Forest Rescoring Faster Decoding with Integrated Language Models Liang Huang David Chiang ACL 2007, Praha, Č eská republika
Statistical Machine Translation Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis translation model (TM) language model (LM) Broken Spanish English competency fluency English What hunger have I Hungry I am so Have I that hunger Que hambre tengo yo I am so hungry I am so hungry How hunger have I ... Huang and Chiang (Knight and Koehn, 2003) Forest Rescoring 2
Statistical Machine Translation Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis translation model (TM) language model (LM) Broken Spanish English competency fluency English n -best rescoring What hunger have I Hungry I am so Have I that hunger Que hambre tengo yo I am so hungry I am so hungry How hunger have I ... Huang and Chiang (Knight and Koehn, 2003) Forest Rescoring 2
Statistical Machine Translation Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis translation model (TM) language model (LM) Broken Spanish English competency fluency English Huang and Chiang Forest Rescoring 3
Statistical Machine Translation Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis translation model (TM) language model (LM) Broken Spanish English competency fluency English decoder integrated decoder I am so hungry Que hambre tengo yo (LM-integrated) computationally challenging! ☹ Huang and Chiang Forest Rescoring 3
Statistical Machine Translation Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis phrase-based TM translation model (TM) language model (LM) Broken n -gram LM Spanish English competency fluency English syntax-based decoder integrated decoder I am so hungry Que hambre tengo yo (LM-integrated) computationally challenging! ☹ Huang and Chiang Forest Rescoring 3
Forest Rescoring Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis phrase-based TM translation model (TM) language model (LM) Broken n -gram LM Spanish English competency fluency English syntax-based packed forest decoder integrated decoder I am so hungry Que hambre tengo yo (LM-integrated) computationally challenging! ☹ Huang and Chiang Forest Rescoring 4
Forest Rescoring Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis phrase-based TM translation model (TM) language model (LM) Broken n -gram LM Spanish English competency fluency English syntax-based packed forest on-the-fly rescoring decoder integrated decoder I am so hungry Que hambre tengo yo (LM-integrated) computationally challenging! ☹ Huang and Chiang Forest Rescoring 4
Forest Rescoring Spanish/English English Bilingual Text Text Statistical Analysis Statistical Analysis phrase-based TM translation model (TM) language model (LM) Broken n -gram LM Spanish English competency fluency English syntax-based packed forest on-the-fly rescoring decoder integrated decoder forest rescorer I am so hungry Que hambre tengo yo (LM-integrated) significant speed-up: 10~30 times faster! ☺ Huang and Chiang Forest Rescoring 4
The Forest Framework unifying phrase- and syntax-based decoding
Phrase-based Decoding source-side: coverage vector 与 沙龙 举行 了 会谈 _ _ ● ● ● yu Shalong juxing le huitan held a talk target-side: grow hypotheses held a talk with Sharon strictly left-to-right ... ... _ _ ● ● ● ● ● ● _ _ _ _ _ ● ● held a talk held a talk with Sharon ... ... Huang and Chiang Forest Rescoring 6
Syntax-based Translation • synchronous context-free grammars (SCFGs) • context-free grammar in two dimensions • generating pairs of strings/trees simultaneously • co-indexed nonterminal further rewritten as a unit PP (1) VP (2) , VP (2) PP (1) VP → VP juxing le huitan , held a meeting → PP yu Shalong , with Sharon → VP VP PP VP VP PP yu Shalong juxing le huitan held a meeting with Sharon Huang and Chiang Forest Rescoring 7
Translation as Parsing • translation with SCFGs => monolingual parsing • parse the source input with the source projection • build the corresponding target sub-strings in parallel PP (1) VP (2) , VP (2) PP (1) VP → VP juxing le huitan , held a meeting → PP yu Shalong , with Sharon → VP 1, 6 VP 3, 6 PP 1, 3 yu Shalong juxing le huitan Huang and Chiang Forest Rescoring 8
Translation as Parsing • translation with SCFGs => monolingual parsing • parse the source input with the source projection • build the corresponding target sub-strings in parallel PP (1) VP (2) , VP (2) PP (1) VP → VP juxing le huitan , held a meeting → PP yu Shalong , with Sharon → VP 1, 6 VP 3, 6 PP 1, 3 yu Shalong juxing le huitan Huang and Chiang Forest Rescoring 8
Translation as Parsing • translation with SCFGs => monolingual parsing • parse the source input with the source projection • build the corresponding target sub-strings in parallel PP (1) VP (2) , VP (2) PP (1) VP → VP juxing le huitan , held a meeting → held a talk with Sharon PP yu Shalong , with Sharon → VP 1, 6 with Sharon held a talk VP 3, 6 PP 1, 3 yu Shalong juxing le huitan Huang and Chiang Forest Rescoring 8
Packed Forest • a compact representation of all translations • has a structure of hypergraph (graph is a special case) phrase-based: graph syntax-based: hypergraph VP 1, 6 ●●●●● _ _ ● _ _ _ _ _ ● ● PP 1, 3 VP 3, 6 Huang and Chiang Forest Rescoring 9
Packed Forest • a compact representation of all translations • has a structure of hypergraph (graph is a special case) phrase-based: graph syntax-based: hypergraph nodes VP 1, 6 ●●●●● _ _ ● _ _ _ _ _ ● ● PP 1, 3 VP 3, 6 (hyper-)edges Huang and Chiang Forest Rescoring 9
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● ... talks _ _ ● ● ● VP 1, 6 PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● with Sharon ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● ... talks _ _ ● ● ● VP 1, 6 PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● with Sharon ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● bigram ... talks _ _ ● ● ● VP 1, 6 PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● with Sharon ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● bigram ... talks _ _ ● ● ● VP 1, 6 held ... talk with ... Sharon PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● with Sharon ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● bigram ... talks _ _ ● ● ● VP 1, 6 held ... talk with ... Sharon bigram PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● with Sharon ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● bigram ... talks _ _ ● held ... Sharon ● ● VP 1, 6 held ... talk with ... Sharon bigram PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Adding a Bigram Model ●●●●● ... Shalong ... meeting _ _ ● ● ● with Sharon ●●●●● ... Sharon ... talk _ _ _ _ _ _ _ ● ● ● bigram ... talks _ _ ● held ... Sharon ● ● hold ... Sharon VP 1, 6 held ... Shalong hold ... Shalong held ... talk with ... Sharon bigram PP 1, 3 VP 3, 6 +LM items with ... Sharon held ... talk along ... Sharon held ... meeting with ... Shalong hold ... talks Huang and Chiang Forest Rescoring 10
Conventional Beam Search VP 1, 6 hyperedge PP 1, 3 VP 3, 6 PP 1, 4 VP 4, 6 NP 1, 4 VP 4, 6 1.0 2.3 1.1 4.6 2.5 7.2 • beam search: only keep top- k +LM items at each node • but there are many ways to derive each node • can we avoid enumerating all combinations? • best-first enumeration? Huang and Chiang Forest Rescoring 11
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