def compile_wfst_from_left_branching_grammar(grammar):
"""
:type grammar: LCFRS
:rtype: Fst, Enumerator
Create a FST from a left-branching hybrid grammar.
The Output of the is a rule tree in `reverse polish notation <https://en.wikipedia.org/wiki/Reverse_Polish_notation>`_
"""
myfst = Fst()
nonterminals = SymbolTable()
sid = myfst.add_state()
nonterminals.add_symbol(INITIAL)
myfst.set_start(sid)
for nont in grammar.nonts():
sid = myfst.add_state()
nonterminals.add_symbol(nont, sid)
if nont == grammar.start():
myfst.set_final(sid)
rules = Enumerator(first_index=1)
for rule in grammar.rules():
rules.object_index(rule)
terminals = SymbolTable()
terminals.add_symbol('<epsilon>', 0)
for rule in grammar.rules():
if rule.rank() == 2:
assert len(rule.lhs().arg(0)) == 2
for rule2 in grammar.lhs_nont_to_rules(rule.rhs_nont(1)):
if len(rule2.rhs()) == 0:
arc = Arc(terminals.add_symbol(rule2.lhs().args()[0][0]),
terminals.add_symbol(
str(rules.object_index(rule2)) + '-' +
str(rules.object_index(rule))),
make_weight(rule.weight() * rule2.weight()),
nonterminals.find(rule.lhs().nont()))
myfst.add_arc(nonterminals.find(rule.rhs_nont(0)), arc)
elif rule.rank() == 0:
assert len(rule.lhs().arg(0)) == 1
arc = Arc(terminals.add_symbol(rule.lhs().args()[0][0]),
terminals.add_symbol(str(rules.object_index(rule))), make_weight(rule.weight()),
nonterminals.find(rule.lhs().nont()))
myfst.add_arc(nonterminals.find(INITIAL), arc)
else:
assert rule.rank() == 1
assert rule.lhs().nont() == grammar.start()
assert len(rule.lhs().arg(0)) == 1
arc = Arc(0, terminals.add_symbol(str(rules.object_index(rule))), make_weight(rule.weight()),
nonterminals.find(grammar.start())
)
myfst.add_arc(nonterminals.find(rule.rhs_nont(0)), arc)
myfst.set_input_symbols(terminals)
myfst.set_output_symbols(terminals)
myfst.optimize(True)
return myfst, rules
def fsa_from_list_of_symbols(input, symbol_table):
"""
:param input:
:type input:
:param symbol_table:
:type symbol_table: SymbolTable
:return: An acceptor for the given list of tokens.
:rtype: Fst
The symbol table gets extended, if new tokens occur in the input.
"""
fsa = Fst()
fsa.set_input_symbols(symbol_table)
fsa.set_output_symbols(symbol_table)
state = fsa.add_state()
fsa.set_start(state)
for x in input:
next_state = fsa.add_state()
try:
arc = Arc(symbol_table.find(x), symbol_table.find(x), 0, next_state)
except KeyError:
arc = Arc(symbol_table.add_symbol(x), symbol_table.add_symbol(x), 0, next_state)
fsa.add_arc(state, arc)
state = next_state
fsa.set_final(state)
return fsa