/// <summary>
/// Transforms grammar according to given grammar options.
/// </summary>
/// <param name="grammar">target grammar</param>
/// <param name="options">set of options</param>
/// <returns>application of options to grammar</returns>
public static CFG ApplyOptions(this CFG grammar, OrderedHashSet <GrammarOption> options)
{
if (options.Contains(GrammarOption.Reversed))
{
return(Reversed(grammar));
}
return(grammar);
}
public CFGSubtypingAPIGenerator(CFG grammar, string grammarName, bool generateFluentAPI)
{
if (!grammar.InGreibachNormalForm())
{
throw new Exception("can generate API only for grammars in Greibach normal form");
}
this.grammar = grammar;
this.grammarName = grammarName;
this.generateFluentAPI = generateFluentAPI;
}
private void Execute(GeneratorExecutionContext context, AdditionalText file)
{
CFG grammar = CFGParser.Parse(file.GetText().ToString());
grammar = grammar.ApplyOptions(GetOptions(context, file)).ToGreibachNormalForm();
CFGSubtypingAPIGenerator generator = new CFGSubtypingAPIGenerator(grammar, Path.GetFileNameWithoutExtension(file.Path),
GetBoolConfiguration("FluentAPI", context, file));
context.AddSource(generator.NamespaceName(), SourceText.From(generator.PrintAPI(), Encoding.UTF8));
}
private static bool HasEpsilonProductionsExceptStartVariable(this CFG grammar)
{
foreach (Production production in grammar.productions)
{
if (production.lhs != grammar.startVariable && production.IsEpsilonProduction())
{
return(true);
}
}
return(false);
}
/// <summary>
/// Reverses context-free grammar, by reversing each of its productions.
/// </summary>
/// <param name="grammar">target grammar</param>
/// <returns>reversed grammar</returns>
public static CFG Reversed(this CFG grammar)
{
OrderedHashSet <Production> productions = new OrderedHashSet <Production>();
foreach (Production production in grammar.productions)
{
string[] reversed = new string[production.rhs.Length];
Array.Copy(production.rhs, reversed, reversed.Length);
Array.Reverse(reversed);
productions.Add(new Production(production.lhs, reversed));
}
return(new CFG(grammar.startVariable, productions));
}
/// <summary>
/// Transforms grammar into Greibach normal form, i.e.,
/// where productions are of the form `v ::= t s1 s2...`, `v` a variable,
/// `t` a terminal.
/// The grammar's start variable may be nullable, `v0 ::= epsilon`.
/// </summary>
/// <param name="grammar">target grammar</param>
/// <returns>grammar in Greibach normal form</returns>
public static CFG ToGreibachNormalForm(this CFG grammar)
{
if (grammar.InGreibachNormalForm())
{
return(grammar);
}
while (true)
{
grammar = grammar.RemoveEpsilonProductions().RemoveLeftRecursion();
OrderedHashSet <Production> productions = new OrderedHashSet <Production>();
foreach (Production production in grammar.productions)
{
if (production.IsEpsilonProduction() || grammar.Terminals().Contains(production.rhs[0]))
{
productions.Add(production);
}
else
{
string rhsStartVariable = production.rhs[0];
foreach (Production otherProduction in grammar.productions)
{
if (otherProduction.lhs == rhsStartVariable)
{
List <string> newRHS = new List <string>();
newRHS.AddRange(otherProduction.rhs);
newRHS.AddRange(production.rhs.Skip(1));
productions.Add(new Production(production.lhs, newRHS.ToArray()));
}
}
}
}
grammar = verifyConsistency(grammar, new CFG(grammar.startVariable, productions));
if (grammar.InGreibachNormalForm())
{
return(grammar);
}
}
}
/// <summary>
/// Checks whether grammar is in Greibach normal form, i.e.,
/// where productions are of the form `v ::= t s1 s2...`, `v` a variable,
/// `t` a terminal.
/// The grammar's start variable may be nullable, `v0 ::= epsilon`.
/// </summary>
/// <param name="grammar">context-free grammar</param>
/// <returns>whether grammar is in Greibach normal form</returns>
public static bool InGreibachNormalForm(this CFG grammar)
{
bool hasNilStartVariable = false;
foreach (Production production in grammar.productions)
{
if (production.IsEpsilonProduction())
{
if (production.lhs == grammar.startVariable)
{
hasNilStartVariable = true;
}
else
{
return(false);
}
}
else if (!grammar.Terminals().Contains(production.rhs[0]))
{
return(false);
}
}
if (hasNilStartVariable)
{
foreach (Production production in grammar.productions)
{
foreach (string symbol in production.rhs)
{
if (grammar.startVariable == symbol)
{
return(false);
}
}
}
}
return(true);
}
/// <summary>
/// Checks whether the grammar has a left-recursive production, i.e.,
/// of the form `v ::= v s1 s2...` (can also be indirect).
/// </summary>
/// <param name="grammar">context-free grammar</param>
/// <returns>whether grammar has left-recursion</returns>
public static bool HasLeftRecursion(this CFG grammar)
{
foreach (string variable in grammar.Variables())
{
OrderedHashSet <string> reachableVariables = new OrderedHashSet <string>();
reachableVariables.Add(variable);
int n = 0;
while (reachableVariables.Count() > n)
{
n = reachableVariables.Count();
OrderedHashSet <string> newReachableVariables = new OrderedHashSet <string>();
foreach (string reachableVariable in reachableVariables)
{
foreach (Production production in grammar.productions)
{
if (production.lhs == reachableVariable && production.rhs.Length > 0)
{
string first = production.rhs[0];
if (first == variable)
{
return(true);
}
else if (grammar.Variables().Contains(first))
{
newReachableVariables.Add(first);
}
}
}
}
foreach (string newVariable in newReachableVariables)
{
reachableVariables.Add(newVariable);
}
}
}
return(false);
}
/// <summary>
/// Remove epsilon productions.
/// If the grammar derives epsilon, then only the start variable may derive epsilon, but then it
/// would not be found on the right-hand side of any production.
/// </summary>
/// <param name="grammar">target grammar</param>
/// <returns>grammar without epsilon productions</returns>
public static CFG RemoveEpsilonProductions(this CFG grammar)
{
bool hasNullableStartSymbolInRHS = grammar.HasNullableStartSymbolInRHS();
if (!grammar.HasEpsilonProductionsExceptStartVariable() && !hasNullableStartSymbolInRHS)
{
return(grammar);
}
if (hasNullableStartSymbolInRHS)
{
OrderedHashSet <Production> newProductions = new OrderedHashSet <Production>();
foreach (Production production in grammar.productions)
{
newProductions.Add(production);
}
string newStartVariable = GenerateAuxiliaryVariable(grammar.Symbols(), grammar.startVariable);
newProductions.Add(new Production(newStartVariable, grammar.startVariable));
newProductions.Add(new Production(newStartVariable));
return(RemoveEpsilonProductions(new CFG(newStartVariable, newProductions)));
}
OrderedHashSet <Production> productions = grammar.productions;
while (true)
{
OrderedHashSet <string> nullableVariables = new OrderedHashSet <string>(), nullVariables = new OrderedHashSet <string>();
foreach (Production production in productions)
{
if (production.IsEpsilonProduction())
{
nullableVariables.Add(production.lhs);
nullVariables.Add(production.lhs);
}
}
foreach (Production production in productions)
{
if (!production.IsEpsilonProduction())
{
nullVariables.Remove(production.lhs);
}
}
if (nullableVariables.Count == 0 || nullableVariables.Count == 1 && nullableVariables.Contains(grammar.startVariable))
{
return(verifyConsistency(grammar, new CFG(grammar.startVariable, productions).Simplify()));
}
OrderedHashSet <Production> newProductions = new OrderedHashSet <Production>();
foreach (Production production in productions)
{
OrderedHashSet <List <string> > rhss = new OrderedHashSet <List <string> >();
rhss.Add(new List <string>());
foreach (string symbol in production.rhs)
{
OrderedHashSet <List <string> > newRHSs = new OrderedHashSet <List <string> >();
foreach (var rhs in rhss)
{
if (nullVariables.Contains(symbol))
{
newRHSs.AddUnique(rhs);
}
else if (!nullableVariables.Contains(symbol))
{
List <string> newRHS = new List <string>();
newRHS.AddRange(rhs);
newRHS.Add(symbol);
newRHSs.Add(newRHS);
}
else
{
newRHSs.AddUnique(rhs);
List <string> newRHS = new List <string>();
newRHS.AddRange(rhs);
newRHS.Add(symbol);
newRHSs.Add(newRHS);
}
}
rhss = newRHSs;
}
foreach (var newRHS in rhss)
{
newProductions.Add(new Production(production.lhs, newRHS.ToArray()));
}
}
foreach (string nullableVariable in nullableVariables)
{
if (nullableVariable != grammar.startVariable)
{
newProductions.Remove(new Production(nullableVariable));
}
}
productions = newProductions;
}
}
/// <summary>
/// Removes left-recursive productions from the grammar.
/// Implementation of Paull's algorithm.
/// </summary>
/// <param name="grammar">target grammar</param>
/// <returns>grammar without left recursion</returns>
public static CFG RemoveLeftRecursion(this CFG grammar)
{
if (!grammar.HasLeftRecursion())
{
return(grammar);
}
grammar = grammar.Simplify().RemoveEpsilonProductions();
if (!grammar.HasLeftRecursion())
{
return(grammar);
}
bool epsilonIsDerived = grammar.productions.Contains(new Production(grammar.startVariable));
OrderedHashSet <Production> productions = new OrderedHashSet <Production>();
foreach (Production production in grammar.productions)
{
productions.Add(production);
}
if (epsilonIsDerived)
{
productions.Remove(new Production(grammar.startVariable));
}
List <string> variables = new List <string>(grammar.Variables());
OrderedHashSet <string> usedSymbols = new OrderedHashSet <string>();
foreach (string symbol in grammar.Symbols())
{
usedSymbols.Add(symbol);
}
for (int i = 0; i < variables.Count; ++i)
{
string vi = variables[i];
for (int j = 0; j < i; ++j)
{
string vj = variables[j];
OrderedHashSet <Production> toRemove = new OrderedHashSet <Production>();
OrderedHashSet <Production> toAdd = new OrderedHashSet <Production>();
foreach (Production production in productions)
{
if (production.lhs == vi && production.rhs.Length > 0 && production.rhs[0] == vj)
{
toRemove.Add(production);
foreach (Production otherProduction in productions)
{
if (otherProduction.lhs == vj)
{
List <string> newRHS = new List <string>();
newRHS.AddRange(otherProduction.rhs);
newRHS.AddRange(production.rhs.Skip(1));
toAdd.Add(new Production(vi, newRHS.ToArray()));
}
}
break;
}
}
foreach (Production addedProduction in toAdd)
{
productions.Add(addedProduction);
}
foreach (Production removedProduction in toRemove)
{
productions.Remove(removedProduction);
}
}
OrderedHashSet <Production> productionsWithoutViDirectLeftRecursion = new OrderedHashSet <Production>();
OrderedHashSet <Production> newViProductions = new OrderedHashSet <Production>();
foreach (Production production in productions)
{
if (production.lhs != vi)
{
productionsWithoutViDirectLeftRecursion.Add(production);
}
else
{
newViProductions.Add(production);
}
}
foreach (Production newViProduction in RemoveDirectLeftRecursion(usedSymbols, newViProductions))
{
productionsWithoutViDirectLeftRecursion.Add(newViProduction);
}
productions = productionsWithoutViDirectLeftRecursion;
}
if (epsilonIsDerived)
{
productions.Add(new Production(grammar.startVariable));
}
return(verifyConsistency(grammar, new CFG(grammar.startVariable, productions)));
}
public static void Main(String[] args)
{
var options = new SomeOptions();
Parser.Default.ParseArguments <SomeOptions>(args).WithParsed(parsed => options = parsed);
if (options.Input == null || options.Name == null)
{
return;
}
CFG grammar = null;
try
{
grammar = CFGParser.Parse(File.ReadAllText(options.Input));
} catch (Exception e)
{
Console.WriteLine($"Could not parse file {options.Input}:\n{e.Message}");
return;
}
try
{
if (options.Fluent)
{
grammar = grammar.Reversed();
}
} catch (Exception e)
{
Console.WriteLine($"Error when reversing grammar:\n{e.Message}");
return;
}
try
{
grammar = grammar.ToGreibachNormalForm();
} catch (Exception e)
{
Console.WriteLine($"Error when converting grammar to Greibach normal form:\n{e.Message}");
return;
}
string result = null;
try
{
result = new CFGSubtypingAPIGenerator(grammar, options.Name, options.Fluent).PrintAPI();
}
catch (Exception e)
{
Console.WriteLine($"Error when generating API:\n{e.Message}");
return;
}
try
{
if (options.Output == null)
{
Console.WriteLine(result);
}
else
{
File.WriteAllText(options.Output, result);
}
}
catch (Exception e)
{
Console.WriteLine($"Error when writing output:\n{e.Message}");
return;
}
}