Class: Rly::LRTable

Inherits:
Object
  • Object
show all
Defined in:
lib/rly/parse/lr_table.rb

Constant Summary collapse

MAXINT =
(2**(0.size * 8 -2) -1)

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(grammar, method = :LALR) ⇒ LRTable

Returns a new instance of LRTable

Raises:

  • (ArgumentError)

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# File 'lib/rly/parse/lr_table.rb', line 10

def initialize(grammar, method=:LALR)
  raise ArgumentError unless [:LALR, :SLR].include?(method)

  @grammar = grammar
  @lr_method = method

  @lr_action = {}
  @lr_goto = {}
  @lr_productions = grammar.productions
  @lr_goto_cache = {}
  @lr0_cidhash = {}

  @add_count = 0

  @sr_conflict = 0
  @rr_conflict = 0
  @conflicts = []

  @sr_conflicts = []
  @rr_conflicts = []

  grammar.build_lritems
  grammar.compute_first
  grammar.compute_follow
end

Instance Attribute Details

#lr_actionObject (readonly)

Returns the value of attribute lr_action


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# File 'lib/rly/parse/lr_table.rb', line 8

def lr_action
  @lr_action
end

#lr_gotoObject (readonly)

Returns the value of attribute lr_goto


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# File 'lib/rly/parse/lr_table.rb', line 8

def lr_goto
  @lr_goto
end

#lr_productionsObject (readonly)

Returns the value of attribute lr_productions


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# File 'lib/rly/parse/lr_table.rb', line 8

def lr_productions
  @lr_productions
end

Instance Method Details

#parse_table(log = PlyDump.stub) ⇒ Object


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# File 'lib/rly/parse/lr_table.rb', line 36

def parse_table(log=PlyDump.stub)
  productions = @grammar.productions
  precedence = @grammar.precedence

  actionp = {}

  log.info("Parsing method: %s", @lr_method)

  c = lr0_items

  add_lalr_lookaheads(c) if @lr_method == :LALR

  # Build the parser table, state by state
  st = 0
  c.each do |i|
    # Loop over each production in I
    actlist = []              # List of actions
    st_action  = {}
    st_actionp = {}
    st_goto    = {}
    log.info("")
    log.info("state %d", st)
    log.info("")
    i.each { |p| log.info("    (%d) %s", p.index, p.to_s) }
    log.info("")

    i.each do |p|
      if p.length == p.lr_index + 1
        if p.name == :"S'"
          # Start symbol. Accept!
          st_action[:"$end"] = 0
          st_actionp[:"$end"] = p
        else
          # We are at the end of a production.  Reduce!
          if @lr_method == :LALR
            laheads = p.lookaheads[st]
          else
            laheads = @grammar.follow[p.name]
          end
          laheads.each do |a|
            actlist << [a, p, sprintf("reduce using rule %d (%s)", p.index, p)]
            r = st_action[a]
            if r
              # Whoa. Have a shift/reduce or reduce/reduce conflict
              if r > 0
                # Need to decide on shift or reduce here
                # By default we favor shifting. Need to add
                # some precedence rules here.
                sprec, slevel = productions[st_actionp[a].index].precedence
                rprec, rlevel = precedence[a] || [:right, 0]
                if (slevel < rlevel) || ((slevel == rlevel) && (rprec == :left))
                  # We really need to reduce here.
                  st_action[a] = -p.index
                  st_actionp[a] = p
                  if ! slevel && ! rlevel
                    log.info("  ! shift/reduce conflict for %s resolved as reduce",a)
                    @sr_conflicts << [st, a, 'reduce']
                  end
                  productions[p.index].reduced += 1
                elsif (slevel == rlevel) && (rprec == :nonassoc)
                  st_action[a] = nil
                else
                  # Hmmm. Guess we'll keep the shift
                  unless rlevel
                    log.info("  ! shift/reduce conflict for %s resolved as shift",a)
                    @sr_conflicts << [st,a,'shift']
                  end
                end
              elsif r < 0
                  # Reduce/reduce conflict.   In this case, we favor the rule
                  # that was defined first in the grammar file
                  oldp = productions[-r]
                  pp = productions[p.index]
                  if oldp.line > pp.line
                    st_action[a] = -p.index
                    st_actionp[a] = p
                    chosenp = pp
                    rejectp = oldp
                    productions[p.index].reduced += 1
                    productions[oldp.index].reduced -= 1
                  else
                    chosenp,rejectp = oldp,pp
                  end
                  @rr_conflicts << [st, chosenp, rejectp]
                  log.info("  ! reduce/reduce conflict for %s resolved using rule %d (%s)", a, st_actionp[a].index, st_actionp[a])
              else
                raise RuntimeError("Unknown conflict in state #{st}")
              end
            else
              st_action[a] = -p.index
              st_actionp[a] = p
              productions[p.index].reduced += 1
            end
          end
        end
      else
        a = p.prod[p.lr_index+1]       # Get symbol right after the "."
        if @grammar.terminals.include?(a)
          g = lr0_goto(i, a)
          j = @lr0_cidhash[g.hash] || -1
          if j >= 0
            # We are in a shift state
            actlist << [a, p, sprintf("shift and go to state %d", j)]
            r = st_action[a]
            if r
              # Whoa have a shift/reduce or shift/shift conflict
              if r > 0
                if r != j
                  raise RuntimeError("Shift/shift conflict in state #{st}")
                end
              elsif r < 0
                # Do a precedence check.
                #   -  if precedence of reduce rule is higher, we reduce.
                #   -  if precedence of reduce is same and left assoc, we reduce.
                #   -  otherwise we shift
                rprec, rlevel = productions[st_actionp[a].index].precedence
                sprec, slevel = precedence[a] || [:right, 0]
                if (slevel > rlevel) || ((slevel == rlevel) && (rprec == :right))
                  # We decide to shift here... highest precedence to shift
                  productions[st_actionp[a].index].reduced -= 1
                  st_action[a] = j
                  st_actionp[a] = p
                  unless rlevel
                    log.info("  ! shift/reduce conflict for %s resolved as shift",a)
                    @sr_conflicts << [st, a, 'shift']
                  end
                elsif (slevel == rlevel) && (rprec == :nonassoc)
                  st_action[a] = nil
                else
                  # Hmmm. Guess we'll keep the reduce
                  if ! slevel && ! rlevel
                    log.info("  ! shift/reduce conflict for %s resolved as reduce",a)
                    @sr_conflicts << [st, a, 'reduce']
                  end
                end
              else
                raise RuntimeError("Unknown conflict in state #{st}")
              end
            else
              st_action[a] = j
              st_actionp[a] = p
            end
          end
        end
      end
    end

    # Print the actions associated with each terminal
    _actprint = {}
    actlist.each do |a, p, m|
      if st_action[a]
        if p == st_actionp[a]
          log.info("    %-15s %s",a,m)
          _actprint[[a,m]] = 1
        end
      end
    end
    log.info("")
    # Print the actions that were not used. (debugging)
    not_used = false
    actlist.each do |a, p, m|
      if st_action[a]
        unless p == st_actionp[a]
          unless _actprint[[a,m]]
            log.debug("  ! %-15s [ %s ]", a, m)
            not_used = true
            _actprint[[a,m]] = 1
          end
        end
      end
    end
    log.debug("") if not_used

    # Construct the goto table for this state

    nkeys = {}
    i.each do |ii|
      ii.usyms.each do |s|
        nkeys[s] = nil if @grammar.nonterminals.include?(s)
      end
    end
    nkeys.each do |n, _|
      g = lr0_goto(i, n)
      j = @lr0_cidhash[g.hash] || -1
      if j >= 0
        st_goto[n] = j
        log.info("    %-30s shift and go to state %d",n,j)
      end
    end

    @lr_action[st] = st_action
    actionp[st] = st_actionp
    @lr_goto[st] = st_goto
    st += 1
  end
end