fixes casting level 0 spell says "not enough mana" (#2568)

Source/control.cpp

@@ -491,7 +491,7 @@ spell_type GetSBookTrans(spell_id ii, bool townok)
 		st = RSPLTYPE_SKILL;
 	}
 	if (st == RSPLTYPE_SPELL) {
-		if (!CheckSpell(MyPlayerId, ii, st, true)) {
+		if (CheckSpell(MyPlayerId, ii, st, true) != SpellCheckResult::Success) {
 			st = RSPLTYPE_INVALID;
 		}
 		if ((char)(myPlayer._pSplLvl[ii] + myPlayer._pISplLvlAdd) <= 0) {
@@ -668,7 +668,7 @@ void DrawSpell(const Surface &out)
 	// BUGFIX: Move the next line into the if statement to avoid OOB (SPL_INVALID is -1) (fixed)
 	if (st == RSPLTYPE_SPELL && spl != SPL_INVALID) {
 		int tlvl = myPlayer._pISplLvlAdd + myPlayer._pSplLvl[spl];
-		if (!CheckSpell(MyPlayerId, spl, st, true))
+		if (CheckSpell(MyPlayerId, spl, st, true) != SpellCheckResult::Success)
 			st = RSPLTYPE_INVALID;
 		if (tlvl <= 0)
 			st = RSPLTYPE_INVALID;

Source/player.cpp

@@ -3556,11 +3556,12 @@ void CheckPlrSpell()
 				return;
 		}
 	}
-
+	SpellCheckResult spellcheck = SpellCheckResult::Success;
 	switch (myPlayer._pRSplType) {
 	case RSPLTYPE_SKILL:
 	case RSPLTYPE_SPELL:
-		addflag = CheckSpell(MyPlayerId, rspell, myPlayer._pRSplType, false);
+		spellcheck = CheckSpell(MyPlayerId, rspell, myPlayer._pRSplType, false);
+		addflag = spellcheck == SpellCheckResult::Success;
 		break;
 	case RSPLTYPE_SCROLL:
 		addflag = UseScroll();
@@ -3574,7 +3575,17 @@ void CheckPlrSpell()
 
 	if (!addflag) {
 		if (myPlayer._pRSplType == RSPLTYPE_SPELL) {
-			myPlayer.Say(HeroSpeech::NotEnoughMana);
+			switch (spellcheck) {
+			case SpellCheckResult::Fail_NoMana:
+				myPlayer.Say(HeroSpeech::NotEnoughMana);
+				break;
+			case SpellCheckResult::Fail_Level0:
+				myPlayer.Say(HeroSpeech::ICantCastThatYet);
+				break;
+			default:
+				myPlayer.Say(HeroSpeech::ICantDoThat);
+				break;
+			}
 			LastMouseButtonAction = MouseActionType::None;
 		}
 		return;

Source/spells.cpp

@@ -196,28 +196,31 @@ void EnsureValidReadiedSpell(PlayerStruct &player)
 	}
 }
 
-bool CheckSpell(int id, spell_id sn, spell_type st, bool manaonly)
+SpellCheckResult CheckSpell(int id, spell_id sn, spell_type st, bool manaonly)
 {
 #ifdef _DEBUG
 	if (debug_mode_key_inverted_v)
-		return true;
+		return SpellCheckResult::Success;
 #endif
 
 	if (!manaonly && pcurs != CURSOR_HAND) {
-		return false;
+		return SpellCheckResult::Fail_Busy;
 	}
 
 	if (st == RSPLTYPE_SKILL) {
-		return true;
+		return SpellCheckResult::Success;
 	}
 
 	if (GetSpellLevel(id, sn) <= 0) {
-		return false;
+		return SpellCheckResult::Fail_Level0;
 	}
 
 	auto &player = Players[id];
+	if (player._pMana < GetManaAmount(player, sn)) {
+		return SpellCheckResult::Fail_NoMana;
+	}
 
-	return player._pMana >= GetManaAmount(player, sn);
+	return SpellCheckResult::Success;
 }
 
 void CastSpell(int id, int spl, int sx, int sy, int dx, int dy, int spllvl)

Source/spells.h

@@ -9,9 +9,16 @@
 
 namespace devilution {
 
+enum class SpellCheckResult : uint8_t {
+	Success,
+	Fail_NoMana,
+	Fail_Level0,
+	Fail_Busy,
+};
+
 int GetManaAmount(PlayerStruct &player, spell_id sn);
 void UseMana(int id, spell_id sn);
-bool CheckSpell(int id, spell_id sn, spell_type st, bool manaonly);
+SpellCheckResult CheckSpell(int id, spell_id sn, spell_type st, bool manaonly);
 void EnsureValidReadiedSpell(PlayerStruct &player);
 void CastSpell(int id, int spl, int sx, int sy, int dx, int dy, int spllvl);
 void DoResurrect(int pnum, int rid);