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more efficient suffix array construction

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This commit is contained in:
Akash Mozumdar 2019-08-08 06:23:15 -04:00
parent c38551905d
commit 488a19134a
1 changed files with 39 additions and 3 deletions
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42
extensions/removerepeatphrase.cpp
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@ -2,6 +2,44 @@
constexpr wchar_t ERASED = 0xe012; // inside Unicode private use area
std::vector<int> GenerateSuffixArray(const std::wstring& text)
{
std::vector<int> identity(text.size());
for (int i = 0; i < text.size(); ++i) identity[i] = i;
std::vector<int> suffixArray = identity;
// The below code is a more efficient way of doing this:
// std::sort(suffixArray.begin(), suffixArray.end(), [&](int a, int b) { return wcscmp(text.c_str() + a, text.c_str() + b) > 0; });
std::stable_sort(suffixArray.begin(), suffixArray.end(), [&](int a, int b) { return text[a] > text[b]; });
std::vector<int> classes(text.begin(), text.end());
for (int length = 1; length < text.size(); length *= 2)
{
// Determine equivalence class up to length, by checking length/2 equivalence of suffixes and their following length/2 suffixes
std::vector<int> oldClasses = classes;
classes[suffixArray[0]] = 0;
for (int i = 1; i < text.size(); ++i)
{
int currentSuffix = suffixArray[i];
int lastSuffix = suffixArray[i - 1];
if (currentSuffix + length < text.size() && oldClasses[currentSuffix] == oldClasses[lastSuffix] &&
oldClasses[currentSuffix + length / 2] == oldClasses.at(lastSuffix + length / 2)) // not completely certain that this will stay in range
classes[currentSuffix] = classes[lastSuffix];
else classes[currentSuffix] = i;
}
// Sort within equivalence class based on order of following suffix after length
// Orders up to length*2
std::vector<int> count = identity;
for (auto suffix : std::vector(suffixArray))
{
int precedingSuffix = suffix - length;
if (precedingSuffix >= 0) suffixArray[count[classes[precedingSuffix]]++] = precedingSuffix;
}
}
for (int i = 0; i + 1 < text.size(); ++i)
assert(wcscmp(text.c_str() + suffixArray[i], text.c_str() + suffixArray[i + 1]) > 0);
return suffixArray;
}
bool ProcessSentence(std::wstring& sentence, SentenceInfo sentenceInfo)
{
if (sentenceInfo["text number"] == 0) return false;
@ -9,9 +47,7 @@ bool ProcessSentence(std::wstring& sentence, SentenceInfo sentenceInfo)
// This algorithm looks for repeating substrings (in other words, common prefixes among the set of suffixes) of the sentence with length > 6
// It then looks for any regions of characters at least twice as long as the substring made up only of characters in the substring, and erases them
// If this results in the common prefix being completely erased from the string, the common prefix is copied to the last location where it was located in the original string
std::vector<int> suffixArray(sentence.size());
for (int i = 0; i < sentence.size(); ++i) suffixArray[i] = i;
std::sort(suffixArray.begin(), suffixArray.end(), [&](int a, int b) { return wcsncmp(sentence.c_str() + a, sentence.c_str() + b, 5000) > 0; });
std::vector<int> suffixArray = GenerateSuffixArray(sentence);
for (int i = 0; i + 1 < sentence.size(); ++i)
{
int commonPrefixLength = 0;