/* mpn_gcdext -- Extended Greatest Common Divisor.

Copyright 1996, 1998, 2000-2005, 2008, 2009, 2012 Free Software Foundation,

Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify

it under the terms of either:

  * the GNU Lesser General Public License as published by the Free

    Software Foundation; either version 3 of the License, or (at your

    option) any later version.

or

  * the GNU General Public License as published by the Free Software

    Foundation; either version 2 of the License, or (at your option) any

    later version.

or both in parallel, as here.

The GNU MP Library 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

for more details.

You should have received copies of the GNU General Public License and the

GNU Lesser General Public License along with the GNU MP Library.  If not,

see https://www.gnu.org/licenses/.  */

#include "gmp-impl.h"

#include "longlong.h"

/* Here, d is the index of the cofactor to update. FIXME: Could use qn

   = 0 for the common case q = 1. */

void

mpn_gcdext_hook (void *p, mp_srcptr gp, mp_size_t gn,

     mp_srcptr qp, mp_size_t qn, int d)

{

  struct gcdext_ctx *ctx = (struct gcdext_ctx *) p;

  mp_size_t un = ctx->un;

  if (gp)

    {

      mp_srcptr up;

      ASSERT (gn > 0);

      ASSERT (gp[gn-1] > 0);

      MPN_COPY (ctx->gp, gp, gn);

      ctx->gn = gn;

      if (d < 0)

  {

    int c;

    /* Must return the smallest cofactor, +u1 or -u0 */

    MPN_CMP (c, ctx->u0, ctx->u1, un);

    ASSERT (c != 0 || (un == 1 && ctx->u0[0] == 1 && ctx->u1[0] == 1));

    d = c < 0;

  }

      up = d ? ctx->u0 : ctx->u1;

      MPN_NORMALIZE (up, un);

      MPN_COPY (ctx->up, up, un);

      *ctx->usize = d ? -un : un;

    }

  else

    {

      mp_limb_t cy;

      mp_ptr u0 = ctx->u0;

      mp_ptr u1 = ctx->u1;

      ASSERT (d >= 0);

      if (d)

  MP_PTR_SWAP (u0, u1);

      qn -= (qp[qn-1] == 0);

      /* Update u0 += q  * u1 */

      if (qn == 1)

  {

    mp_limb_t q = qp[0];

    if (q == 1)

      /* A common case. */

      cy = mpn_add_n (u0, u0, u1, un);

    else

      cy = mpn_addmul_1 (u0, u1, un, q);

  }

      else

  {

    mp_size_t u1n;

    mp_ptr tp;

    u1n = un;

    MPN_NORMALIZE (u1, u1n);

    if (u1n == 0)

      return;

    /* Should always have u1n == un here, and u1 >= u0. The

       reason is that we alternate adding u0 to u1 and u1 to u0

       (corresponding to subtractions a - b and b - a), and we

       can get a large quotient only just after a switch, which

       means that we'll add (a multiple of) the larger u to the

       smaller. */

    tp = ctx->tp;

    if (qn > u1n)

      mpn_mul (tp, qp, qn, u1, u1n);

    else

      mpn_mul (tp, u1, u1n, qp, qn);

    u1n += qn;

    u1n -= tp[u1n-1] == 0;

    if (u1n >= un)

      {

        cy = mpn_add (u0, tp, u1n, u0, un);

        un = u1n;

      }

    else

      /* Note: Unlikely case, maybe never happens? */

      cy = mpn_add (u0, u0, un, tp, u1n);

  }

      u0[un] = cy;

      ctx->un = un + (cy > 0);

    }

}

/* Temporary storage: 3*(n+1) for u. If hgcd2 succeeds, we need n for

   the matrix-vector multiplication adjusting a, b. If hgcd fails, we

   need at most n for the quotient and n+1 for the u update (reusing

   the extra u). In all, 4n + 3. */

mp_size_t

mpn_gcdext_lehmer_n (mp_ptr gp, mp_ptr up, mp_size_t *usize,

         mp_ptr ap, mp_ptr bp, mp_size_t n,

         mp_ptr tp)

{

  mp_size_t ualloc = n + 1;

  /* Keeps track of the second row of the reduction matrix

   *

   *   M = (v0, v1 ; u0, u1)

   *

   * which correspond to the first column of the inverse

   *

   *   M^{-1} = (u1, -v1; -u0, v0)

   *

   * This implies that

   *

   *   a =  u1 A (mod B)

   *   b = -u0 A (mod B)

   *

   * where A, B denotes the input values.

   */

  struct gcdext_ctx ctx;

  mp_size_t un;

  mp_ptr u0;

  mp_ptr u1;

  mp_ptr u2;

  MPN_ZERO (tp, 3*ualloc);

  u0 = tp; tp += ualloc;

  u1 = tp; tp += ualloc;

  u2 = tp; tp += ualloc;

  u1[0] = 1; un = 1;

  ctx.gp = gp;

  ctx.up = up;

  ctx.usize = usize;

  /* FIXME: Handle n == 2 differently, after the loop? */

  while (n >= 2)

    {

      struct hgcd_matrix1 M;

      mp_limb_t ah, al, bh, bl;

      mp_limb_t mask;

      mask = ap[n-1] | bp[n-1];

      ASSERT (mask > 0);

      if (mask & GMP_NUMB_HIGHBIT)

  {

    ah = ap[n-1]; al = ap[n-2];

    bh = bp[n-1]; bl = bp[n-2];

  }

      else if (n == 2)

  {

    /* We use the full inputs without truncation, so we can

       safely shift left. */

    int shift;

    count_leading_zeros (shift, mask);

    ah = MPN_EXTRACT_NUMB (shift, ap[1], ap[0]);

    al = ap[0] << shift;

    bh = MPN_EXTRACT_NUMB (shift, bp[1], bp[0]);

    bl = bp[0] << shift;

  }

      else

  {

    int shift;

    count_leading_zeros (shift, mask);

    ah = MPN_EXTRACT_NUMB (shift, ap[n-1], ap[n-2]);

    al = MPN_EXTRACT_NUMB (shift, ap[n-2], ap[n-3]);

    bh = MPN_EXTRACT_NUMB (shift, bp[n-1], bp[n-2]);

    bl = MPN_EXTRACT_NUMB (shift, bp[n-2], bp[n-3]);

  }

      /* Try an mpn_nhgcd2 step */

      if (mpn_hgcd2 (ah, al, bh, bl, &M))

  {

    n = mpn_matrix22_mul1_inverse_vector (&M, tp, ap, bp, n);

    MP_PTR_SWAP (ap, tp);

    un = mpn_hgcd_mul_matrix1_vector(&M, u2, u0, u1, un);

    MP_PTR_SWAP (u0, u2);

  }

      else

  {

    /* mpn_hgcd2 has failed. Then either one of a or b is very

       small, or the difference is very small. Perform one

       subtraction followed by one division. */

    ctx.u0 = u0;

    ctx.u1 = u1;

    ctx.tp = u2;

    ctx.un = un;

    /* Temporary storage n for the quotient and ualloc for the

       new cofactor. */

    n = mpn_gcd_subdiv_step (ap, bp, n, 0, mpn_gcdext_hook, &ctx, tp);

    if (n == 0)

      return ctx.gn;

    un = ctx.un;

  }

    }

  ASSERT_ALWAYS (ap[0] > 0);

  ASSERT_ALWAYS (bp[0] > 0);

  if (ap[0] == bp[0])

    {

      int c;

      /* Which cofactor to return now? Candidates are +u1 and -u0,

   depending on which of a and b was most recently reduced,

   which we don't keep track of. So compare and get the smallest

   one. */

      gp[0] = ap[0];

      MPN_CMP (c, u0, u1, un);

      ASSERT (c != 0 || (un == 1 && u0[0] == 1 && u1[0] == 1));

      if (c < 0)

  {

    MPN_NORMALIZE (u0, un);

    MPN_COPY (up, u0, un);

    *usize = -un;

  }

      else

  {

    MPN_NORMALIZE_NOT_ZERO (u1, un);

    MPN_COPY (up, u1, un);

    *usize = un;

  }

      return 1;

    }

  else

    {

      mp_limb_t uh, vh;

      mp_limb_signed_t u;

      mp_limb_signed_t v;

      int negate;

      gp[0] = mpn_gcdext_1 (&u, &v, ap[0], bp[0]);

      /* Set up = u u1 - v u0. Keep track of size, un grows by one or

   two limbs. */

      if (u == 0)

  {

    ASSERT (v == 1);

    MPN_NORMALIZE (u0, un);

    MPN_COPY (up, u0, un);

    *usize = -un;

    return 1;

  }

      else if (v == 0)

  {

    ASSERT (u == 1);

    MPN_NORMALIZE (u1, un);

    MPN_COPY (up, u1, un);

    *usize = un;

    return 1;

  }

      else if (u > 0)

  {

    negate = 0;

    ASSERT (v < 0);

    v = -v;

  }

      else

  {

    negate = 1;

    ASSERT (v > 0);

    u = -u;

  }

      uh = mpn_mul_1 (up, u1, un, u);

      vh = mpn_addmul_1 (up, u0, un, v);

      if ( (uh | vh) > 0)

  {

    uh += vh;

    up[un++] = uh;

    if (uh < vh)

      up[un++] = 1;

  }

      MPN_NORMALIZE_NOT_ZERO (up, un);

      *usize = negate ? -un : un;

      return 1;

    }

}