Decoding is process of taking input in French:
…And finding its best English translation under your model:
To decode, we need a model of English sentences conditioned on the French sentence. You did most of the work of creating such a model in Homework 1. In this assignment, we will give you some French sentences and a probabilistic model consisting of a phrase-based translation model and an n-gram language model . Your challenge is to find the most probable English translation under the model. We assume a noisy channel decomposition.
If you have a clone of the repository from homework 1, you can update it from your working directory:
git pull origin master
Alternatively, get a fresh copy:
git clone https://github.com/alopez/en600.468.git
decode directory, you now have simple decoder.
Test it out!
python decode > output
This creates the file
output with translations of
You can compute using
python compute-model-score < output
This command sums over all possible ways that the model could have generated the English from the French, including translations that permute the phrases. This sum is intractable, but the phrase dictionary is fixed and sparse, so we can compute it in a few minutes. It is still easier to do this than it is to find the optimal translation. But if you look at this command you may get some hints about how to do the assignment!
The decoder generates the most probable translations that it can find, using three common approximations.
First, it seeks the Viterbi approximation to the most probable translation. Instead of computing the intractable sum over all alignments for each sentence, we simply find the best single alignment and use its translation.
Second, it translates French phrases into English without
changing their order. So, it only reorders words if
the reordering has been memorized as a phrase pair.
For example, in the first sentence, we see that
is correctly translated as
If we consult
data/tm, we will find that the model
has memorized the phrase
mardi dernier ||| last Tuesday. But in the
second sentence, we see that
is translated as
rather than the more correct
To show that this is a search problem rather than
a modeling problem, we can generate the latter output
by hand and check that the model really prefers it.
head -2 data/input | tail -1 > example python decode -i example | python compute-model-score -i example echo a selection committee was achievement . | python compute-model-score -i example
The scores are reported as log-probabilities, and higher scores (with lower absolute value) are better. We see that the model prefers selection committee, but the decoder does not consider this word order.
Finally, our decoder uses strict pruning. As it consumes the input
sentence from left to right, it keeps only the highest-scoring
output up to that point. You can vary the number of number
of outputs kept at each point in the translation using the
-s parameter. See how this affects the resulting model score.
python decode | python compute-model-score python decode -s 10000 | python compute-model-score
Your task is to find the most probable English translation. Our model assumes that any segmentation of the French sentence into phrases followed by a one-for-one substitution and permutation of those phrases is a valid translation. We make the simplifying assumption that segmentation and ordering probabilities are uniform across all sentences, hence constant. This means that is proportional to the product of the n-gram probabilities in and the phrase translation probabilities in . To avoid numerical underflow we work in logspace, seeking . The baseline decoder works with log probabilities, so you can simply follow what it does.
To pass, you must implement a beam-search decoder like the one we have given you that is also capable of swapping adjacent phrases. To get full credit, you must additionally experiment with another decoding algorithm. Any permutation of phrases is a valid translation, so we strongly suggest searching over all or some part of this larger space. This search is NP-Hard, so it will not be easy. You can trade efficiency for search effectiveness by implementing histogram pruning or threshold pruning, or by using reordering limits as described in the textbook (Chapter 6). Or, you might consider implementing other approaches to solving this combinatorial optimization problem:
These methods all attempt to approximate or solve the Viterbi approximation to decoding. You can also try to approximate directly.
But the sky’s the limit! There are many ways to decode. You can try anything you want as long as you follow the ground rules: