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FATAL: NGramModel: bad ngram model topology

BenoitFavre - 10 Sep 2012 - 09:18

I generated an ngram model from a .arpa file with the following command:

ngramread --ARPA lm.arpa > lm.model

ngramread does not complain, but ngraminfo and trying to load the model from C++ code generate the following error:

FATAL: NGramModel: bad ngram model topology

How can I troubleshoot the problem?

BenoitFavre - 10 Sep 2012 - 09:27

Adding verbosity results in more mystery...

ngraminfo --v=2 lm.model INFO: FstImpl::ReadHeader: source: lm.model, fst_type: vector, arc_type: standard, version: 2, flags: 3 INFO: Incomplete # of ascending n-grams: 1377525 FATAL: NGramModel: bad ngram model topology

BrianRoark - 11 Sep 2012 - 10:33

Hi,

that error is coming from a sanity check that verifies that every state in the language model (other than the start and unigram states) is reached by exactly one 'ascending' arc, that goes from a lower order to a higher order state. ARPA format models can diverge from this, by, for example, having 'holes' (e.g., bigrams pruned but trigrams with that bigram as a suffix retained). But ngramread should plug all of those. maybe duplication? I'll email you about this.

BrianRoark - 18 Sep 2012 - 10:50

Benoit found a case where certain 'holes' from a pruned ARPA model were not being filled appropriately in the conversion. The sanity check routines on loading the model ensured that this anomaly was caught (causing the errors he mentioned), and we were able to find the cases where this was occurring and update the code. The updated conversion functions will be in the forthcoming version update release of the library, within the next month or two. In the meantime, if anyone encounters this problem, let me know and I can provide a workaround.
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-- CyrilAllauzen - 09 Aug 2012

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Topic revision: r4 - 2012-09-18 - BrianRoark
 
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