4kyu: add „Range Extraction“

• Also add missing „Sum of Intervals“ to README

Signed-off-by: Matej Focko <mfocko@redhat.com>

4kyu/range_extraction/solution.cpp

@@ -0,0 +1,68 @@
+#include <cassert>
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <vector>
+
+namespace {
+
+struct range_t {
+  int from;
+  int to;
+
+  range_t(int from, int to) : from(from), to(to) {}
+  void extend() { to++; }
+
+  int size() const { return to - from + 1; }
+  bool prefixes(int x) const { return x == to + 1; }
+};
+
+std::ostream &operator<<(std::ostream &stream, const range_t &range) {
+  if (range.size() >= 3) {
+    stream << range.from << "-" << range.to;
+  } else {
+    stream << range.from;
+
+    for (int i = range.from + 1; i <= range.to; i++) {
+      stream << "," << i;
+    }
+  }
+  return stream;
+}
+
+} // namespace
+
+std::string range_extraction(std::vector<int> args) {
+  if (args.empty()) {
+    return "";
+  }
+
+  std::vector<range_t> ranges;
+
+  for (int x : args) {
+    if (ranges.size() && ranges.back().prefixes(x)) {
+      ranges.back().extend();
+    } else {
+      ranges.emplace_back(x, x);
+    }
+  }
+
+  std::stringstream out;
+
+  auto it = ranges.cbegin();
+  out << (*it);
+
+  for (++it; it != ranges.cend(); ++it) {
+    out << "," << (*it);
+  }
+
+  return out.str();
+}
+
+int main() {
+  assert(range_extraction({-6, -3, -2, -1, 0,  1,  3,  4,  5,  7,
+                           8,  9,  10, 11, 14, 15, 17, 18, 19, 20}) ==
+         "-6,-3-1,3-5,7-11,14,15,17-20");
+  assert(range_extraction({-3, -2, -1, 2, 10, 15, 16, 18, 19, 20}) ==
+         "-3--1,2,10,15,16,18-20");
+}

README.md

@@ -39,6 +39,7 @@
 - [Permutations](https://www.codewars.com/kata/5254ca2719453dcc0b00027d) - [solution](4kyu/permutations)
 - [Matrix Determinant](https://www.codewars.com/kata/52a382ee44408cea2500074c) - [solution](4kyu/matrix_determinant)
 - [Infix to Postfix Converter](https://www.codewars.com/kata/52e864d1ffb6ac25db00017f) - [solution](4kyu/infix_to_postfix_converter)
+- [Range Extraction](https://www.codewars.com/kata/51ba717bb08c1cd60f00002f) - [solution](4kyu/range_extraction)
 
 ### JS
 
@@ -52,6 +53,10 @@
 
 - [Route Calculator](https://www.codewars.com/kata/581bc0629ad9ff9873000316) - [solution](4kyu/route_calculator)
 
+### Java
+
+- [Sum of Intervals](https://www.codewars.com/kata/52b7ed099cdc285c300001cd) - [solution](4kyu/sum_of_intervals)
+
 ## 5 kyu
 
 ### Kotlin