def test_first_empty_in_rhs():
"""
Test FIRST calculation when there are empty derivations in RHS of a
production.
"""
grammar = """
S: A C;
A: B | EMPTY;
terminals
B: "b";
C: "c";
"""
g = Grammar.from_string(grammar)
first_set = first(g)
A = g.get_nonterminal('A')
B = g.get_terminal('B')
C = g.get_terminal('C')
S = g.get_nonterminal('S')
assert EMPTY in first_set[A]
assert B in first_set[A]
assert B in first_set[S]
# 'A' can derive empty, thus 'C' must be in firsts of 'S'.
assert C in first_set[S]
def test_first():
"""
Tests FIRST function.
FIRST function returns a set of terminals that could start the sentence
derived from the given non-terminal.
"""
expression_grammar = get_grammar()
first_set = first(expression_grammar)
assert OPEN in first_set[T]
assert ID in first_set[T]
def test_first_empty_in_rhs():
"""
Test FIRST calculation when there are empty derivations in RHS of a
production.
"""
grammar = """
S: A C;
A: B | EMPTY;
B: "b";
C: "c";
"""
g = Grammar.from_string(grammar)
first_set = first(g)
assert EMPTY in first_set[NonTerminal('A')]
assert Terminal('B') in first_set[NonTerminal('A')]
assert Terminal('B') in first_set[NonTerminal('S')]
# 'A' can derive empty, thus 'C' must be in firsts of 'S'.
assert Terminal('C') in first_set[NonTerminal('S')]
