From 6f8d1f34d57d292f04fa0ffcb27070951fa76a54 Mon Sep 17 00:00:00 2001
From: Vahagn Khachatryan <vahagn.khachatryan@gmail.com>
Date: Fri, 21 Aug 2020 12:29:00 +0100
Subject: [PATCH] : constexpr_sqrt.cpp

---
 puzzles/training/constexpr_sqrt.cpp | 61 +++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 puzzles/training/constexpr_sqrt.cpp

diff --git a/puzzles/training/constexpr_sqrt.cpp b/puzzles/training/constexpr_sqrt.cpp
new file mode 100644
index 0000000..ff4b9dd
--- /dev/null
+++ b/puzzles/training/constexpr_sqrt.cpp
@@ -0,0 +1,61 @@
+/*
+VIM: let b:cf5build="clang -std=c++20 -O2 -pthread -lstdc++ -I. {SRC} -o {OUT}"
+VIM: let b:cf5run="{OUT}"
+*/
+#include <iostream>
+#include <exception>
+#include <chrono>
+
+/* This is from "The C++ Programming Language, Fourth Edition 
+ * by Bjarne Stroustrup"*/
+constexpr int sqrt2(int x, int i = 3, int i_sq = 1){
+    return i_sq <= x ? sqrt2(x, i+2, i_sq + i) : i/2-1;
+}
+
+/* My simplified version:
+ * Basically sum(1+2*i) where i==0..n-1 == n^2
+ */
+constexpr int sqrt(int x, int i = 1, int i_sq = 1){
+    return i_sq <= x ? sqrt(x, i+1, i_sq + (1+2*i)) : i-1;
+}
+
+
+int main ( void )
+{try{
+    auto begin = std::chrono::high_resolution_clock::now();
+
+    std::cout << "sqrt(-1)=" << sqrt(-1) << " vs " << sqrt2(-1) << std::endl;
+    std::cout << "sqrt(0)=" << sqrt(0) << " vs " << sqrt2(0) << std::endl;
+    std::cout << "sqrt(1)=" << sqrt(1) << " vs " << sqrt2(1) << std::endl;
+    std::cout << "sqrt(2)=" << sqrt(2) << " vs " << sqrt2(2) << std::endl;
+    std::cout << "sqrt(3)=" << sqrt(3) << " vs " << sqrt2(3) << std::endl;
+    std::cout << "sqrt(4)=" << sqrt(4) << " vs " << sqrt2(4) << std::endl;
+    std::cout << "sqrt(5)=" << sqrt(5) << " vs " << sqrt2(5) << std::endl;
+    std::cout << "sqrt(6)=" << sqrt(6) << " vs " << sqrt2(6) << std::endl;
+    std::cout << "sqrt(9)=" << sqrt(9) << " vs " << sqrt2(9) << std::endl;
+    std::cout << "sqrt(16)=" << sqrt(16) << " vs " << sqrt2(16) << std::endl;
+    std::cout << "sqrt(24)=" << sqrt(24) << " vs " << sqrt2(24) << std::endl;
+    std::cout << "sqrt(25)=" << sqrt(25) << " vs " << sqrt2(25) << std::endl;
+    std::cout << "sqrt(26)=" << sqrt(26) << " vs " << sqrt2(26) << std::endl;
+    std::cout << "sqrt(36)=" << sqrt(36) << " vs " << sqrt2(36) << std::endl;
+
+    //......
+
+    auto end = std::chrono::high_resolution_clock::now();
+    std::chrono::duration<double> seconds = end - begin;
+    std::cout << "Time: " << seconds.count() << std::endl;
+    return 0;
+}
+catch ( const std::exception& e )
+{
+    std::cerr << std::endl
+            << "std::exception(\"" << e.what() << "\")." << std::endl;
+    return 2;
+}
+catch ( ... )
+{
+    std::cerr  << std::endl
+            << "unknown exception." << std::endl;
+    return 1;
+}}
+