/// <summary>
/// Creates and returns the LR(0) canonical sets for the given <paramref name="grammar"/>.
/// </summary>
/// <param name="grammar">The grammar to build the canonical sets for.</param>
/// <returns>The built canonical sets.</returns>
/// <exception cref="ArgumentNullException"> if <paramref name="grammar"/> is <c>null</c>.</exception>
public static LR0CanonicalSets Build( RestrictedStartSymbolGrammar grammar )
{
if( grammar == null )
throw new ArgumentNullException("grammar");
var terms_and_gramms = new List<Symbol>();
terms_and_gramms.AddRange(grammar.Grammaticals);
terms_and_gramms.AddRange(grammar.Terminals);
var sets = new LR0CanonicalSets();
// Calculate the I0 canonical set from the 'starting' rule
var s0 = new HashSet<LR0Item>();
if( grammar.Rules.Count > 0 )
s0.Add(new LR0Item(grammar.StartRule, 0));
var I0 = Closure(s0, grammar);
sets.Add(I0);
// Generate LR0CanonicalSets with Read()
for( int current_set_id = 0; current_set_id < sets.Sets.Count; ++current_set_id )
// Generate new sets for every symbol read
foreach( var symbol in terms_and_gramms )
{
var new_I = Read(sets.Sets[current_set_id], symbol, grammar);
// If the set is empty then continue
if( new_I.Count == 0 )
continue;
// Identify new_I
int next_set_id = sets.IndexOf(new_I);
// If this is a new CanonicalSet then add to the group
if( next_set_id == -1 )
next_set_id = sets.Add(new_I);
// Record the state transition
sets.SetTransition(current_set_id, symbol, next_set_id);
}
return sets;
}
/// <summary>
/// Calculates and returns the read function on the given <paramref name="set"/>.
/// </summary>
/// <param name="set"></param>
/// <param name="symbol"></param>
/// <param name="grammar"><paramref name="set"/> and <paramref name="symbol"/> belong to this grammar.</param>
/// <returns></returns>
/// <exception cref="ArgumentNullException"> if any of the parameters is <c>null</c>.</exception>
public static ISet<LR0Item> Read( ISet<LR0Item> set, Symbol symbol, RestrictedStartSymbolGrammar grammar )
{
if( object.ReferenceEquals(set, null) )
throw new ArgumentNullException("set");
if( object.ReferenceEquals(symbol, null) )
throw new ArgumentNullException("symbol");
if( object.ReferenceEquals(grammar, null) )
throw new ArgumentNullException("grammar");
var result = new HashSet<LR0Item>();
foreach( var item in set )
if( item.CursorPosition < item.Rule.RightHandSide.Count &&
item.Rule.RightHandSide[item.CursorPosition].Equals(symbol) )
{
// If the next symbol is the reading symbol, advance the CursorPosition
// then add the closure of it to the result.
var new_set = new HashSet<LR0Item>();
new_set.Add(new LR0Item(item.Rule, item.CursorPosition + 1));
result.UnionWith(Closure(new_set, grammar));
}
return result;
}
/// <summary>
/// Calculates and returns the closure of the given canonical set.
/// </summary>
/// <param name="set">Get the closure of this set.</param>
/// <param name="grammar">The set belongs to this grammar.</param>
/// <returns>The closure of the given <paramref name="set"/>.</returns>
/// <exception cref="ArgumentNullException"> if any of the parameters is <c>null</c>.</exception>
public static ISet<LR0Item> Closure( ISet<LR0Item> set, RestrictedStartSymbolGrammar grammar )
{
if( object.ReferenceEquals(set, null) )
throw new ArgumentNullException("set");
if( object.ReferenceEquals(grammar, null) )
throw new ArgumentNullException("grammar");
var closure = new List<LR0Item>(set);
for( int i = 0; i < closure.Count; ++i )
{
var item = closure[i];
// Skip this, if the cursor pos is on the end
if( item.CursorPosition == item.Rule.RightHandSide.Count )
continue;
// Skip this, if the next_symbol is not grammatical
var next_symbol = item.Rule.RightHandSide[item.CursorPosition];
if( !(next_symbol is GrammaticalSymbol) )
continue;
// Get all the rules whit next_symbol on the left hand side
var rules = from rule in grammar.Rules
where rule.LeftHandSide.Equals(next_symbol)
select rule;
// Add all the generated RL(0) items to the closure
foreach( var rule in rules )
{
var new_item = new LR0Item(rule, 0);
if( !closure.Contains(new_item) )
closure.Add(new_item);
}
}
return new HashSet<LR0Item>(closure);
}