use num_complex::Complex;
use std::ops::MulAssign;

use alga::general::Real;
use core::{Matrix, MatrixMN, VectorN, DefaultAllocator, Matrix2x3, Vector2};
use dimension::{Dim, DimMin, DimMinimum, DimSub, DimDiff, U1, U2};
use storage::Storage;
use allocator::Allocator;
use constraint::{ShapeConstraint, SameNumberOfRows};

use linalg::givens;
use linalg::symmetric_eigen;
use linalg::Bidiagonal;
use geometry::UnitComplex;



/// The Singular Value Decomposition of a real matrix.
#[derive(Clone)]
pub struct SVD<N: Real, R: DimMin<C>, C: Dim>
    where DefaultAllocator: Allocator<N, R, C>                +
                            Allocator<N, DimMinimum<R, C>, C> +
                            Allocator<N, R, DimMinimum<R, C>> +
                            Allocator<N, DimMinimum<R, C>> {
    /// The left-singular vectors `U` of this SVD.
    pub u: Option<MatrixMN<N, R, DimMinimum<R, C>>>,
    /// The right-singular vectors `V^t` of this SVD.
    pub v_t: Option<MatrixMN<N, DimMinimum<R, C>, C>>,
    /// The singular values of this SVD.
    pub singular_values: VectorN<N, DimMinimum<R, C>>,
}

impl<N: Real, R: DimMin<C>, C: Dim> SVD<N, R, C>
    where DimMinimum<R, C>: DimSub<U1>, // for Bidiagonal.
          DefaultAllocator: Allocator<N, R, C>                +
                            Allocator<N, C>                   + // for Bidiagonal
                            Allocator<N, R>                   + // for Bidiagonal
                            Allocator<N, DimDiff<DimMinimum<R, C>, U1>> + // for Bidiagonal
                            Allocator<N, DimMinimum<R, C>, C> +
                            Allocator<N, R, DimMinimum<R, C>> +
                            Allocator<N, DimMinimum<R, C>> {

    /// Computes the Singular Value Decomposition of `matrix` using implicit shift.
    pub fn new(matrix: MatrixMN<N, R, C>, compute_u: bool, compute_v: bool) -> Self {
        Self::try_new(matrix, compute_u, compute_v, N::default_epsilon(), 0).unwrap()
    }

    /// Computes the Singular Value Decomposition of `matrix` using implicit shift.
    ///
    /// # Arguments
    ///
    /// * `compute_u` − set this to `true` to enable the computation of left-singular vectors.
    /// * `compute_v` − set this to `true` to enable the computation of left-singular vectors.
    /// * `eps`       − tolerence used to determine when a value converged to 0.
    /// * `max_niter` − maximum total number of iterations performed by the algorithm. If this
    /// number of iteration is exceeded, `None` is returned. If `niter == 0`, then the algorithm
    /// continues indefinitely until convergence.
    pub fn try_new(mut matrix: MatrixMN<N, R, C>,
                   compute_u:  bool,
                   compute_v:  bool,
                   eps:        N,
                   max_niter:  usize)
                   -> Option<Self> {
        assert!(matrix.len() != 0, "Cannot compute the SVD of an empty matrix.");
        let (nrows, ncols)  = matrix.data.shape();
        let min_nrows_ncols = nrows.min(ncols);
        let dim = min_nrows_ncols.value();

        let m_amax = matrix.amax();

        if !m_amax.is_zero() {
            matrix /= m_amax;
        }

        let mut b   = Bidiagonal::new(matrix);
        let mut u   = if compute_u { Some(b.u()) } else { None };
        let mut v_t = if compute_v { Some(b.v_t()) } else { None };

        let mut niter = 0;
        let (mut start, mut end) = Self::delimit_subproblem(&mut b, &mut u, &mut v_t, dim - 1, eps);

        while end != start {
            let subdim = end - start + 1;

            // Solve the subproblem.
            if subdim > 2 {
                let m = end - 1;
                let n = end;

                let mut vec;
                {
                    let dm = b.diagonal[m];
                    let dn = b.diagonal[n];
                    let fm = b.off_diagonal[m];

                    let tmm = dm * dm + b.off_diagonal[m - 1] * b.off_diagonal[m - 1];
                    let tmn = dm * fm;
                    let tnn = dn * dn + fm * fm;

                    let shift = symmetric_eigen::wilkinson_shift(tmm, tnn, tmn);

                    vec = Vector2::new(b.diagonal[start] * b.diagonal[start] - shift,
                                       b.diagonal[start] * b.off_diagonal[start]);
                }


                for k in start .. n {
                    let m12 = if k == n - 1 { N::zero() } else { b.off_diagonal[k + 1] };

                    let mut subm = Matrix2x3::new(
                        b.diagonal[k], b.off_diagonal[k], N::zero(),
                        N::zero(),     b.diagonal[k + 1], m12);

                    if let Some((rot1, norm1)) = givens::cancel_y(&vec) {
                        rot1.conjugate().rotate_rows(&mut subm.fixed_columns_mut::<U2>(0));

                        if k > start {
                            // This is not the first iteration.
                            b.off_diagonal[k - 1] = norm1;
                        }

                        let v = Vector2::new(subm[(0, 0)], subm[(1, 0)]);
                        // FIXME: does the case `v.y == 0` ever happen?
                        let (rot2, norm2) = givens::cancel_y(&v).unwrap_or((UnitComplex::identity(), subm[(0, 0)]));
                        rot2.rotate(&mut subm.fixed_columns_mut::<U2>(1));
                        subm[(0, 0)] = norm2;

                        if let Some(ref mut v_t) = v_t {
                            if b.is_upper_diagonal() {
                                rot1.rotate(&mut v_t.fixed_rows_mut::<U2>(k));
                            }
                            else {
                                rot2.rotate(&mut v_t.fixed_rows_mut::<U2>(k));
                            }
                        }

                        if let Some(ref mut u) = u {
                            if b.is_upper_diagonal() {
                                rot2.inverse().rotate_rows(&mut u.fixed_columns_mut::<U2>(k));
                            }
                            else {
                                rot1.inverse().rotate_rows(&mut u.fixed_columns_mut::<U2>(k));
                            }
                        }

                        b.diagonal[k + 0]     = subm[(0, 0)];
                        b.diagonal[k + 1]     = subm[(1, 1)];
                        b.off_diagonal[k + 0] = subm[(0, 1)];

                        if k != n - 1 {
                            b.off_diagonal[k + 1] = subm[(1, 2)];
                        }

                        vec.x = subm[(0, 1)];
                        vec.y = subm[(0, 2)];
                    }
                    else {
                        break;
                    }
                }
            }
            else if subdim == 2 {
                // Solve the remaining 2x2 subproblem.
                let (u2, s, v2) = Self::compute_2x2_uptrig_svd(
                    b.diagonal[start], b.off_diagonal[start], b.diagonal[start + 1],
                    compute_u && b.is_upper_diagonal() || compute_v && !b.is_upper_diagonal(),
                    compute_v && b.is_upper_diagonal() || compute_u && !b.is_upper_diagonal());

                b.diagonal[start + 0] = s[0];
                b.diagonal[start + 1] = s[1];
                b.off_diagonal[start] = N::zero();

                if let Some(ref mut u) = u {
                    let rot = if b.is_upper_diagonal() { u2.unwrap() } else { v2.unwrap() };
                    rot.rotate_rows(&mut u.fixed_columns_mut::<U2>(start));
                }

                if let Some(ref mut v_t) = v_t {
                    let rot = if b.is_upper_diagonal() { v2.unwrap() } else { u2.unwrap() };
                    rot.inverse().rotate(&mut v_t.fixed_rows_mut::<U2>(start));
                }

                end -= 1;
            }

            // Re-delimit the suproblem in case some decoupling occured.
            let sub = Self::delimit_subproblem(&mut b, &mut u, &mut v_t, end, eps);
            start = sub.0;
            end   = sub.1;

            niter += 1;
            if niter == max_niter {
                return None;
            }
        }

        b.diagonal *= m_amax;

        // Ensure all singular value are non-negative.
        for i in 0 .. dim {
            let sval = b.diagonal[i];
            if sval < N::zero() {
                b.diagonal[i] = -sval;

                if let Some(ref mut u) = u {
                    u.column_mut(i).neg_mut();
                }
            }
        }

        Some(SVD { u: u, v_t: v_t, singular_values: b.diagonal })
    }

    // Explicit formulaes inspired from the paper "Computing the Singular Values of 2-by-2 Complex
    // Matrices", Sanzheng Qiao and Xiaohong Wang.
    // http://www.cas.mcmaster.ca/sqrl/papers/sqrl5.pdf
    fn compute_2x2_uptrig_svd(m11: N, m12: N, m22: N, compute_u: bool, compute_v: bool)
        -> (Option<UnitComplex<N>>, Vector2<N>, Option<UnitComplex<N>>) {

        let two:  N = ::convert(2.0f64);
        let half: N = ::convert(0.5f64);

        let denom = (m11 + m22).hypot(m12) + (m11 - m22).hypot(m12);

        // NOTE: v1 is the singular value that is the closest to m22.
        // This prevents cancellation issues when constructing the vector `csv` bellow. If we chose
        // otherwise, we would have v1 ~= m11 when m12 is small. This would cause catastrofic
        // cancellation on `v1 * v1 - m11 * m11` bellow.
        let v1 = two * m11 * m22 / denom;
        let v2 = half * denom;

        let mut u   = None;
        let mut v_t = None;

        if compute_u || compute_v {
            let csv = Vector2::new(m11 * m12, v1 * v1 - m11 * m11).normalize();

            if compute_v {
                v_t = Some(UnitComplex::new_unchecked(Complex::new(csv.x, csv.y)));
            }

            if compute_u {
                let cu = (m11 * csv.x + m12 * csv.y) / v1;
                let su = (m22 * csv.y) / v1;

                u = Some(UnitComplex::new_unchecked(Complex::new(cu, su)));
            }
        }

        (u, Vector2::new(v1, v2), v_t)
    }

    /*
    fn display_bidiag(b: &Bidiagonal<N, R, C>, begin: usize, end: usize) {
        for i in begin .. end {
            for k in begin .. i {
                print!("    ");
            }
            println!("{}  {}", b.diagonal[i], b.off_diagonal[i]);
        }
        for k in begin .. end {
            print!("    ");
        }
        println!("{}", b.diagonal[end]);
    }
    */

    fn delimit_subproblem(b:   &mut Bidiagonal<N, R, C>,
                          u:   &mut Option<MatrixMN<N, R, DimMinimum<R, C>>>,
                          v_t: &mut Option<MatrixMN<N, DimMinimum<R, C>, C>>,
                          end: usize,
                          eps: N)
                          -> (usize, usize) {
        let mut n = end;

        while n > 0 {
            let m = n - 1;

            if b.off_diagonal[m].is_zero() ||
               b.off_diagonal[m].abs() <= eps * (b.diagonal[n].abs() + b.diagonal[m].abs()) {

                b.off_diagonal[m] = N::zero();
            }
            else if b.diagonal[m].abs() <= eps {
                b.diagonal[m] = N::zero();
                Self::cancel_horizontal_off_diagonal_elt(b, u, v_t, m, m + 1);

                if m != 0 {
                    Self::cancel_vertical_off_diagonal_elt(b, u, v_t, m - 1);
                }
            }
            else if b.diagonal[n].abs() <= eps {
                    b.diagonal[n] = N::zero();
                    Self::cancel_vertical_off_diagonal_elt(b, u, v_t, m);
            }
            else {
                break;
            }

            n -= 1;
        }

        if n == 0 {
            return (0, 0);
        }

        let mut new_start = n - 1;
        while new_start > 0 {
            let m = new_start - 1;

            if b.off_diagonal[m].abs() <= eps * (b.diagonal[new_start].abs() + b.diagonal[m].abs()) {
                b.off_diagonal[m] = N::zero();
                break;
            }
            // FIXME: write a test that enters this case.
            else if b.diagonal[m].abs() <= eps {
                b.diagonal[m] = N::zero();
                Self::cancel_horizontal_off_diagonal_elt(b, u, v_t, m, n);

                if m != 0 {
                    Self::cancel_vertical_off_diagonal_elt(b, u, v_t, m - 1);
                }
                break;
            }

            new_start -= 1;
        }

        (new_start, n)
    }

    // Cancels the i-th off-diagonal element using givens rotations.
    fn cancel_horizontal_off_diagonal_elt(b:   &mut Bidiagonal<N, R, C>,
                               u:   &mut Option<MatrixMN<N, R, DimMinimum<R, C>>>,
                               v_t: &mut Option<MatrixMN<N, DimMinimum<R, C>, C>>,
                               i:        usize,
                               end:      usize) {
        let mut v = Vector2::new(b.off_diagonal[i], b.diagonal[i + 1]);
        b.off_diagonal[i] = N::zero();

        for k in i .. end {
            if let Some((rot, norm)) = givens::cancel_x(&v) {
                b.diagonal[k + 1] = norm;

                if b.is_upper_diagonal() {
                    if let Some(ref mut u) = *u {
                        rot.inverse().rotate_rows(&mut u.fixed_columns_with_step_mut::<U2>(i, k - i + 1));
                    }
                }
                else if let Some(ref mut v_t) = *v_t {
                    rot.rotate(&mut v_t.fixed_rows_with_step_mut::<U2>(i, k - i + 1));
                }

                if k + 1 != end {
                    v.x = -rot.sin_angle() * b.off_diagonal[k + 1];
                    v.y = b.diagonal[k + 2];
                    b.off_diagonal[k + 1] *= rot.cos_angle();
                }
            }
            else {
                break;
            }
        }
    }

    // Cancels the i-th off-diagonal element using givens rotations.
    fn cancel_vertical_off_diagonal_elt(b:   &mut Bidiagonal<N, R, C>,
                                        u:   &mut Option<MatrixMN<N, R, DimMinimum<R, C>>>,
                                        v_t: &mut Option<MatrixMN<N, DimMinimum<R, C>, C>>,
                                        i:   usize) {
        let mut v = Vector2::new(b.diagonal[i], b.off_diagonal[i]);
        b.off_diagonal[i] = N::zero();

        for k in (0 .. i + 1).rev() {
            if let Some((rot, norm)) = givens::cancel_y(&v) {
                b.diagonal[k] = norm;

                if b.is_upper_diagonal() {
                    if let Some(ref mut v_t) = *v_t {
                        rot.rotate(&mut v_t.fixed_rows_with_step_mut::<U2>(k, i + 1 - k));
                    }
                }
                else if let Some(ref mut u) = *u {
                    rot.inverse().rotate_rows(&mut u.fixed_columns_with_step_mut::<U2>(k, i + 1 - k));
                }

                if k > 0 {
                    v.x = b.diagonal[k - 1];
                    v.y = rot.sin_angle() * b.off_diagonal[k - 1];
                    b.off_diagonal[k - 1] *= rot.cos_angle();
                }
            }
            else {
                break;
            }
        }
    }

    /// Computes the rank of the decomposed matrix, i.e., the number of singular values greater
    /// than `eps`.
    pub fn rank(&self, eps: N) -> usize {
        assert!(eps >= N::zero(), "SVD rank: the epsilon must be non-negative.");
        self.singular_values.iter().filter(|e| **e > eps).count()
    }

    /// Rebuild the original matrix.
    ///
    /// This is useful if some of the singular values have been manually modified.  Panics if the
    /// right- and left- singular vectors have not been computed at construction-time.
    pub fn recompose(self) -> MatrixMN<N, R, C> {
        let mut u = self.u.expect("SVD recomposition: U has not been computed.");
        let v_t   = self.v_t.expect("SVD recomposition: V^t has not been computed.");

        for i in 0 .. self.singular_values.len() {
            let val = self.singular_values[i];
            u.column_mut(i).mul_assign(val);
        }

        u * v_t
    }

    /// Computes the pseudo-inverse of the decomposed matrix.
    ///
    /// Any singular value smaller than `eps` is assumed to be zero.
    /// Panics if the right- and left- singular vectors have not been computed at
    /// construction-time.
    pub fn pseudo_inverse(mut self, eps: N) -> MatrixMN<N, C, R>
        where DefaultAllocator: Allocator<N, C, R> {

        assert!(eps >= N::zero(), "SVD pseudo inverse: the epsilon must be non-negative.");
        for i in 0 .. self.singular_values.len() {
            let val = self.singular_values[i];

            if val > eps {
                self.singular_values[i] = N::one() / val;
            }
            else {
                self.singular_values[i] = N::zero();
            }
        }

        self.recompose().transpose()
    }

    /// Solves the system `self * x = b` where `self` is the decomposed matrix and `x` the unknown.
    ///
    /// Any singular value smaller than `eps` is assumed to be zero.
    /// Returns `None` if the singular vectors `U` and `V` have not been computed.
    // FIXME: make this more generic wrt the storage types and the dimensions for `b`.
    pub fn solve<R2: Dim, C2: Dim, S2>(&self, b: &Matrix<N, R2, C2, S2>, eps: N) -> MatrixMN<N, C, C2>
        where S2: Storage<N, R2, C2>,
              DefaultAllocator: Allocator<N, C, C2> +
                                Allocator<N, DimMinimum<R, C>, C2>,
              ShapeConstraint: SameNumberOfRows<R, R2> {

        assert!(eps >= N::zero(), "SVD solve: the epsilon must be non-negative.");
        let u   = self.u.as_ref().expect("SVD solve: U has not been computed.");
        let v_t = self.v_t.as_ref().expect("SVD solve: V^t has not been computed.");

        let mut ut_b = u.tr_mul(b);

        for j in 0 .. ut_b.ncols() {
            let mut col = ut_b.column_mut(j);

            for i in 0 .. self.singular_values.len() {
                let val = self.singular_values[i];
                if val > eps {
                    col[i] /= val;
                }
                else {
                    col[i] = N::zero();
                }
            }
        }

        v_t.tr_mul(&ut_b)
    }
}


impl<N: Real, R: DimMin<C>, C: Dim, S: Storage<N, R, C>> Matrix<N, R, C, S>
    where DimMinimum<R, C>: DimSub<U1>, // for Bidiagonal.
          DefaultAllocator: Allocator<N, R, C>                +
                            Allocator<N, C>                   + // for Bidiagonal
                            Allocator<N, R>                   + // for Bidiagonal
                            Allocator<N, DimDiff<DimMinimum<R, C>, U1>> + // for Bidiagonal
                            Allocator<N, DimMinimum<R, C>, C> +
                            Allocator<N, R, DimMinimum<R, C>> +
                            Allocator<N, DimMinimum<R, C>> {
    /// Computes the singular values of this matrix.
    pub fn singular_values(&self) -> VectorN<N, DimMinimum<R, C>> {
        SVD::new(self.clone_owned(), false, false).singular_values
    }

    /// Computes the rank of this matrix.
    ///
    /// All singular values bellow `eps` are considered equal to 0.
    pub fn rank(&self, eps: N) -> usize {
        let svd = SVD::new(self.clone_owned(), false, false);
        svd.rank(eps)
    }

    /// Computes the pseudo-inverse of this matrix.
    ///
    /// All singular values bellow `eps` are considered equal to 0.
    pub fn pseudo_inverse(self, eps: N) -> MatrixMN<N, C, R>
        where DefaultAllocator: Allocator<N, C, R> {

        SVD::new(self.clone_owned(), true, true).pseudo_inverse(eps)
    }
}