// RUN: %dafny /compile:0 /dprint:"%t.dprint" "%s" > "%t"
// RUN: %diff "%s.expect" "%t"

function Sum(a: seq<int>, s: int, t: int): int
  requires 0 <= s <= t <= |a|;
{
  if s == t then 0 else Sum(a, s, t-1) + a[t-1]
}

method MaxSegSum(a: seq<int>) returns (k: int, m: int)
  ensures 0 <= k <= m <= |a|;
  ensures forall p,q :: 0 <= p <= q <= |a| ==> Sum(a, p, q) <= Sum(a, k, m);
{
  k, m := 0, 0;
  var s := 0;  // invariant s == Sum(a, k, m)
  var n := 0;
  var c, t := 0, 0;  // invariant t == Sum(a, c, n)
  while (n < |a|)
    invariant n <= |a|;
    invariant 0 <= c <= n && t == Sum(a, c, n);
    invariant forall b :: 0 <= b <= n ==> Sum(a, b, n) <= Sum(a, c, n);
    invariant 0 <= k <= m <= n && s == Sum(a, k, m);
    invariant forall p,q :: 0 <= p <= q <= n ==> Sum(a, p, q) <= Sum(a, k, m);
  {
    t, n := t + a[n], n + 1;
    if (t < 0) {
      c, t := n, 0;
    } else if (s < t) {
      k, m, s := c, n, t;
    }
  }
}