(* Author: Lukas Bulwahn, TU Muenchen *)

 theory Imperative_Quicksort

 imports "~~/src/HOL/Imperative_HOL/Imperative_HOL" Subarray Multiset Efficient_Nat

 begin

 text {* We prove QuickSort correct in the Relational Calculus. *}

 definition swap :: "nat array \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> unit Heap"

 where

   "swap arr i j = (

      do

        x \<leftarrow> nth arr i;

        y \<leftarrow> nth arr j;

        upd i y arr;

        upd j x arr;

        return ()

      done)"

 lemma swap_permutes:

   assumes "crel (swap a i j) h h' rs"

   shows "multiset_of (get_array a h') 

   = multiset_of (get_array a h)"

   using assms

   unfolding swap_def

   by (auto simp add: Heap.length_def multiset_of_swap dest: sym [of _ "h'"] elim!: crelE crel_nth crel_return crel_upd)

 function part1 :: "nat array \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat Heap"

 where

   "part1 a left right p = (

      if (right \<le> left) then return right

      else (do

        v \<leftarrow> nth a left;

        (if (v \<le> p) then (part1 a (left + 1) right p)

                     else (do swap a left right;

   part1 a left (right - 1) p done))

      done))"

 by pat_completeness auto

 termination

 by (relation "measure (\<lambda>(_,l,r,_). r - l )") auto

 declare part1.simps[simp del]

 lemma part_permutes:

   assumes "crel (part1 a l r p) h h' rs"

   shows "multiset_of (get_array a h') 

   = multiset_of (get_array a h)"

   using assms

 proof (induct a l r p arbitrary: h h' rs rule:part1.induct)

   case (1 a l r p h h' rs)

   thus ?case

     unfolding part1.simps [of a l r p]

     by (elim crelE crel_if crel_return crel_nth) (auto simp add: swap_permutes)

 qed

 lemma part_returns_index_in_bounds:

   assumes "crel (part1 a l r p) h h' rs"

   assumes "l \<le> r"

   shows "l \<le> rs \<and> rs \<le> r"

 using assms

 proof (induct a l r p arbitrary: h h' rs rule:part1.induct)

   case (1 a l r p h h' rs)

   note cr = `crel (part1 a l r p) h h' rs`

   show ?case

   proof (cases "r \<le> l")

     case True (* Terminating case *)

     with cr `l \<le> r` show ?thesis

       unfolding part1.simps[of a l r p]

       by (elim crelE crel_if crel_return crel_nth) auto

   next

     case False (* recursive case *)

     note rec_condition = this

     let ?v = "get_array a h ! l"

     show ?thesis

     proof (cases "?v \<le> p")

       case True

       with cr False

       have rec1: "crel (part1 a (l + 1) r p) h h' rs"

         unfolding part1.simps[of a l r p]

         by (elim crelE crel_nth crel_if crel_return) auto

       from rec_condition have "l + 1 \<le> r" by arith

       from 1(1)[OF rec_condition True rec1 `l + 1 \<le> r`]

       show ?thesis by simp

     next

       case False

       with rec_condition cr

       obtain h1 where swp: "crel (swap a l r) h h1 ()"

         and rec2: "crel (part1 a l (r - 1) p) h1 h' rs"

         unfolding part1.simps[of a l r p]

         by (elim crelE crel_nth crel_if crel_return) auto

       from rec_condition have "l \<le> r - 1" by arith

       from 1(2) [OF rec_condition False rec2 `l \<le> r - 1`] show ?thesis by fastsimp

     qed

   qed

 qed

 lemma part_length_remains:

   assumes "crel (part1 a l r p) h h' rs"

   shows "Heap.length a h = Heap.length a h'"

 using assms

 proof (induct a l r p arbitrary: h h' rs rule:part1.induct)

   case (1 a l r p h h' rs)

   note cr = `crel (part1 a l r p) h h' rs`

   show ?case

   proof (cases "r \<le> l")

     case True (* Terminating case *)

     with cr show ?thesis

       unfolding part1.simps[of a l r p]

       by (elim crelE crel_if crel_return crel_nth) auto

   next

     case False (* recursive case *)

     with cr 1 show ?thesis

       unfolding part1.simps [of a l r p] swap_def

       by (auto elim!: crelE crel_if crel_nth crel_return crel_upd) fastsimp

   qed

 qed

 lemma part_outer_remains:

   assumes "crel (part1 a l r p) h h' rs"

   shows "\<forall>i. i < l \<or> r < i \<longrightarrow> get_array (a::nat array) h ! i = get_array a h' ! i"

   using assms

 proof (induct a l r p arbitrary: h h' rs rule:part1.induct)

   case (1 a l r p h h' rs)

   note cr = `crel (part1 a l r p) h h' rs`

   show ?case

   proof (cases "r \<le> l")

     case True (* Terminating case *)

     with cr show ?thesis

       unfolding part1.simps[of a l r p]

       by (elim crelE crel_if crel_return crel_nth) auto

   next

     case False (* recursive case *)

     note rec_condition = this

     let ?v = "get_array a h ! l"

     show ?thesis

     proof (cases "?v \<le> p")

       case True

       with cr False

       have rec1: "crel (part1 a (l + 1) r p) h h' rs"

         unfolding part1.simps[of a l r p]

         by (elim crelE crel_nth crel_if crel_return) auto

       from 1(1)[OF rec_condition True rec1]

       show ?thesis by fastsimp

     next

       case False

       with rec_condition cr

       obtain h1 where swp: "crel (swap a l r) h h1 ()"

         and rec2: "crel (part1 a l (r - 1) p) h1 h' rs"

         unfolding part1.simps[of a l r p]

         by (elim crelE crel_nth crel_if crel_return) auto

       from swp rec_condition have

         "\<forall>i. i < l \<or> r < i \<longrightarrow> get_array a h ! i = get_array a h1 ! i"

 	unfolding swap_def

 	by (elim crelE crel_nth crel_upd crel_return) auto

       with 1(2) [OF rec_condition False rec2] show ?thesis by fastsimp

     qed

   qed

 qed

 lemma part_partitions:

   assumes "crel (part1 a l r p) h h' rs"

   shows "(\<forall>i. l \<le> i \<and> i < rs \<longrightarrow> get_array (a::nat array) h' ! i \<le> p)

   \<and> (\<forall>i. rs < i \<and> i \<le> r \<longrightarrow> get_array a h' ! i \<ge> p)"

   using assms

 proof (induct a l r p arbitrary: h h' rs rule:part1.induct)

   case (1 a l r p h h' rs)

   note cr = `crel (part1 a l r p) h h' rs`

   show ?case

   proof (cases "r \<le> l")

     case True (* Terminating case *)

     with cr have "rs = r"

       unfolding part1.simps[of a l r p]

       by (elim crelE crel_if crel_return crel_nth) auto

     with True

     show ?thesis by auto

   next

     case False (* recursive case *)

     note lr = this

     let ?v = "get_array a h ! l"

     show ?thesis

     proof (cases "?v \<le> p")

       case True

       with lr cr

       have rec1: "crel (part1 a (l + 1) r p) h h' rs"

         unfolding part1.simps[of a l r p]

         by (elim crelE crel_nth crel_if crel_return) auto

       from True part_outer_remains[OF rec1] have a_l: "get_array a h' ! l \<le> p"

 	by fastsimp

       have "\<forall>i. (l \<le> i = (l = i \<or> Suc l \<le> i))" by arith

       with 1(1)[OF False True rec1] a_l show ?thesis

 	by auto

     next

       case False

       with lr cr

       obtain h1 where swp: "crel (swap a l r) h h1 ()"

         and rec2: "crel (part1 a l (r - 1) p) h1 h' rs"

         unfolding part1.simps[of a l r p]

         by (elim crelE crel_nth crel_if crel_return) auto

       from swp False have "get_array a h1 ! r \<ge> p"

 	unfolding swap_def

 	by (auto simp add: Heap.length_def elim!: crelE crel_nth crel_upd crel_return)

       with part_outer_remains [OF rec2] lr have a_r: "get_array a h' ! r \<ge> p"

 	by fastsimp

       have "\<forall>i. (i \<le> r = (i = r \<or> i \<le> r - 1))" by arith

       with 1(2)[OF lr False rec2] a_r show ?thesis

 	by auto

     qed

   qed

 qed

 fun partition :: "nat array \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat Heap"

 where

   "partition a left right = (do

      pivot \<leftarrow> nth a right;

      middle \<leftarrow> part1 a left (right - 1) pivot;

      v \<leftarrow> nth a middle;

      m \<leftarrow> return (if (v \<le> pivot) then (middle + 1) else middle);

      swap a m right;

      return m

    done)"

 declare partition.simps[simp del]

 lemma partition_permutes:

   assumes "crel (partition a l r) h h' rs"

   shows "multiset_of (get_array a h') 

   = multiset_of (get_array a h)"

 proof -

     from assms part_permutes swap_permutes show ?thesis

       unfolding partition.simps

       by (elim crelE crel_return crel_nth crel_if crel_upd) auto

 qed

 lemma partition_length_remains:

   assumes "crel (partition a l r) h h' rs"

   shows "Heap.length a h = Heap.length a h'"

 proof -

   from assms part_length_remains show ?thesis

     unfolding partition.simps swap_def

     by (elim crelE crel_return crel_nth crel_if crel_upd) auto

 qed

 lemma partition_outer_remains:

   assumes "crel (partition a l r) h h' rs"

   assumes "l < r"

   shows "\<forall>i. i < l \<or> r < i \<longrightarrow> get_array (a::nat array) h ! i = get_array a h' ! i"

 proof -

   from assms part_outer_remains part_returns_index_in_bounds show ?thesis

     unfolding partition.simps swap_def

     by (elim crelE crel_return crel_nth crel_if crel_upd) fastsimp

 qed

 lemma partition_returns_index_in_bounds:

   assumes crel: "crel (partition a l r) h h' rs"

   assumes "l < r"

   shows "l \<le> rs \<and> rs \<le> r"

 proof -

   from crel obtain middle h'' p where part: "crel (part1 a l (r - 1) p) h h'' middle"

     and rs_equals: "rs = (if get_array a h'' ! middle \<le> get_array a h ! r then middle + 1

          else middle)"

     unfolding partition.simps

     by (elim crelE crel_return crel_nth crel_if crel_upd) simp

   from `l < r` have "l \<le> r - 1" by arith

   from part_returns_index_in_bounds[OF part this] rs_equals `l < r` show ?thesis by auto

 qed

 lemma partition_partitions:

   assumes crel: "crel (partition a l r) h h' rs"

   assumes "l < r"

   shows "(\<forall>i. l \<le> i \<and> i < rs \<longrightarrow> get_array (a::nat array) h' ! i \<le> get_array a h' ! rs) \<and>

   (\<forall>i. rs < i \<and> i \<le> r \<longrightarrow> get_array a h' ! rs \<le> get_array a h' ! i)"

 proof -

   let ?pivot = "get_array a h ! r" 

   from crel obtain middle h1 where part: "crel (part1 a l (r - 1) ?pivot) h h1 middle"

     and swap: "crel (swap a rs r) h1 h' ()"

     and rs_equals: "rs = (if get_array a h1 ! middle \<le> ?pivot then middle + 1

          else middle)"

     unfolding partition.simps

     by (elim crelE crel_return crel_nth crel_if crel_upd) simp

   from swap have h'_def: "h' = Heap.upd a r (get_array a h1 ! rs)

     (Heap.upd a rs (get_array a h1 ! r) h1)"

     unfolding swap_def

     by (elim crelE crel_return crel_nth crel_upd) simp

   from swap have in_bounds: "r < Heap.length a h1 \<and> rs < Heap.length a h1"

     unfolding swap_def

     by (elim crelE crel_return crel_nth crel_upd) simp

   from swap have swap_length_remains: "Heap.length a h1 = Heap.length a h'"

     unfolding swap_def by (elim crelE crel_return crel_nth crel_upd) auto

   from `l < r` have "l \<le> r - 1" by simp 

   note middle_in_bounds = part_returns_index_in_bounds[OF part this]

   from part_outer_remains[OF part] `l < r`

   have "get_array a h ! r = get_array a h1 ! r"

     by fastsimp

   with swap

   have right_remains: "get_array a h ! r = get_array a h' ! rs"

     unfolding swap_def

     by (auto simp add: Heap.length_def elim!: crelE crel_return crel_nth crel_upd) (cases "r = rs", auto)

   from part_partitions [OF part]

   show ?thesis

   proof (cases "get_array a h1 ! middle \<le> ?pivot")

     case True

     with rs_equals have rs_equals: "rs = middle + 1" by simp

     { 

       fix i

       assume i_is_left: "l \<le> i \<and> i < rs"

       with swap_length_remains in_bounds middle_in_bounds rs_equals `l < r`

       have i_props: "i < Heap.length a h'" "i \<noteq> r" "i \<noteq> rs" by auto

       from i_is_left rs_equals have "l \<le> i \<and> i < middle \<or> i = middle" by arith

       with part_partitions[OF part] right_remains True

       have "get_array a h1 ! i \<le> get_array a h' ! rs" by fastsimp

       with i_props h'_def in_bounds have "get_array a h' ! i \<le> get_array a h' ! rs"

 	unfolding Heap.upd_def Heap.length_def by simp

     }

     moreover

     {

       fix i

       assume "rs < i \<and> i \<le> r"

       hence "(rs < i \<and> i \<le> r - 1) \<or> (rs < i \<and> i = r)" by arith

       hence "get_array a h' ! rs \<le> get_array a h' ! i"

       proof

 	assume i_is: "rs < i \<and> i \<le> r - 1"

 	with swap_length_remains in_bounds middle_in_bounds rs_equals

 	have i_props: "i < Heap.length a h'" "i \<noteq> r" "i \<noteq> rs" by auto

 	from part_partitions[OF part] rs_equals right_remains i_is

 	have "get_array a h' ! rs \<le> get_array a h1 ! i"

 	  by fastsimp

 	with i_props h'_def show ?thesis by fastsimp

       next

 	assume i_is: "rs < i \<and> i = r"

 	with rs_equals have "Suc middle \<noteq> r" by arith

 	with middle_in_bounds `l < r` have "Suc middle \<le> r - 1" by arith

 	with part_partitions[OF part] right_remains 

 	have "get_array a h' ! rs \<le> get_array a h1 ! (Suc middle)"

 	  by fastsimp

 	with i_is True rs_equals right_remains h'_def

 	show ?thesis using in_bounds

 	  unfolding Heap.upd_def Heap.length_def

 	  by auto

       qed

     }

     ultimately show ?thesis by auto

   next

     case False

     with rs_equals have rs_equals: "middle = rs" by simp

     { 

       fix i

       assume i_is_left: "l \<le> i \<and> i < rs"

       with swap_length_remains in_bounds middle_in_bounds rs_equals

       have i_props: "i < Heap.length a h'" "i \<noteq> r" "i \<noteq> rs" by auto

       from part_partitions[OF part] rs_equals right_remains i_is_left

       have "get_array a h1 ! i \<le> get_array a h' ! rs" by fastsimp

       with i_props h'_def have "get_array a h' ! i \<le> get_array a h' ! rs"

 	unfolding Heap.upd_def by simp

     }

     moreover

     {

       fix i

       assume "rs < i \<and> i \<le> r"

       hence "(rs < i \<and> i \<le> r - 1) \<or> i = r" by arith

       hence "get_array a h' ! rs \<le> get_array a h' ! i"

       proof

 	assume i_is: "rs < i \<and> i \<le> r - 1"

 	with swap_length_remains in_bounds middle_in_bounds rs_equals

 	have i_props: "i < Heap.length a h'" "i \<noteq> r" "i \<noteq> rs" by auto

 	from part_partitions[OF part] rs_equals right_remains i_is

 	have "get_array a h' ! rs \<le> get_array a h1 ! i"

 	  by fastsimp

 	with i_props h'_def show ?thesis by fastsimp

       next

 	assume i_is: "i = r"

 	from i_is False rs_equals right_remains h'_def

 	show ?thesis using in_bounds

 	  unfolding Heap.upd_def Heap.length_def

 	  by auto

       qed

     }

     ultimately

     show ?thesis by auto

   qed

 qed

 function quicksort :: "nat array \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> unit Heap"

 where

   "quicksort arr left right =

      (if (right > left)  then

         do

           pivotNewIndex \<leftarrow> partition arr left right;

           pivotNewIndex \<leftarrow> assert (\<lambda>x. left \<le> x \<and> x \<le> right) pivotNewIndex;

           quicksort arr left (pivotNewIndex - 1);

           quicksort arr (pivotNewIndex + 1) right

         done

      else return ())"

 by pat_completeness auto

 (* For termination, we must show that the pivotNewIndex is between left and right *) 

 termination

 by (relation "measure (\<lambda>(a, l, r). (r - l))") auto

 declare quicksort.simps[simp del]

 lemma quicksort_permutes:

   assumes "crel (quicksort a l r) h h' rs"

   shows "multiset_of (get_array a h') 

   = multiset_of (get_array a h)"

   using assms

 proof (induct a l r arbitrary: h h' rs rule: quicksort.induct)

   case (1 a l r h h' rs)

   with partition_permutes show ?case

     unfolding quicksort.simps [of a l r]

     by (elim crel_if crelE crel_assert crel_return) auto

 qed

 lemma length_remains:

   assumes "crel (quicksort a l r) h h' rs"

   shows "Heap.length a h = Heap.length a h'"

 using assms

 proof (induct a l r arbitrary: h h' rs rule: quicksort.induct)

   case (1 a l r h h' rs)

   with partition_length_remains show ?case

     unfolding quicksort.simps [of a l r]

     by (elim crel_if crelE crel_assert crel_return) auto

 qed

 lemma quicksort_outer_remains:

   assumes "crel (quicksort a l r) h h' rs"

    shows "\<forall>i. i < l \<or> r < i \<longrightarrow> get_array (a::nat array) h ! i = get_array a h' ! i"

   using assms

 proof (induct a l r arbitrary: h h' rs rule: quicksort.induct)

   case (1 a l r h h' rs)

   note cr = `crel (quicksort a l r) h h' rs`

   thus ?case

   proof (cases "r > l")

     case False

     with cr have "h' = h"

       unfolding quicksort.simps [of a l r]

       by (elim crel_if crel_return) auto

     thus ?thesis by simp

   next

   case True

    { 

       fix h1 h2 p ret1 ret2 i

       assume part: "crel (partition a l r) h h1 p"

       assume qs1: "crel (quicksort a l (p - 1)) h1 h2 ret1"

       assume qs2: "crel (quicksort a (p + 1) r) h2 h' ret2"

       assume pivot: "l \<le> p \<and> p \<le> r"

       assume i_outer: "i < l \<or> r < i"

       from  partition_outer_remains [OF part True] i_outer

       have "get_array a h !i = get_array a h1 ! i" by fastsimp

       moreover

       with 1(1) [OF True pivot qs1] pivot i_outer

       have "get_array a h1 ! i = get_array a h2 ! i" by auto

       moreover

       with qs2 1(2) [of p h2 h' ret2] True pivot i_outer

       have "get_array a h2 ! i = get_array a h' ! i" by auto

       ultimately have "get_array a h ! i= get_array a h' ! i" by simp

     }

     with cr show ?thesis

       unfolding quicksort.simps [of a l r]

       by (elim crel_if crelE crel_assert crel_return) auto

   qed

 qed

 lemma quicksort_is_skip:

   assumes "crel (quicksort a l r) h h' rs"

   shows "r \<le> l \<longrightarrow> h = h'"

   using assms

   unfolding quicksort.simps [of a l r]

   by (elim crel_if crel_return) auto

 lemma quicksort_sorts:

   assumes "crel (quicksort a l r) h h' rs"

   assumes l_r_length: "l < Heap.length a h" "r < Heap.length a h" 

   shows "sorted (subarray l (r + 1) a h')"

   using assms

 proof (induct a l r arbitrary: h h' rs rule: quicksort.induct)

   case (1 a l r h h' rs)

   note cr = `crel (quicksort a l r) h h' rs`

   thus ?case

   proof (cases "r > l")

     case False

     hence "l \<ge> r + 1 \<or> l = r" by arith 

     with length_remains[OF cr] 1(5) show ?thesis

       by (auto simp add: subarray_Nil subarray_single)

   next

     case True

     { 

       fix h1 h2 p

       assume part: "crel (partition a l r) h h1 p"

       assume qs1: "crel (quicksort a l (p - 1)) h1 h2 ()"

       assume qs2: "crel (quicksort a (p + 1) r) h2 h' ()"

       from partition_returns_index_in_bounds [OF part True]

       have pivot: "l\<le> p \<and> p \<le> r" .

      note length_remains = length_remains[OF qs2] length_remains[OF qs1] partition_length_remains[OF part]

       from quicksort_outer_remains [OF qs2] quicksort_outer_remains [OF qs1] pivot quicksort_is_skip[OF qs1]

       have pivot_unchanged: "get_array a h1 ! p = get_array a h' ! p" by (cases p, auto)

         (*-- First of all, by induction hypothesis both sublists are sorted. *)

       from 1(1)[OF True pivot qs1] length_remains pivot 1(5) 

       have IH1: "sorted (subarray l p a h2)"  by (cases p, auto simp add: subarray_Nil)

       from quicksort_outer_remains [OF qs2] length_remains

       have left_subarray_remains: "subarray l p a h2 = subarray l p a h'"

 	by (simp add: subarray_eq_samelength_iff)

       with IH1 have IH1': "sorted (subarray l p a h')" by simp

       from 1(2)[OF True pivot qs2] pivot 1(5) length_remains

       have IH2: "sorted (subarray (p + 1) (r + 1) a h')"

         by (cases "Suc p \<le> r", auto simp add: subarray_Nil)

            (* -- Secondly, both sublists remain partitioned. *)

       from partition_partitions[OF part True]

       have part_conds1: "\<forall>j. j \<in> set (subarray l p a h1) \<longrightarrow> j \<le> get_array a h1 ! p "

         and part_conds2: "\<forall>j. j \<in> set (subarray (p + 1) (r + 1) a h1) \<longrightarrow> get_array a h1 ! p \<le> j"

         by (auto simp add: all_in_set_subarray_conv)

       from quicksort_outer_remains [OF qs1] quicksort_permutes [OF qs1] True

         length_remains 1(5) pivot multiset_of_sublist [of l p "get_array a h1" "get_array a h2"]

       have multiset_partconds1: "multiset_of (subarray l p a h2) = multiset_of (subarray l p a h1)"

 	unfolding Heap.length_def subarray_def by (cases p, auto)

       with left_subarray_remains part_conds1 pivot_unchanged

       have part_conds2': "\<forall>j. j \<in> set (subarray l p a h') \<longrightarrow> j \<le> get_array a h' ! p"

         by (simp, subst set_of_multiset_of[symmetric], simp)

           (* -- These steps are the analogous for the right sublist \<dots> *)

       from quicksort_outer_remains [OF qs1] length_remains

       have right_subarray_remains: "subarray (p + 1) (r + 1) a h1 = subarray (p + 1) (r + 1) a h2"

 	by (auto simp add: subarray_eq_samelength_iff)

       from quicksort_outer_remains [OF qs2] quicksort_permutes [OF qs2] True

         length_remains 1(5) pivot multiset_of_sublist [of "p + 1" "r + 1" "get_array a h2" "get_array a h'"]

       have multiset_partconds2: "multiset_of (subarray (p + 1) (r + 1) a h') = multiset_of (subarray (p + 1) (r + 1) a h2)"

         unfolding Heap.length_def subarray_def by auto

       with right_subarray_remains part_conds2 pivot_unchanged

       have part_conds1': "\<forall>j. j \<in> set (subarray (p + 1) (r + 1) a h') \<longrightarrow> get_array a h' ! p \<le> j"

         by (simp, subst set_of_multiset_of[symmetric], simp)

           (* -- Thirdly and finally, we show that the array is sorted

           following from the facts above. *)

       from True pivot 1(5) length_remains have "subarray l (r + 1) a h' = subarray l p a h' @ [get_array a h' ! p] @ subarray (p + 1) (r + 1) a h'"

 	by (simp add: subarray_nth_array_Cons, cases "l < p") (auto simp add: subarray_append subarray_Nil)

       with IH1' IH2 part_conds1' part_conds2' pivot have ?thesis

 	unfolding subarray_def

 	apply (auto simp add: sorted_append sorted_Cons all_in_set_sublist'_conv)

 	by (auto simp add: set_sublist' dest: le_trans [of _ "get_array a h' ! p"])

     }

     with True cr show ?thesis

       unfolding quicksort.simps [of a l r]

       by (elim crel_if crel_return crelE crel_assert) auto

   qed

 qed

 lemma quicksort_is_sort:

   assumes crel: "crel (quicksort a 0 (Heap.length a h - 1)) h h' rs"

   shows "get_array a h' = sort (get_array a h)"

 proof (cases "get_array a h = []")

   case True

   with quicksort_is_skip[OF crel] show ?thesis

   unfolding Heap.length_def by simp

 next

   case False

   from quicksort_sorts [OF crel] False have "sorted (sublist' 0 (List.length (get_array a h)) (get_array a h'))"

     unfolding Heap.length_def subarray_def by auto

   with length_remains[OF crel] have "sorted (get_array a h')"

     unfolding Heap.length_def by simp

   with quicksort_permutes [OF crel] properties_for_sort show ?thesis by fastsimp

 qed

 subsection {* No Errors in quicksort *}

 text {* We have proved that quicksort sorts (if no exceptions occur).

 We will now show that exceptions do not occur. *}

 lemma noError_part1: 

   assumes "l < Heap.length a h" "r < Heap.length a h"

   shows "noError (part1 a l r p) h"

   using assms

 proof (induct a l r p arbitrary: h rule: part1.induct)

   case (1 a l r p)

   thus ?case

     unfolding part1.simps [of a l r] swap_def

     by (auto intro!: noError_if noErrorI noError_return noError_nth noError_upd elim!: crelE crel_upd crel_nth crel_return)

 qed

 lemma noError_partition:

   assumes "l < r" "l < Heap.length a h" "r < Heap.length a h"

   shows "noError (partition a l r) h"

 using assms

 unfolding partition.simps swap_def

 apply (auto intro!: noError_if noErrorI noError_return noError_nth noError_upd noError_part1 elim!: crelE crel_upd crel_nth crel_return)

 apply (frule part_length_remains)

 apply (frule part_returns_index_in_bounds)

 apply auto

 apply (frule part_length_remains)

 apply (frule part_returns_index_in_bounds)

 apply auto

 apply (frule part_length_remains)

 apply auto

 done

 lemma noError_quicksort:

   assumes "l < Heap.length a h" "r < Heap.length a h"

   shows "noError (quicksort a l r) h"

 using assms

 proof (induct a l r arbitrary: h rule: quicksort.induct)

   case (1 a l ri h)

   thus ?case

     unfolding quicksort.simps [of a l ri]

     apply (auto intro!: noError_if noErrorI noError_return noError_nth noError_upd noError_assert noError_partition)

     apply (frule partition_returns_index_in_bounds)

     apply auto

     apply (frule partition_returns_index_in_bounds)

     apply auto

     apply (auto elim!: crel_assert dest!: partition_length_remains length_remains)

     apply (subgoal_tac "Suc r \<le> ri \<or> r = ri") 

     apply (erule disjE)

     apply auto

     unfolding quicksort.simps [of a "Suc ri" ri]

     apply (auto intro!: noError_if noError_return)

     done

 qed

 subsection {* Example *}

 definition "qsort a = do

     k \<leftarrow> length a;

     quicksort a 0 (k - 1);

     return a

   done"

 ML {* @{code qsort} (Array.fromList [42, 2, 3, 5, 0, 1705, 8, 3, 15]) () *}

 export_code qsort in SML_imp module_name QSort

 export_code qsort in OCaml module_name QSort file -

 export_code qsort in OCaml_imp module_name QSort file -

 export_code qsort in Haskell module_name QSort file -

 end