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/**************************************************************************/
/* */
/* The Why platform for program certification */
/* Copyright (C) 2002-2008 */
/* Romain BARDOU */
/* Jean-Franois COUCHOT */
/* Mehdi DOGGUY */
/* Jean-Christophe FILLITRE */
/* Thierry HUBERT */
/* Claude MARCH */
/* Yannick MOY */
/* Christine PAULIN */
/* Yann RGIS-GIANAS */
/* Nicolas ROUSSET */
/* Xavier URBAIN */
/* */
/* This software is free software; you can redistribute it and/or */
/* modify it under the terms of the GNU General Public */
/* License version 2, as published by the Free Software Foundation. */
/* */
/* This software is distributed in the hope that it will be useful, */
/* but WITHOUT ANY WARRANTY; without even the implied warranty of */
/* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. */
/* */
/* See the GNU General Public License version 2 for more details */
/* (enclosed in the file GPL). */
/* */
/**************************************************************************/
/* Dijkstra's dutch flag */
typedef enum { BLUE, WHITE, RED } color;
/*@ predicate isMonochrome(color t[], int i, int j, color c)
@ { \valid_range(t,i,j) && \forall int k; i <= k && k <= j => t[k] == c }
@*/
/*@ requires \valid_index(t,i) && \valid_index(t,j)
@ assigns t[i],t[j]
@ ensures t[i] == \old(t[j]) && t[j] == \old(t[i])
@*/
void swap(color t[], int i, int j) {
color tmp = t[i];
t[i] = t[j];
t[j] = tmp;
}
/*@ requires
@ n >= 0 && \valid_range(t,0,n-1)
@ assigns t[0 .. n-1]
@ ensures
@ (\exists int b, int r;
@ isMonochrome(t,0,b-1,BLUE) &&
@ isMonochrome(t,b,r-1,WHITE) &&
@ isMonochrome(t,r,n-1,RED))
@*/
void flag(color t[], int n) {
int b = 0;
int i = 0;
int r = n;
/*@ invariant
@ 0 <= b && b <= i && i <= r && r <= n &&
@ isMonochrome(t,0,b-1,BLUE) &&
@ isMonochrome(t,b,i-1,WHITE) &&
@ isMonochrome(t,r,n-1,RED)
@ loop_assigns b,i,r,t[0 .. n-1]
@ variant r - i
@*/
while (i < r) {
switch (t[i]) {
case BLUE:
swap(t, b++, i++);
break;
case WHITE:
i++;
break;
case RED:
swap(t, --r, i);
break;
}
}
}
/*
Local Variables:
compile-command: "make flag_checkenum.enum"
End:
*/
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