Cache.h

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#ifndef CACHE_H
#define CACHE_H
 
#ifndef _WIN32
#include <unistd.h>
#endif
#ifdef _WIN32
#include <direct.h>
#endif
 
#if __cplusplus >= 201703L
#include <filesystem>
#endif
 
namespace bridges {
 
 
    class CacheException : public std::exception {
            const char* whatmsg;
        public:
 
            CacheException(const char* what_msg = "")
                : std::exception(), whatmsg(what_msg) {
 
            }
 
            virtual const char* what() const noexcept {
                return whatmsg;
            }
 
    };
 
    class Cache {
        public:
            virtual bool inCache(const std::string & docName) noexcept(false) = 0;
            virtual std::string getDoc (const std::string & docName) noexcept(false) = 0;
            //store content under docname
            virtual void putDoc (const std::string & docName,
                const std::string & content) noexcept(false) = 0;
    };
 
    class SimpleCache : public Cache {
        private:
            std::string cacheDir;
 
            std::string getFilename(const std::string & docName) {
                return cacheDir + "/" + docName; //TODO: bad things can happen if docName contains / or .. or stuff like that
 
            }
 
            //return whether s is a directory (true) or does not exist (false). all other cases are exception
            bool directoryExist(const std::string &s) {
                struct stat buffer;
                int ret = stat (s.c_str(), &buffer);
 
                if (ret == 0) { //file exist
                    if ((buffer.st_mode & S_IFMT) == S_IFDIR) { //Not using S_ISDIR because VS2017 does not support it.
                        return true;
                    }
                    else {
                        throw CacheException("Expect directory"); //s exist but is not a directory
                    }
                }
 
                return false;
            }
 
            //make a directory called s or throw an exception
            static void makeDirectory (const std::string &s) {
#if __cplusplus >= 201703L
                //C++17 support
 
                //we can use std::filesystem::create_directories to make the directories recursively.
 
                bool ret = std::filesystem::create_directories(s);
                if (!ret)
                    throw CacheException("error in makeDirectory");
 
#else
                //No C++17 support. So no std::filesystem support
                //So forgo recursive creation. Probably not worth the cost of writing the code.
#ifndef _WIN32
                int ret = mkdir(s.c_str(), 0700);
#endif
#ifdef _WIN32
                int ret = _mkdir(s.c_str());
#endif
 
                if (ret != 0)
                    throw CacheException("error in makeDirectory");
 
#endif
 
 
            }
 
        public:
            SimpleCache() {
                //According to XDG, you should put the cache data in
                //$XDG_CACHE_HOME and if not defined in $HOME/.cache
                //However, MS Windows does not set $HOME. So we
                //use $LOCALAPPDATA as if it was $HOME.
                //
                //So we put the data in $XDG_CACHE_HOME/bridges_data/cxx
                //
                //Finally, one can overide everything by setting $FORCE_BRIDGES_CACHEDIR
 
                char * home = getenv("HOME"); // a reasonable location on unixes
                if (home == nullptr)
                    home = getenv("LOCALAPPDATA"); // a reasonnable location on windowses
 
                if (home != nullptr)
                    cacheDir += std::string(home) + "/.cache/";
 
 
                //override the directory of the cache if  is set
                char* xdg_cache_home = getenv("XDG_CACHE_HOME");
                if (xdg_cache_home != nullptr)
                    cacheDir = std::string(xdg_cache_home) + "/";
 
                cacheDir += "bridges_data/cxx/";
                //probably should check directory existence here, but exception in constructors are weird.
 
 
                //override the directory of the cache if FORCE_BRIDGES_CACHEDIR is set
                char* forcedir = getenv("FORCE_BRIDGES_CACHEDIR");
                if (forcedir != nullptr)
                    cacheDir = std::string(forcedir) + "/";
            }
 
            virtual ~SimpleCache() = default;
 
            //is docName in the cache
            virtual bool inCache(const std::string & docName) noexcept(false) override {
                std::string filename = getFilename(docName);
 
                std::ifstream in(filename);
 
                return in.is_open();
            }
 
            //return the content of docName which is in the cache
            virtual std::string getDoc (const std::string & docName) noexcept(false) override {
                std::string filename = getFilename(docName);
 
                std::ifstream in(filename);
 
                if (!in.good() || !(in.is_open()))
                    throw CacheException("Can't open file to read");
 
                std::string contents;
                in.seekg(0, std::ios::end);
                contents.resize(in.tellg());
                in.seekg(0, std::ios::beg);
                in.read(&contents[0], contents.size());
                if (! (in.good()))
                    throw CacheException("Error while reading cache document");
                in.close();
                return (contents);
 
            }
 
            //store content under docname
            virtual void putDoc (const std::string & docName,
                const std::string & content) noexcept(false) override {
                if (!directoryExist(cacheDir))
                    makeDirectory(cacheDir);
 
                std::string filename = getFilename(docName);
 
                std::ofstream out(filename);
                if (!out.good() || !(out.is_open()))
                    throw CacheException("can't open file to store");
 
                out << content.c_str(); //this assumes string isn't binary
                if (!out.good() || !(out.is_open()))
                    throw CacheException("error while writing cache document");
 
            }
 
            bool evict(const std::string& docName) {
                string f = cacheDir;
                f.append(docName);
 
                return std::remove(f.c_str()) == 0;
            }
    };
 
    class lruCache : public Cache {
 
            int maxCache;
            std::vector<std::string> v;
            SimpleCache ca;
        public:
            lruCache(int maxFileNumber = 30)
                : maxCache(maxFileNumber) {
            }
 
            virtual ~lruCache() = default;
 
            virtual std::string getDoc (const std::string& hash_value) override { //returns LRU vector from cache file
                string content;
                getLRU();
                content = ca.getDoc(hash_value);
                updateLRU(hash_value);
                saveLRU();
                return content;
            }
 
            virtual bool inCache(const std::string& hash_value) override {
                if (ca.inCache(hash_value)) {
                    return true;
                }
                return false;
            }
 
            virtual void putDoc(const std::string& hash_value, const std::string& content) override { //puts hash value at front of LRU vector
                getLRU();
                ca.putDoc(hash_value, content);
                updateLRU(hash_value);
 
                //checks size of vector and pops lru off
                if (v.size() >= maxCache + 1) { // keeps maxCache local maps
                    if (ca.evict(v[v.size() - 1]))
                        v.pop_back();
                }
                saveLRU();
                return;
            }
 
        private:
            void updateLRU(std::string hash_value) {
                for (auto it = v.begin(); it != v.end(); ) {
                    if (*it == hash_value) {
                        v.erase(it); //removes old hash vlaue location in vector
                        break;
                    }
                    else {
                        ++it;
                    }
                }
                v.insert(v.begin(), hash_value); //puts hash value in the front of the vector
                return;
            }
            void getLRU() {
                v.clear();
                if (ca.inCache("lru")) {
                    string vector_string = ca.getDoc("lru"); //Imported LRU
                    std::istringstream ss(vector_string);
                    std::string token;
                    //Parses string and turns it into vector
                    while (std::getline(ss, token, ',')) {
                        v.push_back(token);
                    }
                }
                return;
            }
 
            void saveLRU() {
                //Saves the vector to file
                string out_vector;
                int x = 0;
                for (auto s : v) {
                    if (x == 0) { //prevents , from being first character
                        out_vector = s;
                        x++;
                    }
                    else {
                        out_vector = out_vector + "," + s;
                    }
                }
                ca.putDoc("lru", out_vector);
            }
    };
}
 
#endif