Use log probabilities in beam search.#23
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| child_beam.pr_text = parent_beam.pr_text * bigram_prob # probability of char sequence | ||
| bigram_prob = lm_factor * np.log(lm.get_char_bigram(c1, c2)) | ||
| if parent_beam.is_empty(): | ||
| child_beam.pr_text = bigram_prob # first char in beam |
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no special case needed (no if else), as an empty beam is initialized with pr_text=log(1)=0, so adding parent_beam.pr_text + bigram_prob is OK
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Thanks for picking this up.
| last.entries[labeling] = BeamEntry() | ||
| last.entries[labeling].pr_blank = 1 | ||
| last.entries[labeling].pr_total = 1 | ||
| last.entries[labeling].pr_blank = LOG_ZERO |
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should be log(1)==0 instead of LOG_ZERO which is -inf
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That's true, that allows the below if statements to be cleaned up also, thanks for picking this up.
| # in case of non-empty beam | ||
| if labeling: | ||
| # probability of paths with repeated last char at the end | ||
| pr_non_blank = last.entries[labeling].pr_non_blank * mat[t, labeling[-1]] |
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"cannot add to -inf" -> I think this is due to initialization to log(0) instead of log(1).
-inf + x == -inf, which makes sense, because 0*x=0.
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| # probability of paths ending with a blank | ||
| pr_blank = last.entries[labeling].pr_total * mat[t, blank_idx] | ||
| if last.entries[labeling].pr_total == LOG_ZERO: |
| # probability of paths ending with a blank | ||
| pr_blank = last.entries[labeling].pr_total * mat[t, blank_idx] | ||
| if last.entries[labeling].pr_total == LOG_ZERO: | ||
| pr_blank = np.log(mat[t, blank_idx]) # cannot add to -inf |
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directly using np.log creates warning outputs for np.log(0).
Define some wrapper function which takes care of that:
def log(x):
with np.errstate(divide='ignore'):
return np.log(x)
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thanks for your contribution. Using log-probs is a good idea 👍. |
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I anyway will push some changes to the repo, so I'll integrate your changes and will do the small modifications myself. |
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Thanks @githubharald for accepting the pull request. I've taken a look at the changes you've implemented since and they look good, thanks for your improvements and for maintaining the repo. |
2 participants
Implemented log-space operations in beam search to avoid underflow when dealing with regular probabilities.