start of the day in UTC.
 
 ### Limits
 
 Time, memory, and other limits to be imposed on submissions.
 A map with the following keys:
 
-Key                  | Comments                   | Default        | Typical system default
--------------------- | -------------------------- | -------------- | ----------------------
-`time_multipliers`   | optional                   | see below      |
-`time_limit`         | optional float, in seconds | see below      |
-`time_resolution`    | optional float, in seconds | 1.0            |
-`memory`             | optional, in MiB           | system default | 2048
-`output`             | optional, in MiB           | system default | 8
-`code`               | optional, in KiB           | system default | 128
-`compilation_time`   | optional, in seconds       | system default | 60
-`compilation_memory` | optional, in MiB           | system default | 2048
-`validation_time`    | optional, in seconds       | system default | 60
-`validation_memory`  | optional, in MiB           | system default | 2048
-`validation_output`  | optional, in MiB           | system default | 8
-`validation_passes`  | optional                   | 2              |
+Key                  | Comments                       | Default        | Typical system default
+-------------------- | ------------------------------ | -------------- | ----------------------
+`time_multipliers`   | optional                       | see below      |
+`time_limit`         | optional float > 0, in seconds | see below      |
+`time_resolution`    | optional float > 0, in seconds | 1.0            |
+`memory`             | optional int > 0, in MiB       | system default | 2048
+`output`             | optional int > 0, in MiB       | system default | 8
+`code`               | optional int > 0, in KiB       | system default | 128
+`compilation_time`   | optional int > 0, in seconds   | system default | 60
+`compilation_memory` | optional int > 0, in MiB       | system default | 2048
+`validation_time`    | optional int > 0, in seconds   | system default | 60
+`validation_memory`  | optional int > 0, in MiB       | system default | 2048
+`validation_output`  | optional int > 0, in MiB       | system default | 8
+`validation_passes`  | optional int >= 1              | 2              |
 
 For most keys, the system default will be used if nothing is specified.
 This can vary, but you **should** assume that it's reasonable.
@@ -347,10 +348,10 @@ Only specify limits when the problem needs a specific limit, but do specify limi
 
 `time_multipliers` is a map with the following keys:
 
-Key                 | Comments | Default
-------------------- | -------- | -------
-`ac_to_time_limit`  | float    | 2.0
-`time_limit_to_tle` | float    | 1.5
+Key                 | Comments   | Default
+------------------- | ---------- | -------
+`ac_to_time_limit`  | float >= 1 | 2.0
+`time_limit_to_tle` | float >= 1 | 1.5
 
 The value of `time_limit` is an integer or floating-point problem time limit in seconds.
 The time multipliers specify safety margins relative to the slowest accepted submission, `T_ac`, and fastest time_limit_exceeded submission, `T_tle`.
@@ -359,7 +360,7 @@ In these calculations, `T_tle` is treated as infinity if the problem does not pr
 
 If no `time_limit` is provided, the default value is the smallest integer multiple of `time_resolution` that satisfies the above inequalities.
 It is an error if no such multiple exists.
-The `time_resolution` key is ignored if the problem provides an explicit time limit (and in particular, 
+The `time_resolution` key is ignored if the problem provides an explicit time limit (and in particular,
 the time limit is not required to be a multiple of the resolution).
 Since time multipliers are more future-proof than absolute time limits, avoid specifying `time_limit` whenever practical.
 
@@ -369,12 +370,11 @@ and should warn when importing a problem whose time limit is specified with prec
 ### Keywords
 
 List of keywords describing the problem.
-Keywords should not contain spaces.
 
 ### Languages
 
-List of programming languages codes from the [languages table](../appendix/languages.md) or the string `all`.
-If the value is not `all`, the problem may only be solved using the listed programming languages.
+List of one or more programming languages codes from the [languages table](../appendix/languages.md) or the string `all`.
+If the value is not `all`, the problem may only be solved using any supported programming languages.
 
 File endings in parenthesis are not used for determining language.