Tomita‘s Parser Tihomira Panayotova Paolina Teneva Seminar für Sprachwissenschaft

A simple overview LR(0) conflicts Tomita‘s method summarized Complications Two optimiziations A moderately ambiguous grammar Stack duplication Combinig equal states Combinig equal stack prefixes Discussion Summary

LR(0) Conflicts LR Parser: 1.handle recognizing FS automaton 2. no inadequate states There exist grammars for which the automaton has some inadequate states

LR(0) Conflicts

Tomita‘s method summarized Simple definition: – A breadth-first search over those parsing decisions that are not solved by the LR automaton – It gives an efficient and very effective approach to grammars for which the automaton has some inadequate states

Tomita‘s method summarized How does the parser act when it encounters an inadequate state on the top of the stack? Step1. It duplicates the stack and splits the parse into a different process for each copy: One copy is reserved for the REDUCE step The other copy is reserved for the SHIFT step Step2. Stacks that have a right-most state that does not allow a shift on the next input token are DISCARDED

Tomita‘s method summarized SHIFT step: push a new symbol and the new state onto the stack REDUCE step: removes part of the right end of the stack and replaces it with a non-terminal; using this non-terminal as a move in the automaton, we find a new state to put on the top of the stack Conclusion: Every time we encounter an inadequate state on the top of the stack, the duplication process is repeated untill all reduces have been treated.

Complications The repetition of the duplication process can cause a proliferation of stacks. A great number will be copied and subsequently discarded If all stacks are discarded in Step2 => the input was in error Grammars with loops : A->B B->A - the process may not terminate

Complications Some ideas to cope with the complications: 1. Use of look-ahead to decide which reduces can be made in Step1 2.Grammar with loops: 2.1. upon creating a stack, check if it is already there (and then ignore it) 2.2 check the grammar in advance for loops (and then reject it).

Two optimizations Combining equal states Combibing equal stack prefixes

A moderately ambiguous grammar S S -> E # E -> E + E E -> d Figure 9.38 A moderately ambiguous grammar

LR(0) automaton for the grammar

Stack Duplication a. 1 d+d+d# shift b. 1 d 2 +d+d# reduce c. 1 E 3 +d+d# shift d. 1 E d+d# shift e. 1 E d 2 +d# reduce f. 1 E E 5 +d# duplicate to g1 and g2 g1. 1 E E 5 +d# REDUCE; reduce to g1.1 g2. 1 E E 5 +d# SHIFT; shift to g1.2 g1.1 1 E 3 +d# shift to h1 g1.2 1 E E d# shift to h2 h1 1 E d# shift to h1.1 h2 1 E E d 2 # reduce to h1.2 h1.1 1 E d 2 # reduce to i h1.2 1 E E E 5 # duplicate to i1 and i2 i 1 E E 5 # duplicate to j1 and j2

Stack Duplication i1 1 E E E 5 #REDUCE, reduce to k1 I2 1 E E E 5 #SHIFT - DISCARDED j1 1 E E 5 #REDUCE, reduce to k2 j2 1 E E 5 #SHIFT - DISCARDED k1 1 E E 5 # reduce to l1 k2 1 E 3 # shift to l2 l1 1 E 3 # shift to m1 l2 1 E 3 # 6 reduce to m2 m1 1 E 3 # 6 reduce to n m2 1 S ACCEPT n 1 S ACCEPT

Parse trees

Combining equal states Examine the following: Both stacks have the same state on top=>further actions on both stacks will be identical Combine the two stacks to avoid duplicate work

Combining equal states f) 1. 1 E d 2 # both 2. 1 E E d 2 # REDUCE to g

Combining equal states f) 1. 1 E d 2 # both 2. 1 E E d 2 # REDUCE to g g) 1. 1 E E 5 # duplicate to 2. 1 E E E 5 # g’ and g ’’

Combining equal states f) 1. 1 E d 2 # both 2. 1 E E d 2 # REDUCE to g g) 1. 1 E E 5 # duplicate to 2. 1 E E E 5 # g’ and g ’’ g’) 1. 1 E E 5 # for REDUCE 2. 1 E E E 5 #

Combining equal states f) 1. 1 E d 2 # both 2. 1 E E d 2 # REDUCE to g g) 1. 1 E E 5 # duplicate to 2. 1 E E E 5 # g’ and g ’’ g’) 1. 1 E E 5 # for REDUCE 2. 1 E E E 5 # g’’) 1. 1 E E 5 # copy to h3) 2. 1 E E E 5 # for SHIFT

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1)

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1) g’.2) 1 E E E 5 # REDUCE to h.2)

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1) g’.2) 1 E E E 5 # REDUCE to h.2) h.1 ) 1 E 5 # SHIFT

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1) g’.2) 1 E E E 5 # REDUCE to h.2) h.1 ) 1 E 5 # SHIFT h2 ) 1 E E 5 # REDUCE to h2.1) and h2.2)

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1) g’.2) 1 E E E 5 # REDUCE to h.2) h.1 ) 1 E 5 # SHIFT h2 ) 1 E E 5 # REDUCE to h2.1) and h2.2) h2.1) 1 E 5 # SHIFT

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1) g’.2) 1 E E E 5 # REDUCE to h.2) h.1 ) 1 E 5 # SHIFT h2 ) 1 E E 5 # REDUCE to h2.1) and h2.2) h2.1) 1 E 5 # SHIFT h2.2) 1 E E 5 # SHIFT

Combining equal states g’.1) 1 E E 5 # REDUCE to h.1) g’.2) 1 E E E 5 # REDUCE to h.2) h.1 ) 1 E 5 # SHIFT h2 ) 1 E E 5 # REDUCE to h2.1) and h2.2) h2.1) 1 E 5 # SHIFT h2.2) 1 E E 5 # SHIFT h3) 1. 1 E E 5 # SHIFT 2. 1 E E E 5

Combining equal states Now we have five stacks (h1, h2.1, h2.2, h3). h1) and h2.1) carry state (3) on top h2.2) and h3) carry state (5) on top h1 ) 1 E 3 # SHIFT h2 ) 1 E E 5 # REDUCE to h2.1), copy to h2.2) h2.1) 1 E 3 # SHIFT h2.2) 1 E E 5 # SHIFT h3) 1. 1 E E 5 # SHIFT 2. 1 E E E 5

Combining equal states We combine the stacks with identical states on top into two bundles h’ and h’’. h’) h1) 1 E 3 #copy to i) h2.1) 1 E 3

Combining equal states We combine the stacks with identical states on top into two bundles h’ and h’’. h’) h1) 1 E 3 #copy to i) h2.1) 1 E 3 h’’) h3) 1. 1 E E E E E 5 #discard h2) 1 E E 5

Combining equal states i) 1) 1 E 3 3 # 6 2) 1 E 3

Combining equal states i) 1) 1 E 3 3 # 6 2) 1 E 3 i’) 1 E 3 # 6 REDUCE to j1)

Combining equal states i) 1) 1 E 3 3 # 6 2) 1 E 3 i’) 1 E 3 # 6 REDUCE to j1) i”) 1 E 3 # 6 REDUCE to j2)

Combining equal states i) 1) 1 E 3 3 # 6 2) 1 E 3 i’) 1 E 3 # 6 REDUCE to j1) i”) 1 E 3 # 6 REDUCE to j2) j1) 1 S ACCEPT

Combining equal states i) 1) 1 E 3 3 # 6 2) 1 E 3 i’) 1 E 3 # 6 REDUCE to j1) i”) 1 E 3 # 6 REDUCE to j2) j1) 1 S ACCEPT j2) 1 S ACCEPT

Combining equal stack prefixes When the parser makes the call for the stack to be copied,there is no actual need to copy the entire stack! It is enough to copy the top state suffixes

Combining Equal Stack-Prefixes If we observe the example : e) 1 E E 5 +d#

Combining Equal Stack-Prefixes If we observe the example : e) 1 E E 5 +d# When we duplicate the stack we have two copies of It and REDUCE is applied only to one of the copies and only “so much” of the stack is copied:

Combining Equal Stack-Prefixes If we observe the example : e) 1 E E 5 +d# When we duplicate the stack we have two copies of it and REDUCE is applied only to one of the copies and only “so much” of the stack is copied: e’) 1 E 3 +d# SHIFT e’’) 1 E E 5 +d# SHIFT

Discussion Table characteristics: -the method can work with every bottom-up table -the weaker the table, the more non-determinism will have to be resolved by breadth-first search Time requirements: - in theory – exponential - in practice - linear or slightly more than linear

SUMMARY Breadth-first search over those parsing decisions that are not solved by the LR automaton Important notions that should be memorized: – stack duplication (inadequate states, reduce, shift, discarded) – combining equal states – combining equal stack prefixes

References Dick Grune & Ceriel Jacobs (1990). Parsing Techniques