Finished create with range implementation

thorsten/ExaArray#5 (comment)

ExaArray/ExaArray1D.Factories.cs

@@ -32,14 +32,33 @@ namespace Exa
             return next;
         }
 
+        /// <summary>
+        /// Creates a new ExaArray1D from another, respecting the given range.
+        /// </summary>
+        /// <remarks>
+        /// When <c>T</c> is a value type, data gets copied as values. When <c>T</c> is a reference type, the pointers
+        /// to the original objects are copied. Thus, this factory method does not create a deep copy.
+        ///
+        /// Performance: O(n)
+        ///
+        /// The indices are inclusive.
+        /// </remarks>
+        /// <param name="other">The instance from which the new instance is to be created.</param>
+        /// <param name="indexFrom">The first source element which should be part of the new array.</param>
+        /// <param name="indexTo">The last source element which should be part of the new array.</param>
+        /// <returns>The new instance</returns>
+        /// <exception cref="IndexOutOfRangeException">Throws, when one or both of the indices are out of range.</exception>
         public static ExaArray1D<T> CreateFrom(ExaArray1D<T> other, ulong indexFrom, ulong indexTo)
         {
             if (indexTo < indexFrom)
                 throw new IndexOutOfRangeException("Index to must be greater than index from.");
             
             if (indexTo >= other.Length || indexFrom >= other.Length)
-                throw new IndexOutOfRangeException("Index to must be greater than index from.");
+                throw new IndexOutOfRangeException("Index to must be within the range of the source.");
             
+            //
+            // Determine the source start chunk and index.
+            //
             int sourceChunkIndexTo = -1;
             int sourceElementIndexTo = -1;
             switch (other.OptimizationStrategy)
@@ -54,6 +73,9 @@ namespace Exa
                     break;
             }
             
+            //
+            // Determine the source end chunk and index.
+            //
             int sourceChunkIndexFrom = -1;
             int sourceElementIndexFrom = -1;
             switch (other.OptimizationStrategy)
@@ -68,7 +90,12 @@ namespace Exa
                     break;
             }
             
+            // How many element we have to copy?
             ulong newRange = indexTo - indexFrom + 1;
+            
+            //
+            // Determine, how many total chunks we need for the copy.
+            //
             int destChunks = -1;
             switch (other.OptimizationStrategy)
             {
@@ -80,20 +107,29 @@ namespace Exa
                     break;
             }
 
+            // Create the copy and allocate the needed number of outer chunk.
             var next = new ExaArray1D<T>(other.OptimizationStrategy)
             {
                 Length = newRange,
                 chunks = new T[destChunks][],
             };
-
-            int sourceChunkIndex = sourceChunkIndexFrom;
-            int destinChunkIndex = 0;
-            int sourceElementIndex = sourceElementIndexFrom;
-            int destinElementIndex = 0;
-            ulong leftOver = newRange;
             
+            //
+            // Variables for the copy process.
+            //
+            
+            int sourceChunkIndex = sourceChunkIndexFrom;
+            int destinationChunkIndex = 0;
+            int sourceElementIndex = sourceElementIndexFrom;
+            int destinationElementIndex = 0;
+            ulong leftOverTotal = newRange;
+
             do
             {
+                //
+                // Determine how many elements we copy next.
+                //
+
                 uint numberToCopy = 0;
                 
                 // Case: small number of elements from first chunk only
@@ -105,36 +141,47 @@ namespace Exa
                     numberToCopy = (uint) (other.maxArrayCapacity - sourceElementIndex);
                     
                 // Case: multiple chunks + we are in the middle of huge copy process + next chunk does __not__ exist
-                else if (next.chunks[destinChunkIndex] == null && sourceElementIndex == 0 && leftOver >= other.maxArrayCapacity)
+                else if (next.chunks[destinationChunkIndex] == null && sourceElementIndex == 0 && leftOverTotal >= other.maxArrayCapacity)
                     numberToCopy = other.maxArrayCapacity;
                 
                 // Case: multiple chunks + we are in the middle of huge copy process + next chunk does exist
-                else if (next.chunks[destinChunkIndex] != null && sourceElementIndex == 0 && leftOver >= other.maxArrayCapacity)
+                else if (next.chunks[destinationChunkIndex] != null && sourceElementIndex == 0 && leftOverTotal >= other.maxArrayCapacity)
-                    numberToCopy = (uint) (other.maxArrayCapacity - next.chunks[destinChunkIndex].Length);
+                    numberToCopy = (uint) (other.maxArrayCapacity - next.chunks[destinationChunkIndex].Length);
                 
                 // Case: multiple chunks + this seems to be the last chunk
-                else if (sourceElementIndex == 0 && leftOver < other.maxArrayCapacity)
+                else if (sourceElementIndex == 0 && leftOverTotal < other.maxArrayCapacity)
-                    numberToCopy = (uint) leftOver;
+                    numberToCopy = (uint) leftOverTotal;
                 
-                if(next.chunks[destinChunkIndex] == null)
+                //
-                    next.chunks[destinChunkIndex] = new T[numberToCopy];
+                // Can we allocate an entire chunk or do we have to fill up the existing
+                // chunk first?
+                //
+                if(next.chunks[destinationChunkIndex] == null)
+                    next.chunks[destinationChunkIndex] = new T[numberToCopy];
                 else
                 {
-                    var extended = new T[next.chunks[destinChunkIndex].Length + numberToCopy];
+                    var extended = new T[next.chunks[destinationChunkIndex].Length + numberToCopy];
-                    Array.Copy(next.chunks[destinChunkIndex], extended, next.chunks[destinChunkIndex].Length);
+                    Array.Copy(next.chunks[destinationChunkIndex], extended, next.chunks[destinationChunkIndex].Length);
-                    next.chunks[destinChunkIndex] = extended;
+                    next.chunks[destinationChunkIndex] = extended;
                 }
                 
-                Array.Copy(other.chunks[sourceChunkIndex], sourceElementIndex, next.chunks[destinChunkIndex], destinElementIndex, numberToCopy);
+                // Copy the data:
+                Array.Copy(other.chunks[sourceChunkIndex], sourceElementIndex, next.chunks[destinationChunkIndex], destinationElementIndex, numberToCopy);
 
-                var needNewChunk = next.chunks[destinChunkIndex].Length == next.maxArrayCapacity;
+                //
-                leftOver -= numberToCopy;
+                // Update the state machine.
-                sourceChunkIndex++;
+                //
-                destinChunkIndex = needNewChunk ? destinChunkIndex + 1 : destinChunkIndex;
+                var needNewDestinationChunk = next.chunks[destinationChunkIndex].Length == next.maxArrayCapacity;
-                sourceElementIndex = 0;
+                var readNextSourceChunk = sourceElementIndex + numberToCopy == other.maxArrayCapacity;
-                destinElementIndex = (int) (needNewChunk ? 0 : numberToCopy);
+                var leftOverCurrentSourceChunk = (int) (readNextSourceChunk ? 0 : other.maxArrayCapacity - sourceElementIndex - numberToCopy);
+                
+                leftOverTotal -= numberToCopy;
+                sourceChunkIndex = readNextSourceChunk ? sourceChunkIndex + 1 : sourceChunkIndex;
+                destinationChunkIndex = needNewDestinationChunk ? destinationChunkIndex + 1 : destinationChunkIndex;
+                sourceElementIndex = leftOverCurrentSourceChunk;
+                destinationElementIndex = (int) (needNewDestinationChunk ? 0 : numberToCopy);
 
-            } while (leftOver > 0);
+            } while (leftOverTotal > 0);
             return next;
         }
     }