Commits on Jul 7, 2024

1. Bugfix ChainedVector constructor performance …

   Currently the ChainedVector constructor has bad performance if the args have empty entries.
   
   ```
   import Random
   struct MyType
       x::Float64
       y::Float64
   end
   
   x1 = [ MyType[MyType(rand(), rand()) for _ in 1:Random.rand(1:6)] for _ in 1:1_000_000];
   @time ChainedVector(x1);  # 0.017 secs
   
   x2 = [ MyType[MyType(rand(), rand()) for _ in 1:Random.rand(0:5)] for _ in 1:1_000_000];
   @time ChainedVector(x2);  # 24 secs
   ```
   
   Note that the difference comes only from the fact that x2 might have empty entries
   whereas x1 doesn't.
   
   The problem is that the function cleanup! has runtime proportional to the number of
   elements times the number of elements which are empty vectors.
   
   ```
   function cleanup_original!(arrays, inds)
       @Assert length(arrays) == length(inds)
       for i = length(arrays):-1:1
           if !isassigned(arrays, i) || length(arrays[i]) == 0
               deleteat!(arrays, i)
               deleteat!(inds, i)
           end
       end
       return
   end
   
   function cleanup_new!(arrays, inds)
       @Assert length(arrays) == length(inds)
       mask_it = Base.Iterators.filter(i->isassigned(arrays, i) && length(arrays[i])>0, 1:length(arrays))
       n = 0
       for k in mask_it
           n += 1
           k > n || continue
           arrays[n] = arrays[k]
           inds[n] = inds[k]
       end
       resize!(arrays, n)
       resize!(inds, n)
       return
   end
   ```
   
   We can check that the new version is faster and returns the same result
   
   ```
   x = [ MyType[MyType(rand(), rand()) for _ in 1:Random.rand(0:5)] for _ in 1:1_000_000];
   arrays = x;
   inds = Vector{Int}(undef, length(arrays));
   
   x1 = copy(x);
   inds1 = copy(inds);
   x2 = copy(x);
   inds2 = copy(inds);
   
   @time cleanup_original!(x1, inds1)  # 20 secs
   @time cleanup_new!(x2, inds2)  # 0.04 secs
   
   all(x1 .== x2)  # true
   all(inds1 .== inds2)  # true
   ```

   

   JoaoAparicio authored and quinnj committed Jul 7, 2024
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