#include "TopingSystem.h"
#include "VoxelWorld.h"
#include <cmath>
#include <cstring>

namespace voxel {

// ── Edge definitions for +Y face ────────────────────────────────
// Each edge sits on one side of the unit square [0,1]² (the XZ plane at y=1).
// The bevel strip runs along the edge, with a wedge cross-section
// rising from the voxel face (y=1) to a peak (y=1+h) and sloping
// inward by width w.
//
// Cross-section (looking along the strip):
//
//         peak (edge, 1+h)
//         /|
//        / |
//       /  |  slope face (visible from above)
//      /   |
//     /    |  outer wall (visible from the side)
//    /     |
//   inner  outer
//   (1-w,1) (edge,1)
//
struct EdgeDef {
    float sx, sz;    // strip start point (on the voxel face)
    float ex, ez;    // strip end point
    float ix, iz;    // inward direction (unit, perpendicular to strip)
    float nx, nz;    // outer wall normal (points outward)
};

static const EdgeDef kEdges[4] = {
    //  sx    sz    ex    ez    ix    iz    nx    nz
    { 1.0f, 0.0f, 1.0f, 1.0f,-1.0f, 0.0f, 1.0f, 0.0f }, // bit 0: +X edge
    { 0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 0.0f,-1.0f, 0.0f }, // bit 1: -X edge
    { 0.0f, 1.0f, 1.0f, 1.0f, 0.0f,-1.0f, 0.0f, 1.0f }, // bit 2: +Z edge
    { 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f,-1.0f }, // bit 3: -Z edge
};

// ── Helper: emit one triangle with auto-corrected winding ───────
// Compares the geometric normal (from vertex cross product) to the
// desired shading normal. If they disagree, swaps B and C to flip
// the winding so the triangle is front-facing (CCW in Wicked Engine).
static void emitTri(std::vector<TopingVertex>& v,
    float ax, float ay, float az,
    float bx, float by, float bz,
    float cx, float cy, float cz,
    float nx, float ny, float nz)
{
    // Geometric normal = cross(AB, AC)
    float abx = bx - ax, aby = by - ay, abz = bz - az;
    float acx = cx - ax, acy = cy - ay, acz = cz - az;
    float gnx = aby * acz - abz * acy;
    float gny = abz * acx - abx * acz;
    float gnz = abx * acy - aby * acx;

    // If geometric normal disagrees with desired normal, swap B↔C
    if (gnx * nx + gny * ny + gnz * nz > 0.0f) {
        v.push_back({ ax, ay, az, nx, ny, nz });
        v.push_back({ cx, cy, cz, nx, ny, nz });
        v.push_back({ bx, by, bz, nx, ny, nz });
    } else {
        v.push_back({ ax, ay, az, nx, ny, nz });
        v.push_back({ bx, by, bz, nx, ny, nz });
        v.push_back({ cx, cy, cz, nx, ny, nz });
    }
}

// ═════════════════════════════════════════════════════════════════
// TopingSystem implementation
// ═════════════════════════════════════════════════════════════════

void TopingSystem::initialize() {
    registerDefs();
    generateMeshes();
}

// ── Register toping types ───────────────────────────────────────
void TopingSystem::registerDefs() {
    defs_.clear();

    // Type 0: Stone bevel — clean angular ridge along open edges
    // Applied to stone (materialID=3), face +Y
    // Priority 0 (low): yields to grass at material boundaries
    {
        TopingDef def{};
        def.materialID = 3;
        def.face       = FACE_POS_Y;
        def.priority   = 0;
        def.height     = 0.06f;  // subtle bevel
        def.width      = 0.12f;
        def.segments   = 1;      // single smooth segment per edge
        defs_.push_back(def);
    }

    // Type 1: Grass edge — organic bumpy tufts along open edges
    // Applied to grass (materialID=1), face +Y
    // Priority 1 (high): generates bevels over stone at boundaries
    {
        TopingDef def{};
        def.materialID = 1;
        def.face       = FACE_POS_Y;
        def.priority   = 1;
        def.height     = 0.12f;  // taller, more visible
        def.width      = 0.18f;
        def.segments   = 4;      // subdivided for bumpy profile
        defs_.push_back(def);
    }
}

// ── Generate all 16 mesh variants per def ───────────────────────
void TopingSystem::generateMeshes() {
    vertices_.clear();
    for (auto& def : defs_) {
        for (int bitmask = 0; bitmask < 16; bitmask++) {
            generateVariant(def, (uint8_t)bitmask);
        }
    }
}

// ── Generate mesh for one (def, bitmask) pair ───────────────────
// An UNSET bit means the edge is open → add bevel strip.
// A SET bit means a neighbor is present → no bevel (toping connects).
void TopingSystem::generateVariant(TopingDef& def, uint8_t bitmask) {
    const uint32_t startOffset = (uint32_t)vertices_.size();

    for (int edge = 0; edge < 4; edge++) {
        if (bitmask & (1 << edge)) continue; // neighbor present → skip

        const EdgeDef& e = kEdges[edge];
        const float w    = def.width;
        const int   segs = def.segments;

        // Build height profile along the strip
        std::vector<float> heights(segs + 1);
        if (segs <= 1) {
            // Stone: constant height (smooth bevel)
            heights[0] = def.height;
            heights[1] = def.height;
        } else {
            // Grass: sinusoidal bumps, phase offset per edge for variety
            for (int j = 0; j <= segs; j++) {
                float t    = (float)j / segs;
                float bump = sinf((t * 2.5f + edge * 0.31f) * 3.14159f);
                heights[j] = def.height * (0.5f + 0.5f * std::abs(bump));
                if (heights[j] < 0.02f) heights[j] = 0.02f;
            }
        }

        // Strip direction
        const float dx = e.ex - e.sx;
        const float dz = e.ez - e.sz;

        for (int i = 0; i < segs; i++) {
            const float t0 = (float)i / segs;
            const float t1 = (float)(i + 1) / segs;
            const float h0 = heights[i];
            const float h1 = heights[i + 1];

            // Points at t0 along the strip (all at y=1, the voxel face)
            const float x0 = e.sx + t0 * dx;
            const float z0 = e.sz + t0 * dz;
            // Points at t1
            const float x1 = e.sx + t1 * dx;
            const float z1 = e.sz + t1 * dz;

            // Cross-section at t0:
            //   outerBot = (x0, 1, z0)           — on the voxel face, at the edge
            //   peak     = (x0, 1+h0, z0)        — raised at the edge
            //   inner    = (x0+w*ix, 1, z0+w*iz) — on the face, inward
            const float pk0y = 1.0f + h0;
            const float in0x = x0 + w * e.ix;
            const float in0z = z0 + w * e.iz;

            // Cross-section at t1:
            const float pk1y = 1.0f + h1;
            const float in1x = x1 + w * e.ix;
            const float in1z = z1 + w * e.iz;

            // ── Outer wall face (vertical, facing outward) ──────
            // Normal points outward: (nx, 0, nz)
            // Winding: emit both orderings — CW from outside view
            // Empirically CW = front-facing in our engine (see CLAUDE.md)
            emitTri(vertices_,
                x0, pk0y, z0,   x1, pk1y, z1,   x0, 1.0f, z0,
                e.nx, 0.0f, e.nz);
            emitTri(vertices_,
                x1, pk1y, z1,   x1, 1.0f, z1,   x0, 1.0f, z0,
                e.nx, 0.0f, e.nz);

            // ── Slope face (from peak down to inner edge) ───────
            // Slope normal: the slope rises from inner (y=1) to peak (y=1+h) at the edge.
            // Since it tilts outward, the normal points INWARD and UP.
            // Normal = inward_dir * (h/L) + up * (w/L), where L = sqrt(h²+w²).
            const float avgH = (h0 + h1) * 0.5f;
            float slopeLen = sqrtf(avgH * avgH + w * w);
            if (slopeLen < 0.0001f) slopeLen = 1.0f;
            float cnx = e.ix * (avgH / slopeLen);
            float cny = w / slopeLen;
            float cnz = e.iz * (avgH / slopeLen);

            // Slope: peak → inner, strip direction
            // CW winding from normal direction
            emitTri(vertices_,
                x0, pk0y, z0,   in0x, 1.0f, in0z,   x1, pk1y, z1,
                cnx, cny, cnz);
            emitTri(vertices_,
                x1, pk1y, z1,   in0x, 1.0f, in0z,   in1x, 1.0f, in1z,
                cnx, cny, cnz);
        }
    }

    const uint32_t count = (uint32_t)vertices_.size() - startOffset;
    def.variants[bitmask] = { startOffset, count };
}

// ── Collect toping instances from the world ─────────────────────
// Scans every exposed voxel face that matches a registered TopingDef,
// computes the 4-bit adjacency bitmask, and emits a TopingInstance.
//
// Currently only supports FACE_POS_Y (top face). For other faces,
// the adjacency directions would need to be adapted to the face plane.
void TopingSystem::collectInstances(const VoxelWorld& world) {
    instances_.clear();

    // Quick lookup: material → toping def index (-1 if none)
    int8_t matToDef[256];
    memset(matToDef, -1, sizeof(matToDef));
    for (size_t i = 0; i < defs_.size(); i++) {
        matToDef[defs_[i].materialID] = (int8_t)i;
    }

    world.forEachChunk([&](const ChunkPos& cpos, const Chunk& chunk) {
        for (int z = 0; z < CHUNK_SIZE; z++) {
            for (int y = 0; y < CHUNK_SIZE; y++) {
                for (int x = 0; x < CHUNK_SIZE; x++) {
                    const VoxelData& v = chunk.at(x, y, z);
                    if (v.isEmpty()) continue;

                    const uint8_t mat = v.getMaterialID();
                    const int8_t defIdx = matToDef[mat];
                    if (defIdx < 0) continue;

                    const TopingDef& def = defs_[defIdx];

                    // World coordinates
                    const int wx = cpos.x * CHUNK_SIZE + x;
                    const int wy = cpos.y * CHUNK_SIZE + y;
                    const int wz = cpos.z * CHUNK_SIZE + z;

                    // Check if the target face is exposed (neighbor in face direction is empty)
                    // Currently only FACE_POS_Y is supported
                    if (def.face == FACE_POS_Y) {
                        if (!world.getVoxel(wx, wy + 1, wz).isEmpty()) continue;

                        // Face exposed. Compute 4-bit adjacency bitmask.
                        // A neighbor "connects" (bit SET → no bevel on that edge) if:
                        //   - Same material AND its +Y face is also exposed (same toping type)
                        //   - OR different material with a toping of HIGHER or EQUAL priority
                        //     AND its +Y face is exposed (the dominant toping wins at the boundary)
                        uint8_t adj = 0;
                        const uint8_t myPriority = def.priority;

                        auto checkNeighbor = [&](int nx, int nz) -> bool {
                            uint8_t nMat = world.getVoxel(nx, wy, nz).getMaterialID();
                            if (nMat == 0) return false; // empty
                            if (!world.getVoxel(nx, wy + 1, nz).isEmpty()) return false; // +Y not exposed
                            // Same material → connect (no bevel)
                            if (nMat == mat) return true;
                            // Different material with toping: check priority
                            int8_t nDefIdx = matToDef[nMat];
                            if (nDefIdx >= 0 && defs_[nDefIdx].priority >= myPriority) return true;
                            return false;
                        };

                        if (checkNeighbor(wx + 1, wz)) adj |= 1; // +X
                        if (checkNeighbor(wx - 1, wz)) adj |= 2; // -X
                        if (checkNeighbor(wx, wz + 1)) adj |= 4; // +Z
                        if (checkNeighbor(wx, wz - 1)) adj |= 8; // -Z

                        instances_.push_back({
                            (float)wx, (float)wy, (float)wz,
                            (uint16_t)defIdx, adj
                        });
                    }
                    // TODO: support other face directions (FACE_NEG_Y, FACE_POS_X, etc.)
                    // Each face direction needs different adjacency directions in its plane.
                }
            }
        }
    });
}

} // namespace voxel