theory Lsp
imports Main
begin

fun lsp :: "('a \<Rightarrow> ('b::linorder)) \<Rightarrow> 'a list \<Rightarrow> ('a list \<times> 'a list \<times> 'a list)"
where 
   "lsp f [] = ([], [], [])" |
   "lsp f [x] = ([], [x], [])" |
   "lsp f (x#xs) = (case (lsp f xs) of
                     (l, [], r) \<Rightarrow> ([], [x], []) |
                     (l, y#ys, r) \<Rightarrow> if f x \<ge> f y then ([], [x], xs) else (x#l, y#ys, r))"

inductive lsp_p :: "('a \<Rightarrow> ('b::linorder)) \<Rightarrow> 'a list \<Rightarrow> ('a list \<times> 'a list \<times> 'a list) \<Rightarrow> bool"
for f :: "('a \<Rightarrow> ('b::linorder))"
where
  lsp_nil [intro]: "lsp_p f [] ([], [], [])" |
  lsp_single [intro]: "lsp_p f [x] ([], [x], [])" |
  lsp_cons_1 [intro]: "\<lbrakk>xs \<noteq> []; lsp_p f xs (l, [m], r); f x \<ge> f m\<rbrakk> \<Longrightarrow> lsp_p f (x#xs) ([], [x], xs)" |
  lsp_cons_2 [intro]: "\<lbrakk>xs \<noteq> []; lsp_p f xs (l, [m], r); f x < f m\<rbrakk> \<Longrightarrow> lsp_p f (x#xs) (x#l, [m], r)"

lemma lsp_p_lsp_1: "lsp_p f x y \<Longrightarrow> y = lsp f x"
proof (induct rule:lsp_p.induct)
  case (lsp_cons_1 xs  l m r x)
  assume lsp_xs [symmetric]: "(l, [m], r) = lsp f xs"
    and le_mx: "f m \<le> f x"
  show ?case (is "?L = ?R")
  proof(cases xs, simp)
    case (Cons v vs)
    show ?thesis
      apply (simp add:Cons)
      apply (fold Cons)
      by (simp add:lsp_xs le_mx)
  qed
next
  case (lsp_cons_2 xs l m r x)
  assume lsp_xs [symmetric]: "(l, [m], r) = lsp f xs"
    and lt_xm: "f x < f m"
  show ?case (is "?L = ?R")
  proof(cases xs)
    case (Cons v vs)
    show ?thesis
      apply (simp add:Cons)
      apply (fold Cons)
      apply (simp add:lsp_xs)
      by (insert lt_xm, auto)
  next
    case Nil
    from prems show ?thesis by simp
  qed
qed auto

lemma lsp_mid_nil: "lsp f xs = (a, [], c) \<Longrightarrow> xs = []"
  apply (induct xs arbitrary:a c, auto)
  apply (case_tac xs, auto)
  by (case_tac "(lsp f (ab # list))", auto split:if_splits list.splits)


lemma lsp_mid_length: "lsp f x = (u, v, w) \<Longrightarrow> length v \<le> 1"
proof(induct x arbitrary:u v w, simp)
  case (Cons x xs)
  assume ih: "\<And> u v w. lsp f xs = (u, v, w) \<Longrightarrow> length v \<le> 1"
  and h: "lsp f (x # xs) = (u, v, w)"
  show "length v \<le> 1" using h
  proof(cases xs, simp add:h)
    case (Cons z zs)
    assume eq_xs: "xs = z # zs"
    show ?thesis
    proof(cases "lsp f xs")
      fix l m r
      assume eq_lsp: "lsp f xs = (l, m, r)"
      show ?thesis
      proof(cases m)
        case Nil
        from Nil and eq_lsp have "lsp f xs = (l, [], r)" by simp
        from lsp_mid_nil [OF this] have "xs = []" .
        with h show ?thesis by auto
      next
        case (Cons y ys)
        assume eq_m: "m = y # ys"
        from ih [OF eq_lsp] have eq_xs_1: "length m \<le> 1" .
        show ?thesis
        proof(cases "f x \<ge> f y")
          case True
          from eq_xs eq_xs_1 True h eq_lsp show ?thesis 
            by (auto split:list.splits if_splits)
        next
          case False
          from eq_xs eq_xs_1 False h eq_lsp show ?thesis 
             by (auto split:list.splits if_splits)
        qed
      qed
    qed
  next
    assume "[] = u \<and> [x] = v \<and> [] = w"
    hence "v = [x]" by simp
    thus "length v \<le> Suc 0" by simp
  qed
qed

lemma lsp_p_lsp_2: "lsp_p f x (lsp f x)"
proof(induct x, auto)
  case (Cons x xs)
  assume ih: "lsp_p f xs (lsp f xs)"
  show ?case
  proof(cases xs)
    case Nil
    thus ?thesis by auto
  next
    case (Cons v vs)
    show ?thesis
    proof(cases "xs")
      case Nil
      thus ?thesis by auto
    next
      case (Cons v vs)
      assume eq_xs: "xs = v # vs"
      show ?thesis
      proof(cases "lsp f xs")
        fix l m r
        assume eq_lsp_xs: "lsp f xs = (l, m, r)"
        show ?thesis
        proof(cases m)
          case Nil
          from eq_lsp_xs and Nil have "lsp f xs = (l, [], r)" by simp
          from lsp_mid_nil [OF this] have eq_xs: "xs = []" .
          hence "lsp f (x#xs) = ([], [x], [])" by simp
          with eq_xs show ?thesis by auto
        next
          case (Cons y ys)
          assume eq_m: "m = y # ys"
          show ?thesis
          proof(cases "f x \<ge> f y")
            case True
            from eq_xs eq_lsp_xs Cons True
            have eq_lsp: "lsp f (x#xs) = ([], [x], v # vs)" by simp
            show ?thesis
            proof (simp add:eq_lsp)
              show "lsp_p f (x # xs) ([], [x], v # vs)"
              proof(fold eq_xs, rule lsp_cons_1 [OF _])
                from eq_xs show "xs \<noteq> []" by simp
              next
                from lsp_mid_length [OF eq_lsp_xs] and Cons
                have "m = [y]" by simp
                with eq_lsp_xs have "lsp f xs = (l, [y], r)" by simp
                with ih show "lsp_p f xs (l, [y], r)" by simp
              next
                from True show "f y \<le> f x" by simp
              qed
            qed
          next
            case False
            from eq_xs eq_lsp_xs Cons False
            have eq_lsp: "lsp f (x#xs) = (x # l, y # ys, r) " by simp
            show ?thesis
            proof (simp add:eq_lsp)
              from lsp_mid_length [OF eq_lsp_xs] and eq_m
              have "ys = []" by simp
              moreover have "lsp_p f (x # xs) (x # l, [y], r)"
              proof(rule lsp_cons_2)
                from eq_xs show "xs \<noteq> []" by simp
              next
                from lsp_mid_length [OF eq_lsp_xs] and Cons
                have "m = [y]" by simp
                with eq_lsp_xs have "lsp f xs = (l, [y], r)" by simp
                with ih show "lsp_p f xs (l, [y], r)" by simp
              next
                from False show "f x < f y" by simp
              qed
              ultimately show "lsp_p f (x # xs) (x # l, y # ys, r)" by simp
            qed
          qed
        qed
      qed
    qed
  qed
qed

lemma lsp_induct:
  fixes f x1 x2 P
  assumes h: "lsp f x1 = x2"
  and p1: "P [] ([], [], [])"
  and p2: "\<And>x. P [x] ([], [x], [])"
  and p3: "\<And>xs l m r x. \<lbrakk>xs \<noteq> []; lsp f xs = (l, [m], r); P xs (l, [m], r); f m \<le> f x\<rbrakk> \<Longrightarrow> P (x # xs) ([], [x], xs)"
  and p4: "\<And>xs l m r x. \<lbrakk>xs \<noteq> []; lsp f xs = (l, [m], r); P xs (l, [m], r); f x < f m\<rbrakk> \<Longrightarrow> P (x # xs) (x # l, [m], r)"
  shows "P x1 x2"
proof(rule lsp_p.induct)
  from lsp_p_lsp_2 and h
  show "lsp_p f x1 x2" by metis
next
  from p1 show "P [] ([], [], [])" by metis
next
  from p2 show "\<And>x. P [x] ([], [x], [])" by metis
next
  fix xs l m r x 
  assume h1: "xs \<noteq> []" and h2: "lsp_p f xs (l, [m], r)" and h3: "P xs (l, [m], r)" and h4: "f m \<le> f x"
  show "P (x # xs) ([], [x], xs)" 
  proof(rule p3 [OF h1 _ h3 h4])
    from h2 and lsp_p_lsp_1 show "lsp f xs = (l, [m], r)" by metis
  qed
next
  fix xs l m r x 
  assume h1: "xs \<noteq> []" and h2: "lsp_p f xs (l, [m], r)" and h3: "P xs (l, [m], r)" and h4: "f x < f m"
  show "P (x # xs) (x # l, [m], r)"
  proof(rule p4 [OF h1 _ h3 h4])
    from h2 and lsp_p_lsp_1 show "lsp f xs = (l, [m], r)" by metis
  qed
qed

lemma lsp_set_eq: 
  fixes f x u v w
  assumes h: "lsp f x = (u, v, w)"
  shows "x = u@v@w"
proof -
  have "\<And> f x r. lsp f x = r \<Longrightarrow> \<forall> u v w. (r = (u, v, w) \<longrightarrow> x = u@v@w)" 
    by (erule lsp_induct, simp+)
  from this [rule_format, OF h] show ?thesis by simp
qed

lemma lsp_set: 
  assumes h: "(u, v, w) = lsp f x"
  shows "set (u@v@w) = set x"
proof -
  from lsp_set_eq [OF h[symmetric]] 
  show ?thesis by simp
qed

lemma max_insert_gt:
  fixes S fx
  assumes h: "fx < Max S"
  and np: "S \<noteq> {}"
  and fn: "finite S" 
  shows "Max S = Max (insert fx S)"
proof -
  from Max_insert [OF fn np]
  have "Max (insert fx S) = max fx (Max S)" .
  moreover have "\<dots> = Max S"
  proof(cases "fx \<le> Max S")
    case False
    with h
    show ?thesis by (simp add:max_def)
  next
    case True
    thus ?thesis by (simp add:max_def)
  qed
  ultimately show ?thesis by simp
qed

lemma max_insert_le: 
  fixes S fx
  assumes h: "Max S \<le> fx"
  and fn: "finite S"
  shows "fx = Max (insert fx S)"
proof(cases "S = {}")
  case True
  thus ?thesis by simp
next
  case False
  from Max_insert [OF fn False]
  have "Max (insert fx S) = max fx (Max S)" .
  moreover have "\<dots> = fx"
  proof(cases "fx \<le> Max S")
    case False
    thus ?thesis by (simp add:max_def)
  next
    case True
    have hh: "\<And> x y. \<lbrakk> x \<le> (y::('a::linorder)); y \<le> x\<rbrakk> \<Longrightarrow> x = y" by auto
    from hh [OF True h]
    have "fx = Max S" .
    thus ?thesis by simp
  qed
  ultimately show ?thesis by simp
qed
  
lemma lsp_max: 
  fixes f x u m w
  assumes h: "lsp f x = (u, [m], w)"
  shows "f m = Max (f ` (set x))"
proof -
  { fix y
    have "lsp f x = y \<Longrightarrow> \<forall> u m w. y = (u, [m], w) \<longrightarrow> f m = Max (f ` (set x))"
    proof(erule lsp_induct, simp)
      { fix x u m w
        assume "(([]::'a list), ([x]::'a list), ([]::'a list)) = (u, [m], w)"
        hence "f m = Max (f ` set [x])"  by simp
      } thus "\<And>x. \<forall>u m w. ([], [x], []) = (u, [m], w) \<longrightarrow> f m = Max (f ` set [x])" by simp
    next
      fix xs l m r x
      assume h1: "xs \<noteq> []"
        and h2: " lsp f xs = (l, [m], r)"
        and h3: "\<forall>u ma w. (l, [m], r) = (u, [ma], w) \<longrightarrow> f ma = Max (f ` set xs)"
        and h4: "f m \<le> f x"
      show " \<forall>u m w. ([], [x], xs) = (u, [m], w) \<longrightarrow> f m = Max (f ` set (x # xs))"
      proof -
        have "f x = Max (f ` set (x # xs))"
        proof -
          from h2 h3 have "f m = Max (f ` set xs)" by simp
          with h4 show ?thesis
            apply auto
            by (rule_tac max_insert_le, auto)
        qed
        thus ?thesis by simp
      qed
    next
      fix xs l m r x
      assume h1: "xs \<noteq> []"
        and h2: " lsp f xs = (l, [m], r)"
        and h3: " \<forall>u ma w. (l, [m], r) = (u, [ma], w) \<longrightarrow> f ma = Max (f ` set xs)"
        and h4: "f x < f m"
      show "\<forall>u ma w. (x # l, [m], r) = (u, [ma], w) \<longrightarrow> f ma = Max (f ` set (x # xs))"
      proof -
        from h2 h3 have "f m = Max (f ` set xs)" by simp
        with h4
        have "f m =  Max (f ` set (x # xs))"
          apply auto
          apply (rule_tac max_insert_gt, simp+)
          by (insert h1, simp+)
        thus ?thesis by auto
      qed
    qed
  } with h show ?thesis by metis
qed

end