test: added y_combinator test (#131)

* test: added `y_combinator` test

* fix: formatting

Author: BrandonPacewic

tests/cpl/y_combinator/test.cpp

@@ -0,0 +1,66 @@
+// Copyright (c) Brandon Pacewic
+// SPDX-License-Identifier: MIT
+
+#include <cassert>
+#include <vector>
+
+#include "xutility.h"
+
+int main() {
+    using namespace cpl;
+    {
+        auto factorial = y_combinator([](auto self, int n) -> int { return n <= 1 ? 1 : n * self(n - 1); });
+
+        assert(factorial(0) == 1);
+        assert(factorial(1) == 1);
+        assert(factorial(5) == 120);
+        assert(factorial(10) == 3628800);
+    }
+    {
+        auto gcd = y_combinator([](auto self, int a, int b) -> int { return b == 0 ? a : self(b, a % b); });
+
+        assert(gcd(20, 30) == 10);
+        assert(gcd(48, 18) == 6);
+        assert(gcd(0, 5) == 5);
+        assert(gcd(17, 13) == 1);
+    }
+    {
+        auto fib = y_combinator([](auto self, int n) -> int { return n <= 1 ? n : self(n - 1) + self(n - 2); });
+
+        assert(fib(0) == 0);
+        assert(fib(1) == 1);
+        assert(fib(10) == 55);
+        assert(fib(15) == 610);
+    }
+    {
+        std::vector<std::vector<int>> adj = {{1, 2}, {3, 4}, {}, {}, {}};
+        std::vector<int>              visited;
+
+        auto dfs = y_combinator([&](auto self, int node) -> void {
+            visited.push_back(node);
+            for (int next : adj[node]) {
+                self(next);
+            }
+        });
+
+        dfs(0);
+        assert(visited.size() == 5);
+        assert(visited[0] == 0);
+        assert(visited[1] == 1);
+        assert(visited[2] == 3);
+        assert(visited[3] == 4);
+        assert(visited[4] == 2);
+    }
+    {
+        int  call_count = 0;
+        auto counter    = y_combinator([&](auto self, int n) -> int {
+            ++call_count;
+            return n <= 0 ? 0 : self(n - 1);
+        });
+
+        counter(5);
+        assert(call_count == 6);
+    }
+
+    return 0;
+}