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#ifndef LM_COMMON_COMPARE_H
#define LM_COMMON_COMPARE_H
#include "ngram.hh"
#include "../word_index.hh"
#include <functional>
#include <string>
namespace lm {
/**
* Abstract parent class for defining custom n-gram comparators.
*/
template <class Child> class Comparator : public std::binary_function<const void *, const void *, bool> {
public:
/**
* Constructs a comparator capable of comparing two n-grams.
*
* @param order Number of words in each n-gram
*/
explicit Comparator(std::size_t order) : order_(order) {}
/**
* Applies the comparator using the Compare method that must be defined in any class that inherits from this class.
*
* @param lhs A pointer to the n-gram on the left-hand side of the comparison
* @param rhs A pointer to the n-gram on the right-hand side of the comparison
*
* @see ContextOrder::Compare
* @see PrefixOrder::Compare
* @see SuffixOrder::Compare
*/
inline bool operator()(const void *lhs, const void *rhs) const {
return static_cast<const Child*>(this)->Compare(static_cast<const WordIndex*>(lhs), static_cast<const WordIndex*>(rhs));
}
/** Gets the n-gram order defined for this comparator. */
std::size_t Order() const { return order_; }
protected:
std::size_t order_;
};
/**
* N-gram comparator that compares n-grams according to their reverse (suffix) order.
*
* This comparator compares n-grams lexicographically, one word at a time,
* beginning with the last word of each n-gram and ending with the first word of each n-gram.
*
* Some examples of n-gram comparisons as defined by this comparator:
* - a b c == a b c
* - a b c < a b d
* - a b c > a d b
* - a b c > a b b
* - a b c > x a c
* - a b c < x y z
*/
class SuffixOrder : public Comparator<SuffixOrder> {
public:
/**
* Constructs a comparator capable of comparing two n-grams.
*
* @param order Number of words in each n-gram
*/
explicit SuffixOrder(std::size_t order) : Comparator<SuffixOrder>(order) {}
/**
* Compares two n-grams lexicographically, one word at a time,
* beginning with the last word of each n-gram and ending with the first word of each n-gram.
*
* @param lhs A pointer to the n-gram on the left-hand side of the comparison
* @param rhs A pointer to the n-gram on the right-hand side of the comparison
*/
inline bool Compare(const WordIndex *lhs, const WordIndex *rhs) const {
for (std::size_t i = order_ - 1; i != 0; --i) {
if (lhs[i] != rhs[i])
return lhs[i] < rhs[i];
}
return lhs[0] < rhs[0];
}
static const unsigned kMatchOffset = 1;
};
/**
* N-gram comparator that compares n-grams according to the reverse (suffix) order of the n-gram context.
*
* This comparator compares n-grams lexicographically, one word at a time,
* beginning with the penultimate word of each n-gram and ending with the first word of each n-gram;
* finally, this comparator compares the last word of each n-gram.
*
* Some examples of n-gram comparisons as defined by this comparator:
* - a b c == a b c
* - a b c < a b d
* - a b c < a d b
* - a b c > a b b
* - a b c > x a c
* - a b c < x y z
*/
class ContextOrder : public Comparator<ContextOrder> {
public:
/**
* Constructs a comparator capable of comparing two n-grams.
*
* @param order Number of words in each n-gram
*/
explicit ContextOrder(std::size_t order) : Comparator<ContextOrder>(order) {}
/**
* Compares two n-grams lexicographically, one word at a time,
* beginning with the penultimate word of each n-gram and ending with the first word of each n-gram;
* finally, this comparator compares the last word of each n-gram.
*
* @param lhs A pointer to the n-gram on the left-hand side of the comparison
* @param rhs A pointer to the n-gram on the right-hand side of the comparison
*/
inline bool Compare(const WordIndex *lhs, const WordIndex *rhs) const {
for (int i = order_ - 2; i >= 0; --i) {
if (lhs[i] != rhs[i])
return lhs[i] < rhs[i];
}
return lhs[order_ - 1] < rhs[order_ - 1];
}
};
/**
* N-gram comparator that compares n-grams according to their natural (prefix) order.
*
* This comparator compares n-grams lexicographically, one word at a time,
* beginning with the first word of each n-gram and ending with the last word of each n-gram.
*
* Some examples of n-gram comparisons as defined by this comparator:
* - a b c == a b c
* - a b c < a b d
* - a b c < a d b
* - a b c > a b b
* - a b c < x a c
* - a b c < x y z
*/
class PrefixOrder : public Comparator<PrefixOrder> {
public:
/**
* Constructs a comparator capable of comparing two n-grams.
*
* @param order Number of words in each n-gram
*/
explicit PrefixOrder(std::size_t order) : Comparator<PrefixOrder>(order) {}
/**
* Compares two n-grams lexicographically, one word at a time,
* beginning with the first word of each n-gram and ending with the last word of each n-gram.
*
* @param lhs A pointer to the n-gram on the left-hand side of the comparison
* @param rhs A pointer to the n-gram on the right-hand side of the comparison
*/
inline bool Compare(const WordIndex *lhs, const WordIndex *rhs) const {
for (std::size_t i = 0; i < order_; ++i) {
if (lhs[i] != rhs[i])
return lhs[i] < rhs[i];
}
return false;
}
static const unsigned kMatchOffset = 0;
};
template <class Range> struct SuffixLexicographicLess : public std::binary_function<const Range, const Range, bool> {
bool operator()(const Range first, const Range second) const {
for (const WordIndex *f = first.end() - 1, *s = second.end() - 1; f >= first.begin() && s >= second.begin(); --f, --s) {
if (*f < *s) return true;
if (*f > *s) return false;
}
return first.size() < second.size();
}
};
} // namespace lm
#endif // LM_COMMON_COMPARE_H
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