Add inventory count conditions and item removal actions

src/ItemCountParser.cs

@@ -0,0 +1,36 @@
+namespace Cthangover.Mods.Items
+{
+    using System;
+    using Cthangover.Core.Scenes;
+    using Cthangover.Core.Scenes.Parsing;
+
+    /// <summary>Adds item('id') OP count to the shared condition language, usable by quests, choices and map pages alike.</summary>
+    public class ItemCountParser : ILeafParser
+    {
+        /// <inheritdoc />
+        public bool CanParse(TokenPeeker peeker) => !peeker.AtEnd
+            && peeker.Peek.Type == ConditionParser.TokenType.Ident && peeker.Peek.Value == "item"
+            && peeker.PeekAhead(1).Type == ConditionParser.TokenType.LParen;
+
+        /// <inheritdoc />
+        public ICondition Parse(TokenPeeker peeker, Func<ICondition> parseSubExpr)
+        {
+            peeker.Advance();
+            peeker.Expect(ConditionParser.TokenType.LParen, "expected '(' after item");
+            if (!peeker.Check(ConditionParser.TokenType.String) && !peeker.Check(ConditionParser.TokenType.Ident))
+                throw new FormatException("item() expects an item id");
+            var id = peeker.Advance().Value;
+            peeker.Expect(ConditionParser.TokenType.RParen, "expected ')' after item id");
+            var op = peeker.Peek.Type switch
+            {
+                ConditionParser.TokenType.Eq => "==", ConditionParser.TokenType.Neq => "!=",
+                ConditionParser.TokenType.Ge => ">=", ConditionParser.TokenType.Le => "<=",
+                ConditionParser.TokenType.Gt => ">", ConditionParser.TokenType.Lt => "<",
+                _ => throw new FormatException("item() expects a comparison")
+            };
+            peeker.Advance();
+            var count = peeker.Expect(ConditionParser.TokenType.Number, "item() expects a whole-number count");
+            return new ItemCountCondition(id, op, int.Parse(count.Value));
+        }
+    }
+}