KumiScript/interpreter/Environment.cs

using System.Diagnostics.CodeAnalysis;

namespace KumiScript.Interpreter
{
    public class Environment
    {
        readonly Environment? _outer;
        readonly Dictionary<Symbol, Expression> _bindings;
        public Environment()
        {
            _bindings = new Dictionary<Symbol, Expression>(SymbolComparer.GetInstance());
        }
        public Environment(Environment outer)
        {
            _bindings = new Dictionary<Symbol, Expression>(SymbolComparer.GetInstance());
            _outer = outer;
        }

        public Expression Lookup(Symbol symbol)
        {
            Expression? result;
            _bindings.TryGetValue(symbol, out result);

            if (result is not null)
                return result;
            
            if (_outer is null)
                throw new InterpreterUnboundSymbolException();
            
            return _outer.Lookup(symbol);
        }
        
        public void AddSymbol(Symbol symbol, Expression value)
        {
            _bindings.Add(symbol, value);
            return;
        }

        public void RedefineSymbol(Symbol symbol, Expression value)
        {
            if (_bindings.ContainsKey(symbol))
            {
                _bindings.Remove(symbol);
                _bindings.Add(symbol, value);
                return;
            }
            
            if (_outer is null)
                throw new InterpreterUnboundSymbolException(symbol.ToString());
            
            _outer.RedefineSymbol(symbol, value);
        }
    }

    internal class SymbolComparer : IEqualityComparer<Symbol>
    {
        private static SymbolComparer? _instance;
        private SymbolComparer()
        {
        }

        public static SymbolComparer GetInstance()
        {
            if (_instance is null)
                _instance = new SymbolComparer();
            
            return _instance;
        }

        public bool Equals(Symbol? x, Symbol? y)
        {
            if (x is not Symbol || y is not Symbol)
                return false;
            
            return x.Equals(y);
        }

        public int GetHashCode([DisallowNull] Symbol obj)
        {
            return obj.ToString().GetHashCode();
        }
    }
}