/// <summary>
/// Join a sequence of arrays along an existing axis.
/// </summary>
/// <param name="axis">The axis along which the arrays will be joined. If axis is None, arrays are flattened before use. Default is 0.</param>
/// <param name="arrays">The arrays must have the same shape, except in the dimension corresponding to axis (the first, by default).</param>
/// <returns>The concatenated array.</returns>
/// <remarks>https://docs.scipy.org/doc/numpy/reference/generated/numpy.concatenate.html</remarks>
public static NDArray concatenate(NDArray[] arrays, int axis = 0)
{
//What we do is we have the axis which is the only dimension that is allowed to be different
//We need to perform a check if the dimensions actually match.
//After we have the axis ax=1 where shape is (3,ax,3) - ax is the only dimension that can vary.
//So if we got input of (3,5,3) and (3,1,3), we create a return shape of (3,6,3).
//We perform the assignment by iterating a slice: (:,n,:) on src and dst where dst while n of dst grows as we iterate all arrays.
if (arrays == null)
{
throw new ArgumentNullException(nameof(arrays));
}
if (arrays.Length == 0)
{
throw new ArgumentException("Value cannot be an empty collection.", nameof(arrays));
}
if (arrays.Length == 1)
{
return(arrays[0]);
}
var first = arrays[0];
var firstShape = (int[])first.shape.Clone();
while (axis < 0)
{
axis = first.ndim + axis; //translate negative axis
}
int i, j;
int axisSize = 0; //accumulated shape[axis] size for return shape.
NPTypeCode retType = first.GetTypeCode;
foreach (var src in arrays)
{
//accumulate the concatenated axis
var shape = src.shape;
axisSize += shape[axis];
if (ReferenceEquals(src, first))
{
continue;
}
var srcType = src.GetTypeCode;
//resolve what the return type should be and should we perform casting.
if (first.GetTypeCode != srcType)
{
if (srcType.CompareTo(retType) == 1)
{
retType = srcType;
}
}
if (shape.Length != first.ndim)
{
throw new IncorrectShapeException("all the input arrays must have same number of dimensions.");
}
//verify the shapes are equal
for (j = 0; j < shape.Length; j++)
{
if (axis == j)
{
continue;
}
if (shape[j] != firstShape[j])
{
throw new IncorrectShapeException("all the input array dimensions except for the concatenation axis must match exactly.");
}
}
}
//prepare return shape
firstShape[axis] = axisSize;
var retShape = new Shape(firstShape);
var dst = new NDArray(retType, retShape);
var accessorDst = new Slice[retShape.NDim];
var accessorSrc = new Slice[retShape.NDim];
for (i = 0; i < accessorDst.Length; i++)
{
accessorSrc[i] = accessorDst[i] = Slice.All;
}
accessorSrc[axis] = Slice.Index(0);
accessorDst[axis] = Slice.Index(0);
foreach (var src in arrays)
{
var len = src.shape[axis];
for (i = 0; i < len; i++)
{
var writeTo = dst[accessorDst];
var writeFrom = src[accessorSrc];
MultiIterator.Assign(writeTo.Storage, writeFrom.Storage);
accessorSrc[axis]++;
accessorDst[axis]++; //increment every step
}
accessorSrc[axis] = Slice.Index(0); //reset src
}
return(dst);
}
