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/*
Challenge 098
TASK #1 › Read N-characters
Submitted by: Mohammad S Anwar
You are given file $FILE.
Create subroutine readN($FILE, $number) returns the first n-characters and
moves the pointer to the (n+1)th character.
Example:
Input: Suppose the file (input.txt) contains "1234567890"
Output:
print readN("input.txt", 4); # returns "1234"
print readN("input.txt", 4); # returns "5678"
print readN("input.txt", 4); # returns "90"
*/
#include <algorithm>
#include <array>
#include <iostream>
#include <fstream>
#include <string>
#include <cassert>
#include <cstring>
// store list of open files
class Files {
public:
std::string readN(const std::string filename, int n) {
std::ifstream& ifs = add_file(filename);
char* buffer = new char[n + 1];
memset(buffer, 0, n + 1);
ifs.read(buffer, n);
auto text = std::string(buffer);
delete[] buffer;
return text;
}
private:
// store std::ifstream in a pre-aloocated array as storing pointers to
// std::ifstream in an unordered_map was causing a heap corruption in
// GCC - probably a compiler/stdlib bug
static const int MAX_FILES = 20;
std::array<std::ifstream, MAX_FILES> files;
std::array<std::string, MAX_FILES> filenames;
size_t num_files{ 0 };
std::ifstream& add_file(const std::string filename) {
auto end = filenames.begin() + num_files;
auto found = std::find(filenames.begin(), end, filename);
if (found != end) { // found in filename
size_t i = std::distance(filenames.begin(), found);
return files[i];
}
else { // not found, must create new entry
if (num_files >= MAX_FILES) {
std::cerr << "too many files" << std::endl;
exit(EXIT_FAILURE);
}
filenames[num_files] = filename;
files[num_files].open(filename);
if (!files[num_files].is_open()) {
std::cerr << "open file " << filename << " failed" << std::endl;
exit(EXIT_FAILURE);
}
return files[num_files++];
}
}
};
int main(int argc, char* argv[]) {
Files f;
for (int i = 1; i + 1 < argc; i += 2)
std::cout << f.readN(argv[i], atoi(argv[i + 1])) << std::endl;
}
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