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src/__Libraries/StellaOps.DistroIntel/DistroDerivative.cs

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+// -----------------------------------------------------------------------------
+// DistroDerivative.cs
+// Sprint: SPRINT_20251230_001_BE_backport_resolver (BP-301, BP-302, BP-303, BP-304)
+// Task: Create DistroDerivativeMapping model for cross-distro OVAL/CSAF evidence
+// -----------------------------------------------------------------------------
+
+using System.Collections.Frozen;
+using System.Collections.Immutable;
+
+namespace StellaOps.DistroIntel;
+
+/// <summary>
+/// Confidence level for derivative distro mappings.
+/// Determines the confidence penalty applied when using evidence from a derivative.
+/// </summary>
+public enum DerivativeConfidence
+{
+    /// <summary>
+    /// High confidence - ABI-compatible rebuilds.
+    /// Examples: AlmaLinux/Rocky → RHEL, CentOS → RHEL
+    /// Confidence multiplier: 0.95
+    /// </summary>
+    High,
+
+    /// <summary>
+    /// Medium confidence - Modified derivatives with some customizations.
+    /// Examples: Linux Mint → Ubuntu, Ubuntu → Debian
+    /// Confidence multiplier: 0.80
+    /// </summary>
+    Medium
+}
+
+/// <summary>
+/// Represents a relationship between a canonical (parent) distro and a derivative.
+/// Used for Tier 1 evidence fallback when native OVAL/CSAF is unavailable.
+/// </summary>
+/// <param name="CanonicalDistro">The parent/upstream distro identifier (e.g., "rhel", "debian").</param>
+/// <param name="DerivativeDistro">The derivative distro identifier (e.g., "almalinux", "ubuntu").</param>
+/// <param name="MajorRelease">The major release version for which this mapping applies.</param>
+/// <param name="Confidence">Confidence level of the derivative relationship.</param>
+public sealed record DistroDerivative(
+    string CanonicalDistro,
+    string DerivativeDistro,
+    int MajorRelease,
+    DerivativeConfidence Confidence);
+
+/// <summary>
+/// Static registry of distro derivative mappings for cross-distro evidence sharing.
+/// </summary>
+public static class DistroMappings
+{
+    /// <summary>
+    /// All known distro derivative relationships.
+    /// Maps parent distros to their derivatives for OVAL/CSAF fallback.
+    /// </summary>
+    public static readonly ImmutableArray<DistroDerivative> Derivatives =
+    [
+        // RHEL family - High confidence (ABI-compatible rebuilds)
+        new DistroDerivative("rhel", "almalinux", 8, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "almalinux", 9, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "almalinux", 10, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "rocky", 8, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "rocky", 9, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "rocky", 10, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "centos", 7, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "centos", 8, DerivativeConfidence.High), // CentOS 8 (EOL)
+        new DistroDerivative("rhel", "oracle", 7, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "oracle", 8, DerivativeConfidence.High),
+        new DistroDerivative("rhel", "oracle", 9, DerivativeConfidence.High),
+
+        // Debian family - Medium confidence (modified derivatives)
+        new DistroDerivative("debian", "ubuntu", 10, DerivativeConfidence.Medium), // Debian 10 -> Ubuntu derivation
+        new DistroDerivative("debian", "ubuntu", 11, DerivativeConfidence.Medium), // Debian 11 (Bullseye)
+        new DistroDerivative("debian", "ubuntu", 12, DerivativeConfidence.Medium), // Debian 12 (Bookworm)
+
+        // Ubuntu derivatives - Medium confidence
+        new DistroDerivative("ubuntu", "linuxmint", 20, DerivativeConfidence.Medium), // Ubuntu 20.04 base
+        new DistroDerivative("ubuntu", "linuxmint", 21, DerivativeConfidence.Medium), // Ubuntu 21.04 base
+        new DistroDerivative("ubuntu", "linuxmint", 22, DerivativeConfidence.Medium), // Ubuntu 22.04 base
+        new DistroDerivative("ubuntu", "pop", 20, DerivativeConfidence.Medium),       // Pop!_OS
+        new DistroDerivative("ubuntu", "pop", 22, DerivativeConfidence.Medium),
+
+        // SUSE family
+        new DistroDerivative("sles", "opensuse-leap", 15, DerivativeConfidence.High),
+    ];
+
+    private static readonly FrozenDictionary<(string, int), ImmutableArray<DistroDerivative>> _byCanonicalIndex =
+        Derivatives
+            .GroupBy(d => (d.CanonicalDistro.ToLowerInvariant(), d.MajorRelease))
+            .ToFrozenDictionary(
+                g => g.Key,
+                g => g.ToImmutableArray());
+
+    private static readonly FrozenDictionary<(string, int), DistroDerivative?> _byDerivativeIndex =
+        Derivatives
+            .ToFrozenDictionary(
+                d => (d.DerivativeDistro.ToLowerInvariant(), d.MajorRelease),
+                d => (DistroDerivative?)d);
+
+    /// <summary>
+    /// Finds derivatives for a canonical (parent) distro at a specific major release.
+    /// Use this to find alternative evidence sources when native OVAL/CSAF is unavailable.
+    /// </summary>
+    /// <param name="canonicalDistro">The canonical distro identifier (e.g., "rhel").</param>
+    /// <param name="majorRelease">The major release version.</param>
+    /// <returns>Matching derivative mappings, ordered by confidence (High first).</returns>
+    /// <example>
+    /// var derivatives = DistroMappings.FindDerivativesFor("rhel", 9);
+    /// // Returns: [(rhel, almalinux, 9, High), (rhel, rocky, 9, High), (rhel, oracle, 9, High)]
+    /// </example>
+    public static IEnumerable<DistroDerivative> FindDerivativesFor(string canonicalDistro, int majorRelease)
+    {
+        var key = (canonicalDistro.ToLowerInvariant(), majorRelease);
+        if (_byCanonicalIndex.TryGetValue(key, out var derivatives))
+        {
+            return derivatives.OrderByDescending(d => d.Confidence);
+        }
+        return [];
+    }
+
+    /// <summary>
+    /// Finds the canonical (parent) distro for a derivative distro.
+    /// Use this to map a derivative back to its upstream source.
+    /// </summary>
+    /// <param name="derivativeDistro">The derivative distro identifier (e.g., "almalinux").</param>
+    /// <param name="majorRelease">The major release version.</param>
+    /// <returns>The canonical mapping if found, null otherwise.</returns>
+    /// <example>
+    /// var canonical = DistroMappings.FindCanonicalFor("almalinux", 9);
+    /// // Returns: (rhel, almalinux, 9, High)
+    /// </example>
+    public static DistroDerivative? FindCanonicalFor(string derivativeDistro, int majorRelease)
+    {
+        var key = (derivativeDistro.ToLowerInvariant(), majorRelease);
+        return _byDerivativeIndex.GetValueOrDefault(key);
+    }
+
+    /// <summary>
+    /// Gets the confidence multiplier for a derivative relationship.
+    /// Apply this to the base confidence when using derivative evidence.
+    /// </summary>
+    /// <param name="confidence">The derivative confidence level.</param>
+    /// <returns>Multiplier value (0.95 for High, 0.80 for Medium).</returns>
+    public static decimal GetConfidenceMultiplier(DerivativeConfidence confidence)
+    {
+        return confidence switch
+        {
+            DerivativeConfidence.High => 0.95m,
+            DerivativeConfidence.Medium => 0.80m,
+            _ => 0.70m // Unknown - conservative
+        };
+    }
+
+    /// <summary>
+    /// Checks if a distro is a known canonical (parent) distro.
+    /// </summary>
+    /// <param name="distro">The distro identifier to check.</param>
+    /// <returns>True if the distro is a known canonical distro.</returns>
+    public static bool IsCanonicalDistro(string distro)
+    {
+        var lower = distro.ToLowerInvariant();
+        return lower is "rhel" or "debian" or "ubuntu" or "sles" or "alpine";
+    }
+
+    /// <summary>
+    /// Normalizes a distro name to its canonical form.
+    /// </summary>
+    /// <param name="distro">The distro name to normalize.</param>
+    /// <returns>Lowercase canonical form.</returns>
+    public static string NormalizeDistroName(string distro)
+    {
+        var lower = distro.ToLowerInvariant();
+        return lower switch
+        {
+            "redhat" or "red hat" or "red-hat" => "rhel",
+            "alma" or "almalinux-os" => "almalinux",
+            "rockylinux" or "rocky-linux" => "rocky",
+            "oracle linux" or "oraclelinux" => "oracle",
+            "opensuse" or "opensuse-tumbleweed" => "opensuse-leap",
+            "mint" => "linuxmint",
+            "popos" or "pop_os" => "pop",
+            _ => lower
+        };
+    }
+}

src/__Libraries/StellaOps.DistroIntel/StellaOps.DistroIntel.csproj

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+<?xml version="1.0" encoding="utf-8"?>
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
+    <RootNamespace>StellaOps.DistroIntel</RootNamespace>
+  </PropertyGroup>
+</Project>