(.module:
  lux
  (lux (data [library
              [number]]
             (coll [list "list/" Mix<List> Monoid<List>]
                   ["s" set])))
  (luxc (lang ["la" analysis]
              ["ls" synthesis]
              ["[0]L" variable {"+" [Variable]}])))

(def: (bound-vars path)
  (-> ls.Path (List Variable))
  (case path
    {#ls.BindP register}
    (list (.int register))

    (^or {#ls.SeqP pre post} {#ls.AltP pre post})
    (list/composite (bound-vars pre) (bound-vars post))
    
    _
    (list)))

(def: (path-bodies path)
  (-> ls.Path (List ls.Synthesis))
  (case path
    {#ls.ExecP body}
    (list body)

    {#ls.SeqP pre post}
    (path-bodies post)

    {#ls.AltP pre post}
    (list/composite (path-bodies pre) (path-bodies post))
    
    _
    (list)))

(def: (non-arg? arity var)
  (-> ls.Arity Variable Bit)
  (and (variableL.local? var)
       (n/> arity (.nat var))))

(type: Tracker (s.Set Variable))

(def: init-tracker Tracker (s.new number.Hash<Int>))

(def: (unused-vars current-arity bound exprS)
  (-> ls.Arity (List Variable) ls.Synthesis (List Variable))
  (let [tracker (loop [exprS exprS
                       tracker (list/mix s.has init-tracker bound)]
                  (case exprS
                    {#ls.Variable var}
                    (if (non-arg? current-arity var)
                      (s.lacks var tracker)
                      tracker)
                    
                    {#ls.Variant tag last? memberS}
                    (recur memberS tracker)

                    {#ls.Tuple membersS}
                    (list/mix recur tracker membersS)

                    {#ls.Call funcS argsS}
                    (list/mix recur (recur funcS tracker) argsS)
                    
                    (^or {#ls.Recur argsS}
                         {#ls.Procedure name argsS})
                    (list/mix recur tracker argsS)

                    {#ls.Let offset inputS outputS}
                    (|> tracker (recur inputS) (recur outputS))

                    {#ls.If testS thenS elseS}
                    (|> tracker (recur testS) (recur thenS) (recur elseS))

                    {#ls.Loop offset initsS bodyS}
                    (recur bodyS (list/mix recur tracker initsS))

                    {#ls.Case inputS outputPS}
                    (let [tracker' (list/mix s.has
                                             (recur inputS tracker)
                                             (bound-vars outputPS))]
                      (list/mix recur tracker' (path-bodies outputPS)))

                    {#ls.Function arity env bodyS}
                    (list/mix s.lacks tracker env)

                    _
                    tracker
                    ))]
    (s.to-list tracker)))

... (def: (optimize-register-use current-arity [pathS bodyS])
...   (-> ls.Arity [ls.Path ls.Synthesis] [ls.Path ls.Synthesis])
...   (let [bound (bound-vars pathS)
...         unused (unused-vars current-arity bound bodyS)
...         adjusted (adjust-vars unused bound)]
...     [(|> pathS (clean-pattern adjusted) simplify-pattern)
...      (clean-expression adjusted bodyS)]))