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<Paper uid="P91-1012">
  <Title>COMPOSE-REWUCE PARSING</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
0. INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> The work reported here grew out of our attempt to improve on the o (n 2) performance of the SIMD parallel parser described in (Thompson 1991).</Paragraph>
    <Paragraph position="1"> Rather than start with a commitment to a specific SIMD architecture, as that work had, we agreed that the best place to start was with a more abstract architecture-independent consideration of the CF-PSG parsing problem-given arbitrary resources, what algorithms could one envisage which could recognise and/or parse atomic category phrase-structure grammars in o (n) ? In the end, two quite different approaches emerged. One took as its starting point non-deterministic shift-reduce parsing, and sought to achieve linear (indeed real-time) complexity by performing a constant-time step per word of the input. The other took as its starting point tabular parsing (Earley, C KY), and sought to achieve linear complexity by performing a constant-time step for the identification/construction of constituents of each length from 0 to n. The latter route has been widely canvassed, although to our knowledge has not yet been implemented--see (Nijholt 1989, 90) for extensive references. The former route, whereby real-time parsing is achieved by processor forking at non-deterministic choice points in an extended shill-reduce parser, is to our knowledge new. In this paper we present outlines of two such parsers, which we call compose-reduce parsers.</Paragraph>
  </Section>
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