From 6744c38f0df5adea467f9f58ee133c832c034715 Mon Sep 17 00:00:00 2001
From: Vahagn Khachatryan <vahagn.khachatryan@gmail.com>
Date: Sat, 29 Aug 2020 22:18:30 +0100
Subject: [PATCH] permutation.cpp

---
 puzzles/training/permutation.cpp | 70 ++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 puzzles/training/permutation.cpp

diff --git a/puzzles/training/permutation.cpp b/puzzles/training/permutation.cpp
new file mode 100644
index 0000000..8d3f245
--- /dev/null
+++ b/puzzles/training/permutation.cpp
@@ -0,0 +1,70 @@
+/*
+VIM: let b:cf5build="clang++ -std=c++20 -O2 -pthread -I. {SRC} -o {OUT}"
+VIM: let b:cf5run="{OUT}"
+*/
+#include <iostream>
+#include <exception>
+#include <chrono>
+
+void permutate(auto b, auto e)
+{
+    auto bt = std::reverse_iterator(e);
+    auto et = std::reverse_iterator(b);
+
+    auto it = bt+1;
+    for ( ; it != et && *it >= *(it-1); ++it);
+
+    for (auto qt = bt, rt = it-1; rt > qt; qt++, rt--){
+        std::swap(*qt, *rt);
+    }
+
+    if (it != et){
+        auto pt = it-1;
+        for( ; pt != bt && *it >= *pt; --pt);
+        std::swap(*it, *pt);
+    }
+}
+
+void test(std::string char_set)
+{
+    std::sort(char_set.begin(), char_set.end());
+    std::string perm = char_set;
+    int i = 0;
+    do {
+        ++i;
+        std::cout << perm << '\n';
+        permutate(perm.begin(), perm.end());
+    } while (perm != char_set);
+
+    std::cout << i << '\n' << "-------------------------------------\n";
+}
+
+int main ( void )
+{try{
+    auto begin = std::chrono::high_resolution_clock::now();
+
+    test( "abcd" );
+    test( "aab" );
+    test( "aabb" );
+    test( "aaabbb" );
+
+    //......
+
+    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;
+}}
+