Add comments in decompression.jl

src/decompression.jl

@@ -299,30 +299,37 @@ end
 function decompress_aux!(
     A::AbstractMatrix{R}, B::AbstractMatrix{R}, result::TreeSetColoringResult
 ) where {R<:Real}
+    n = checksquare(A)
     A .= zero(R)
     S = get_matrix(result)
     color = column_colors(result)
-    forest = result.tree_set.forest
 
+    # forest is a structure DisjointSets from DataStructures.jl
+    # - forest.intmap: a dictionary that maps an edge (i, j) to an integer k
+    # - forest.revmap: a dictionary that does the reverse of intmap, mapping an integer k to an edge (i, j)
+    # - forest.internal.ngroups: the number of trees in the forest
+    forest = result.tree_set.forest
     ntrees = forest.internal.ngroups
-    for edge in forest.revmap
-        # ensure that all paths are compressed
-        find_root!(forest, edge)
-    end
-    roots = forest.internal.parents
-    unique_roots = unique(roots)
 
+    # vector of trees where each tree contains the indices of its edges
     trees = [Int[] for i in 1:ntrees]
+
+    # dictionary that maps a tree's root to the index of the tree
+    roots = Dict{Int, Int}()
+
     k = 0
-    for root in unique_roots
-        k += 1
-        for (pos, val) in enumerate(roots)
-            if root == val
-                push!(trees[k], pos)
-            end
+    for edge in forest.revmap
+        root_edge = find_root!(forest, edge)
+        root = forest.intmap[root_edge]
+        if !haskey(roots, root)
+            k += 1
+            roots[root] = k
         end
+        index_tree = roots[root]
+        push!(trees[index_tree], forest.intmap[edge])
     end
 
+    # vector of dictionaries where each dictionary stores the degree of each vertex in a tree
     degrees = [Dict{Int,Int}() for k in 1:ntrees]
     for k in 1:ntrees
         tree = trees[k]
@@ -336,7 +343,8 @@ function decompress_aux!(
         end
     end
 
-    n = checksquare(A)
+    # stored_values holds the sum of edge values for subtrees in a tree.
+    # For each vertex i, stored_values[i] is the sum of edge values in the subtree rooted at i.
     stored_values = Vector{R}(undef, n)
 
     # Recover the diagonal coefficients of A