fix: Pretty print elements with intrinsic values on a single line (#296)

Elements using the empty-string CodingKey ("") for intrinsic text
content were being pretty-printed with the value on a separate line:

    <element ref="id">
    120000.0
    </element>

This happened because the pretty printer only checked containsTextNodes,
which isn't set for intrinsic values encoded via keyed containers. Add a
check for elements whose children are all inline (key is empty) and
don't wrap named child elements, treating them the same as text nodes.

Author: weppos

Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift

@@ -314,7 +314,10 @@ struct XMLCoderElement: Equatable, Sendable {
         }
 
         if !elements.isEmpty || formatting.contains(.noEmptyElements) {
-            let prettyPrintElements = prettyPrinted && !containsTextNodes
+            let hasOnlyIntrinsicContent = elements.allSatisfy { element in
+                element.key.isEmpty && !element.elements.contains { !$0.key.isEmpty }
+            }
+            let prettyPrintElements = prettyPrinted && !containsTextNodes && !hasOnlyIntrinsicContent
             if !key.isEmpty {
                 string += prettyPrintElements ? ">\n" : ">"
             }

Tests/XMLCoderTests/PrettyPrintTest.swift

@@ -14,6 +14,58 @@ private struct NestedContainer: Encodable {
     let values: [String]
 }
 
+/// Element with an attribute and intrinsic text value (empty-string CodingKey).
+private struct Measurement: Codable, Equatable {
+    @Attribute var ref: String?
+    var value: Double
+
+    enum CodingKeys: String, CodingKey {
+        case ref
+        case value = ""
+    }
+
+    init(value: Double, ref: String? = nil) {
+        self._ref = Attribute(ref)
+        self.value = value
+    }
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        _ref = try container.decodeIfPresent(Attribute<String?>.self, forKey: .ref) ?? Attribute(nil)
+        value = try container.decode(Double.self, forKey: .value)
+    }
+}
+
+private struct Sample: Codable, Equatable {
+    var depth: Double
+    var readings: [Measurement]
+
+    enum CodingKeys: String, CodingKey {
+        case depth
+        case readings = "measurement"
+    }
+
+    init(depth: Double, readings: [Measurement]) {
+        self.depth = depth
+        self.readings = readings
+    }
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        depth = try container.decode(Double.self, forKey: .depth)
+        readings = try container.decodeIfPresent([Measurement].self, forKey: .readings) ?? []
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(depth, forKey: .depth)
+        for reading in readings {
+            let enc = container.superEncoder(forKey: .readings)
+            try reading.encode(to: enc)
+        }
+    }
+}
+
 final class PrettyPrintTest: XCTestCase {
     private let testContainer = TopContainer(nested: NestedContainer(values: ["foor", "bar"]))
 
@@ -56,6 +108,56 @@ final class PrettyPrintTest: XCTestCase {
         )
     }
 
+    func testIntrinsicValueWithoutAttribute() throws {
+        let encoder = XMLEncoder()
+        encoder.outputFormatting = [.prettyPrinted]
+        encoder.prettyPrintIndentation = .spaces(4)
+
+        let sample = Sample(depth: 6.0, readings: [
+            Measurement(value: 118000.0),
+        ])
+        let encoded = try encoder.encode(sample, withRootKey: "sample")
+
+        XCTAssertEqual(
+            String(data: encoded, encoding: .utf8)!,
+            """
+            <sample>
+                <depth>6.0</depth>
+                <measurement>118000.0</measurement>
+            </sample>
+            """
+        )
+
+        let decoded = try XMLDecoder().decode(Sample.self, from: encoded)
+        XCTAssertEqual(decoded, sample)
+    }
+
+    func testIntrinsicValueWithAttribute() throws {
+        let encoder = XMLEncoder()
+        encoder.outputFormatting = [.prettyPrinted]
+        encoder.prettyPrintIndentation = .spaces(4)
+
+        let sample = Sample(depth: 30.0, readings: [
+            Measurement(value: 120000.0, ref: "sensor-1"),
+            Measurement(value: 122000.0, ref: "sensor-2"),
+        ])
+        let encoded = try encoder.encode(sample, withRootKey: "sample")
+
+        XCTAssertEqual(
+            String(data: encoded, encoding: .utf8)!,
+            """
+            <sample>
+                <depth>30.0</depth>
+                <measurement ref="sensor-1">120000.0</measurement>
+                <measurement ref="sensor-2">122000.0</measurement>
+            </sample>
+            """
+        )
+
+        let decoded = try XMLDecoder().decode(Sample.self, from: encoded)
+        XCTAssertEqual(decoded, sample)
+    }
+
     func testTabs() throws {
         let encoder = XMLEncoder()
         encoder.outputFormatting = [.prettyPrinted]