Added compat header

Author: spoutn1k

src/chunk.cpp

@@ -1,4 +1,5 @@
 #include "./chunk.h"
+#include <compat.hpp>
 #include <functional>
 
 namespace mcmap {

src/helper.h

@@ -70,23 +70,4 @@ fs::path getHome();
 fs::path getSaveDir();
 fs::path getTempDir();
 
-// TODO Finesse this atrocity out of existence
-template <typename F>
-typename std::map<int, F>::const_iterator
-compatible(const std::map<int, F> &hash, int version) {
-  auto it = hash.rbegin();
-  int compatible = 0;
-
-  while (it != hash.rend()) {
-    if (it->first <= version) {
-      compatible = it->first;
-      break;
-    }
-
-    it++;
-  }
-
-  return hash.find(compatible);
-}
-
 #endif // HELPER_H_

src/include/compat.hpp

@@ -0,0 +1,31 @@
+#pragma once
+#include <map>
+
+/* Loosely inspired from lower_bound in
+ * /usr/include/c++/11.1.0/bits/stl_algobase.h
+ * Returns a const_iterator to the highest integer that is inferior or equal to
+ * `version` from the keys of a map<int, F> */
+template <typename F>
+typename std::map<int, F>::const_iterator
+compatible(const std::map<int, F> &hash, int version) {
+  typedef typename std::map<int, F>::const_iterator _Iterator;
+  typedef typename _Iterator::difference_type _DistanceType;
+
+  _Iterator __first = hash.begin(), __end = hash.end();
+  _DistanceType __len = std::distance(__first, __end);
+
+  while (__len > 1) {
+    _DistanceType __half = __len >> 1;
+    _Iterator __middle = __first;
+    std::advance(__middle, __half);
+
+    if (__middle->first > version)
+      __end = __middle;
+    else
+      __first = __middle;
+
+    __len = __len - __half;
+  }
+
+  return __first;
+}

src/section.cpp

@@ -1,4 +1,5 @@
 #include "./section.h"
+#include <compat.hpp>
 #include <functional>
 
 namespace versions {

tests/test_compat.cpp

@@ -0,0 +1,21 @@
+#include <cmath>
+#include <compat.hpp>
+#include <gtest/gtest.h>
+
+TEST(TestCompat, TestReturn) {
+  std::map<int, int> versions = {
+      {0, 0}, {1, 0}, {2, 0}, {4, 0}, {8, 0}, {16, 0},
+  };
+
+  std::map<int, int>::const_iterator it;
+
+  it = compatible(versions, 0);
+  ASSERT_NE(it, versions.end());
+  ASSERT_EQ(it->first, 0);
+
+  for (int i = 1; i < 20; i++) {
+    it = compatible(versions, i);
+    ASSERT_NE(it, versions.end());
+    ASSERT_EQ(it->first, 1 << int(std::log2(double(i))));
+  }
+}