[SOAR-0013] Idiomatic naming strategy (#683)

### Motivation

See proposal

### Modifications

N/A

### Result

N/A

### Test Plan

N/A

Sources/swift-openapi-generator/Documentation.docc/Proposals/Proposals.md

@@ -54,3 +54,4 @@ If you have any questions, tag [Honza Dvorsky](https://github.com/czechboy0) or
 - <doc:SOAR-0010>
 - <doc:SOAR-0011>
 - <doc:SOAR-0012>
+- <doc:SOAR-0013>

Sources/swift-openapi-generator/Documentation.docc/Proposals/SOAR-0013.md

@@ -0,0 +1,170 @@
+# SOAR-0013: Idiomatic naming strategy
+
+Introduce an alternative naming strategy for more idiomatic Swift identifiers, including a way to provide custom name overrides.
+
+## Overview
+
+- Proposal: SOAR-0013
+- Author(s): [Honza Dvorsky](https://github.com/czechboy0), [Si Beaumont](https://github.com/simonjbeaumont)
+- Status: **Implemented (1.6.0)**
+- Versions:
+    - v1.0 (2024-11-27): Initial version
+    - v1.1 (2024-11-28): Also handle "/", "{", and "}" for better synthesized operation names
+    - v1.2 (2024-12-10): Treat "/" as a word separator and improve handling of the lowercasing of acronyms at the start of an identifier.
+    - v1.3 (2024-12-17): Add "+" as a word separator, clarify that defensive strategy always post-processes the output of the idiomatic strategy, clarify that content type names also change with the idiomatic strategy
+- Issues:
+    - [apple/swift-openapi-generator#112][issuePlugin]
+    - [apple/swift-openapi-generator#107][issue1]
+    - [apple/swift-openapi-generator#503][issue2]
+    - [apple/swift-openapi-generator#244][issue3]
+    - [apple/swift-openapi-generator#405][issue4]
+- Implementation:
+    - [apple/swift-openapi-generator#679][pr]
+- New configuration options:
+    - `namingStrategy`
+    - `nameOverrides`
+- Affected components:
+    - generator
+
+### Introduction
+
+Introduce a new naming strategy as an opt-in feature, instructing the generator to produce more conventional Swift names, and offer a way to completely customize how any OpenAPI identifier gets projected to a Swift identifier.
+
+### Motivation
+
+The purpose of Swift OpenAPI Generator is to generate Swift code from OpenAPI documents. As part of that process, names specified in the OpenAPI document have to be converted to names in Swift code - and there are many ways to do that. We call these "naming strategies" in this proposal.
+
+When Swift OpenAPI Generator 0.1.0 went open-source in May 2023, it had a simple naming strategy that produced relatively conventional Swift identifiers from OpenAPI names.
+
+However, when tested on a large test corpus of around 3000 OpenAPI documents, it produced an unacceptably high number of non-compiling packages due to naming conflicts. The root cause of conflicts are the different allowed character sets for OpenAPI names and Swift identifiers. OpenAPI has a more flexible allowed character set than Swift identifiers.
+
+In response to these findings, [SOAR-0001: Improved mapping of identifiers][soar0001], which shipped in 0.2.0, changed the naming strategy to avoid these conflicts, allowing hundreds of additional OpenAPI documents to be correctly handled by Swift OpenAPI Generator. This addressed all issues related to naming conflicts in the test corpus. This is the existing naming strategy today. This strategy also avoids changing the character casing, as we discovered OpenAPI documents with properties within an object schema that only differed by case.
+
+The way the conflicts are avoided in the naming strategy from SOAR-0001 is by turning any special characters (any characters that aren't letters, numbers, or an underscore) into words, resulting in identifiers like:
+
+```
+User -> User
+User_1 -> User_1
+user-name -> user_hyphen_name
+my.org.User -> my_period_org_period_User
+```
+
+Our priority was to support as many valid OpenAPI documents as possible. However, we've also [heard][issue1] [from][issue2] [adopters][issue3] [who][issue4] would prefer more idiomatic generated code and don't benefit from the defensive naming strategy.
+
+### Proposed solution
+
+We propose to introduce a second, opt-in naming strategy, which produces idiomatic Swift identifiers from arbitrary OpenAPI names, and a way to fully customize the conversion from an OpenAPI name to a Swift identifier using a string -> string map.
+
+For clarity, we'll refer to the existing naming strategy as the "defensive" naming strategy, and to the new proposed strategy as the "idiomatic" naming strategy. The names reflect the strengths of each strategy - the defensive strategy can handle any OpenAPI document and produce compiling Swift code, the idiomatic naming strategy produces prettier names, but does not work for all documents.
+
+Part of the new strategy is adjusting the capitalization, and producing `UpperCamelCase` names for types, and `lowerCamelCase` names for members, as is common in hand-written Swift code.
+
+This strategy will pre-process the input string, and then still apply the defensive strategy on the output, to handle any illegal characters that are no explicitly handled by the idiomatic strategy.
+
+The configurable naming strategy will affect not only general names from the OpenAPI document ([SOAR-0001][soar0001]), but also content type names ([SOAR-0002][soar0002]). 
+
+The second feature introduced as part of this proposal is a way to provide a string -> string map to fully override only specific OpenAPI names and provide their exact Swift identifiers. This is the ultimate escape hatch when both naming strategies fail to provide the desired result for the adopter.
+
+> Note: Because the idiomatic naming strategy can change capitalization, in rare cases it can still generate non-compiling code from a valid OpenAPI document. We recommend using the idiomatic naming strategy and, if it produces conflicts, address them using name overrides, or switch back to the defensive naming strategy.
+
+#### Examples
+
+To get a sense for the proposed change, check out the table below that compares the existing defensive strategy against the proposed idiomatic strategy on a set of examples:
+
+| OpenAPI name | Defensive | Idiomatic (capitalized) | Idiomatic (non-capitalized) |
+| ------------ | --------- | ------------------------ | ---------------------------- |
+| `foo` | `foo` | `Foo` | `foo` |
+| `Hello world` | `Hello_space_world` | `HelloWorld` | `helloWorld` |
+| `My_URL_value` | `My_URL_value` | `MyURLValue` | `myURLValue` |
+| `Retry-After` | `Retry_hyphen_After` | `RetryAfter` | `retryAfter` |
+| `NOT_AVAILABLE` | `NOT_AVAILABLE` | `NotAvailable` | `notAvailable` |
+| `version 2.0` | `version_space_2_period_0` | `Version2_0` | `version2_0` |
+| `naïve café` | `naïve_space_café` | `NaïveCafé` | `naïveCafé` |
+| `__user` | `__user` | `__User` | `__user` |
+| `get/pets/{petId}` _Changed in v1.2_ | `get_sol_pets_sol__lcub_petId_rcub_` | `GetPetsPetId` | `getPetsPetId` |
+| `HTTPProxy` _Added in v1.2_ | `HTTPProxy` | `HTTPProxy` | `httpProxy` |
+| `application/myformat+json` _Added in v1.3_ | `application_myformat_plus_json` | - | `applicationMyformatJson` | 
+| `order#123` | `order_num_123` | `order_num_123` | `order_num_123` |
+
+Notice that in the last example, since the OpenAPI name contains the pound (`#`) character, the idiomatic naming strategy lets the defensive naming strategy handle the illegal character. In all the other cases, however, the resulting names are more idiomatic Swift identifiers.
+
+> Tip: For more examples, check out the updated [test suite](https://github.com/czechboy0/swift-openapi-generator/blob/hd-naming-strategy-optimistic/Tests/OpenAPIGeneratorCoreTests/Extensions/Test_SwiftSafeNames.swift).
+
+### Detailed design
+
+This section goes into detail of the [draft implementation][pr] that you can already check out and try to run on your OpenAPI document.
+
+> Note: To enable it, you'll need to add `namingStrategy: idiomatic` to your `openapi-generator-config.yaml` file.
+
+#### Naming logic
+
+The idiomatic naming strategy (check out the current code [here][impl], look for the method `safeForSwiftCode_idiomatic`) is built around the decision to _only_ optimize for names that include the following:
+
+- letters
+- numbers
+- periods (`.`, ASCII: `0x2e`)
+- dashes (`-`, ASCII: `0x2d`)
+- underscores (`_`, ASCII: `0x5f`)
+- spaces (` `, ASCII: `0x20`)
+- slashes (`/`, ASCII: `0x2f`) _Added in v1.1, changed meaning in 1.2_
+- curly braces (`{` and `}`, ASCII: `0x7b` and `0x7d`) _Added in v1.1_
+- pluses (`+`, ASCII: `0x2b`) _Added in v1.3_
+
+> Note: We let [`Swift.String.isLetter`](https://developer.apple.com/documentation/swift/character/isletter) decide whether a character is a letter, which has the advantage of including letters in the non-ASCII range. Swift identifiers also support a [wide range](https://docs.swift.org/swift-book/documentation/the-swift-programming-language/lexicalstructure/#Identifiers) of alphanumeric characters.
+
+If the OpenAPI name includes any _other_ characters, the idiomatic naming strategy lets the defensive strategy handle those characters.
+
+There's a second special case for handling all uppercased names, such as `NOT_AVAILABLE` - if this situation is detected, the idiomatic naming strategy turns it into `NotAvailable` for types and `notAvailable` for members.
+
+The best way to understand the detailed logic is to check out the [code][impl], feel free to leave comments on the pull request.
+
+#### Naming strategy configuration
+
+Since Swift OpenAPI Generator is on a stable 1.x version, we cannot change the naming strategy for everyone, as it would be considered an API break. So this new naming strategy is fully opt-in using a new configuration key called `namingStrategy`, with the following allowed values:
+
+- `defensive`: the existing naming strategy introduced in 0.2.0
+- `idiomatic`: the new naming strategy proposed here
+- not specified: defaults to `defensive` for backwards compatibility
+
+Enabling this feature in the configuration file would look like this:
+
+```yaml
+namingStrategy: idiomatic
+```
+
+#### Name overrides
+
+While the new naming strategy produces much improved Swift names, there are still cases when the adopter knows better how they'd like a specific OpenAPI name be translated to a Swift identifier.
+
+A good examples are the `+1` and `-1` properties in the GitHub OpenAPI document: using both strategies, the names would be `_plus_1` and `_hyphen_1`, respectively. While such names aren't too confusing, the adopter might want to customize them to, for example: `thumbsUp` and `thumbsDown`.
+
+Enabling this feature in the configuration file would look like this:
+
+```yaml
+nameOverrides:
+  '+1': 'thumbsUp'
+  '-1': 'thumbsDown'
+```
+
+### API stability
+
+Both the new naming strategy and name overrides are purely additive, and require the adopter to explicitly opt-in.
+
+### Future directions
+
+With this proposal, we plan to abandon the ["naming extensions" idea][issuePlugin], as we consider the solution in this proposal to solve the name conversion problem for Swift OpenAPI Generator 1.x for all use cases.
+
+### Alternatives considered
+
+- ["Naming extensions"][issuePlugin], however that'd require the community to build and maintain custom naming strategies, and it was not clear that this feature would be possible in SwiftPM using only current features.
+- Not changing anything, this was the status quo since 0.2.0, but adopters have made it clear that there is room to improve the naming strategy through the several filed issues linked at the top of the proposal, so we feel that some action here is justified.
+
+[soar0001]: https://swiftpackageindex.com/apple/swift-openapi-generator/documentation/swift-openapi-generator/soar-0001
+[soar0002]: https://swiftpackageindex.com/apple/swift-openapi-generator/documentation/swift-openapi-generator/soar-0002
+[issue1]: https://github.com/apple/swift-openapi-generator/issues/107
+[issue2]: https://github.com/apple/swift-openapi-generator/issues/503
+[issue3]: https://github.com/apple/swift-openapi-generator/issues/244
+[issue4]: https://github.com/apple/swift-openapi-generator/issues/405
+[issuePlugin]: https://github.com/apple/swift-openapi-generator/issues/112
+[pr]: https://github.com/apple/swift-openapi-generator/pull/679
+[impl]: https://github.com/czechboy0/swift-openapi-generator/blob/hd-naming-strategy-optimistic/Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/SwiftSafeNames.swift