Introduce URIEncoder and URIDecoder types (#41)

Introduce URIEncoder and URIDecoder types

### Motivation

Fixes apple/swift-openapi-generator#192.

For refactoring how we encode:
- path parameters
- query parameters
- headers
- (new feature, coming later) `application/x-www-form-urlencoded` bodies

Supports:
- form + explode
- form + unexplode
- simple + explode
- simple + unexplode
- form where space is encoded as + instead of %20 (for bodies) + explode
- form where space is encoded as + instead of %20 (for bodies) + unexplode

### Modifications

First step - introduce two new types: `URIEncoder` and `URIDecoder`.

They're configurable types that can handle the URI Template (RFC 6570) form and simple styles, refined by OpenAPI 3.0.3, and also the `application/x-www-form-urlencoded` format (mostly identical to URI Template).

### Result

The types can be used now, subsequent PRs will integrate them.

### Test Plan

Added unit tests for the individual parts of the coder, but also roundtrip tests.


Reviewed by: glbrntt

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (api breakage) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#41

Author: czechboy0

NOTICE.txt

@@ -32,3 +32,12 @@ This product contains derivations of various scripts and templates from SwiftNIO
     * https://www.apache.org/licenses/LICENSE-2.0
   * HOMEPAGE:
     * https://github.com/apple/swift-nio
+
+---
+
+This product contains coder implementations inspired by swift-http-structured-headers.
+
+  * LICENSE (Apache License 2.0):
+    * https://www.apache.org/licenses/LICENSE-2.0
+  * HOMEPAGE:
+    * https://github.com/apple/swift-http-structured-headers

Sources/OpenAPIRuntime/URICoder/Common/URICodeCodingKey.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// The coding key used by the URI encoder and decoder.
+struct URICoderCodingKey {
+
+    /// The string to use in a named collection (e.g. a string-keyed dictionary).
+    var stringValue: String
+
+    /// The value to use in an integer-indexed collection (e.g. an int-keyed
+    /// dictionary).
+    var intValue: Int?
+
+    /// Creates a new key with the same string and int value as the provided key.
+    /// - Parameter key: The key whose values to copy.
+    init(_ key: some CodingKey) {
+        self.stringValue = key.stringValue
+        self.intValue = key.intValue
+    }
+}
+
+extension URICoderCodingKey: CodingKey {
+
+    init(stringValue: String) {
+        self.stringValue = stringValue
+        self.intValue = nil
+    }
+
+    init(intValue: Int) {
+        self.stringValue = "\(intValue)"
+        self.intValue = intValue
+    }
+}

Sources/OpenAPIRuntime/URICoder/Common/URICoderConfiguration.swift

@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// A bag of configuration values used by the URI encoder and decoder.
+struct URICoderConfiguration {
+
+    /// A variable expansion style as described by RFC 6570 and OpenAPI 3.0.3.
+    enum Style {
+
+        /// A style for simple string variable expansion.
+        case simple
+
+        /// A style for form-based URI expansion.
+        case form
+    }
+
+    /// A character used to escape the space character.
+    enum SpaceEscapingCharacter: String {
+
+        /// A percent encoded value for the space character.
+        case percentEncoded = "%20"
+
+        /// The plus character.
+        case plus = "+"
+    }
+
+    /// The variable expansion style.
+    var style: Style
+
+    /// A Boolean value indicating whether the key should be repeated with
+    /// each value, as described by RFC 6570 and OpenAPI 3.0.3.
+    var explode: Bool
+
+    /// The character used to escape the space character.
+    var spaceEscapingCharacter: SpaceEscapingCharacter
+
+    /// The coder used for serializing the Date type.
+    var dateTranscoder: any DateTranscoder
+}

Sources/OpenAPIRuntime/URICoder/Common/URIEncodedNode.swift

@@ -0,0 +1,149 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// A node produced by `URIValueToNodeEncoder`.
+enum URIEncodedNode: Equatable {
+
+    /// No value.
+    case unset
+
+    /// A single primitive value.
+    case primitive(Primitive)
+
+    /// An array of nodes.
+    case array([Self])
+
+    /// A dictionary with node values.
+    case dictionary([String: Self])
+
+    /// A primitive value.
+    enum Primitive: Equatable {
+
+        /// A boolean value.
+        case bool(Bool)
+
+        /// A string value.
+        case string(String)
+
+        /// An integer value.
+        case integer(Int)
+
+        /// A floating-point value.
+        case double(Double)
+
+        /// A date value.
+        case date(Date)
+    }
+}
+
+extension URIEncodedNode {
+
+    /// An error thrown by the methods modifying `URIEncodedNode`.
+    enum InsertionError: Swift.Error {
+
+        /// The encoder encoded a second primitive value.
+        case settingPrimitiveValueAgain
+
+        /// The encoder set a single value on a container.
+        case settingValueOnAContainer
+
+        /// The encoder appended to a node that wasn't an array.
+        case appendingToNonArrayContainer
+
+        /// The encoder inserted a value for key into a node that wasn't
+        /// a dictionary.
+        case insertingChildValueIntoNonContainer
+
+        /// The encoder added a value to an array, but the key was not a valid
+        /// integer key.
+        case insertingChildValueIntoArrayUsingNonIntValueKey
+    }
+
+    /// Sets the node to be a primitive node with the provided value.
+    /// - Parameter value: The primitive value to set into the node.
+    /// - Throws: If the node is already set.
+    mutating func set(_ value: Primitive) throws {
+        switch self {
+        case .unset:
+            self = .primitive(value)
+        case .primitive:
+            throw InsertionError.settingPrimitiveValueAgain
+        case .array, .dictionary:
+            throw InsertionError.settingValueOnAContainer
+        }
+    }
+
+    /// Inserts a value for a key into the node, which is interpreted as a
+    /// dictionary.
+    /// - Parameters:
+    ///   - childValue: The value to save under the provided key.
+    ///   - key: The key to save the value for into the dictionary.
+    /// - Throws: If the node is already set to be anything else but a
+    /// dictionary.
+    mutating func insert<Key: CodingKey>(
+        _ childValue: Self,
+        atKey key: Key
+    ) throws {
+        switch self {
+        case .dictionary(var dictionary):
+            self = .unset
+            dictionary[key.stringValue] = childValue
+            self = .dictionary(dictionary)
+        case .array(var array):
+            // Check that this is a valid key for an unkeyed container,
+            // but don't actually extract the index, we only support appending
+            // here.
+            guard let intValue = key.intValue else {
+                throw InsertionError.insertingChildValueIntoArrayUsingNonIntValueKey
+            }
+            precondition(
+                intValue == array.count,
+                "Unkeyed container inserting at an incorrect index"
+            )
+            self = .unset
+            array.append(childValue)
+            self = .array(array)
+        case .unset:
+            if let intValue = key.intValue {
+                precondition(
+                    intValue == 0,
+                    "Unkeyed container inserting at an incorrect index"
+                )
+                self = .array([childValue])
+            } else {
+                self = .dictionary([key.stringValue: childValue])
+            }
+        default:
+            throw InsertionError.insertingChildValueIntoNonContainer
+        }
+    }
+
+    /// Appends a value to the array node.
+    /// - Parameter childValue: The node to append to the underlying array.
+    /// - Throws: If the node is already set to be anything else but an array.
+    mutating func append(_ childValue: Self) throws {
+        switch self {
+        case .array(var items):
+            self = .unset
+            items.append(childValue)
+            self = .array(items)
+        case .unset:
+            self = .array([childValue])
+        default:
+            throw InsertionError.appendingToNonArrayContainer
+        }
+    }
+}

Sources/OpenAPIRuntime/URICoder/Common/URIParsedNode.swift

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// The type used for keys by `URIParser`.
+typealias URIParsedKey = String.SubSequence
+
+/// The type used for values by `URIParser`.
+typealias URIParsedValue = String.SubSequence
+
+/// The type used for an array of values by `URIParser`.
+typealias URIParsedValueArray = [URIParsedValue]
+
+/// The type used for a node and a dictionary by `URIParser`.
+typealias URIParsedNode = [URIParsedKey: URIParsedValueArray]