From f9867c2281218340b0962ddce257f3c8a1eb5456 Mon Sep 17 00:00:00 2001
From: John Wellbelove <john.wellbelove@etlcpp.com>
Date: Mon, 13 May 2024 09:07:30 +0100
Subject: [PATCH] Added specialisations of etl::rotate for different iterator
 types

---
 include/etl/algorithm.h | 106 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 97 insertions(+), 9 deletions(-)

diff --git a/include/etl/algorithm.h b/include/etl/algorithm.h
index 43ec1668..ff785df4 100644
--- a/include/etl/algorithm.h
+++ b/include/etl/algorithm.h
@@ -43,6 +43,7 @@ SOFTWARE.
 #include "iterator.h"
 #include "functional.h"
 #include "utility.h"
+#include "gcd.h"
 
 #include <stdint.h>
 #include <string.h>
@@ -1157,20 +1158,94 @@ namespace etl
   namespace private_algorithm
   {
     //*********************************
+    // For random access iterators
     template <typename TIterator>
     ETL_CONSTEXPR14
-    TIterator rotate_general(TIterator first, TIterator middle, TIterator last)
+    typename etl::enable_if<etl::is_random_access_iterator<TIterator>::value, TIterator>::type
+      rotate_general(TIterator first, TIterator middle, TIterator last) 
     {
+      if (first == middle || middle == last)
+      {
+        return first;
+      }
+
+      int n = last - first;
+      int m = middle - first;
+      int gcd_nm = etl::gcd(n, m);
+
+      TIterator result = first + (last - middle);
+
+      for (int i = 0; i < gcd_nm; i++) 
+      {
+        auto temp = *(first + i);
+        int j = i;
+        
+        while (true) 
+        {
+          int k = j + m;
+          
+          if (k >= n)
+          {
+            k = k - n;
+          }
+          
+          if (k == i)
+          {
+            break;
+          }
+
+          *(first + j) = *(first + k);
+          j = k;
+        }
+
+        *(first + j) = temp;
+      }
+
+      return result;
+    }
+
+    //*********************************
+    // For bidirectional iterators
+    template <typename TIterator>
+    ETL_CONSTEXPR14
+    typename etl::enable_if<etl::is_bidirectional_iterator<TIterator>::value, TIterator>::type
+      rotate_general(TIterator first, TIterator middle, TIterator last)
+    {
+      if (first == middle || middle == last)
+      {
+        return first;
+      }
+
+      TIterator result = first;
+      etl::advance(result, etl::distance(middle, last));
+
+      reverse(first, middle);
+      reverse(middle, last);
+      reverse(first, last);
+
+      return result;
+    }
+
+    //*********************************
+    // For forward iterators
+    template <typename TIterator>
+    ETL_CONSTEXPR14
+    typename etl::enable_if<etl::is_forward_iterator<TIterator>::value, TIterator>::type
+      rotate_general(TIterator first, TIterator middle, TIterator last)
+    {
+      if (first == middle || middle == last)
+      {
+        return first;
+      }
+
       TIterator next = middle;
+      TIterator result = first;
+      std::advance(result, std::distance(middle, last));
 
       while (first != next)
       {
-        using ETL_OR_STD::swap; // Allow ADL
-
-        swap(*first, *next);
-
-        ++first;
-        ++next;
+        using ETL_OR_STD::swap;
+        swap(*first++, *next++);
 
         if (next == last)
         {
@@ -1182,10 +1257,11 @@ namespace etl
         }
       }
 
-      return first;
+      return result;
     }
 
     //*********************************
+    // Simplified algorithm for rotate left by one
     template <typename TIterator>
     ETL_CONSTEXPR14
     TIterator rotate_left_by_one(TIterator first, TIterator last)
@@ -1206,6 +1282,7 @@ namespace etl
     }
 
     //*********************************
+    // Simplified for algorithm rotate right by one
     template <typename TIterator>
     ETL_CONSTEXPR14
     TIterator rotate_right_by_one(TIterator first, TIterator last)
@@ -1239,7 +1316,18 @@ namespace etl
 
     if (etl::next(middle) == last)
     {
-      return private_algorithm::rotate_right_by_one(first, last);
+#if ETL_USING_CPP20
+      if ETL_IF_CONSTEXPR(etl::is_forward_iterator<TIterator>::value)
+      {
+        return private_algorithm::rotate_general(first, middle, last);
+      }
+      else
+      {
+        return private_algorithm::rotate_right_by_one(first, last);
+      }
+#else
+      return private_algorithm::rotate_general(first, middle, last);
+#endif
     }
 
     return private_algorithm::rotate_general(first, middle, last);