/*
    Copyright (C) 2011, 2012 Sebastian Pancratz

    This file is part of FLINT.

    FLINT is free software: you can redistribute it and/or modify it under
    the terms of the GNU Lesser General Public License (LGPL) as published
    by the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.  See <https://www.gnu.org/licenses/>.
*/

#include "padic.h"

void padic_sub(padic_t rop, const padic_t op1, const padic_t op2,
               const padic_ctx_t ctx)
{
    if (padic_prec(rop) <= FLINT_MIN(padic_val(op1), padic_val(op2)))
    {
        padic_zero(rop);
        return;
    }

    if (padic_is_zero(op1))
    {
        padic_neg(rop, op2, ctx);
    }
    else if (padic_is_zero(op2))
    {
        padic_set(rop, op1, ctx);
    }
    else
    {
        if (padic_val(op1) == padic_val(op2))
        {
            fmpz_sub(padic_unit(rop), padic_unit(op1), padic_unit(op2));
            padic_val(rop) = padic_val(op1);

            _padic_canonicalise(rop, ctx);

            if (padic_prec(rop) <= padic_val(rop))
            {
                padic_zero(rop);
                return;
            }
        }
        else if (padic_val(op1) < padic_val(op2))
        {
            fmpz_t f;

            fmpz_init(f);
            fmpz_pow_ui(f, ctx->p, padic_val(op2) - padic_val(op1));
            if (rop != op2)
            {
                fmpz_set(padic_unit(rop), padic_unit(op1));
                fmpz_submul(padic_unit(rop), f, padic_unit(op2));
            }
            else
            {
                fmpz_mul(padic_unit(rop), f, padic_unit(op2));
                fmpz_sub(padic_unit(rop), padic_unit(rop), padic_unit(op1));
                fmpz_neg(padic_unit(rop), padic_unit(rop));
            }
            fmpz_clear(f);

            padic_val(rop) = padic_val(op1);
        }
        else  /* padic_val(op1) > padic_val(op2) */
        {
            fmpz_t f;

            fmpz_init(f);
            fmpz_pow_ui(f, ctx->p, padic_val(op1) - padic_val(op2));
            if (rop != op1)
            {
                fmpz_neg(padic_unit(rop), padic_unit(op2));
                fmpz_addmul(padic_unit(rop), f, padic_unit(op1));
            }
            else
            {
                fmpz_mul(padic_unit(rop), f, padic_unit(op1));
                fmpz_sub(padic_unit(rop), padic_unit(rop), padic_unit(op2));
            }
            fmpz_clear(f);

            padic_val(rop) = padic_val(op2);
        }

        {
            int alloc;
            fmpz_t pow;

            alloc = _padic_ctx_pow_ui(pow, padic_prec(rop) - padic_val(rop), ctx);

            fmpz_mod(padic_unit(rop), padic_unit(rop), pow);

            if (fmpz_is_zero(padic_unit(rop)))
                padic_val(rop) = 0;

            if (alloc)
                fmpz_clear(pow);
        }
    }
}
