Implement basic AI player research

Right now they just randomly pick techs to research without any particular strategy, but this will get us closer to some niceties, like tech trading and era advancement.

#391

C7Engine/AI/PlayerAI.cs

@@ -13,7 +13,7 @@ namespace C7Engine {
 	public class PlayerAI {
 		private static ILogger log = Log.ForContext<PlayerAI>();
 
-		public static void PlayTurn(Player player, Random rng) {
+		public static void PlayTurn(Player player, Random rng, List<Tech> techs) {
 			if (player.isHuman || player.isBarbarians) {
 				return;
 			}
@@ -32,6 +32,17 @@ public static void PlayTurn(Player player, Random rng) {
 				player.turnsUntilPriorityReevaluation--;
 			}
 
+			if (player.currentlyResearchedTech == null) {
+				Tech toResearch = PickTechToResearch(player, techs);
+				if (toResearch == null) {
+					log.Information($"Player {player.civilization.name} has no techs available to research.");
+					player.SetCurrentlyResearchedTech(null);
+				} else {
+					log.Information($"Player {player.civilization.name} is researching {toResearch.Name}.");
+					player.SetCurrentlyResearchedTech(toResearch.id);
+				}
+			}
+
 			//Do things with units.  Copy into an array first to avoid collection-was-modified exception
 			foreach (MapUnit unit in player.units.ToArray()) {
 				//For each unit, if there's already an AI task assigned, it will attempt to complete its goal.
@@ -253,5 +264,33 @@ public static UnitAI getAIForUnitStrategy(UnitAIData aiData) {
 			}
 			throw new Exception("AI data not recognized" + aiData);
 		}
+
+		private static Tech PickTechToResearch(Player player, List<Tech> techs) {
+			List<Tech> possibleTechs = new();
+
+			// Figure out what techs we're allowed to research.
+			foreach (Tech tech in techs) {
+				if (tech.EraCivilopediaName != player.eraCivilopediaName) {
+					continue;
+				}
+				bool prereqsKnown = true;
+				foreach (Tech prereq in tech.Prerequisites) {
+					if (!player.knownTechs.Contains(prereq.id)) {
+						prereqsKnown = false;
+						break;
+					}
+				}
+				if (!prereqsKnown) {
+					continue;
+				}
+				possibleTechs.Add(tech);
+			}
+
+			if (possibleTechs.Count == 0) {
+				return null;
+			}
+
+			return possibleTechs[(int)GameData.rng.NextInt64(possibleTechs.Count)];
+		}
 	}
 }

C7Engine/EntryPoints/TurnHandling.cs

@@ -86,7 +86,7 @@ private static bool PlayPlayerTurns(GameData gameData, bool firstTurn) {
 						new BarbarianAI().PlayTurn(player, gameData);
 						player.hasPlayedThisTurn = true;
 					} else if (!player.isHuman) {
-						PlayerAI.PlayTurn(player, GameData.rng);
+						PlayerAI.PlayTurn(player, GameData.rng, gameData.techs);
 						player.hasPlayedThisTurn = true;
 					} else if (player.id != EngineStorage.uiControllerID) {
 						player.hasPlayedThisTurn = true;