GNU Core Utils cat in less than 100 lines of code

cat.cpp

#include <iostream>
#include <string>
#include <fstream>
#include <iomanip>
#include <iterator>
#include <regex>
#include <vector>
#include <unistd.h>

namespace {
bool show_numbers    = false;
bool show_line_end   = false;
bool show_tabs       = false;
bool show_non_print  = false;
bool squash_blank    = false;
bool number_nonblank = false;

void cat(std::istream& stream) {
	int blank_lines = 0;
	for (std::string line{}; std::getline(stream, line);) {
		if (squash_blank) { blank_lines = (line.empty() ? ++blank_lines : 0); }
		if (number_nonblank && line.empty()) {
			if (squash_blank && blank_lines == 1) { std::cout << (show_line_end ? "$" : "") << std::endl; continue; }
		}
		if (show_numbers) {
			static int count = 0;
			if (blank_lines <= 1 && !(number_nonblank && line.empty())) {
				std::cout << std::right << std::setw(6) << ++count << std::resetiosflags(std::ios_base::basefield) << "\t";
			}
		}
		if (show_tabs) { line = std::regex_replace(line, std::regex{"\t"}, "^I"); }
		if (show_line_end) { line.append("$"); }
		if (show_non_print) {
			auto const regex = std::regex{ "[^\t-~]" };
			auto start = std::sregex_iterator{ std::begin(line), std::end(line), regex }; auto end = std::sregex_iterator{};
			for (auto i = start; i != end; ++i) {  std::cout << ((*i).str(0)) << std::endl; }
		}
		if (blank_lines <= 1) {	std::cout << line << std::endl;	}
	}
}
}

int main(int argc, char* argv[]) {
	for(int opt = 0; opt != -1; opt = getopt(argc, argv, "ubsEhnvT")) {
		switch (opt) {
		case 'u': break;
		case 's': squash_blank    = true; break;
		case 'b': number_nonblank = true; show_numbers = true; break;
		case 'n': show_numbers    = true; break;
		case 'E': show_line_end   = true; break;
		case 'v': show_non_print  = true; break;
		case 'T': show_tabs       = true; break;
		case 'h': std::cout << "Usage: cat [OPTION]... [FILE]...\nConcatenate FILE(s) to standard output.\n\n\
With no FILE, or when FILE is -, read standard input.\n\n  -A, --show-all           equivalent to -vET\n\
  -b, --number-nonblank    number nonempty output lines, overrides -n\n  -e                       equivalent to -vE\n\
  -E, --show-ends          display $ at end of each line\n  -n, --number             number all output lines\n\
  -s, --squeeze-blank      suppress repeated empty output lines\n  -t                       equivalent to -vT\n\
  -T, --show-tabs          display TAB characters as ^I\n  -u                       (ignored)\n\
  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB\n      --help     display this help and exit\n\
      --version  output version information and exit\n\nExamples:\n\
  cat f - g  Output f\'s contents, then standard input, then g\'s contents.\n  cat        Copy standard input to standard output.\n\n\
GNU coreutils online help: <http://www.gnu.org/software/coreutils/>\nFull documentation at: <http://www.gnu.org/software/coreutils/cat>\n\
or available locally via: info \'(coreutils) cat invocation\'\n"; return 0;
				  break;
		}
	}
	if (optind == argc) { cat(std::cin); }
	for (int index = optind; index < argc; ++index) {
		if (auto filename = std::string{ argv[index] }; filename == "-") { cat(std::cin); }
		else { auto file = std::ifstream{ argv[index] }; cat(file); }
	}
	return 0;
}