Adds helper function to find what Pokemon evolves into what

PkmnLib.Static/Libraries/SpeciesLibrary.cs

@@ -13,6 +13,10 @@ public interface IReadOnlySpeciesLibrary : IEnumerable<ISpecies>
     /// Tries to get a species from the library. Returns false if the species is not found.
     /// </summary>
     bool TryGet(StringKey key, [MaybeNullWhen(false)] out ISpecies value);
+    
+    /// <summary>
+    /// Tried to get a species from the library by its national dex number. Returns false if the species is not found.
+    /// </summary>
     bool TryGetById(int id, [MaybeNullWhen(false)] out ISpecies value);
 
     /// <summary>
@@ -29,11 +33,19 @@ public interface IReadOnlySpeciesLibrary : IEnumerable<ISpecies>
     /// Whether the library is empty.
     /// </summary>
     bool IsEmpty { get; }
+
+    /// <summary>
+    /// Finds the Pokémon that evolves into the given species. If the species does not evolve, returns null.
+    /// If multiple species evolve into the given species, returns the first one found.
+    /// </summary>
+    ISpecies? FindPreEvolution(ISpecies species);
 }
 
 /// <inheritdoc cref="IReadOnlySpeciesLibrary"/>
 public class SpeciesLibrary : DataLibrary<ISpecies>, IReadOnlySpeciesLibrary
 {
+    private Dictionary<ISpecies, ISpecies> _preEvolutionCache = new();
+    
     /// <inheritdoc />
     public bool TryGetById(int id, [MaybeNullWhen(false)] out ISpecies value)
     {
@@ -41,4 +53,19 @@ public class SpeciesLibrary : DataLibrary<ISpecies>, IReadOnlySpeciesLibrary
             ? (value = species) != null
             : (value = default) != null;
     }
+
+    /// <inheritdoc />
+    public ISpecies? FindPreEvolution(ISpecies species)
+    {
+        if (_preEvolutionCache.TryGetValue(species, out var preEvolution))
+            return preEvolution;
+        foreach (var s in this)
+        {
+            if (s.EvolutionData.All(e => e.ToSpecies != species.Name)) 
+                continue;
+            _preEvolutionCache[species] = s;
+            return s;
+        }
+        return null;
+    }
 }