def build_grammar(grammar_txt: str):
"""
Transform a string in this format:
S --> A B
A --> a A | epsilon
B --> b B | epsilon
to a Grammar object
"""
try:
nonTerminals, terminals, P = [], [], []
lines = [
l.strip() for l in grammar_txt.splitlines() if l.strip() != ''
]
head = None
for l in lines:
if '->' not in l:
l = f'{head} -> {l}'
head, bodies = l.split('->')
head = head.split()[0]
nonTerminals.append(head)
for body in bodies.split('|'):
if body.strip() != '':
P.append({'Head': head, 'Body': list(body.split())})
terminals.extend(P[-1]['Body'])
set_terminals, set_nonTerminals = set(terminals).difference(
nonTerminals + ['epsilon']), set(nonTerminals)
N, T = [], []
for nt in nonTerminals:
if nt in set_nonTerminals and set_nonTerminals.discard(
nt) == None:
N.append(nt)
for t in terminals:
if t in set_terminals and set_terminals.discard(t) == None:
T.append(t)
data = json.dumps({
'Terminals': T,
'NonTerminals': N,
'Productions': P
})
G = Grammar.from_json(data)
# print(data)
G.startSymbol = G.nonTerminals[0]
return G
except:
return None
def parseGrammar(self, text: str):
terminals, nonTerminals, productions = [], [], []
try:
lines = text.split('\r\n')
for line in lines:
head, bodies = line.split('->')
head, = head.split()
if len(head[0]) > 1:
raise BadGrammarException()
nonTerminals.append(head)
for body in bodies.split('|'):
productions.append({
'Head': head,
'Body': list(body.split())
})
terminals.extend(productions[-1]['Body'])
except:
raise BadGrammarException()
nonTerminals = set(nonTerminals)
terminals = set(
[t for t in terminals if t not in nonTerminals and t != 'epsilon'])
data = json.dumps({
'NonTerminals': [nt for nt in nonTerminals],
'Terminals': [t for t in terminals],
'Productions': productions
})
return Grammar.from_json(data)