Sort a dict by its value pairs - sorting

Question

I have a dict which looks like this:
Unsorted:
12 {12 489} 29 {89 12} 27 {301 302} 26 {489 329} 8 {89 302} 55 {44 301}
I would like to sort it like this:
55 {44 301} 27 {301 302} 8 {89 302} 29 {89 12} 12 {12 489} 26 {489 329}
As you can see, most of the time the second key value of the preceding entry is identical to the first key entry of the following entry. ( 12 and 489 in the last two entries)
This although is no requirement. The 302 of the second and third entry also fullfills the requirement of a "chain" as it exists in both the second and the third entry.
The only thing I want to do is sorting these entries in such a way, that the values in braces form a uninterupted chain.
It does not matter if the result looks like in the example or if it is mirrored.
From TCL 8.6 on I could do something similar to Sort Tcl dict by value using stride. But I'm stuck with this (Tcl8.5.9) version. What is the easiest way to this?

Answer 1

I don't know if this is the easiest way:
set x [dict create 12 {12 489} 29 {89 12} 27 {301 302} 26 {489 329} 8 {89 302} 55 {44 301}]
# transform the dict into a list of lists
dict for {k v} $x {lappend unsorted [list $k $v]}
lappend sorted [lindex $unsorted 0]
set unsorted [lrange $unsorted 1 end]
# keep going until there's nothing more to add to the sorted list
while {[llength $unsorted] != 0} {
set changed false
for {set idx 0} {$idx < [llength $unsorted]} {incr idx} {
set elem [lindex $unsorted $idx]
lassign [lindex $elem end] a b
set head [lindex $sorted 0 end]
set tail [lindex $sorted end end]
if {$a in $head || $b in $head} {
set sorted [linsert $sorted 0 $elem]
set changed true
} elseif {$a in $tail || $b in $tail} {
lappend sorted $elem
set changed true
}
if {$changed} {
set unsorted [lreplace $unsorted $idx $idx]
break
}
}
# avoid infinite loop if the unsorted list is not empty, but
# contains nothing to add to the sorted list
if {! $changed} break
}
foreach elem $sorted {dict set y {*}$elem}
puts "Unsorted: $x"
puts "Sorted: $y"
Unsorted: 12 {12 489} 29 {89 12} 27 {301 302} 26 {489 329} 8 {89 302} 55 {44 301}
Sorted: 55 {44 301} 27 {301 302} 8 {89 302} 29 {89 12} 12 {12 489} 26 {489 329}