module Sqrt

  use array.Array
  use map.Map
  use mach.c.C
  use ref.Ref
  use mach.int.Int32
  use import mach.int.UInt64GMP as Limb
  use int.EuclideanDivision
  use int.Int
  use int.Power
  use types.Types
  use types.Int32Eq
  use types.UInt64Eq
  use lemmas.Lemmas
  use compare.Compare
  use util.UtilOld
  use add_1.Add_1
  use add.AddOld
  use sub_1.Sub_1
  use sub.SubOld
  use mul.Mul
  use logical.LogicalUtil
  use logical.Logical
  use div.Div
  use sqrt.Sqrt1
  use ptralias.Alias

  use real.ExpLog (* have to use real to be able to remove log2/log10... *)
  meta remove_logic function log10 (* kills CVC3 *)
  meta remove_logic function log2  (* same *)

  let lemma same_mod (a b:int)
    requires { 0 <= a }
    requires { 0 < b }
    ensures  { ComputerDivision.mod a b = EuclideanDivision.mod a b }
  = ()

  meta remove_prop axiom same_mod

  let wmpn_sqrtrem2 (sp rp np: ptr limb) : limb
    requires { valid rp 1 }
    requires { valid sp 1 }
    requires { valid np 2 }
    requires { (pelts np)[offset np + 1] >= power 2 (Limb.length - 2) }
    requires { writable sp /\ writable rp }
    ensures  { value np 2
               = (pelts sp)[offset sp] * (pelts sp)[offset sp]
                  + result * radix + (pelts rp)[offset rp] }
    ensures  { (pelts rp)[offset rp] + result * radix <= 2 * (pelts sp)[offset sp] }
    ensures  { 0 <= result <= 1 }
  =
    let np0 = C.get np in
    let ghost np1 = C.get_ofs np 1 in
    let ref sp0 = sqrt1 rp (C.get_ofs np 1) in
    let ref rp0 = C.get rp in
    let ghost orp = pure { rp0 } in
    let ghost osp = pure { sp0 } in
    let prec = (Limb.of_int Limb.length) / 2 in (* prec = 32 *)
    assert { power 2 prec * power 2 prec = radix };
    assert { sp0 * sp0 + rp0 = np1 };
    assert { sp0 >= power 2 (prec - 1)
             by np1 >= power 2 (Limb.length - 2)
             so ((sp0 + 1) * (sp0 + 1) > np1
                by (sp0 + 1) * (sp0 + 1) > sp0 * sp0 + 2 * sp0 >= np1)
             so (power 2 (prec - 1)) * (power 2 (prec - 1))
                 = power 2 (Limb.length - 2) };
    assert { sp0 < power 2 prec
             by sp0 * sp0 <= np1 < radix = power 2 (Limb.length)
             so (power 2 prec) * (power 2 prec) = power 2 (Limb.length) };
    let nph = lsr_mod np0 (prec + 1) in
    assert { nph < power 2 (prec - 1)
             by nph = div np0 (power 2 (prec + 1))
             so nph * power 2 (prec + 1) <= np0 < radix
             so power 2 (prec - 1) * power 2 (prec + 1) = radix };
    assert { power 2 (prec - 1) * rp0 + nph < radix
             by rp0 < power 2 (prec + 1)
             so rp0 <= power 2 (prec + 1) - 1
             so power 2 (prec - 1) * rp0
                <= power 2 (prec + prec) - power 2 (prec - 1)
                = radix - power 2 (prec - 1)
             so nph < power 2 (prec - 1) };
    rp0 <- lsl rp0 (prec - 1) + nph;
    label Div in
    let ref q = Limb.(/) rp0 sp0 in
    assert { q <= power 2 prec
             by rp0 = power 2 (prec - 1) * orp + nph
             so orp <= 2 * sp0
             so nph < power 2 (prec - 1) <= sp0
             so rp0 < power 2 prec * sp0 + sp0
                     = (power 2 prec + 1) * sp0
             so 0 <= mod rp0 sp0
             so rp0 = sp0 * q + mod rp0 sp0
             so q * sp0 <= rp0
             so q * sp0 < (power 2 prec + 1) * sp0
             so q < power 2 prec + 1 };
    assert { q = div rp0 osp };
    begin
      ensures { if old q = power 2 prec
                then q = power 2 prec - 1
                else q = old q }
      ensures { q < power 2 prec }
      let rq = lsr_mod q prec in
      assert { q = power 2 prec -> rq = div q q = 1 by mod q q = 0 };
      q <- q - rq
    end;
    assert { q * sp0 < radix by q < power 2 prec so sp0 < power 2 prec
             so q * sp0 < power 2 prec * power 2 prec = radix };
    assert { rp0 - q * sp0 >= 0
             by q <= div rp0 sp0
             so rp0 = div rp0 sp0 * sp0 + mod rp0 sp0
             so 0 <= mod rp0 sp0
             so rp0 >= div rp0 sp0 * sp0
             so q * sp0 <= div rp0 sp0 * sp0 };
    let u = rp0 - (q * sp0) in
    assert { sp0 * power 2 prec < radix - q
             by sp0 <= power 2 prec - 1
             so sp0 * power 2 prec <= power 2 prec * power 2 prec - power 2 prec
                = radix - power 2 prec < radix - q };
    assert { q <> power 2 prec - 1 -> u <= osp - 1
             by div rp0 osp = q
             so rp0 = osp * div rp0 osp + mod rp0 osp
             so u = mod rp0 osp < osp };
    assert { q = power 2 prec - 1 -> u <= osp + nph
             by rp0 = power 2 (prec - 1) * orp + nph
             so orp <= 2 * osp
             so rp0 <= power 2 prec * osp + nph
             so q = power 2 prec - 1
             so u = rp0 - (power 2 prec - 1) * osp };
    sp0 <- lsl sp0 prec + q;
    assert { sp0 = osp * power 2 prec + q };
    let uh = lsr_mod u (prec - 1) in
    assert { uh <= power 2 (prec + 1)
             by uh = div u (power 2 (prec - 1))
             so uh * power 2 (prec - 1) = u - mod u (power 2 (prec - 1))
                <= u < radix
             so power 2 (prec + 1) * power 2 (prec - 1)
                = power 2 (prec + 1 + prec - 1) = radix };
    let ref cc = to_int64 uh in
    let npl = np0 % (lsl 1 (prec + 1)) in
    assert { np0 = power 2 (prec + 1) * nph + npl
             by np0 = (power 2 (prec + 1)) * div np0 (power 2 (prec + 1))
                      + mod np0 (power 2 (prec + 1))
             so npl = mod np0 (power 2 (prec + 1)) };
    let ul = lsl_mod_ext u (prec + 1) in
    rp0 <- ul + npl;
    assert { q * q < radix by q < power 2 prec
             so q * q < power 2 prec * power 2 prec = radix };
    let q2 = q * q in
    assert { ul + radix * uh = power 2 (prec + 1) * u
             by
               let p = u * power 2 (prec + 1) in
               let m = mod u (power 2 (prec - 1)) in
               mod p radix = ul
               so m < power 2 (prec - 1)
               so power 2 (prec + 1) * m
                  < power 2 (prec + 1) * power 2 (prec - 1) = radix
               so u = power 2 (prec - 1) * uh + m
               so p = power 2 (prec + 1) * power 2 (prec - 1) * uh
                      + power 2 (prec + 1) * m
                    = uh * radix + power 2 (prec + 1) * m
                    < uh * radix + radix
               so uh * radix <= p
               so div p radix = uh
               so p = uh * radix + ul };
    assert { rp0 + radix * cc = npl + power 2 (prec + 1) * u
             by rp0 + radix * cc = npl + ul + radix * uh };
    begin ensures { rp0 + radix * cc = old (rp0 + radix * cc) - q2 }
      label S in
      if rp0 < q2 then cc <- Int64.(-) cc 1;
      rp0 <- sub_mod rp0 q2;
    end;
    assert { sp0 * sp0 + rp0 + radix * cc = np0 + radix * np1
             by rp0 + radix * cc = (power 2 (prec + 1)) * u + npl - q * q
             so sp0 * sp0 = ((power 2 prec) * osp + q)
                            * ((power 2 prec) * osp + q)
                          = power 2 prec * power 2 prec * osp * osp
                            + q * q
                            + 2 * power 2 prec * osp * q
                          = radix * osp * osp + q * q
                            + 2 * power 2 prec * osp * q
             so osp * q = rp0 at Div - u
             so osp * osp = np1 - orp
             so rp0 at Div = power 2 (prec - 1) * orp + nph };
    assert { rp0 + radix * cc <= 2 * sp0
             by rp0 + radix * cc = power 2 (prec + 1) * u + npl - q * q
             so 2 * sp0 = 2 * (power 2 prec * osp + q)
                        >= power 2 (prec + 1) * osp
             so npl < power 2 (prec + 1)
             so if q = power 2 prec - 1
                then
                  u <= osp + nph
                  so power 2 (prec + 1) * u
                     <= power 2 (prec + 1) * (osp + nph)
                  so rp0 + radix * cc
                     <= power 2 (prec + 1) * osp
                        + power 2 (prec + 1) * nph + npl - q * q
                      = power 2 (prec + 1) * osp + np0 - q * q
                  so 2 * sp0 = power 2 (prec + 1) * osp + 2 * q
                  so q * q = (power 2 prec - 1) * (power 2 prec - 1)
                           = radix - power 2 (prec + 1) + 1
                  so rp0 + radix * cc - 2 * sp0
                     <= np0 - q * q - 2 * q
                     <= radix - 1 - q * q - 2 * q
                     = power 2 (prec + 1) - 2 - 2 * q
                     = 0
                else
                  rp0 + radix * cc <= power 2 (prec + 1) * (osp - 1) + npl
                                   <= power 2 (prec + 1) * osp };
    label Adjust in
    let ghost sp0a = pure { sp0 } in
    if Int64.(<) cc 0 (* cc = -1 *)
    then begin
      assert { cc = -1 };
      assert { sp0 + sp0 > radix
               by sp0 * sp0 + rp0 - radix = np0 + radix * np1
               so np1 >= power 2 (Limb.length - 2)
               so rp0 < radix
               so sp0 * sp0 > np0 + radix * np1 >= radix * np1
                  >= power 2 (Limb.length) * power 2 (Limb.length - 2)
                  = power 2 (Limb.length + Limb.length - 2)
                  = power 2 (Limb.length - 1) * power 2 (Limb.length - 1)
               so sp0 > power 2 (Limb.length - 1) };
      begin ensures { rp0 + radix * cc = old (rp0 + radix * cc) + sp0 }
            ensures { cc >= 0 \/ rp0 = old rp0 + sp0 }
        rp0 <- add_mod rp0 sp0;
        if rp0 < sp0 then cc <- Int64.(+) cc 1;
      end;
      sp0 <- sp0 - 1;
      begin ensures { rp0 + radix * cc = old (rp0 + radix * cc) + sp0 }
            ensures { cc >= 0 }
        label A2 in
        rp0 <- add_mod rp0 sp0;
        if rp0 < sp0 then cc <- Int64.(+) cc 1
      end;
      assert { sp0 * sp0 + rp0 + radix * cc
               = (sp0 * sp0 + rp0 + radix * cc) at Adjust
               by sp0 = sp0a - 1
               so sp0 * sp0 = sp0a * sp0a - sp0a - sp0a + 1
               so rp0 + radix * cc = rp0 + radix * cc at Adjust + sp0 + sp0a };
    end;
    C.set rp rp0;
    C.set sp sp0;
    assert { value np 2 = np0 + radix * np1 };
    of_int64 cc

  use toom.Toom

  let rec wmpn_dc_sqrtrem (sp np: ptr limb) (n:int32) (scratch: ptr limb) : limb
    requires { valid np (n+n) }
    requires { valid sp n }
    requires { 1 <= n }
    requires { valid scratch (1 + div n 2) }
    requires { (pelts np)[offset np + n + n - 1] >= power 2 (Limb.length - 2) }
    requires { writable sp /\ writable scratch /\ writable np }
    requires { 4 * n < max_int32 }
(*    writes   { np, sp, scratch }*)
    ensures  { (value sp n) * (value sp n)
               + value np n + (power radix n) * result
               = old value np (n+n) }
    ensures  { value np n + power radix n * result <= 2 * value sp n }
    ensures  { (pelts sp)[offset sp + n-1] >= power 2 (Limb.length - 1) }
    ensures  { 0 <= result <= 1 }
    ensures  { max np = old max np }
    ensures  { min np = old min np }
    ensures  { plength np = old plength np }
    ensures  { max scratch = old max scratch }
    ensures  { min scratch = old min scratch }
    ensures  { plength scratch = old plength scratch }
    ensures  { max sp = old max sp }
    ensures  { min sp = old min sp }
    ensures  { plength sp = old plength sp }
    variant  { n }
  = label Start in
    if n = 1
    then
      let r = wmpn_sqrtrem2 sp scratch np in
      C.set np (C.get scratch);
      r
    else begin
      let l = n / 2 in
      assert { 1 <= l };
      let h = n - l in
      let ghost vn = value np (int32'int n + int32'int n) in
      value_concat np (l+l) (n+n);
      let np' = C.incr_split np (l+l) in
      value_concat np l (l+l);
      let ghost n0 = value np (int32'int l) in
      let ghost n1 = value_sub (pelts np) (offset np + int32'int l)
                               (offset np + int32'int l + int32'int l) in
      let ghost n'' = pure { n0 + power radix l * n1 } in
      let ghost n' = pure { value np' (h+h) } in
      assert { value np (l+l) = n''};
      assert { vn = n'' + power radix (l+l) * n' };
      begin ensures { power radix (n+n) <= 4 * vn }
        value_tail np (n+n-1);
        assert { 4 * vn >= power radix (n+n)
                 by vn = value np (n+n-1)
                         + power radix (n+n-1) * (pelts np)[offset np + (n+n-1)]
                       >= power radix (n+n-1) * (pelts np)[offset np + (n+n-1)]
                       >= power radix (n+n-1) * power 2 (Limb.length - 2)
                 so 4 * power 2 (Limb.length - 2) = radix
                 so 4 * vn >= power radix (n+n-1) * radix = power radix (n+n) };
      end;
      let spl = C.incr_split sp l in
      label Rec in
      let ref q = wmpn_dc_sqrtrem spl np' h scratch in
      assert { n' = value spl h * value spl h
                    + value np' h + power radix h * q };
      begin ensures { power radix l <= 2 * value spl h }
        assert { power radix (h+h) * power radix (l+l)
                 = power radix (n+n)
                 <= 4 * (n'' + power radix (l+l) * n')
                 < 4 * (n' + 1) * power radix (l+l)
                 by n'' < power radix (l+l) };
        assert { power radix (h+h) <= 4 * n' };
        assert { (value spl h + 1) * (value spl h + 1) > n' };
        let ghost ts = pure { 2 * (value spl h + 1) } in
        assert { power radix h < ts
                 by power radix h * power radix h <= 4 * n' < ts * ts
                 so 0 < power radix h * power radix h < ts * ts
                 so 0 < power radix h so 0 < ts
                 so 0 < (ts - power radix h) * (ts + power radix h)
                 so 0 < ts + power radix h
                 so 0 < ts - power radix h };
        assert { power radix l <= 2 * value spl h
                 by power radix l <= power radix h
                 so power radix h < ts
                 so power radix l < ts};
      end;
      let ghost r' = value np' (int32'int h) + power radix (int32'int h) * (l2i q) in
      assert { r' <= 2 * value spl h };
      label Sub in
      begin ensures {    (q = 1 /\ value np' h = r' - value spl h)
                      \/ (q = 0 /\ value np' h = r') }
      if (q <> 0) then begin
        assert { q = 1 };
        assert { value np' h = r' - power radix h };
        assert { value np' h < value spl h
                 by value np' h + power radix h = value np' h + power radix h * q
                    <= 2 * value spl h
                 so value np' h < power radix h
                 so value np' h + value np' h < 2 * value spl h };
        let ghost b = wmpn_sub_n_in_place np' spl h in
        assert { b = 1 };
        assert { value np' h = r' - value spl h
                 by value np' h - power radix h = value np' h at Sub - value spl h
                    = r' - power radix h - value spl h };
        end
      end;
      label Join1 in
      let ghost onp = { np } in
      let ghost onp' = { np' } in
      join np np';
      value_sub_frame (pelts np) (pelts onp') (offset np + p2i l + p2i l)
                                      (offset np + p2i l + p2i l + p2i h);
      value_sub_frame (pelts np) (pelts onp) (offset np) (offset np + p2i l);
      value_sub_frame (pelts np) (pelts onp) (offset np + p2i l) (offset np + p2i l + p2i l);
      assert { value_sub (pelts np) (offset np + l + l) (offset np + l + l + h)
               = value onp' h };
      let npl = C.incr_split np l in
      assert { value_sub (pelts npl) (offset npl + l) (offset npl + l + h)
               = value np' h at Join1 };
      value_concat npl l n;
      assert { value npl n = n1 + power radix l * value onp' h };
      label DivS in
      wmpn_tdiv_qr_in_place scratch 0 npl n spl h;
      assert { n1 + power radix l * (r' - q * value spl h)
               = value scratch (l+1) * value spl h + value npl h };
      value_tail scratch l;
      value_tail spl h;
      assert { 0 <= (pelts scratch)[offset scratch + l] <= 1
               by value (npl at DivS) n < power radix n
               so (pelts spl)[offset spl + h-1] >= power 2 (Limb.length - 1)
               so 2 * (pelts spl)[offset spl + h - 1] >= radix
               so value spl h
                  >= power radix (h-1) * (pelts spl)[offset spl + h-1]
               so 2 * value spl h
                  >= power radix (h-1) * 2 * (pelts spl)[offset spl + h - 1]
                  >= power radix (h-1) * radix = power radix h
               so 0 <= value npl h
               so value spl h * value scratch (l+1) < power radix n
                  = power radix h * power radix l
                  <= value spl h * 2 * power radix l
               so value scratch (l+1) < 2 * power radix l
               so value scratch (l+1)
                  = value scratch l + power radix l
                    * (pelts scratch)[offset scratch + l]
                  >= power radix l * (pelts scratch)[offset scratch + l]
               so (pelts scratch)[offset scratch + l] < 2 };
      let sl = get_ofs scratch l in
      value_concat scratch l (l+1);
      assert { value scratch (l+1) = value scratch l + power radix l * sl };
      q <- q + sl;
      assert { 0 <= q <= 2 };
      assert { n1 + power radix l * r'
               = (value scratch l + power radix l * q) * value spl h
                 + value npl h };
      let sh = C.get scratch in
      let ref c = to_int64 (sh % 2) in
      value_concat scratch 1 l;
      assert { c = mod (value scratch l) 2
               by let st = value_sub (pelts scratch) (offset scratch + 1) (offset scratch + l) in
                  value scratch l = sh + radix * st
               so sh = value scratch 1
               so let q = div sh 2 in
                  c = mod sh 2
               so sh = c + 2 * q
               so value scratch l = c + 2 * q + radix * st
                  = c + 2 * (q + power 2 (Limb.length - 1) * st) };
      let ghost r = wmpn_rshift_sep sp scratch l 1 in
      label Div2 in
      assert { 2 * value sp l + c = value scratch l
               by r + radix * value sp l = value scratch l * power 2 (Limb.length - 1)
               so let p = power 2 (Limb.length - 1) in
                  2 * p = radix
               so p * (2 * value sp l) + r = p * value scratch l };
      let st = C.get_ofs sp (l-1) in
      value_tail sp (l-1);
      assert { value sp l = value sp (l-1) + power radix (l-1) * st };
      assert { st + power 2 (Limb.length - 1) < radix
               by 2 * value sp l <= value scratch l < power radix l
               so value sp l  >= power radix (l-1) * st
               so (2 * st) * power radix (l-1) < power radix l
                     = radix * power radix (l-1)
               so 2 * st < radix
               so st < power 2 (Limb.length - 1) };
      let ql = lsl_mod_ext q (Limb.of_int Limb.length - 1) in
      let qh = lsr_mod q 1 in
      assert { 0 <= qh <= 1
               by 0 <= q <= 2
               so qh = div q 2 };
      assert { ql + radix * qh = power 2 (Limb.length - 1) * q
               by qh = div q 2
               so q = 2 * qh + mod q 2
               so power 2 (Limb.length - 1) * q
                  = radix * qh + power 2 (Limb.length - 1) * mod q 2
               so (0 <= power 2 (Limb.length - 1) * mod q 2 < radix
                   by mod q 2 = 0 \/ mod q 2 = 1)
               so ql = mod (q * power 2 (Limb.length - 1)) radix
                     = mod (radix * qh + power 2 (Limb.length - 1) * mod q 2) radix
                     = mod (power 2 (Limb.length - 1) * mod q 2) radix
                     = power 2 (Limb.length - 1) * mod q 2
               };
      value_sub_update_no_change (pelts sp) (sp.offset + p2i l - 1)
                                 (sp.offset) (sp.offset + p2i l - 1) (st + ql);
      C.set_ofs sp (l-1) (st + ql);
      value_tail sp (l-1);
      assert { value sp l = value sp l at Div2 + power radix (l-1) * ql
               by value sp l = value sp (l-1) + power radix (l-1) * (st + ql)
               so value sp (l-1) = value sp (l-1) at Div2 };
      (* TODO if (UNLIKELY ((sp[0] & approx) != 0)) /* (sp[0] & mask) > 1 */
	return 1; /* Remainder is non-zero */ *)
      q <- qh;
      assert { 2 * (value sp l + power radix l * q) + c
                 = value scratch l + power radix l * (q at Div2) };
      assert { n1 + power radix l * r'
               = 2 * value spl h * (value sp l + power radix l * q)
                 + value spl h * c
                 + value npl h
               by 2 * value spl h * (value sp l + power radix l * q)
                  + value spl h * c
                  = value spl h * (2 * (value sp l + power radix l * q) + c)
                  = value spl h
                    * (value scratch l + power radix l * (q at Div2)) };
      assert { value npl h < value spl h };
      begin
        ensures { n1 + power radix l * r'
                  = 2 * value spl h * (value sp l + power radix l * q)
                    + value npl h + power radix h * c }
        ensures { 0 <= c <= 1 }
        ensures { 0 <= value npl h + power radix h * c < 2 * value spl h }
        if not (Int64.(=) c 0)
        then begin
          assert { c = 1 };
          assert { n1 + power radix l * r'
               = 2 * value spl h * (value sp l + power radix l * q)
                 + value spl h + value npl h };
          let c' = wmpn_add_n_in_place npl spl h in
          c <- to_int64 c';
          end
      end;
      let ghost dq = pure { value sp l + power radix l * q } in
      let ghost s' = pure { value spl h } in
      let ghost r'' = pure { value npl h + power radix h * c } in
      assert { n1 + power radix l * r'
                  = (2 * s') * dq + r''};
      assert { r'' < 2 * s' };
      assert { 0 <= dq <= power radix l
               by n1 < power radix l <= 2 * s'
               so r' <= 2 * s'
               so n1 + power radix l * r'
                  < 2 * s' + power radix l * r'
                  <= 2 * s' + power radix l * (2 * s')
                  = 2 * s' * (1 + power radix l)
               so 0 <= r''
               so (2 * s') * dq <= (2 * s') * dq + r'' = n1 + power radix l * r'
               so (2 * s') * dq < (2 * s') * (1 + power radix l)
               so dq < 1 + power radix l
               so 0 <= value sp l
               so 0 <= q };
      let ghost onp = pure { np } in
      let ghost onpl = pure { npl } in
      join np npl;
      value_sub_frame (pelts np) (pelts onpl)
                      (offset np + p2i l) (offset np + p2i n);
      value_sub_frame (pelts np) (pelts onp) (offset np) (offset np + p2i l);
      assert { value_sub (pelts np) (offset np + l) (offset np + n)
               = value onpl h by offset npl + h = offset np + n};
      assert { value np l = value onp l = n0 };
      value_concat np l n;
      assert { value np n + power radix n * c = n0 + power radix l * r''
               by value np n = n0 + power radix l * value onpl h
               so power radix n = power radix (l+h)
                  = power radix l * power radix h
               so power radix n * c = power radix l * (power radix h * c)
               so value np n + power radix n * c
                  = n0 + power radix l * value onpl h
                    + power radix l * (power radix h * c) };
      let npn = C.incr_split np n in
      let ghost _ = wmpn_mul npn sp l sp l 64 in
      let ll = 2 * l in
      assert { value npn ll + power radix ll * q = dq * dq
               by 0 <= q <= 1 so q * q = q
               so dq <= power radix l
               so power radix ll = power radix l * power radix l
               so value sp l + power radix l * q <= power radix l
               so (value sp l = 0 \/ q = 0
                   by 0 <= value sp l
                   so if q = 1
                      then value sp l = 0
                      else q = 0)
               so value sp l * q = 0
               so dq * dq = value sp l * value sp l
                  + (power radix l * q) * (power radix l * q)
                  = value sp l * value sp l + power radix ll * q };
      label Sub2 in
      let ghost onp = pure { np } in
      value_concat np ll n;
      let bo = wmpn_sub_n_in_place np npn ll in
      value_concat np ll n;
      value_sub_frame (pelts np) (pelts onp) (offset np + int32'int ll)
                                             (offset np + int32'int n);
      let b = q + bo in
      assert { value np ll - power radix ll * b
               = value np ll at Sub2 - dq * dq };
      assert { value np n - power radix ll * b
               = value np n at Sub2 - dq * dq
               by value np n = value np ll + power radix ll
                    * value_sub (pelts np) (offset np + ll) (offset np + n)
               so value_sub (pelts np) (offset np + ll) (offset np + n)
                  = value_sub (pelts onp) (offset np + ll) (offset np + n) };
      begin ensures { value np n + power radix n * c
                      = n0 + power radix l * r'' - dq * dq }
            ensures { - 1 <= c <= 1 }
        if l = h
        then begin
          assert { n = ll };
          assert { value np n - power radix n * b
                   = value np n at Sub2 - dq * dq };
          c <- Int64.(-) c (to_int64 b);
          assert { value np n + power radix n * c
                   = value np n - power radix n * b
                     + power radix n * (c at Sub2)
                   = (value np n + power radix n * c at Sub2) - dq * dq
                   = n0 + power radix l * r'' - dq * dq };
          assert { -1 <= c
                   by dq * dq <= power radix l * power radix l = power radix n
                   so 0 <= n0 so 0 <= r''
                   so 0 <= n0 + power radix l * r''
                   so - power radix n <= n0 + power radix l * r'' - dq * dq
                   so value np n < power radix n
                   so - power radix n <= value np n + power radix n * c
                      < power radix n + power radix n * c
                   so - 2 * power radix n < power radix n * c
                   so -2 < c };
        end
        else begin
          assert { h = l + 1
                   by n = 2 * l + ComputerDivision.mod n 2
                   so h = l + ComputerDivision.mod n 2
                   so h <= l + 1 };
          assert { n = ll + 1 };
          let nll = C.incr np ll in
          label Borrow in
          let ghost onp = pure { np } in
          value_concat np ll n;
          let bo = wmpn_sub_1_in_place nll 1 b in
          value_sub_frame (pelts np) (pelts onp) (offset np)
                          (offset np + int32'int ll);
          value_concat np ll n;
          assert { value nll 1 = value_sub (pelts np) (offset np + ll) (offset np + n) };
          assert { value np n - power radix n * bo
                   = value np n at Borrow - power radix ll * b
                   by value np ll = value np ll at Borrow
                   so value nll 1 - radix * bo = value nll 1 at Borrow - b
                   so value np n - power radix n * bo
                      = value np ll + power radix ll * value nll 1 - power radix n * bo
                      = value np ll + power radix ll * (value nll 1 - radix * bo)
                      = value np ll + power radix ll * (value nll 1 at Borrow - b)};
          c <- Int64.(-) c (to_int64 bo);
          assert { value np n + power radix n * c
                   = value np n - power radix n * bo
                     + power radix n * (c at Sub2)
                   = value np n at Borrow - power radix ll * b
                     + power radix n * c at Sub2
                   = (value np n + power radix n * c at Sub2) - dq * dq
                   = n0 + power radix l * r'' - dq * dq };
        end
      end;
      let ghost vs = pure { dq + power radix l * s' } in
      let ghost vr = pure { power radix l * r'' + n0 - dq * dq } in
      assert { vn = vs * vs + vr
               by vn = n' * power radix (l+l) + n1 * power radix l + n0
               so n' = s' * s' + r'
               so power radix (l+l) = power radix l * power radix l
               so vn = s' * s' * power radix (l+l) + r' * power radix (l+l)
                     + n1 * power radix l + n0
                     = (s' * power radix l) * (s' * power radix l)
                       + r' * power radix (l+l) + n1 * power radix l + n0
                     = (s' * power radix l) * (s' * power radix l)
                       + power radix l * (r' * power radix l + n1) + n0
               so r' * power radix l + n1 = 2 * s' * dq + r''
               so vn = (s' * power radix l) * (s' * power radix l)
                       + power radix l * (2 * s' * dq + r'') + n0
                     = (s' * power radix l) * (s' * power radix l)
                       + 2 * (s' * power radix l * dq)
                       + dq * dq + power radix l * r'' + n0 - dq * dq
               so (s' * power radix l) * (s' * power radix l)
                       + 2 * (s' * power radix l * dq)
                       + dq * dq
                   = vs * vs
               so vn = vs * vs + power radix l * r'' + n0 - dq * dq };
      assert { vr <= 2 * vs
               by n0 < power radix l
               so r'' <= 2 * s' - 1
               so r'' * power radix l + n0
                    < (2 * s' - 1) * power radix l + power radix l
                    = 2 * s' * power radix l
               so dq * dq >= 0
               so vr <= r'' * power radix l + n0 };
      label Adjust in
      assert { dq = value sp l + power radix l * q };
      assert { vr = value np n + power radix n * c };
      assert { value spl h = s' };
      assert { power radix n = power radix l * power radix h };
      assert { (vs - 1) * (vs - 1) <= vn
               by vn = (vs - 1) * (vs - 1) + vr + 2 * vs - 1
               so dq - 1 <= power radix l <= 2 * s'
               so (dq - 1) * (dq - 1) <= (2 * s') * (power radix l)
               so 0 <= n0
               so power radix l * r'' >= 0
               so n0 >= 0
               so vr + 2 * vs - 1
                  = power radix l * r'' + n0 - dq * dq + 2 * dq
                    + 2 * s' * power radix l - 1
                  = power radix l * r'' + n0
                    + 2 * s' * power radix l - (dq - 1) * (dq - 1)
                  >= power radix l * r'' + n0
                  >= 0 };
      assert { vs <= power radix n
               by (vs - 1) * (vs - 1) <= vn
                  < power radix (n+n) = power radix n *  power radix n
               so if vs - 1 < power radix n
                  then true
                  else false
                       by power radix n * power radix n <= (vs - 1) * (vs - 1) };
      if (Int64.(<) c 0)
      then begin
        assert { vr < 0
                 by value np n < power radix n
                 so power radix n * c <= - power radix n };
        q <- wmpn_add_1_in_place spl h q;
        assert { q = 0 \/ q = 1 };
        assert { value sp l + power radix l * value spl h + power radix n * q
                   = vs
                 by value spl h + power radix h * q
                    = s' + q at Adjust
                 so value sp l + power radix l * value spl h + power radix n * q
                    = value sp l
                      + power radix l * (value spl h + power radix h * q)
                    = value sp l + power radix l * s'
                      + power radix l * (q at Adjust)
                    = dq + power radix l * s' = vs };
        let ghost osp = pure { sp } in
        let ghost ospl = pure { spl } in
        join sp spl;
        value_sub_frame (pelts sp) (pelts osp) (offset sp)
                                (offset sp + int32'int l);
        value_sub_frame (pelts sp) (pelts ospl) (offset sp + int32'int l)
                                                (offset sp + int32'int n);
        value_concat sp l n;
        assert { value sp n = value osp l + power radix l * value ospl h
                 by value ospl h = value_sub (pelts ospl) (offset sp + l)
                    (offset sp + n)};
        assert { value sp n + power radix n * q = vs };
        assert { q = 0 \/ value sp n = 0
                 by 0 <= value sp n
                 so if q = 1
                    then value sp n = 0
                    else q = 0 };
        let c' = wmpn_addmul_1 np sp n 2 in
        assert { c' = 0 \/ (q = 0 /\ c' <= 2)
                 by if q = 1
                    then value sp n = 0
                         so value np n + power radix n * c'
                            = value np n at Adjust < power radix n
                         so power radix n * c' < power radix n * 1
                         so c' = 0
                    else value np n + power radix n * c'
                           = value np n at Adjust + 2 * vs
                         so value np n at Adjust < power radix n
                         so vs <= power radix n
                         so value np n + power radix n * c' < 3 * power radix n
                         so 0 <= value np n
                         so c' < 3 };
        c <- Int64.(+) c (to_int64 (2 * q + c'));
        assert { value np n + power radix n * c
                 = value np n at Adjust + 2 * value sp n
                   + power radix n * (2 * q)
                   + power radix n * (c at Adjust)
                 = vr + power radix n * (2 * q)
                      + 2 * value sp n
                 = vr + 2 * vs };
        c <- Int64.(-) c (to_int64 (wmpn_sub_1_in_place np n 1));
        assert { value np n + power radix n * c = vr + 2 * vs - 1 };
        assert { 0 <= c
                 by 0 <= vr + 2 * vs - 1
                 so 0 <= value np n + power radix n * c
                 so value np n < power radix n
                 so -1 < c };
        label AdjS in
        let bo = wmpn_sub_1_in_place sp n 1 in
        assert { bo = 1 -> q = 1
                 by value sp n - power radix n * bo
                    = value sp n at AdjS - 1
                    so value sp n < power radix n
                    so value sp n - power radix n * bo < 0
                    so value sp n at AdjS = 0
                    so vs = power radix n * q
                    so 0 < vs };
        assert { q = 1 -> bo = 1
                 by value sp n at AdjS + power radix n = vs <= power radix n
                 so value sp n at AdjS = 0
                 so value sp n - power radix n * bo = - 1
                 so 0 <= value sp n };
        q <- q - bo;
        assert { q = 0 };
        assert { value sp n = vs - 1 };
        assert { (value sp n) * (value sp n) + value np n + power radix n * c
                   = vn
                 by (value sp n) * (value sp n) = (vs - 1) * (vs - 1)
                    = vs * vs - 2 * vs + 1
                 so value np n + power radix n * c = vr + 2 * vs - 1 };
        assert { value np n + power radix n * c <= 2 * value sp n
                 by value np n + power radix n * c = vr + 2 * vs - 1
                    <= 2 * vs - 1 };
      end
      else begin
        assert { 0 <= vr
                 by 0 <= value np n
                 so 0 <= power radix n * c };
        let ghost osp = pure { sp } in
        let ghost ospl = pure { spl } in
        join sp spl;
        value_sub_frame (pelts sp) (pelts osp) (offset sp)
                                 (offset sp + int32'int l);
        value_sub_frame (pelts sp) (pelts ospl) (offset sp + int32'int l)
                                                (offset sp + int32'int n);
        value_concat sp l n;
        assert { value sp n = value osp l + power radix l * s'
                 by s' = value ospl h
                    = value_sub (pelts ospl) (offset sp + l) (offset sp + n) };
        assert { vs = value sp n + power radix l * q };
        assert { dq * dq < power radix l * (r'' + 1)
                 by 0 <= vr
                 so dq * dq <= power radix l * r'' + n0
                 so n0 < power radix l
                 so power radix l * r'' + n0 < power radix l * (r'' + 1) };
        assert { q = 1 -> dq = power radix l };
        assert { q = 1 -> r'' < power radix l
                 by r' * power radix l + n1 = 2 * s' * dq + r''
                 so dq = power radix l
                 so r' * power radix l + n1 = (2 * s') * power radix l + r''
                 so r' <= 2 * s'
                 so r' * power radix l <= (2 * s') * power radix l
                 so r'' <= n1 < power radix l };
        assert { q = 1 -> false
                 by r'' + 1 <= power radix l
                 so power radix l * power radix l = dq * dq
                    < power radix l * (r'' + 1)
                    <= power radix l * power radix l };
        assert { q = 0 };
        assert { vs = value sp n };
      end;
      let ghost onp = pure { np } in
      join np npn;
      value_sub_frame (pelts np) (pelts onp) (offset np)
                              (offset np + int32'int n);
      assert { value np n = value onp n };
      value_tail sp (n-1);
      let ghost ms = C.get_ofs sp (n-1) in
      let ghost sqrt = pure { value sp n } in
      assert { sqrt = value sp (n-1) + power radix (n-1) * ms };
      assert { (sqrt + 1) * (sqrt + 1) > vn };
      assert { ms >= power 2 (Limb.length - 1)
               by power radix (n+n) <= 4 * vn
               so if (2 * (sqrt + 1)) <= power radix n
                  then (false
                        by 4 * vn <= (2 * (sqrt + 1)) * (2 * (sqrt + 1))
                                  <= power radix n * power radix n
                                  = power radix (n+n))
                  else true
               so power radix n < 2 * (sqrt + 1)
               so value sp (n-1) < power radix (n-1)
               so 1 + value sp (n-1) <= power radix (n-1)
               so sqrt + 1 <= power radix (n-1) * (ms + 1)
               so power radix n = power radix (n-1) * radix
               so power radix (n-1) * radix < power radix (n-1) * (2 * (ms + 1))
               so 0 < power radix (n-1)
               so 0 <= radix so 0 <= 2 * (ms + 1)
               so radix < 2 * (ms + 1)
               so radix = 2 * power 2 (Limb.length - 1) };
      of_int64 c
    end

  let ghost function ceilhalf (n:int)
    ensures { 2 * result >= n }
    ensures { 2 * (result + 1) > n }
  = ComputerDivision.div (n+1) 2

  (* TODO rp = NULL case? *)

  let lemma sqrt_norm (n nn c s s0 s1 : int)
    requires { 0 <= c }
    requires { 0 < n }
    requires { 0 <= s }
    requires { 0 <= s0 < power 2 c }
    requires { nn = power 2 (2 * c) * n }
    requires { s1 = power 2 c * s + s0 }
    requires { s1 * s1 <= nn < (s1 + 1) * (s1 + 1) }
    ensures  { s * s <= n < (s+1) * (s+1) }
  =
    assert { power 2 (2 * c) = power 2 c * power 2 c };
    assert { s * s <= n < (s + 1) * (s + 1)
             by 0 <= s so 0 <= power 2 c
             so power 2 (2 * c) * (s * s)
                = (power 2 c * s) * (power 2 c * s)
                <= s1 * s1 <= nn = power 2 (2 * c) * n
             so power 2 (2 * c) * (s * s) <= power 2 (2 * c) * n
             so 0 <= power 2 (2 * c) so 0 <= s * s so 0 <= n
             so s * s <= n
             so s0 < power 2 c
             so (s1 + 1) = power 2 c * s + s0 + 1
                <= power 2 c * s + power 2 c
                = power 2 c * (s + 1)
             so power 2 (2 * c) * n = nn < (s1 + 1) * (s1 + 1)
                <= (power 2 c * (s + 1)) * (power 2 c * (s + 1))
                = power 2 (2 * c) * ((s + 1) * (s + 1))
             so power 2 (2 * c) * n
                < power 2 (2 * c) * ((s + 1) * (s + 1))
             so 0 < power 2 (2 * c) so 0 < (s + 1) * (s + 1) so 0 < n
             so n < (s + 1) * (s + 1) }

  let rec wmpn_sqrtrem (sp rp np: ptr limb) (n: int32) : int32
    requires { valid sp (ceilhalf n) }
    requires { valid rp n }
    requires { valid np n }
    requires { writable sp /\ writable rp /\ writable np }
    requires { 1 <= n }
    requires { 4 * n < max_int32 }
    requires { (pelts np)[offset np + n - 1] > 0 }
    ensures  { value np n = value sp (ceilhalf n) * value sp (ceilhalf n)
                            + value rp result }
    ensures  { 0 <= result <= n }
    ensures  { value rp result <= 2 * value sp (ceilhalf n) }
    ensures  { result > 0 -> (pelts rp)[offset rp + result - 1] > 0 }
    ensures  { forall j. (pelts np)[j] = old (pelts np)[j] }
    ensures  { max np = old max np }
    ensures  { min np = old min np }
    ensures  { plength np = old plength np }
    ensures  { max rp = old max rp }
    ensures  { min rp = old min rp }
    ensures  { plength rp = old plength rp }
    ensures  { max sp = old max sp }
    ensures  { min sp = old min sp }
    ensures  { plength sp = old plength sp }
    variant  { n }
  =
    label Start in
    let ghost k = ceilhalf (int32'int n) in
    let high = C.get_ofs np (n-1) in
    let ref c = (of_int32 (count_leading_zeros high)) / 2 in
    assert { power 2 (2 * c) * high < radix };
    assert { power 2 (2 * c) * high <= radix - power 2 (2 * c)
             by let p = power 2 (2 * c) in
                let q = power 2 (64 - 2 * c) in
                let r = p * high in
                radix = p * q
                so mod r p = mod (p * high + 0) p = 0
                so r = p * div r p
                so r < p * q
                so div r p < q
                so r <= p * (q - 1) = radix - power 2 (2 * c) };
    assert { 4 * power 2 (2 * c) * high >= radix };
    if n = 1
    then begin
      assert { k = 1 };
      value_tail np 0;
      assert { value np n = high };
      if c = 0
      (* TODO if high & 0xc000_0000_0000_0000 *)
      then begin
        let s = sqrt1 rp high in
        C.set sp s;
        value_tail sp 0;
        assert { value sp k = s };
      end
      else begin
        let nh = lsl high (2 * c) in
        assert { nh = power 2 (2 * c) * high
                 so 4 * nh >= radix   };
        let ncc = sqrt1 rp nh in
        let cc = lsr_mod ncc c in
        let ghost s0 = pure { mod ncc (power 2 c) } in
        assert { ncc = power 2 c * cc + s0 };
        assert { power 2 c * cc <= ncc
                 by 0 <= s0 };
        sqrt_norm (uint64'int high) (uint64'int nh) (uint64'int c)
                  (uint64'int cc) s0 (uint64'int ncc);
        C.set sp cc;
        value_tail sp 0;
        assert { value sp k = cc };
        C.set rp (high - cc * cc);
        assert { value rp 1 = high - cc * cc };
      end;
      let res = if C.get rp = 0 then 0 else 1 in
      value_tail rp 0;
      assert { value rp res = value rp 1 = (pelts rp)[offset rp] };
      return res
    end;
    let ref tn = (n + 1) / 2 in
    assert { tn = k };
    let ref rn : int32 = 0 in
    let adj = to_int32 ((of_int32 n) % 2) in
    assert { 2 * tn = n + adj };
    let scratch = salloc (UInt32.(+) (UInt32.(/) (UInt32.of_int32 tn) 2) 1) in
    if (adj <> 0 || c <> 0)
    then begin
      let ref tp = salloc (UInt32.(*) 2 (UInt32.of_int32 tn)) in
      C.set tp 0;
      begin ensures { value tp (n+adj)
                      = power 2 (2 * c) * power radix adj * value np n }
            ensures { 4 * value tp (n+adj) >= power radix (n+adj) }
            ensures { max tp = old max tp }
            ensures { plength tp = old plength tp }
            ensures { min np = old min np /\ max np = old max np
                      /\ plength np = old plength np }
            ensures  { forall j. (pelts np)[j] = old (pelts np)[j] }
        assert { value tp adj = 0
                 by value tp 1 = 0
                 so adj = 0 \/ adj = 1 };
        let ghost otp = pure { tp } in
        let tpa = C.incr_split tp adj in
        label Shift in
        (if c <> 0
        then begin
          value_tail np (n-1);
          assert { value np n * power 2 (2 * c) < power radix n
                   by value np n = value np (n-1) + power radix (n-1) * high
                   so high * power 2 (2 * c) <= radix - power 2 (2 * c)
                   so power 2 (2 * c) * value np (n-1)
                      < power 2 (2 * c) * power radix (n-1)
                   so power radix (n-1) * (high * power 2 (2 * c))
                      <= power radix (n-1) * (radix - power 2 (2 * c))
                   so value np n * power 2 (2 * c)
                      = value np (n-1) * power 2 (2 * c)
                        + power radix (n-1) * high * power 2 (2 * c)
                      < power 2 (2 * c) * power radix (n-1)
                        + power radix (n-1) * (radix - power 2 (2 * c))
                      = power radix (n-1) * radix = power radix n };
          label Shift in
          let ghost h = wmpn_lshift_sep tpa np n (2 * c) in
          value_sub_frame (pelts np) (pure { pelts np at Shift })
                          (offset np) (offset np + int32'int n);
          assert { value np n = value np n at Shift };
          assert { h = 0
                   by value np n * power 2 (2 * c) < power radix n
                   so value tpa n + power radix n * h < power radix n
                   so 0 <= value tpa n
                   so h < 1 };
          assert { 4 * value tpa n >= power radix n
                   by value np n >= power radix (n-1) * high
                   so value tpa n
                      = power 2 (2 * c) * value np n
                      >= power 2 (2 * c) * power radix (n-1) * high
                   so 4 * power 2 (2 * c) * high >= radix
                   so power radix n
                      = power radix (n-1) * radix
                      <= power radix (n-1) * (4 * power 2 (2 * c) * high)
                      <= 4 * value tpa n };
        end
        else begin
          wmpn_copyi tpa np n;
          assert { 4 * high >= radix };
          assert { 4 * value tpa n >= power radix n
                   by value np n >= power radix (n-1) * high
                   so power radix n
                      = power radix (n-1) * radix
                      <= power radix (n-1) * 4 * high
                      <= 4 * value np n = 4 * value tpa n };
          assert { value tpa n = value np n };
        end);
        let otpa = pure { tpa } in
        join tp tpa;
        value_sub_frame (pelts tp) (pelts otp) 0 (int32'int adj);
        value_sub_frame (pelts tp) (pelts otpa) (int32'int adj)
                                  (int32'int adj + int32'int n);
        assert { value_sub (pelts tp) adj (n+adj) = value otpa n };
        assert { value tp adj = 0 };
        value_concat tp adj (n+adj);
        assert { value tp (n+adj) = power radix adj * value otpa n };
        assert { 4 * value tp (n+adj) >= power radix (n + adj)
                 by power radix (n+adj) = power radix adj * power radix n
                 <= power radix adj * (4 * value otpa n)
                 = 4 * value tp (n+adj) };
      end;
      c <- c + (if adj <> 0 then 32 else 0);
      assert { 0 <= c <= 63 };
      assert { value tp (n+adj) = power 2 (2 * c) * value np n };
      (*let mask = lsl 1 c - 1 in*)
      value_tail tp (tn + tn - 1);
      let ghost h = pure { (pelts tp)[tn + tn - 1] } in
      assert { h >= power 2 (Limb.length - 2)
               by value tp (n+adj)
                  = value tp (tn + tn - 1) + power radix (tn + tn - 1) * h
                  < power radix (tn + tn - 1) + power radix (tn + tn - 1) * h
                  = power radix (tn + tn - 1) * (h+1)
               so power radix (tn + tn) <= 4 * value tp (n+adj)
                  < power radix (tn + tn - 1) * 4 * (h+1)
               so power radix (tn + tn) = power radix (tn + tn - 1) * radix
               so power radix (tn + tn - 1) * radix
                  < power radix (tn + tn - 1) * (4 * (h+1))
               so radix < 4 * (h+1)
               so radix = 4 * power 2 (Limb.length - 2)
               so power 2 (Limb.length - 2) < h+1 };
      let ghost vn = pure { value np n } in
      let ghost vn1 = pure { value tp (n+adj) } in
      assert { vn1 = power 2 (2 * c) * vn };
      let ref rl = wmpn_dc_sqrtrem sp tp tn scratch in
      let ghost vs = pure { value sp tn } in
      let ghost vr = pure { value tp tn + power radix tn * rl } in
      assert { 0 <= vr
               by 0 <= value tp tn
               so 0 <= rl
               so 0 <= power radix tn };
      assert { vn1 = vs * vs + vr };
      let ghost vs0 = pure { mod vs (power 2 c) } in
      assert { vn1 = (vs - vs0) * (vs - vs0) + 2 * vs0 * vs - vs0 * vs0 + vr };
      let s0 = salloc 1 in
      value_concat sp 1 tn;
      let s00 = (C.get sp) % (lsl 1 c) in
      assert { s00 = vs0
               by radix = power 2 Limb.length
                  = power 2 c * power 2 (Limb.length - c)
               so let q = value_sub (pelts sp) (offset sp + 1)
                                              (offset sp + tn) in
                  vs = value sp tn = (pelts sp)[offset sp] + radix * q
                     = power 2 c * (power 2 (Limb.length - c) * q)
                       + (pelts sp)[offset sp]
                  so mod vs (power 2 c)
                     = mod (power 2 c * (power 2 (Limb.length - c) * q)
                            + (pelts sp)[offset sp])
                           (power 2 c)
                     = mod (pelts sp)[offset sp] (power 2 c)
                     = s00 };
      C.set s0 s00;
      assert { value s0 1 = s00 };
      let rc = wmpn_addmul_1 tp sp tn (2 * s00) in
      assert { value tp tn + power radix tn * (rl + rc) = vr + 2 * vs0 * vs };
      assert { rl + rc < radix
               by vr <= 2 * vs
               so vr + 2 * vs0 * vs <= 2 * vs * (vs0 + 1)
               so vs0 < power 2 c <= power 2 63
               so 2 * vs * (vs0 + 1) <= 2 * vs * power 2 63 = radix * vs
               so vs < power radix tn
               so radix * vs < radix * power radix tn
               so power radix tn * radix > value tp tn + power radix tn * (rl + rc)
                  >= power radix tn * (rl + rc)
               so power radix tn * (rl + rc) < power radix tn * radix };
      rl <- rl + rc;
      assert { value tp tn + power radix tn * rl = vr + 2 * vs0 * vs };
      value_concat tp 1 tn;
      let ghost otp = pure { tp } in
      let ref cc = wmpn_submul_1 tp s0 1 s00 in
      value_sub_frame (pelts tp) (pelts otp) (offset tp + 1)
                                  (offset tp + int32'int tn);
      value_concat tp 1 tn;
      assert { value tp tn - radix * cc = value otp tn - s00 * s00 };
      assert { value tp tn + power radix tn * rl - radix * cc
               = vr + 2 * vs0 * vs - vs0 * vs0 };
      begin ensures { value tp tn + power radix tn * rl
                      = vr + 2 * vs0 * vs - vs0 * vs0 }
        if tn > 1
        then begin
          label Sub in
          value_concat tp 1 tn;
          let tp1 = C.incr tp 1 in
          let ghost otp = pure { tp } in
          let ghost otp1 = pure { tp1 } in
          assert { value tp tn = value tp 1 + radix * value tp1 (tn-1) };
          cc <- wmpn_sub_1_in_place tp1 (tn - 1) cc;
          value_sub_frame (pelts tp) (pelts otp1) (offset tp)
                                             (offset tp + 1);
          assert { value tp 1 = value tp 1 at Sub };
          value_concat tp 1 tn;
          assert { value tp tn - power radix tn * cc
                   = value otp tn - radix * (cc at Sub)
                   by value tp1 (tn - 1) - power radix (tn - 1) * cc
                      = value otp1 (tn - 1) - cc at Sub
                   so value tp tn = value tp 1 + radix * value tp1 (tn - 1)
                   so power radix tn = radix * power radix (tn - 1)
                   so value tp tn - power radix tn * cc
                      = value tp 1
                        + radix * (value tp1 (tn - 1) - power radix (tn-1) * cc)
                      = value tp 1 + radix * (value otp1 (tn-1) - (cc at Sub))
                      = value tp 1 + radix * value otp1 (tn-1)
                        - radix * (cc at Sub)
                      = value otp tn - radix * (cc at Sub) };
        end
        else begin
          assert { tn = 1 };
        end;
        assert { value tp tn + power radix tn * (rl - cc)
                   = vr + 2 * vs0 * vs - vs0 * vs0 };
        assert { 0 <= rl - cc
                 by (vs0 = mod vs (power 2 c) <= vs
                     by vs = div vs (power 2 c) * power 2 c + vs0
                     so div vs (power 2 c) >= 0
                     so power 2 c >= 0
                     so div vs (power 2 c) * power 2 c >= 0)
                 so vs0 * vs0 <= vs0 * vs
                 so 2 * vs0 * vs - vs0 * vs0 >= 0
                 so 0 <= vr
                 so 0 <= value tp tn + power radix tn * (rl - cc)
                 so value tp tn < power radix tn
                 so power radix tn * (rl - cc) >= - (power radix tn)
                 so rl - cc > - 1 };
        rl <- rl - cc
      end;
      let ghost r = wmpn_rshift sp sp tn c in
      let ghost vsq = pure { div vs (power 2 c) } in
      assert { vs = vsq * power 2 c + vs0 };
      assert { value sp tn * radix + r = vs * power 2 (Limb.length - c) };
      assert { mod r (power 2 (Limb.length - c)) = 0
               by let q = power 2 (Limb.length - c) in
                  let p = power 2 c in
                  p * q = radix
               so r = vs * q - value sp tn * p * q
                    = q * (vs - value sp tn * p)
               so mod r q
                  = mod (q * (vs - value sp tn * p) + 0) q
                  = 0 };
      let ghost q = pure { div r (power 2 (Limb.length - c)) } in
      assert { r = power 2 (Limb.length - c) * q
               by let p = power 2 (Limb.length - c) in
               r = p * div r p + mod r p
               so div r p = q so mod r p = 0 };
      assert { value sp tn * power 2 c + q = vs
               by radix = power 2 c * power 2 (Limb.length - c)
               so value sp tn * power 2 c * power 2 (Limb.length - c)
                    + power 2 (Limb.length - c) * q
                  = (value sp tn * power 2 c + q) * power 2 (Limb.length - c)
                  = value sp tn * radix + power 2 (Limb.length - c) * q
                  = vs * power 2 (Limb.length - c)
               so (value sp tn * power 2 c + q) * power 2 (Limb.length - c)
                  = vs * power 2 (Limb.length - c)
               so power 2 (Limb.length - c) <> 0 };
      assert { q = vs0
               by vs0 = mod vs (power 2 c)
                  = mod (value sp tn * power 2 c + q) (power 2 c)
                  = mod q (power 2 c)
               so 0 <= q
               so q * power 2 (Limb.length - c) = r < radix
                  = power 2 c * power 2 (Limb.length - c)
               so let p = power 2 (Limb.length - c) in
                  q * p < power 2 c * p
               so 0 <= q so 0 < p
               so q < power 2 c
               so div q (power 2 c) = 0
               so mod q (power 2 c) = q };
      assert { value sp tn * power 2 c = vs - vs0 };
      assert { value tp tn + power radix tn * rl
               = vr + 2 * vs0 * vs - vs0 * vs0 };
      value_sub_update_no_change (pelts tp) (offset tp + int32'int tn)
                                 (offset tp) (offset tp + int32'int tn) rl;
      label Set in
      C.set_ofs tp tn rl;
      value_tail tp tn;
      assert { value tp (tn + 1) = vr + 2 * vs0 * vs - vs0 * vs0
               by value tp (tn + 1) = value tp tn + power radix tn * rl
                  = value (tp at Set) tn + power radix tn * rl };
      assert { vn1 = value tp (tn + 1) + (vs - vs0) * (vs - vs0) };
      assert { power 2 (2 * c) * vn = value tp (tn + 1)
               + power 2 (2 * c) * value sp tn * value sp tn
               by power 2 (2 * c) = power 2 c * power 2 c
               so power 2 (2 * c) * value sp tn * value sp tn
               = (vs - vs0) * (vs - vs0) };
      let ghost vsp = pure { value sp tn } in
      begin ensures { 0 < vn }
        value_tail np (n-1);
        assert { 0 < vn
                 by vn = value np (n-1)
                       + power radix (n-1) * (pelts np)[offset np + n - 1]
                 so 0 <= value np (n-1)
                 so 0 < (pelts np at Start)[offset np + n - 1]
                      = (pelts np)[offset np + n - 1]
                 so 0 < power radix (n-1)
                 so 0 < power radix (n-1) * (pelts np)[offset np + n - 1] };
      end;
      sqrt_norm vn vn1 (uint64'int c) vsp vs0 vs;
      let ref c2 = lsl c 1 in
      assert { c2 = 2 * c };
      assert { value tp (tn + 1) = power 2 c2 * (vn - vsp * vsp) };
      begin ensures { power 2 c2 * (vn - vsp * vsp)
                      = value tp tn }
            ensures { 0 <= c2 < 64 }
            ensures { valid tp tn }
            ensures { 0 < tn <= k+1 }
        if c2 < 64
        then tn <- tn + 1
        else begin
          value_concat tp 1 (tn + 1);
          let tp1 = C.incr tp 1 in
          assert { value tp (tn + 1) = value tp 1 + radix * value tp1 tn };
          assert { power 2 c2 = radix * power 2 (c2 - 64)
                   by radix = power 2 64 so radix * power 2 (c2 - 64) = power 2 c2 };
          assert { value tp (tn + 1)
                   = power 2 c2 * (vn - vsp * vsp)
                   = radix * power 2 (c2 - 64) * (vn - vsp * vsp) };
          assert { value tp 1 = 0
                   by value tp (tn + 1)
                      = radix * power 2 (c2 - 64) * (vn - vsp * vsp)
                   so mod (value tp (tn + 1)) radix
                      = mod (radix * (power 2 (c2 - 64) * (vn - vsp * vsp)) + 0)
                            radix
                      = 0
                   so mod (value tp (tn + 1)) radix
                      = mod (value tp 1) radix
                   so 0 <= value tp 1 < radix
                   so value tp 1 = mod (value tp 1) radix };
          assert { value tp1 tn = power 2 (c2 - 64) * (vn - vsp * vsp)
                   by radix * value tp1 tn
                      = radix * (power 2 (c2 - 64) * (vn - vsp * vsp)) };
          c2 <- c2 - 64;
          tp <- tp1
        end
      end;
      begin ensures { value rp rn = vn - vsp * vsp }
            ensures { 0 < rn <= k+1 }
            ensures { min rp = old min rp /\ max rp = old max rp
                      /\ plength rp = old plength rp }
        if (not (c2 = 0))
        then begin
          label Shift in
          let ghost b = wmpn_rshift_sep rp tp tn c2 in
          value_sub_frame (pelts tp) (pure { pelts tp at Shift })
                          (offset tp) (offset tp + int32'int tn);
          assert { value tp tn = power 2 c2 * value rp tn
                   by radix = power 2 c2 * power 2 (64 - c2)
                   so b + radix * value rp tn
                      = value tp tn * power 2 (64 - c2)
                      = power 2 c2 * (vn - vsp * vsp) * power 2 (64 - c2)
                      = radix * (vn - vsp * vsp)
                   so mod (radix * (vn - vsp * vsp)) radix = 0
                   so 0 = mod (b + radix * value rp tn) radix
                      = mod b radix
                   so 0 <= b < radix
                   so b = mod b radix = 0
                   so value rp tn * power 2 c2 * power 2 (64 - c2)
                      = radix * value rp tn
                      = value tp tn * power 2 (64 - c2) };
          assert { value rp tn = vn - vsp * vsp
                   by value tp tn = power 2 c2 * value rp tn
                   so value tp tn = power 2 c2 * (vn - vsp * vsp) };
        end
        else wmpn_copyi rp tp tn;
        rn <- tn
      end
    end
    else begin
      wmpn_copyi rp np n;
      assert { (pelts rp)[offset rp + tn + tn - 1] >= power 2 (Limb.length - 2)
               by tn + tn = n
               so c = 0
               so (pelts rp)[offset rp + tn + tn - 1]
                  = (pelts rp)[offset rp + (n - 1)]
                  = (pelts np)[offset np + (n - 1)] = high
               so 4 * high >= radix };
      assert { value np n = value rp (tn + tn)
               by tn + tn = n };
      let h = wmpn_dc_sqrtrem sp rp tn scratch in
      value_sub_update_no_change (pelts rp) (offset rp + int32'int tn) (offset rp)
                                 (offset rp + int32'int tn) h;
      C.set_ofs rp tn h;
      value_tail rp tn;
      assert { value rp (tn+1) + value sp tn * value sp tn = value np n
               by value np n = value sp tn * value sp tn + value rp tn + power radix tn * h
               so value rp (tn + 1) = value rp tn + power radix tn * h };
      assert { value rp (tn+h) = value rp (tn + 1)
               by [@case_split] (h = 0 \/ h = 1) };
      rn <- tn + to_int32 h;
    end;
    let ghost orp = pure { rp } in
    let ghost orn = pure { rn } in
    assert { value np n = value sp k * value sp k + value orp orn };
    assert { 1 <= rn <= n };
    while C.get_ofs rp (rn - 1) = 0 do
      invariant { value rp rn = value orp orn }
      invariant { 1 <= rn <= orn }
      variant   { rn }
      value_tail rp (rn-1);
      assert { value rp (rn - 1) = value rp rn };
      rn <- rn - 1;
      if rn = 0
      then begin
        assert { value orp orn = 0
                 by 0 = value rp 0 = value rp 1 = value orp orn };
        break
      end
    done;
    rn

end