Quick sort and related Arrays utilities

src/Collections/Arrays/Arrays.dfy

@@ -0,0 +1,375 @@
+// RUN: %dafny /compile:0 "%s"
+
+/*******************************************************************************
+*  Copyright by the contributors to the Dafny Project
+*  SPDX-License-Identifier: MIT 
+*******************************************************************************/
+
+include "../Sets/Sets.dfy"
+include "../../Comparison.dfy"
+
+module Arrays{
+
+import opened Sets
+import opened Comparison
+import opened Wrappers
+
+  trait Sorter<T> {
+    ghost const C := Comparison(Cmp)
+
+    function method Cmp(t0: T, t1: T): Cmp
+
+    twostate predicate UnchangedSlice(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      reads arr
+    {
+      arr[lo..hi] == old(arr[lo..hi])
+    }
+
+    twostate lemma UnchangedSliceSplit(arr: array<T>, lo: int, mid: int, hi: int)
+      requires 0 <= lo <= mid <= hi <= arr.Length
+      requires UnchangedSlice(arr, lo, hi)
+      ensures UnchangedSlice(arr, lo, mid)
+      ensures UnchangedSlice(arr, mid, hi)
+    {
+      assert arr[lo..mid] == arr[lo..hi][..mid-lo];
+    }
+
+    twostate lemma UnchangedSliceShuffled(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      requires UnchangedSlice(arr, lo, hi)
+      ensures Shuffled(arr, lo, hi)
+    {}
+
+    twostate predicate UnchangedExceptSliceShuffled(arr: array<T>, lo: int, hi: int)
+      reads arr
+      requires 0 <= lo <= hi <= arr.Length
+    {
+      && UnchangedSlice(arr, 0, lo)
+      && Shuffled(arr, lo, hi)
+      && UnchangedSlice(arr, hi, arr.Length)
+    }
+
+    twostate lemma UnchangedExceptSliceShuffledExtend(arr: array<T>, lo: int, lo': int, hi': int, hi: int)
+      requires 0 <= lo <= lo' <= hi' <= hi <= arr.Length
+      requires UnchangedExceptSliceShuffled(arr, lo', hi')
+      ensures UnchangedExceptSliceShuffled(arr, lo, hi)
+    {
+      assert UnchangedExceptSliceShuffled(arr, lo', hi');
+      assert UnchangedSlice(arr, 0, lo') && Shuffled(arr, lo', hi') && UnchangedSlice(arr, hi', arr.Length);
+      UnchangedSliceSplit(arr, 0, lo, lo'); UnchangedSliceSplit(arr, hi', hi, arr.Length);
+      assert && UnchangedSlice(arr, 0, lo) && UnchangedSlice(arr, lo, lo')
+             && Shuffled(arr, lo', hi')
+             && UnchangedSlice(arr, hi', hi) && UnchangedSlice(arr, hi, arr.Length);
+      UnchangedSliceShuffled(arr, lo, lo'); UnchangedSliceShuffled(arr, hi', hi);
+      assert && UnchangedSlice(arr, 0, lo) && Shuffled(arr, lo, lo')
+             && Shuffled(arr, lo', hi')
+             && Shuffled(arr, hi', hi) && UnchangedSlice(arr, hi, arr.Length);
+      ShuffledConcat(arr, lo, lo', hi'); ShuffledConcat(arr, lo, hi', hi);
+      assert UnchangedSlice(arr, 0, lo) && Shuffled(arr, lo, hi) && UnchangedSlice(arr, hi, arr.Length);
+      assert UnchangedExceptSliceShuffled(arr, lo, hi);
+    }
+
+    twostate predicate Shuffled(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      reads arr
+    {
+      multiset(arr[lo..hi]) == old(multiset(arr[lo..hi]))
+    }
+
+    predicate Sortable(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      reads arr
+    {
+      reveal C.Valid(); // Leaks through
+      C.Valid(Sets.OfSlice(arr, lo, hi))
+    }
+
+    twostate lemma SetOfSliceShuffled(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      requires Shuffled(arr, lo, hi)
+      ensures Sets.OfSlice(arr, lo, hi) == old(Sets.OfSlice(arr, lo, hi))
+    {
+      calc {
+        old(Sets.OfSlice(arr, lo, hi));
+        set x | x in old(arr[lo..hi]);
+        set x | x in old(multiset(arr[lo..hi]));
+        set x | x in multiset(arr[lo..hi]);
+        set x | x in arr[lo..hi];
+        Sets.OfSlice(arr, lo, hi);
+      }
+    }
+
+    twostate lemma SortableUnchangedExceptSliceShuffled(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      requires old(Sortable(arr, lo, hi))
+      requires UnchangedExceptSliceShuffled(arr, lo, hi)
+      ensures Sortable(arr, lo, hi)
+    {
+      SetOfSliceShuffled(arr, lo, hi);
+    }
+
+    twostate lemma ShuffledConcat(arr: array<T>, lo: int, mid: int, hi: int)
+      requires 0 <= lo <= mid <= hi <= arr.Length
+      requires Shuffled(arr, lo, mid)
+      requires Shuffled(arr, mid, hi)
+      ensures Shuffled(arr, lo, hi)
+    {
+      calc {
+        old(multiset(arr[lo..hi]));
+        { assert old(arr[lo..hi] == arr[lo..mid] + arr[mid..hi]); }
+        old(multiset(arr[lo..mid] + arr[mid..hi]));
+        old(multiset(arr[lo..mid])) + old(multiset(arr[mid..hi]));
+        multiset(arr[lo..mid]) + multiset(arr[mid..hi]);
+        { assert arr[lo..hi] == arr[lo..mid] + arr[mid..hi]; }
+        multiset(arr[lo..hi]);
+      }
+    }
+
+    lemma SortableSubslice(arr: array<T>, lo: int, hi: int, lo': int, hi': int)
+      requires 0 <= lo <= lo' <= hi' <= hi <= arr.Length
+      requires Sortable(arr, lo, hi)
+      ensures Sortable(arr, lo', hi')
+    {}
+
+    predicate Sorted(arr: array<T>, lo: int, hi: int)
+      reads arr
+      requires 0 <= lo <= hi <= arr.Length
+    {
+      C.Sorted(arr[lo..hi])
+    }
+
+    lemma StripedInit(sq: seq<T>, pivot: T, lo: int, hi: int)
+      requires 0 <= lo <= hi <= |sq|
+      ensures C.Striped(sq, pivot, lo, lo, lo, hi, hi)
+    {
+      reveal C.Striped();
+    }
+
+    lemma StripedNonEmpty(sq: seq<T>, pivot: T, lo: int, left: int, mid: int, right: int, hi: int)
+      requires 0 <= lo <= left <= mid <= right <= hi <= |sq|
+      requires C.Striped(sq, pivot, lo, left, mid, right, hi)
+      requires C.Valid(Sets.OfSeq(sq[lo..hi]))
+      requires pivot in sq[lo..hi]
+      ensures left < right
+    {
+      reveal C.Valid();
+      var idx :| lo <= idx < hi && sq[idx] == pivot;
+      C.AlwaysReflexive(pivot);
+      reveal C.Striped();
+    }
+
+    twostate lemma StripedUnchangedExceptSliceShuffled(arr: array<T>, pivot: T, lo: int, left: int, right: int, hi: int)
+      requires 0 <= lo <= left <= right <= hi <= |arr[..]|
+      requires Shuffled(arr, lo, left)
+      requires UnchangedSlice(arr, left, right)
+      requires Shuffled(arr, right, hi)
+      requires old(C.Striped(arr[..], pivot, lo, left, right, right, hi))
+      ensures C.Striped(arr[..], pivot, lo, left, right, right, hi)
+    {
+      reveal C.Striped();
+      forall i | lo    <= i < left ensures Cmp(arr[..][i], pivot).Lt? {
+        assert arr[..][i] == arr[lo..left][i - lo];
+        assert arr[..][i] in multiset(arr[lo..left]);
+      }
+      forall i | left  <= i < right ensures Cmp(arr[..][i], pivot).Eq? {
+        assert arr[..][i] == arr[left..right][i - left];
+      }
+      forall i | right <= i < hi    ensures Cmp(arr[..][i], pivot).Gt? {
+        assert arr[..][i] == arr[right..hi][i - right];
+        assert arr[..][i] in multiset(arr[right..hi]);
+      }
+    }
+
+    method Swap(arr: array<T>, ghost lo: int, i: int, j: int, ghost hi: int)
+      requires 0 <= lo <= i <= j < hi <= arr.Length
+      modifies arr
+      ensures UnchangedExceptSliceShuffled(arr, lo, hi)
+      ensures arr[..] == old(arr[..][i := arr[j]][j := arr[i]])
+    {
+      arr[i], arr[j] := arr[j], arr[i];
+    }
+
+    method SwapLt(
+      arr: array<T>, pivot: T,
+      lo: int, left: int, mid: int, right: int, hi: int
+    )
+      requires 0 <= lo <= left <= mid < right <= hi <= arr.Length
+      requires C.Striped(arr[..], pivot, lo, left, mid, right, hi)
+      requires Cmp(arr[mid], pivot).Lt?
+      modifies arr
+      ensures UnchangedExceptSliceShuffled(arr, lo, hi)
+      ensures arr[..] == old(arr[..][left := arr[mid]][mid := arr[left]])
+      ensures C.Striped(arr[..], pivot, lo, left + 1, mid + 1, right, hi)
+    {
+      reveal C.Striped();
+      Swap(arr, lo, left, mid, hi);
+    }
+
+    method SwapGt(
+      arr: array<T>, pivot: T,
+      lo: int, left: int, mid: int, right: int, hi: int
+    )
+      requires 0 <= lo <= left <= mid < right <= hi <= arr.Length
+      requires C.Striped(arr[..], pivot, lo, left, mid, right, hi)
+      requires Cmp(arr[mid], pivot).Gt?
+      modifies arr
+      ensures UnchangedExceptSliceShuffled(arr, lo, hi)
+      ensures arr[..] == old(arr[..][mid := arr[right - 1]][right - 1 := arr[mid]])
+      ensures C.Striped(arr[..], pivot, lo, left, mid, right - 1, hi)
+    {
+      reveal C.Striped();
+      Swap(arr, lo, mid, right - 1, hi);
+    }
+
+    lemma SortedStripedMiddle(arr: array<T>, pivot: T, lo: int, left: int, right: int, hi: int)
+      requires 0 <= lo <= left < right <= hi <= arr.Length
+      requires Sortable(arr, lo, hi)
+      requires pivot in multiset(arr[lo..hi])
+      requires C.Striped(arr[..], pivot, lo, left, right, right, hi)
+      ensures Sorted(arr, left, right)
+    {
+      reveal C.Striped();
+      forall i, j | left  <= i < j < right ensures Cmp(arr[i], arr[j]).Le? {
+        var idx :| lo <= idx < hi && arr[idx] == pivot;
+        assert Cmp(arr[i], pivot) == Eq;
+        assert C.CompleteonPair(arr[j], pivot);
+        assert C.Transitive'(arr[i], pivot, arr[j]);
+      }
+    }
+
+    lemma SortedConcat(arr: array<T>, lo: int, mid: int, hi: int)
+      requires 0 <= lo <= mid <= hi <= arr.Length
+      requires Sorted(arr, lo, mid)
+      requires Sorted(arr, mid, hi)
+      requires forall i, j | lo <= i < mid <= j < hi :: Cmp(arr[i], arr[j]).Le?
+      ensures Sorted(arr, lo, hi)
+    {
+      assert arr[lo..mid] + arr[mid..hi] == arr[lo..hi];
+      C.SortedConcat(arr[lo..mid], arr[mid..hi]);
+    }
+
+    twostate lemma SortedUnchangedSlice(arr: array<T>, lo: int, hi: int)
+      requires 0 <= lo <= hi <= arr.Length
+      requires old(Sorted(arr, lo, hi))
+      requires UnchangedSlice(arr, lo, hi)
+      ensures Sorted(arr, lo, hi)
+    {
+      assert arr[lo..hi] == old(arr[lo..hi]);
+      forall i, j | lo <= i < j < hi ensures Cmp(arr[i], arr[j]).Le? {
+        assert arr[i] == arr[lo..hi][i - lo];
+        assert arr[j] == arr[lo..hi][j - lo];
+      }
+    }
+
+    lemma SortedDutchFlag(arr: array<T>, pivot: T, lo: int, left: int, right: int, hi: int)
+      requires 0 <= lo <= left < right <= hi <= arr.Length
+      requires Sortable(arr, lo, hi)
+      requires pivot in multiset(arr[lo..hi])
+      requires Sorted(arr, lo, left)
+      requires Sorted(arr, right, hi)
+      requires C.Striped(arr[..], pivot, lo, left, right, right, hi)
+      ensures Sorted(arr, lo, hi)
+    {
+      reveal C.Striped();
+      forall i, j | lo <= i < left <= j < right ensures Cmp(arr[i], arr[j]).Le? {
+        var idx :| lo <= idx < hi && arr[idx] == pivot;
+        assert Cmp(arr[i], pivot).Le?;
+        assert Cmp(arr[j], pivot).Eq?;
+        assert C.CompleteonPair(arr[j], pivot);
+        assert C.Transitive'(arr[i], pivot, arr[j]);
+      }
+      SortedStripedMiddle(arr, pivot, lo, left, right, hi);
+      SortedConcat(arr, lo, left, right);
+      forall i, j | lo <= i < right <= j < hi ensures Cmp(arr[i], arr[j]).Le? {
+        assert Cmp(arr[i], pivot).Le?;
+        assert Cmp(arr[j], pivot).Gt?;
+        assert C.CompleteonPair(arr[i], pivot);
+        assert C.CompleteonPair(arr[j], pivot);
+        assert C.Transitive'(arr[i], pivot, arr[j]);
+      }
+      SortedConcat(arr, lo, right, hi);
+    }
+
+    // Doesn't have the O(n log n) worst case runtime yet, need MedianofMeians algorithm implemented for that.
+    method QuickSort(arr: array<T>, lo: int := 0, hi: int := arr.Length)
+      requires 0 <= lo <= hi <= arr.Length
+      requires Sortable(arr, lo, hi)
+      decreases hi - lo
+      modifies arr
+      ensures Sortable(arr, lo, hi)
+      ensures Sorted(arr, lo, hi)
+      ensures UnchangedExceptSliceShuffled(arr, lo, hi)
+    {
+      if hi - lo > 1 {
+        var pivot := MedianOfMedians(arr, lo, hi);
+        var left, right := DutchFlag(arr, pivot, lo, hi);
+
+        label left:
+        SortableUnchangedExceptSliceShuffled(arr, lo, hi);
+        SortableSubslice(arr, lo, hi, lo, left);
+        StripedNonEmpty(arr[..], pivot, lo, left, right, right, hi);
+        QuickSort(arr, lo, left);
+
+        label right:
+        UnchangedExceptSliceShuffledExtend@left(arr, lo, lo, left, hi);
+        SortableUnchangedExceptSliceShuffled(arr, lo, hi);
+        SortableSubslice(arr, lo, hi, right, hi);
+        UnchangedSliceSplit@left(arr, left, hi, arr.Length);
+        UnchangedSliceSplit@left(arr, left, right, hi);
+        UnchangedSliceShuffled@left(arr, right, hi);
+        StripedUnchangedExceptSliceShuffled@left(arr, pivot, lo, left, right, hi);
+        QuickSort(arr, right, hi);
+
+        UnchangedExceptSliceShuffledExtend@right(arr, lo, right, hi, hi);
+        SortableUnchangedExceptSliceShuffled(arr, lo, hi);
+        UnchangedSliceSplit@right(arr, lo, left, right);
+        SortedUnchangedSlice@right(arr, lo, left);
+        StripedUnchangedExceptSliceShuffled@right(arr, pivot, lo, left, right, hi);
+        SortedDutchFlag(arr, pivot, lo, left, right, hi);
+      }
+    }
+
+    function method MedianOfMedians(arr: array<T>, lo: int, hi: int): (t: T)
+      requires 0 <= lo < hi <= arr.Length
+      reads arr
+      ensures t in arr[lo..hi]
+    {
+      arr[lo] // TODO
+    }
+
+    method DutchFlag(arr: array<T>, pivot: T, lo: int, hi: int) returns (left: int, right: int)
+      requires 0 <= lo < hi <= arr.Length
+      requires pivot in multiset(arr[lo..hi])
+      modifies arr
+      ensures UnchangedExceptSliceShuffled(arr, lo, hi)
+      ensures lo <= left <= right <= hi
+      ensures C.Striped(arr[..], pivot, lo, left, right, right, hi)
+    {
+      left, right := lo, hi;
+      var mid := lo;
+      StripedInit(arr[..], pivot, lo, hi);
+
+      while mid < right
+        invariant pivot in multiset(arr[lo..hi])
+        invariant lo <= left <= mid <= right <= hi
+        invariant UnchangedExceptSliceShuffled(arr, lo, hi)
+        invariant C.Striped(arr[..], pivot, lo, left, mid, right, hi)
+      {
+        reveal C.Striped();
+        match Cmp(arr[mid], pivot) {
+          case Lt =>
+            SwapLt(arr, pivot, lo, left, mid, right, hi);
+            left := left + 1;
+            mid := mid + 1;
+          case Gt =>
+            SwapGt(arr, pivot, lo, left, mid, right, hi);
+            right := right - 1;
+          case Eq =>
+            mid := mid + 1;
+        }
+      }
+
+    }
+  }
+}

src/Collections/Sets/Sets.dfy

@@ -193,4 +193,15 @@ module Sets {
     LemmaSubsetSize(x, range);
   }
 
-}
+  function method OfSeq<T(==)>(sq: seq<T>): set<T> {
+    set x | x in sq
+  }
+
+  function method OfSlice<T(==)>(arr: array<T>, lo: int, hi: int): set<T>
+    requires 0 <= lo <= hi <= arr.Length
+    reads arr
+  {
+    OfSeq(arr[lo..hi])
+  }
+
+}