# -*- coding: utf-8 -*-

from pyfr.integrators.std.base import BaseStdIntegrator


class BaseStdStepper(BaseStdIntegrator):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # Add kernel cache
        self._axnpby_kerns = {}

    def collect_stats(self, stats):
        super().collect_stats(stats)

        # Total number of RHS evaluations
        stats.set('solver-time-integrator', 'nfevals', self._stepper_nfevals)


class StdEulerStepper(BaseStdStepper):
    stepper_name = 'euler'

    @property
    def _stepper_has_errest(self):
        return False

    @property
    def _stepper_nfevals(self):
        return self.nsteps

    @property
    def _stepper_nregs(self):
        return 2

    @property
    def _stepper_order(self):
        return 1

    def step(self, t, dt):
        add, rhs = self._add, self.system.rhs
        ut, f = self._regidx

        rhs(t, ut, f)
        add(1.0, ut, dt, f)

        return ut


class StdTVDRK3Stepper(BaseStdStepper):
    stepper_name = 'tvd-rk3'

    @property
    def _stepper_has_errest(self):
        return False

    @property
    def _stepper_nfevals(self):
        return 3*self.nsteps

    @property
    def _stepper_nregs(self):
        return 3

    @property
    def _stepper_order(self):
        return 3

    def step(self, t, dt):
        add, rhs = self._add, self.system.rhs

        # Get the bank indices for each register (n, n+1, rhs)
        r0, r1, r2 = self._regidx

        # Ensure r0 references the bank containing u(t)
        if r0 != self._idxcurr:
            r0, r1 = r1, r0

        # First stage; r2 = -∇·f(r0); r1 = r0 + dt*r2
        rhs(t, r0, r2)
        add(0.0, r1, 1.0, r0, dt, r2)

        # Second stage; r2 = -∇·f(r1); r1 = 0.75*r0 + 0.25*r1 + 0.25*dt*r2
        rhs(t + dt, r1, r2)
        add(0.25, r1, 0.75, r0, 0.25*dt, r2)

        # Third stage; r2 = -∇·f(r1);
        #              r1 = 1.0/3.0*r0 + 2.0/3.0*r1 + 2.0/3.0*dt*r2
        rhs(t + 0.5*dt, r1, r2)
        add(2.0/3.0, r1, 1.0/3.0, r0, 2.0/3.0*dt, r2)

        # Return the index of the bank containing u(t + dt)
        return r1


class StdRK4Stepper(BaseStdStepper):
    stepper_name = 'rk4'

    @property
    def _stepper_has_errest(self):
        return False

    @property
    def _stepper_nfevals(self):
        return 4*self.nsteps

    @property
    def _stepper_nregs(self):
        return 3

    @property
    def _stepper_order(self):
        return 4

    def step(self, t, dt):
        add, rhs = self._add, self.system.rhs

        # Get the bank indices for each register
        r0, r1, r2 = self._regidx

        # Ensure r0 references the bank containing u(t)
        if r0 != self._idxcurr:
            r0, r1 = r1, r0

        # First stage; r1 = -∇·f(r0)
        rhs(t, r0, r1)

        # Second stage; r2 = r0 + dt/2*r1; r2 = -∇·f(r2)
        add(0.0, r2, 1.0, r0, dt/2.0, r1)
        rhs(t + dt/2.0, r2, r2)

        # As no subsequent stages depend on the first stage we can
        # reuse its register to start accumulating the solution with
        # r1 = r0 + dt/6*r1 + dt/3*r2
        add(dt/6.0, r1, 1.0, r0, dt/3.0, r2)

        # Third stage; here we reuse the r2 register
        # r2 = r0 + dt/2*r2
        # r2 = -∇·f(r2)
        add(dt/2.0, r2, 1.0, r0)
        rhs(t + dt/2.0, r2, r2)

        # Accumulate; r1 = r1 + dt/3*r2
        add(1.0, r1, dt/3.0, r2)

        # Fourth stage; again we reuse r2
        # r2 = r0 + dt*r2
        # r2 = -∇·f(r2)
        add(dt, r2, 1.0, r0)
        rhs(t + dt, r2, r2)

        # Final accumulation r1 = r1 + dt/6*r2 = u(t + dt)
        add(1.0, r1, dt/6.0, r2)

        # Return the index of the bank containing u(t + dt)
        return r1


class StdRKVdH2RStepper(BaseStdStepper):
    # Coefficients
    a = []
    b = []
    bhat = []

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # Compute the c and error coeffs
        self.c = [0.0] + [sum(self.b[:i]) + ai for i, ai in enumerate(self.a)]
        self.e = [b - bh for b, bh in zip(self.b, self.bhat)]

        self._nstages = len(self.c)

    @property
    def _stepper_has_errest(self):
        return self._controller_needs_errest and len(self.bhat)

    @property
    def _stepper_nfevals(self):
        return len(self.b)*self.nsteps

    @property
    def _stepper_nregs(self):
        return 4 if self._stepper_has_errest else 2

    def step(self, t, dt):
        add, rhs = self._add, self.system.rhs
        errest = self._stepper_has_errest

        r1 = self._idxcurr

        if errest:
            r2, rold, rerr = set(self._regidx) - {r1}

            # Save the current solution
            add(0.0, rold, 1.0, r1)
        else:
            r2, = set(self._regidx) - {r1}

        # Evaluate the stages in the scheme
        for i in range(self._nstages):
            # Compute -∇·f
            rhs(t + self.c[i]*dt, r2 if i > 0 else r1, r2)

            # Accumulate the error term in rerr
            if errest:
                add(1.0 if i > 0 else 0.0, rerr, self.e[i]*dt, r2)

            # Sum (special-casing the final stage)
            if i < self._nstages - 1:
                add(1.0, r1, self.a[i]*dt, r2)
                add((self.b[i] - self.a[i])*dt, r2, 1.0, r1)
            else:
                add(1.0, r1, self.b[i]*dt, r2)

            # Swap
            r1, r2 = r2, r1

        # Return
        return (r2, rold, rerr) if errest else r2


class StdRK34Stepper(StdRKVdH2RStepper):
    stepper_name = 'rk34'

    a = [
        11847461282814 / 36547543011857,
        3943225443063 / 7078155732230,
        -346793006927 / 4029903576067
    ]

    b = [
        1017324711453 / 9774461848756,
        8237718856693 / 13685301971492,
        57731312506979 / 19404895981398,
        -101169746363290 / 37734290219643
    ]

    bhat = [
        15763415370699 / 46270243929542,
        514528521746 / 5659431552419,
        27030193851939 / 9429696342944,
        -69544964788955 / 30262026368149
    ]

    @property
    def _stepper_order(self):
        return 3


class StdRK45Stepper(StdRKVdH2RStepper):
    stepper_name = 'rk45'

    a = [
        970286171893 / 4311952581923,
        6584761158862 / 12103376702013,
        2251764453980 / 15575788980749,
        26877169314380 / 34165994151039
    ]

    b = [
        1153189308089 / 22510343858157,
        1772645290293 / 4653164025191,
        -1672844663538 / 4480602732383,
        2114624349019 / 3568978502595,
        5198255086312 / 14908931495163
    ]

    bhat = [
        1016888040809 / 7410784769900,
        11231460423587 / 58533540763752,
        -1563879915014 / 6823010717585,
        606302364029 / 971179775848,
        1097981568119 / 3980877426909
    ]

    @property
    def _stepper_order(self):
        return 4
