def chi2_spamprob(self, wordstream, evidence=False):
"""Return best-guess probability that wordstream is spam.
wordstream is an iterable object producing words.
The return value is a float in [0.0, 1.0].
If optional arg evidence is True, the return value is a pair
probability, evidence
where evidence is a list of (word, probability) pairs.
"""
from math import frexp, log as ln
H = S = 1.0
Hexp = Sexp = 0
clues = self._getclues(wordstream)
for prob, word, record in clues:
S *= 1.0 - prob
H *= prob
if S < 1e-200: # prevent underflow
S, e = frexp(S)
Sexp += e
if H < 1e-200: # prevent underflow
H, e = frexp(H)
Hexp += e
S = ln(S) + Sexp * LN2
H = ln(H) + Hexp * LN2
n = len(clues)
if n:
S = 1.0 - chi2Q(-2.0 * S, 2*n)
H = 1.0 - chi2Q(-2.0 * H, 2*n)
prob = (S-H + 1.0) / 2.0
else:
prob = 0.5
if evidence:
clues = [(w, p) for p, w, r in clues]
clues.sort(lambda a, b: cmp(a[1], b[1]))
clues.insert(0, ('*S*', S))
clues.insert(0, ('*H*', H))
return prob, clues
else:
return prob
def chi2_spamprob(self, wordstream, evidence=False):
"""Return best-guess probability that wordstream is spam.
wordstream is an iterable object producing words.
The return value is a float in [0.0, 1.0].
If optional arg evidence is True, the return value is a pair
probability, evidence
where evidence is a list of (word, probability) pairs.
"""
from math import frexp, log as ln
# We compute two chi-squared statistics, one for ham and one for
# spam. The sum-of-the-logs business is more sensitive to probs
# near 0 than to probs near 1, so the spam measure uses 1-p (so
# that high-spamprob words have greatest effect), and the ham
# measure uses p directly (so that lo-spamprob words have greatest
# effect).
#
# For optimization, sum-of-logs == log-of-product, and f.p.
# multiplication is a lot cheaper than calling ln(). It's easy
# to underflow to 0.0, though, so we simulate unbounded dynamic
# range via frexp. The real product H = this H * 2**Hexp, and
# likewise the real product S = this S * 2**Sexp.
H = S = 1.0
Hexp = Sexp = 0
clues = self._getclues(wordstream)
for prob, word, record in clues:
S *= 1.0 - prob
H *= prob
if S < 1e-200: # prevent underflow
S, e = frexp(S)
Sexp += e
if H < 1e-200: # prevent underflow
H, e = frexp(H)
Hexp += e
# Compute the natural log of the product = sum of the logs:
# ln(x * 2**i) = ln(x) + i * ln(2).
S = ln(S) + Sexp * LN2
H = ln(H) + Hexp * LN2
n = len(clues)
if n:
S = 1.0 - chi2Q(-2.0 * S, 2*n)
H = 1.0 - chi2Q(-2.0 * H, 2*n)
# How to combine these into a single spam score? We originally
# used (S-H)/(S+H) scaled into [0., 1.], which equals S/(S+H). A
# systematic problem is that we could end up being near-certain
# a thing was (for example) spam, even if S was small, provided
# that H was much smaller.
# Rob Hooft stared at these problems and invented the measure
# we use now, the simpler S-H, scaled into [0., 1.].
prob = (S-H + 1.0) / 2.0
else:
prob = 0.5
if evidence:
clues = [(w, p) for p, w, _r in clues]
clues.sort(lambda a, b: cmp(a[1], b[1]))
clues.insert(0, ('*S*', S))
clues.insert(0, ('*H*', H))
return prob, clues
else:
return prob
def chi2_spamprob(self, wordstream, evidence=False):
"""Return best-guess probability that wordstream is spam.
wordstream is an iterable object producing words.
The return value is a float in [0.0, 1.0].
If optional arg evidence is True, the return value is a pair
probability, evidence
where evidence is a list of (word, probability) pairs.
"""
from math import frexp, log as ln
# We compute two chi-squared statistics, one for ham and one for
# spam. The sum-of-the-logs business is more sensitive to probs
# near 0 than to probs near 1, so the spam measure uses 1-p (so
# that high-spamprob words have greatest effect), and the ham
# measure uses p directly (so that lo-spamprob words have greatest
# effect).
#
# For optimization, sum-of-logs == log-of-product, and f.p.
# multiplication is a lot cheaper than calling ln(). It's easy
# to underflow to 0.0, though, so we simulate unbounded dynamic
# range via frexp. The real product H = this H * 2**Hexp, and
# likewise the real product S = this S * 2**Sexp.
H = S = 1.0
Hexp = Sexp = 0
clues = self._getclues(wordstream)
"""
wordstream.allclues = list(set(wordstream.allclues + clues))
"""
for prob, word, record in clues:
S *= 1.0 - prob
H *= prob
if S < 1e-200: # prevent underflow
S, e = frexp(S)
Sexp += e
if H < 1e-200: # prevent underflow
H, e = frexp(H)
Hexp += e
# Compute the natural log of the product = sum of the logs:
# ln(x * 2**i) = ln(x) + i * ln(2).
S = ln(S) + Sexp * LN2
H = ln(H) + Hexp * LN2
n = len(clues)
if n:
S = 1.0 - chi2Q(-2.0 * S, 2 * n)
H = 1.0 - chi2Q(-2.0 * H, 2 * n)
# How to combine these into a single spam score? We originally
# used (S-H)/(S+H) scaled into [0., 1.], which equals S/(S+H). A
# systematic problem is that we could end up being near-certain
# a thing was (for example) spam, even if S was small, provided
# that H was much smaller.
# Rob Hooft stared at these problems and invented the measure
# we use now, the simpler S-H, scaled into [0., 1.].
prob = (S - H + 1.0) / 2.0
else:
prob = 0.5
if evidence:
clues = [(w, p) for p, w, _r in clues]
clues.sort(lambda a, b: cmp(a[1], b[1]))
clues.insert(0, ('*S*', S))
clues.insert(0, ('*H*', H))
wordstream.prob = prob
return prob, clues
else:
wordstream.prob = prob
return prob
