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    // Copyright 2015 The Ramp Developers
    //
    //    Licensed under the Apache License, Version 2.0 (the "License");
    //    you may not use this file except in compliance with the License.
    //    You may obtain a copy of the License at
    //
    //        http://www.apache.org/licenses/LICENSE-2.0
    //
    //    Unless required by applicable law or agreed to in writing, software
    //    distributed under the License is distributed on an "AS IS" BASIS,
    //    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    //    See the License for the specific language governing permissions and
    //    limitations under the License.

    use ll::limb::{Limb, BaseInt};
    use ll::{same_or_decr, same_or_incr};

    use ll::limb_ptr::{Limbs, LimbsMut};

    /**
     * Performs a bit-shift of the limbs in {xp, xs}, left by `cnt` bits storing the result in {rp,
     * rs}. The top-most shifted bits are returned.
     *
     * If `cnt` is greater than or equal to the number of bits in a limb, the result is undefined.
     */
    pub unsafe fn shl(mut rp: LimbsMut, mut xp: Limbs, mut xs: i32, cnt: u32) -> Limb {
        debug_assert!(xs >= 1);
        debug_assert!(cnt >= 1);
        debug_assert!(cnt < Limb::BITS as u32);
        debug_assert!(same_or_decr(rp, xs, xp, xs));

        let cnt = cnt as usize;

        rp = rp.offset((xs - 1) as isize);
        xp = xp.offset((xs - 1) as isize);

        let inv_cnt = Limb::BITS - cnt;

        let l = *xp;
        let ret = l >> inv_cnt;
        let mut high_limb = l << cnt;

        xs -= 1;
        while xs != 0 {
            xp = xp.offset(-1);
            let low = *xp;

            *rp = high_limb | (low >> inv_cnt);
            high_limb = low << cnt;

            rp = rp.offset(-1);
            xs -= 1;
        }

        *rp = high_limb;

        return ret;
    }

    /**
     * Performs a bit-shift of the limbs in {xp, xs}, right by `cnt` bits storing the result in {rp,
     * rs}. The bottom-most shifted bits are returned.
     *
     * If `cnt` is greater than or equal to the number of bits in a limb, the result is undefined.
     */
    pub unsafe fn shr(mut rp: LimbsMut, mut xp: Limbs, mut xs: i32, cnt: u32) -> Limb {
        debug_assert!(xs >= 1);
        debug_assert!(cnt >= 1);
        debug_assert!(cnt < Limb::BITS as u32);
        debug_assert!(same_or_incr(rp, xs, xp, xs));

        let cnt = cnt as usize;

        let inv_cnt = Limb::BITS - cnt;

        let h = *xp;
        let ret = h << inv_cnt;
        let mut low_limb = h >> cnt;

        xp = xp.offset(1);

        xs -= 1;
        while xs != 0 {
            let high = *xp;
            xp = xp.offset(1);

            *rp = low_limb | (high << inv_cnt);
            low_limb = high >> cnt;
            rp = rp.offset(1);

            xs -= 1;
        }

        *rp = low_limb;

        return ret;
    }

    // Common function for the operations below, since they're all essentially the same
    #[inline(always)]
    unsafe fn bitop<F: Fn(Limb, Limb) -> Limb>(mut wp: LimbsMut,
                                               mut xp: Limbs, mut yp: Limbs,
                                               n: i32, op: F) {
        debug_assert!(same_or_incr(wp, n, xp, n));
        debug_assert!(same_or_incr(wp, n, yp, n));

        let mut i = 0;
        while i < n {
            *wp = op(*xp, *yp);
            wp = wp.offset(1);
            xp = xp.offset(1);
            yp = yp.offset(1);
            i += 1;
        }
    }

    /**
     * Performs a bitwise "and" (`&`) of the n least signficant limbs of `xp` and `yp`, storing the
     * result in `wp`
     */
    pub unsafe fn and_n(wp: LimbsMut,
                        xp: Limbs, yp: Limbs,
                        n: i32) {
        bitop(wp, xp, yp, n, |x, y| x & y);
    }

    /**
     * Performs a bitwise and of the n least signficant limbs of `xp` and `yp`, with the limbs of `yp`
     * being first inverted. The result is stored in `wp`.
     *
     * The operation is x & !y
     */
    pub unsafe fn and_not_n(wp: LimbsMut,
                         xp: Limbs, yp: Limbs,
                         n: i32) {
        bitop(wp, xp, yp, n, |x, y| x & !y);
    }

    /**
     * Performs a bitwise "nand" of the n least signficant limbs of `xp` and `yp`, storing the
     * result in `wp`
     *
     * The operation is !(x & y)
     */
    pub unsafe fn nand_n(wp: LimbsMut,
                         xp: Limbs, yp: Limbs,
                         n: i32) {
        bitop(wp, xp, yp, n, |x, y| !(x & y));
    }

    /**
     * Performs a bitwise "or" (`|`) of the n least signficant limbs of `xp` and `yp`, storing the
     * result in `wp`
     */
    pub unsafe fn or_n(wp: LimbsMut,
                        xp: Limbs, yp: Limbs,
                        n: i32) {
        bitop(wp, xp, yp, n, |x, y| x | y);
    }

    /**
     * Performs a bitwise "or" of the n least signficant limbs of `xp` and `yp`, with the limbs of `yp`
     * being first inverted. The result is stored in `wp`.
     */
    pub unsafe fn or_not_n(wp: LimbsMut,
                        xp: Limbs, yp: Limbs,
                        n: i32) {
        bitop(wp, xp, yp, n, |x, y| x | !y);
    }

    /**
     * Performs a bitwise "nor" of the n least signficant limbs of `xp` and `yp`, storing the
     * result in `wp`
     *
     * The operation is !(x | y)
     */
    pub unsafe fn nor_n(wp: LimbsMut,
                        xp: Limbs, yp: Limbs,
                        n: i32) {
        bitop(wp, xp, yp, n, |x, y| !(x | y));
    }

    /**
     * Performs a bitwise "xor" (`^`) of the n least signficant limbs of `xp` and `yp`, storing the
     * result in `wp`
     */
    pub unsafe fn xor_n(wp: LimbsMut,
                        xp: Limbs, yp: Limbs,
                        n: i32) {
        bitop(wp, xp, yp, n, |x, y| x ^ y);
    }

    /**
     * Performs a bitwise inversion ("not") of the n least signficant limbs of `xp`, storing the
     * result in `wp`
     */
    pub unsafe fn not(mut wp: LimbsMut, mut xp: Limbs, n: i32) {
        debug_assert!(same_or_incr(wp, n, xp, n));

        let mut i = 0;
        while i < n {
            *wp = !*xp;
            wp = wp.offset(1);
            xp = xp.offset(1);
            i += 1;
        }
    }

    /**
     * Computes the two's complement of the `xs` least significant words
     * of `xp`. The result is stored the result in `wp`, and a carry is
     * returned, if there is one.
     */
    pub unsafe fn twos_complement(mut wp: LimbsMut, mut xp: Limbs, xs: i32) -> Limb {
        let mut i = 0;
        let mut carry = Limb(1);

        while i < xs {
            let flipped = !*xp;
            *wp = flipped + carry;
            xp = xp.offset(1);
            wp = wp.offset(1);
            i += 1;
            carry = carry & Limb((flipped == !0) as BaseInt);
        }

        carry
    }

    /**
     * Scans for the first 1 bit starting from the least-significant bit the the most, returning
     * the bit index.
     */
    pub unsafe fn scan_1(mut xp: Limbs, mut xs: i32) -> u32 {
        debug_assert!(xs > 0);
        let mut cnt = 0u32;

        while *xp == 0 {
            cnt += Limb::BITS as u32;
            xp = xp.offset(1);
            xs -= 1;
            if xs == 0 { return cnt; }
        }
        cnt += (*xp).trailing_zeros() as u32;

        return cnt;
    }

    /**
     * Scans for the first 0 bit starting from the least-significant bit the the most, returning
     * the bit index.
     */
    pub unsafe fn scan_0(mut xp: Limbs, mut xs: i32) -> u32 {
        debug_assert!(xs > 0);
        let mut cnt = 0u32;

        while *xp == !0 {
            cnt += Limb::BITS as u32;
            xp = xp.offset(1);
            xs -= 1;
            if xs == 0 { return cnt; }
        }
        let mut last = (*xp).0;
        while last & 1 != 0 {
            cnt += 1;
            last >>= 1;
        }

        return cnt;
    }

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