Calculate chainidxs inside the loop

src/sample.jl

@@ -397,17 +397,9 @@ function mcmcsample(
     models = [deepcopy(model) for _ in interval]
     samplers = [deepcopy(sampler) for _ in interval]
 
-    # Distribute chains amongst the chunks. If nchains/nchunks = m with
-    # remainder n, then the first n chunks will have m + 1 chains, and the rest
-    # will have m chains.
+    # If nchains/nchunks = m with remainder n, then the first n chunks will
+    # have m + 1 chains, and the rest will have m chains.
     m, n = divrem(nchains, nchunks)
-    chain_index_groups = UnitRange{Int}[]
-    current_index = 1
-    for i in interval
-        nchains_this_chunk = i <= n ? m + 1 : m
-        push!(chain_index_groups, current_index:(current_index + nchains_this_chunk - 1))
-        current_index += nchains_this_chunk
-    end
 
     # Create a seed for each chain using the provided random number generator.
     seeds = rand(rng, UInt, nchains)
@@ -447,8 +439,18 @@ function mcmcsample(
 
             Distributed.@async begin
                 try
-                    Distributed.@sync for (chainidxs, _rng, _model, _sampler) in
-                                          zip(chain_index_groups, rngs, models, samplers)
+                    Distributed.@sync for (i, _rng, _model, _sampler) in
+                                          zip(interval, rngs, models, samplers)
+                        if i <= n
+                            chainidx_hi = i * (m + 1)
+                            nchains_chunk = m + 1
+                        else
+                            chainidx_hi = n * (m + 1) + (i - n) * m
+                            nchains_chunk = m
+                        end
+                        chainidx_lo = chainidx_hi - nchains_chunk + 1
+                        chainidxs = chainidx_lo:chainidx_hi
+
                         Threads.@spawn for chainidx in chainidxs
                             # Seed the chunk-specific random number generator with the pre-made seed.
                             Random.seed!(_rng, seeds[chainidx])