Further accelerate fast stack method

src/preprocessing.jl

@@ -1,22 +1,25 @@
 function _fast_stack_sparse(vecs::Vector{SparseVector{T1, T2}}) where {T1 <: Real, T2 <: Integer}
     """Fast method for stacking sparse columns"""
-    n_rows = length(vecs[1])
-    @assert all(length(x) == n_rows for x in vecs)
+    n_rows_total = length(vecs[1])
+    @assert all(length(x) == n_rows_total for x in vecs)
+    nnz_total = sum(nnz(x) for x in vecs)
+    rids, cids, nzvals = zeros(T2, nnz_total), zeros(T2, nnz_total), zeros(T1, nnz_total)
 
-    rids, cids, nzvals = Int[], Int[], T1[]
-
-    for (col_i, v) in enumerate(vecs)
+    nnz_i = 1
+    @inbounds for (col_i, v) in enumerate(vecs)
         n_val = nnz(v)
 
         if n_val > 0
-            append!(rids, rowvals(v))
-            append!(cids, repeat([col_i], n_val))
-            append!(nzvals, nonzeros(v))
+            nnz_range = nnz_i:nnz_i+n_val-1
+            rids[nnz_range] .= rowvals(v)
+            cids[nnz_range] .= col_i
+            nzvals[nnz_range] .= nonzeros(v)
+            nnz_i += n_val 
         end
     end
-
+    
     n_cols = length(vecs)
-    return sparse(rids, cids, nzvals, n_rows, n_cols)
+    return sparse(rids, cids, nzvals, n_rows_total, n_cols)
 end
 
 function stack_or_hcat(vecs::AbstractVector{<:AbstractArray})