/// <summary>
/// Update a value in the symbol table.
/// If the symbol is not defined, return false.
/// Cache the symbol location if not already done.
/// </summary>
/// <param name="symbol">The symbol to update.</param>
/// <param name="val">The new value.</param>
/// <param name="level">The number of levels to travel to find the definition</param>
/// <returns>The new value, null if not found.</returns>
internal SchemeObject Update(Symbol symbol, SchemeObject val, int level)
{
var entry = new NameValuePair(symbol.SymbolName, val);
lock (this.lockObj)
{
if (symbol.Located)
{
if (level != symbol.Level)
{
return null;
}
this.entries[symbol.Pos] = entry;
return val;
}
var index = this.FindSymbol(symbol);
if (!SymbolFound(index))
{
return null;
}
this.entries[index] = entry;
symbol.SetLocation(index, level);
return val;
}
}
/// <summary>
/// Add a symbol and its value to the environment.
/// If the symbol is not defined in the environment, add it.
/// If the symbol is already defined in the envhronment, update its value.
/// The caller is responsible for locking.
/// </summary>
/// <param name="symbol">The symbol name.</param>
/// <param name="val">The value.</param>
private void AddUnlocked(Symbol symbol, SchemeObject val)
{
var index = this.FindSymbol(symbol);
if (!SymbolFound(index))
{
this.entries.Add(new NameValuePair(symbol.SymbolName, val));
index = this.entries.Count - 1;
}
else
{
this.entries[index] = new NameValuePair(symbol.SymbolName, val);
}
symbol.SetLocation(index, 0);
}
/// <summary>
/// Look up a symbol given its name.
/// If the symbol is not found at this level, then return null and we will continue to search
/// through the parent environment chain.
/// </summary>
/// <param name="symbol">The symbol to look up.</param>
/// <param name="level">The number of levels to travel to find the definition. Cached in the symbol the
/// first time it is looked up.</param>
/// <returns>The value of the object looked up, null if not found.</returns>
internal SchemeObject Lookup(Symbol symbol, int level)
{
lock (this.lockObj)
{
if (symbol.Located)
{
Debug.Assert(level == symbol.Level, "SymbolTable:Lookup level -- symbol.Level");
return this.entries[symbol.Pos].Value;
}
var index = this.FindSymbol(symbol);
if (!SymbolFound(index))
{
return null;
}
symbol.SetLocation(index, level);
return this.entries[index].Value;
}
}