grammar-inference-engine/bex/grammar.py

"""AST — canonical grammar representation.

Node types: Symbol, Concat, Alt, Plus, Optional, Star, Epsilon, Empty.
AST is the ONLY representation. No SORE strings exist anywhere.
"""

import math
from functools import lru_cache


class Symbol:
    __slots__ = ('value',)
    def __init__(self, value): self.value = value
    def __eq__(self, other): return isinstance(other, Symbol) and self.value == other.value
    def __hash__(self): return hash(('Sym', self.value))
    def __repr__(self): return f"Symbol({self.value!r})"


class Concat:
    __slots__ = ('parts',)
    def __init__(self, parts): self.parts = list(parts)
    def __eq__(self, other): return isinstance(other, Concat) and self.parts == other.parts
    def __hash__(self): return hash(('Concat', tuple(self.parts)))
    def __repr__(self): return f"Concat({self.parts!r})"


class Alt:
    __slots__ = ('parts',)
    def __init__(self, parts): self.parts = list(parts)
    def __eq__(self, other): return isinstance(other, Alt) and self.parts == other.parts
    def __hash__(self): return hash(('Alt', tuple(self.parts)))
    def __repr__(self): return f"Alt({self.parts!r})"


class Plus:
    __slots__ = ('child',)
    def __init__(self, child): self.child = child
    def __eq__(self, other): return isinstance(other, Plus) and self.child == other.child
    def __hash__(self): return hash(('Plus', self.child))
    def __repr__(self): return f"Plus({self.child!r})"


class Optional:
    __slots__ = ('child',)
    def __init__(self, child): self.child = child
    def __eq__(self, other): return isinstance(other, Optional) and self.child == other.child
    def __hash__(self): return hash(('Optional', self.child))
    def __repr__(self): return f"Optional({self.child!r})"


class Star:
    __slots__ = ('child',)
    def __init__(self, child): self.child = child
    def __eq__(self, other): return isinstance(other, Star) and self.child == other.child
    def __hash__(self): return hash(('Star', self.child))
    def __repr__(self): return f"Star({self.child!r})"


class Epsilon:
    __slots__ = ()
    def __eq__(self, other): return isinstance(other, Epsilon)
    def __hash__(self): return hash('Epsilon')
    def __repr__(self): return 'Epsilon()'


class Empty:
    __slots__ = ()
    def __eq__(self, other): return isinstance(other, Empty)
    def __hash__(self): return hash('Empty')
    def __repr__(self): return 'Empty()'


# ---------------------------------------------------------------------------
# AST operations
# ---------------------------------------------------------------------------

def alphabet(node):
    """Collect all Symbol values from an AST."""
    if isinstance(node, Symbol):
        return {node.value}
    if isinstance(node, (Epsilon, Empty)):
        return set()
    if isinstance(node, (Plus, Optional, Star)):
        return alphabet(node.child)
    if isinstance(node, (Concat, Alt)):
        result = set()
        for p in node.parts:
            result |= alphabet(p)
        return result
    return set()


# ---------------------------------------------------------------------------
# Matching
# ---------------------------------------------------------------------------

def match(node, seq):
    """Check if seq matches the grammar defined by node."""
    ends = _match_set(node, seq, 0)
    return len(seq) in ends


def _match_set(node, seq, pos):
    """Return set of positions reachable from pos after matching node."""
    if isinstance(node, Symbol):
        if pos < len(seq) and seq[pos] == node.value:
            return {pos + 1}
        return set()
    if isinstance(node, Epsilon):
        return {pos}
    if isinstance(node, Empty):
        return set()
    if isinstance(node, Concat):
        current = {pos}
        for part in node.parts:
            next_set = set()
            for p in current:
                next_set |= _match_set(part, seq, p)
            current = next_set
            if not current:
                break
        return current
    if isinstance(node, Alt):
        result = set()
        for part in node.parts:
            result |= _match_set(part, seq, pos)
        return result
    if isinstance(node, Plus):
        return _match_rep(node.child, seq, pos, min_rep=1)
    if isinstance(node, Optional):
        return _match_set(node.child, seq, pos) | {pos}
    if isinstance(node, Star):
        return _match_rep(node.child, seq, pos, min_rep=0)
    return set()


def _match_rep(child, seq, pos, min_rep):
    """Match child repeated min_rep or more times."""
    if min_rep == 0:
        accept = {pos}
    else:
        accept = set()
    current = {pos}
    for _ in range(min_rep):
        next_set = set()
        for p in current:
            next_set |= _match_set(child, seq, p)
        current = next_set
        if not current:
            break
        if min_rep == 0:
            accept |= current
    seen = set()
    frontier = current
    while frontier:
        frontier_next = set()
        for p in frontier:
            if p in seen:
                continue
            seen.add(p)
            accept.add(p)
            frontier_next |= _match_set(child, seq, p)
        frontier = frontier_next - seen
    if min_rep > 0:
        accept |= current
    return accept


# ---------------------------------------------------------------------------
# Counting (for MDL scoring)
# ---------------------------------------------------------------------------

_COUNT_CAP = 10 ** 30

@lru_cache(maxsize=None)
def count_words(node, length):
    """Count how many words of exactly `length` are in L(node).

    Capped at _COUNT_CAP to prevent combinatorial explosion on
    deeply nested CRX grammars with large alphabets.
    """
    if length < 0:
        return 0
    if isinstance(node, Symbol):
        return 1 if length == 1 else 0
    if isinstance(node, Epsilon):
        return 1 if length == 0 else 0
    if isinstance(node, Empty):
        return 0
    if isinstance(node, Concat):
        return _count_concat(tuple(node.parts), length)
    if isinstance(node, Alt):
        total = 0
        for p in node.parts:
            total += count_words(p, length)
            if total >= _COUNT_CAP:
                return _COUNT_CAP
        return total
    if isinstance(node, Plus):
        return _count_rep(node.child, length, 1)
    if isinstance(node, Optional):
        return count_words(node.child, length) + (1 if length == 0 else 0)
    if isinstance(node, Star):
        return _count_rep(node.child, length, 0)
    return 0


@lru_cache(maxsize=None)
def _count_concat(parts, length):
    if not parts:
        return 1 if length == 0 else 0
    first = parts[0]
    rest = parts[1:]
    total = 0
    for take in range(length + 1):
        cnt = count_words(first, take)
        if cnt:
            total += cnt * _count_concat(rest, length - take)
            if total >= _COUNT_CAP:
                return _COUNT_CAP
    return total


@lru_cache(maxsize=None)
def _count_rep(child, length, min_rep):
    total = 0
    for rep in range(min_rep, length + 1):
        total += _count_repeat(child, rep, length)
        if total >= _COUNT_CAP:
            return _COUNT_CAP
    return total


@lru_cache(maxsize=None)
def _count_repeat(child, rep, length):
    if rep == 0:
        return 1 if length == 0 else 0
    total = 0
    for take in range(length + 1):
        cnt = count_words(child, take)
        if cnt:
            total += cnt * _count_repeat(child, rep - 1, length - take)
            if total >= _COUNT_CAP:
                return _COUNT_CAP
    return total


def lang_size(node, n=None):
    """|L(r)≤n| — number of words of length ≤ n."""
    if isinstance(node, Empty):
        return 0
    if isinstance(node, Epsilon):
        return 1
    if n is None:
        n = 2 * model_cost(node) + 1
    return sum(count_words(node, l) for l in range(n + 1))


def model_cost(node):
    """|r| — number of alphabet symbol occurrences in expression."""
    if isinstance(node, Symbol):
        return 1
    if isinstance(node, (Epsilon, Empty)):
        return 0
    if isinstance(node, (Plus, Optional, Star)):
        return model_cost(node.child)
    if isinstance(node, (Concat, Alt)):
        return sum(model_cost(p) for p in node.parts)
    return 0