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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;
+}