fix(parsing): a zero followed by a sign is text, not an octal integer

Int(_, radix:) accepts a sign, so 0-3 (a CPU range, a port range) was read as the integer -3, and 0x-1, 0b-1 and 0o-1 as negative numbers. After a radix prefix only digits may follow. Octal, hex, binary and signed integers read as before.

CHANGELOG.md

@@ -401,6 +401,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A plain scalar that starts with `0` and continues with a sign is text, not an
+  octal integer. `Int(_, radix:)` accepts a leading sign, so `0-3` (a CPU set, a
+  port range) read as the integer -3, and `0x-1`, `0b-1` and `0o-1` as negative
+  numbers. After a radix prefix only digits may follow. Octal (`0755`),
+  hexadecimal, binary and signed integers read as before.
+
 - A string the emitter wrote without quotes could be read back as something
   else, so the string `yes` (and `no`, `on`, `off` in every case), `0x1F`,
   `0b1`, `0o17`, `1_000`, `.inf` and `.nan` did not survive a dump and parse.

Sources/Parsing/Parsing.PlainScalar.swift

@@ -69,7 +69,8 @@ extension PureYAML.Parsing {
 
             for radixPrefix in radixPrefixes where unsigned.hasPrefix(radixPrefix.prefix) {
                 let digits = String(unsigned.dropFirst(radixPrefix.prefix.count))
-                guard !digits.isEmpty, let magnitude = Int(digits, radix: radixPrefix.radix) else {
+                // `Int(_, radix:)` accepts a sign, so `0-3` would read as minus three: after the prefix only digits may follow
+                guard let first = digits.first, first != "-", first != "+", let magnitude = Int(digits, radix: radixPrefix.radix) else {
                     return nil
                 }
                 return negative ? -magnitude : magnitude

Tests/SignedOctalLookalikeTests.swift

@@ -0,0 +1,25 @@
+@testable import PureYAML
+import Testing
+
+/// A plain scalar that starts with `0` is an octal integer only when everything after the `0` is octal digits. A sign after the leading zero makes it text:
+/// `0-3` is the range "0 to 3" (a CPU set, a port range), not minus three.
+@Suite("Plain scalars that begin with a zero")
+struct SignedOctalLookalikeTests {
+    @Test("A zero followed by a sign is text", arguments: ["0-3", "0-15", "0+1", "00-1", "0x-1", "0b-1", "0o-1"])
+    func test_signAfterTheLeadingZeroIsText(_ scalar: String) throws {
+        #expect(try PureYAML.parse("key: \(scalar)\n") == .mapping(.init([.init(key: "key", value: .string(scalar))])))
+    }
+
+    @Test("Octal, hexadecimal, binary and signed integers stay integers", arguments: [
+        ("0755", 493), ("0", 0), ("-0", 0), ("0x1F", 31), ("0b101", 5), ("0o17", 15), ("-12", -12), ("+7", 7), ("-0755", -493),
+    ])
+    func test_integersAreStillIntegers(_ scalar: String, _ expected: Int) throws {
+        #expect(try PureYAML.parse("key: \(scalar)\n") == .mapping(.init([.init(key: "key", value: .int(expected))])))
+    }
+
+    @Test("A range written with a leading zero survives a round trip", arguments: ["0-3", "0-15"])
+    func test_rangeRoundTrips(_ scalar: String) throws {
+        let value = PureYAML.Model.Value.mapping(.init([.init(key: "cpuset", value: .string(scalar))]))
+        #expect(try PureYAML.parse(PureYAML.dump(value)) == value)
+    }
+}