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  3. 11 Dec, 2017 1 commit
    • Justin Ridgewell's avatar
      Implement DFA Unicode Decoder · cedec225
      Justin Ridgewell authored
      This is a separation of the DFA Unicode Decoder from
      https://chromium-review.googlesource.com/c/v8/v8/+/789560
      
      I attempted to make the DFA's table a bit more explicit in this CL. Still, the
      linter prevents me from letting me present the array as a "table" in source
      code. For a better representation, please refer to
      https://docs.google.com/spreadsheets/d/1L9STtkmWs-A7HdK5ZmZ-wPZ_VBjQ3-Jj_xN9c6_hLKA
      
      - - - - -
      
      Now for a big copy-paste from 789560:
      
      Essentially, reworks a standard FSM (imagine an
      array of structs) and flattens it out into a single-dimension array.
      Using Table 3-7 of the Unicode 10.0.0 standard (page 126 of
      http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf), we can nicely
      map all bytes into one of 12 character classes:
      
      00. 0x00-0x7F
      01. 0x80-0x8F (split from general continuation because this range is not
          valid after a 0xF0 leading byte)
      02. 0x90-0x9F (split from general continuation because this range is not
          valid after a 0xE0 nor a 0xF4 leading byte)
      03. 0xA0-0xBF (the rest of the continuation range)
      04. 0xC0-0xC1, 0xF5-0xFF (the joined range of invalid bytes, notice this
          includes 255 which we use as a known bad byte during hex-to-int
              decoding)
      05. 0xC2-0xDF (leading bytes which require any continuation byte
          afterwards)
      06. 0xE0 (leading byte which requires a 0xA0-0xBF afterwards then any
          continuation byte after that)
      07. 0xE1-0xEC, 0xEE-0xEF (leading bytes which requires any continuation
          afterwards then any continuation byte after that)
      08. 0xED (leading byte which requires a 0x80-0x9F afterwards then any
          continuation byte after that)
      09. 0xF1-F3 (leading bytes which requires any continuation byte
          afterwards then any continuation byte then any continuation byte)
      10. 0xF0 (leading bytes which requires a 0x90-0xBF afterwards then any
          continuation byte then any continuation byte)
      11. 0xF4 (leading bytes which requires a 0x80-0x8F afterwards then any
          continuation byte then any continuation byte)
      
      Note that 0xF0 and 0xF1-0xF3 were swapped so that fewer bytes were
      needed to represent the transition state ("9, 10, 10, 10" vs.
      "10, 9, 9, 9").
      
      Using these 12 classes as "transitions", we can map from one state to
      the next. Each state is defined as some multiple of 12, so that we're
      always starting at the 0th column of each row of the FSM. From each
      state, we add the transition and get a index of the new row the FSM is
      entering.
      
      If at any point we encounter a bad byte, the state + bad-byte-transition
      is guaranteed to map us into the first row of the FSM (which contains no
      valid exiting transitions).
      
      The key differences from Björn's original (or his self-modified) DFA is
      the "bad" state is now mapped to 0 (or the first row of the FSM) instead
      of 12 (the second row). This saves ~50 bytes when gzipping, and also
      speeds up determining if a string is properly encoded (see his sample
      code at http://bjoern.hoehrmann.de/utf-8/decoder/dfa/#performance).
      
      Finally, I've replace his ternary check with an array access, to make
      the algorithm branchless. This places a requirement on the caller to 0
      out the code point between successful decodings, which it could always
      have done because it's already branching.
      
      R=marja@google.com
      
      Bug: 
      Change-Id: I574f208a84dc5d06caba17127b0d41f7ce1a3395
      Reviewed-on: https://chromium-review.googlesource.com/805357
      Commit-Queue: Justin Ridgewell <jridgewell@google.com>
      Reviewed-by: 's avatarMarja Hölttä <marja@chromium.org>
      Reviewed-by: 's avatarMathias Bynens <mathias@chromium.org>
      Cr-Commit-Position: refs/heads/master@{#50012}
      cedec225