Theory LinearSearch

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(*
 * Authors : Burkhart Wolff, Frédéric Tuong
 *)

chapter ‹ A Clean Semantics Example : Linear Search›

text‹The following show-case introduces subsequently a non-trivial example involving
local and global variable declarations, declarations of operations with pre-post conditions as
well as direct-recursive operations (i.e. C-like functions with side-effects on global and
local variables. ›

theory LinearSearch
  imports Clean.Clean
          Clean.Hoare_MonadSE
begin


section‹The LinearSearch Example›

definition bool2int where "bool2int x = (if x then 1::int else 0)"

global_vars (state)
    t :: "int list"

global_vars ("2")
    tt :: "int list"

find_theorems (160) name:"2" name:"Linear"

function_spec linearsearch (x::int, n::int) returns int
pre          "‹ 0 ≤ n ∧ n < int(length t) ∧ sorted t›"    
post         "‹λres::int. res = bool2int (∃ i ∈ {0 ..< length t}. t!i = x) ›" 
local_vars   i  :: int
defines      " ‹i := 0 › ;- ‹tt :=  [] ›;-
               while⇩C ‹i < n › 
                 do if⇩C ‹t ! (nat i) < x›  
                      then  ‹i := i + 1 ›
                      else return⇩C result_value_update ‹bool2int(t!(nat i) = x)› 
                    fi 
                 od " 

(*
C‹
/*@ requires "n >= 0"
    requires "valid(t+(0..n-1))"
    requires "(forall integer i,j; 0<=i<=j<n ==> t[i] <= t[j])"
    ensures "exists integer i; (0<=i<n && t[i] == x) <==> result == 1"
    ensures "(forall integer i; 0<=i<n ==> t[i] != x) <==> result == 0"
    assigns nothing
 */

int linearsearch(int x, int t[], int n) {
  int i = 0;

  /*@ loop invariant "0<=i<=n"
      loop invariant "forall integer j; 0<=j<i ==> (t[j] != x)"
      loop assigns i
      loop variant "n-i"
   */
  while (i < n) {
    if (t[i] < x) {
      i++;
    } else {
      return (t[i] == x);
    }
  }

  return 0;
}
›

*)

end