/*
 * Copyright 2018 Anton Khirnov <anton@khirnov.net>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program 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 a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <float.h>
#include <errno.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "common.h"
#include "ell_relax.h"
#include "log.h"

#define FD_STENCIL_MAX 1

struct EllRelaxInternal {
    ptrdiff_t stride;
    double *u_base;
    double *rhs_base;
    double *residual_base;
    double *diff_coeffs_base[ELL_RELAX_DIFF_COEFF_NB];
};

static const struct {
    unsigned int stride_idx;
    unsigned int is_upper;
} boundary_def[] = {
    [ELL_RELAX_BOUNDARY_0L] = {
        .stride_idx = 0,
        .is_upper   = 0
    },
    [ELL_RELAX_BOUNDARY_0U] = {
        .stride_idx = 0,
        .is_upper   = 1,
    },
    [ELL_RELAX_BOUNDARY_1L] = {
        .stride_idx = 1,
        .is_upper   = 0,
    },
    [ELL_RELAX_BOUNDARY_1U] = {
        .stride_idx = 1,
        .is_upper   = 1,
    },
};

static inline void
derivatives_calc(EllRelaxContext *ctx, double *dst, ptrdiff_t idx, ptrdiff_t stride)
{
    const double *u = ctx->u;
    const double dx = ctx->step[0];
    const double dy = ctx->step[1];

    dst[ELL_RELAX_DIFF_COEFF_00] = u[idx];
    dst[ELL_RELAX_DIFF_COEFF_10] = (u[idx + 1]      - u[idx - 1])      / (2.0 * dx);
    dst[ELL_RELAX_DIFF_COEFF_01] = (u[idx + stride] - u[idx - stride]) / (2.0 * dy);

    dst[ELL_RELAX_DIFF_COEFF_20] = (u[idx + 1]      - 2.0 * u[idx] + u[idx - 1])      / (SQR(dx));
    dst[ELL_RELAX_DIFF_COEFF_02] = (u[idx + stride] - 2.0 * u[idx] + u[idx - stride]) / (SQR(dy));

    dst[ELL_RELAX_DIFF_COEFF_11] = (u[idx + 1 + stride] - u[idx + stride - 1] - u[idx - stride + 1] + u[idx - stride - 1]) / (4.0 * dx * dy);
}

static void residual_calc(EllRelaxContext *ctx)
{
    EllRelaxInternal *priv = ctx->priv;

    for (int idx1 = 0; idx1 < ctx->domain_size[1]; idx1++)
        for (int idx0 = 0; idx0 < ctx->domain_size[0]; idx0++) {
            ptrdiff_t idx = idx1 * priv->stride + idx0;

            double u[ELL_RELAX_DIFF_COEFF_NB];
            double res;

            derivatives_calc(ctx, u, idx, priv->stride);

            res = -ctx->rhs[idx];
            for (int i = 0; i < ARRAY_ELEMS(u); i++)
                res += u[i] * ctx->diff_coeffs[i][idx];

            ctx->residual[idx] = res;
        }

    for (int i = 0; i < ARRAY_ELEMS(ctx->boundaries); i++) {
        const ptrdiff_t strides[2] = { 1, priv->stride };
        const int si = boundary_def[i].stride_idx;
        const ptrdiff_t stride_boundary = strides[si];
        const ptrdiff_t stride_offset   = strides[!si];
        const size_t size_boundary      = ctx->domain_size[si];
        const size_t size_offset        = ctx->domain_size[!si];

        double *res = ctx->residual + boundary_def[i].is_upper * ((size_offset - 1) * stride_offset);

        if (ctx->boundaries[i].type == ELL_RELAX_BC_TYPE_FIXVAL) {
            for (size_t j = 0; j < size_boundary; j++) {
                *res = 0.0;
                res += stride_boundary;
            }
        }
    }
}

static void boundaries_apply_fixval(double *dst, ptrdiff_t dst_stride,
                                    const double *src,
                                    size_t boundary_size, ptrdiff_t boundary_stride,
                                    int is_upper)
{
    for (size_t i = 0; i < boundary_size; i++) {
        *dst = *src;
        dst += dst_stride;
        src++;
    }
}

static void boundaries_apply_fixdiff(double *dst, ptrdiff_t dst_stride,
                                    const double *src,
                                    size_t boundary_size, ptrdiff_t boundary_stride,
                                    int is_upper)
{
    if (!is_upper)
        boundary_stride *= -1;

    for (size_t i = 0; i < boundary_size; i++) {
        for (int i = 1; i <= FD_STENCIL_MAX; i++)
            dst[boundary_stride * i] = dst[-boundary_stride * i];

        dst += dst_stride;
    }
}

static void boundaries_apply(EllRelaxContext *ctx)
{
    EllRelaxInternal *priv = ctx->priv;
    const ptrdiff_t strides[2] = { 1, priv->stride };

    for (int i = 0; i < ARRAY_ELEMS(ctx->boundaries); i++) {
        const int si = boundary_def[i].stride_idx;
        const ptrdiff_t stride_boundary = strides[si];
        const ptrdiff_t stride_offset   = strides[!si];
        const size_t size_boundary      = ctx->domain_size[si];
        const size_t size_offset        = ctx->domain_size[!si];

        double *dst = ctx->u + boundary_def[i].is_upper * ((size_offset - 1) * stride_offset);

        switch (ctx->boundaries[i].type) {
        case ELL_RELAX_BC_TYPE_FIXVAL:
            boundaries_apply_fixval(dst, stride_boundary, ctx->boundaries[i].val, size_boundary,
                                    stride_offset, boundary_def[i].is_upper);
            break;
        case ELL_RELAX_BC_TYPE_FIXDIFF:
            boundaries_apply_fixdiff(dst, stride_boundary, ctx->boundaries[i].val, size_boundary,
                                     stride_offset, boundary_def[i].is_upper);
            break;
        }
    }
}

int mg2di_ell_relax_step(EllRelaxContext *ctx)
{
    EllRelaxInternal *priv = ctx->priv;
    const double cfl_fac = (1.0 / 5.0) * ctx->step[0] * ctx->step[1];

    for (int idx1 = 0; idx1 < ctx->domain_size[1]; idx1++)
        for (int idx0 = 0; idx0 < ctx->domain_size[0]; idx0++) {
            ptrdiff_t idx = idx1 * priv->stride + idx0;

            ctx->u[idx] += cfl_fac * ctx->residual[idx];
        }

    boundaries_apply(ctx);
    residual_calc(ctx);

    return 0;
}

int mg2di_ell_relax_init(EllRelaxContext *ctx)
{
    EllRelaxInternal *priv = ctx->priv;
    int ret;

    if (ctx->fd_stencil == 0 || ctx->fd_stencil > FD_STENCIL_MAX) {
        mg2di_log(&ctx->logger, 0, "Invalid finite difference stencil: %zd\n",
                  ctx->fd_stencil);
        return -EINVAL;
    }

    if (ctx->step[0] <= DBL_EPSILON || ctx->step[1] <= DBL_EPSILON) {
        mg2di_log(&ctx->logger, 0, "Spatial step size too small\n");
        return -EINVAL;
    }

    for (int i = 0; i < ARRAY_ELEMS(ctx->boundaries); i++) {
        if (ctx->boundaries[i].type == ELL_RELAX_BC_TYPE_FIXDIFF) {
           for (int k = 0; k < ctx->domain_size[boundary_def[i].stride_idx]; k++)
               if (ctx->boundaries[i].val[k] != 0.0) {
                   mg2di_log(&ctx->logger, 0, "Only zero boundary derivative supported\n");
                   return -ENOSYS;
               }
        }
    }

    boundaries_apply(ctx);
    residual_calc(ctx);

    return 0;
}

static int ell_relax_arrays_alloc(EllRelaxContext *ctx, const size_t domain_size[2])
{
    EllRelaxInternal *priv = ctx->priv;

    const size_t ghosts = FD_STENCIL_MAX;
    const size_t size_padded[2] = {
        domain_size[0] + 2 * ghosts,
        domain_size[1] + 2 * ghosts,
    };
    const size_t stride = size_padded[0];
    const size_t start_offset = ghosts * stride + ghosts;
    const size_t arr_size = size_padded[0] * size_padded[1];
    int ret;

    ret = posix_memalign((void**)&priv->u_base, 32, sizeof(*priv->u_base) * arr_size);
    if (ret != 0)
        return -ret;
    ctx->u        = priv->u_base + start_offset;
    ctx->u_stride = stride;

    ret = posix_memalign((void**)&priv->rhs_base, 32, sizeof(*priv->rhs_base) * arr_size);
    if (ret != 0)
        return -ret;
    ctx->rhs        = priv->rhs_base + start_offset;
    ctx->rhs_stride = stride;

    ret = posix_memalign((void**)&priv->residual_base, 32, sizeof(*priv->residual_base) * arr_size);
    if (ret != 0)
        return -ret;
    memset(priv->residual_base, 0, sizeof(*priv->residual_base) * arr_size);
    ctx->residual        = priv->residual_base + start_offset;
    ctx->residual_stride = stride;

    for (int i = 0; i < ARRAY_ELEMS(ctx->diff_coeffs); i++) {
        ret = posix_memalign((void**)&priv->diff_coeffs_base[i], 32,
                             sizeof(*priv->diff_coeffs_base[i]) * arr_size);
        if (ret != 0)
            return -ret;
        ctx->diff_coeffs[i] = priv->diff_coeffs_base[i] + start_offset;
    }
    ctx->diff_coeffs_stride = stride;

    priv->stride = stride;

    for (int i = 0; i < ARRAY_ELEMS(ctx->boundaries); i++) {
        const size_t boundary_size = domain_size[boundary_def[i].stride_idx];
        ret = posix_memalign((void**)&ctx->boundaries[i].val, 32,
                             sizeof(*ctx->boundaries[i].val) * boundary_size);
        if (ret != 0)
            return -ret;
        ctx->boundaries[i].val_len = boundary_size;
    }

    return 0;
}

EllRelaxContext *mg2di_ell_relax_alloc(size_t domain_size[2])
{
    EllRelaxContext *ctx;
    EllRelaxInternal *priv;
    int ret;

    ctx = calloc(1, sizeof(*ctx));
    if (!ctx)
        return NULL;

    ctx->priv = calloc(1, sizeof(*ctx->priv));
    if (!ctx->priv)
        goto fail;
    priv = ctx->priv;

    if (!domain_size[0] || !domain_size[1] ||
        domain_size[0] > SIZE_MAX / domain_size[1])
        goto fail;

    ret = ell_relax_arrays_alloc(ctx, domain_size);
    if (ret < 0)
        goto fail;

    ctx->domain_size[0] = domain_size[0];
    ctx->domain_size[1] = domain_size[1];

    return ctx;
fail:
    mg2di_ell_relax_free(&ctx);
    return NULL;
}

void mg2di_ell_relax_free(EllRelaxContext **pctx)
{
    EllRelaxContext *ctx = *pctx;

    if (!ctx)
        return;

    free(ctx->priv->u_base);
    free(ctx->priv->rhs_base);
    free(ctx->priv->residual_base);
    for (int i = 0; i < ARRAY_ELEMS(ctx->priv->diff_coeffs_base); i++)
        free(ctx->priv->diff_coeffs_base[i]);
    free(ctx->priv);

    for (int i = 0; i < ARRAY_ELEMS(ctx->boundaries); i++)
        free(ctx->boundaries[i].val);

    free(ctx);
    *pctx = NULL;
}