#include <cmath>
#include <cstdint>
#include <vector>
#include <algorithm>

namespace ipm
{
    // =========================
    // Basic Data Structures
    // =========================

    struct Vec3
    {
        double x;
        double y;
        double z;
    };

    struct GroundPoint
    {
        double X;     // World coordinate X (left-right)
        double Y;     // World coordinate Y (front-back)
        bool valid;   // Whether there is a valid intersection with the ground
    };

    struct CameraParam
    {
        // Focal length (in pixel units)
        // If you only have one d, you can set fx = fy = d
        double fx;
        double fy;

        // Principal point (usually the image center)
        double cx;
        double cy;

        // Camera height above ground, e.g., in cm
        double H;

        // Camera downward pitch angle (radians)
        double pitch;
    };

    struct IPMParam
    {
        // Output top view size
        int outWidth;
        int outHeight;

        // World coordinate range (same unit as H, e.g., cm)
        // X: left-right range
        // Y: front-back range
        double minX;
        double maxX;
        double minY;
        double maxY;
    };

    // =========================
    // Utility Functions
    // =========================

    inline double clampDouble(double v, double lo, double hi)
    {
        return (v < lo) ? lo : ((v > hi) ? hi : v);
    }

    inline uint8_t clampToByte(double v)
    {
        if (v < 0.0) return 0;
        if (v > 255.0) return 255;
        return static_cast<uint8_t>(v + 0.5);
    }

    // Rotation around X-axis: transform the direction in the camera coordinate system to the world coordinate system
    // Here it is assumed:
    // - World Z-axis points upward
    // - Camera optical axis defaults to the positive world Y direction
    // - pitch > 0 means the camera is looking downward
    //
    // To match the image coordinate (v downward), construct a commonly used mapping in engineering:
    //
    // Camera ray rc = [x, y, 1]
    // First map to the world direction "without pitch":
    //   x -> Xw
    //   y -> -Zw
    //   z -> Yw
    //
    // Then rotate around the world X-axis by pitch
    //
    inline Vec3 cameraRayToWorldRay(const Vec3& rc, double pitch)
    {
        // World direction without pitch
        // Camera right -> World right
        // Camera down -> World negative up
        // Camera forward -> World forward
        const double X0 = rc.x;
        const double Y0 = rc.z;
        const double Z0 = -rc.y;

        const double c = std::cos(pitch);
        const double s = std::sin(pitch);

        // Rotate around X-axis
        Vec3 rw;
        rw.x = X0;
        rw.y = c * Y0 - s * Z0;
        rw.z = s * Y0 + c * Z0;

        return rw;
    }

    // =========================
    // Pixel Point -> Ground World Coordinates
    // =========================
    //
    // Input pixel point (u, v), calculate its corresponding world point (X, Y) on the ground Z=0
    //
    // Note:
    // 1. If this ray points upward or is parallel to the ground, it is invalid
    // 2. fx, fy are in pixel units
    // 3. The unit of H determines the unit of the output world coordinates
    //
    inline GroundPoint imagePixelToGround(
        double u,
        double v,
        const CameraParam& cam)
    {
        // 1) Pixel coordinates -> Camera normalized coordinates
        Vec3 rc;
        rc.x = (u - cam.cx) / cam.fx;
        rc.y = (v - cam.cy) / cam.fy;
        rc.z = 1.0;

        // 2) Camera ray -> World ray
        Vec3 rw = cameraRayToWorldRay(rc, cam.pitch);

        // 3) Camera center position in world coordinates
        // Cw = (0, 0, H)
        // Ray equation: P(t) = Cw + t * rw
        //
        // Intersection with ground Zw = 0:
        // H + t * rw.z = 0  =>  t = -H / rw.z
        //
        GroundPoint gp{};
        gp.valid = false;

        // Ray does not point to the ground, or is almost parallel to the ground
        if (std::abs(rw.z) < 1e-12)
            return gp;

        const double t = -cam.H / rw.z;

        // Only accept "forward" intersections
        if (t <= 0.0)
            return gp;

        gp.X = t * rw.x;
        gp.Y = t * rw.y;
        gp.valid = true;
        return gp;
    }

    // =========================
    // World Coordinates -> Top View Pixel
    // =========================
    //
    // Map the ground point (X, Y) to (bx, by) in the output top view
    //
    // Output image convention:
    // - Left is minX, right is maxX
    // - Top is maxY (farther away)
    // - Bottom is minY (closer)
    //
    inline bool groundToBirdPixel(
        double X, double Y,
        const IPMParam& ipmParam,
        double& bx, double& by)
    {
        if (X < ipmParam.minX || X > ipmParam.maxX ||
            Y < ipmParam.minY || Y > ipmParam.maxY)
        {
            return false;
        }

        const double xRatio =
            (X - ipmParam.minX) / (ipmParam.maxX - ipmParam.minX);

        const double yRatio =
            (Y - ipmParam.minY) / (ipmParam.maxY - ipmParam.minY);

        // X from left to right
        bx = xRatio * (ipmParam.outWidth - 1);

        // Want "far away at the top of the image"
        by = (1.0 - yRatio) * (ipmParam.outHeight - 1);

        return true;
    }

    // =========================
    // Bilinear Sampling (Grayscale)
    // =========================
    inline uint8_t bilinearSampleGray(
        const uint8_t* src,
        int width,
        int height,
        int stride,
        double u,
        double v)
    {
        if (u < 0.0 || v < 0.0 || u > width - 1.0 || v > height - 1.0)
            return 0;

        const int x0 = static_cast<int>(std::floor(u));
        const int y0 = static_cast<int>(std::floor(v));
        const int x1 = std::min(x0 + 1, width - 1);
        const int y1 = std::min(y0 + 1, height - 1);

        const double dx = u - x0;
        const double dy = v - y0;

        const double p00 = src[y0 * stride + x0];
        const double p10 = src[y0 * stride + x1];
        const double p01 = src[y1 * stride + x0];
        const double p11 = src[y1 * stride + x1];

        const double v0 = p00 * (1.0 - dx) + p10 * dx;
        const double v1 = p01 * (1.0 - dx) + p11 * dx;
        const double val = v0 * (1.0 - dy) + v1 * dy;

        return clampToByte(val);
    }

    // =========================
    // Bilinear Sampling (RGB Three Channels)
    // 3 bytes per pixel, RGBRGB...
    // =========================
    inline void bilinearSampleRGB(
        const uint8_t* src,
        int width,
        int height,
        int stride,
        double u,
        double v,
        uint8_t outRGB[3])
    {
        if (u < 0.0 || v < 0.0 || u > width - 1.0 || v > height - 1.0)
        {
            outRGB[0] = outRGB[1] = outRGB[2] = 0;
            return;
        }

        const int x0 = static_cast<int>(std::floor(u));
        const int y0 = static_cast<int>(std::floor(v));
        const int x1 = std::min(x0 + 1, width - 1);
        const int y1 = std::min(y0 + 1, height - 1);

        const double dx = u - x0;
        const double dy = v - y0;

        const uint8_t* p00 = src + y0 * stride + x0 * 3;
        const uint8_t* p10 = src + y0 * stride + x1 * 3;
        const uint8_t* p01 = src + y1 * stride + x0 * 3;
        const uint8_t* p11 = src + y1 * stride + x1 * 3;

        for (int c = 0; c < 3; ++c)
        {
            const double v0 = p00[c] * (1.0 - dx) + p10[c] * dx;
            const double v1 = p01[c] * (1.0 - dx) + p11[c] * dx;
            const double val = v0 * (1.0 - dy) + v1 * dy;
            outRGB[c] = clampToByte(val);
        }
    }

    // =========================
    // Top View Pixel -> World Coordinates
    // =========================
    //
    // This is the key to doing "inverse mapping":
    // For each pixel of the output top view, first find its point on the world ground,
    // then back-calculate its position in the original image, and finally sample from the original image.
    //
    inline void birdPixelToGround(
        double bx,
        double by,
        const IPMParam& ipmParam,
        double& X,
        double& Y)
    {
        const double xRatio = bx / (ipmParam.outWidth - 1);
        const double yRatio = 1.0 - by / (ipmParam.outHeight - 1);

        X = ipmParam.minX + xRatio * (ipmParam.maxX - ipmParam.minX);
        Y = ipmParam.minY + yRatio * (ipmParam.maxY - ipmParam.minY);
    }

    // =========================
    // World Ground Point -> Original Image Pixel
    // =========================
    //
    // Given the world point (X, Y, 0), back-project it to the input image for inverse mapping sampling.
    //
    inline bool groundToImagePixel(
        double X,
        double Y,
        const CameraParam& cam,
        double& u,
        double& v)
    {
        // World point Pw = (X, Y, 0)
        // Camera center Cw = (0, 0, H)
        // World direction vector d_w = Pw - Cw = (X, Y, -H)
        const double dwx = X;
        const double dwy = Y;
        const double dwz = -cam.H;

        // Need to transform the world direction back to the camera direction
        // cameraRayToWorldRay uses: Rw = Rx(pitch) * base
        // So here we do the inverse rotation: Rx(-pitch)
        const double c = std::cos(cam.pitch);
        const double s = std::sin(cam.pitch);

        // First, inversely rotate to the state without pitch
        const double X0 = dwx;
        const double Y0 = c * dwy + s * dwz;
        const double Z0 = -s * dwy + c * dwz;

        // Then map back to camera coordinates
        // base: [X0, Y0, Z0] = [xc, zc, -yc]
        const double xc = X0;
        const double yc = -Z0;
        const double zc = Y0;

        // Behind the camera, invalid
        if (zc <= 1e-12)
            return false;

        u = cam.fx * (xc / zc) + cam.cx;
        v = cam.fy * (yc / zc) + cam.cy;
        return true;
    }

    // =========================
    // Grayscale Image IPM
    // =========================
    //
    // src: input grayscale image
    // dst: output grayscale image, must be allocated externally with outHeight * dstStride bytes
    //
    inline void warpIPMGray(
        const uint8_t* src,
        int srcWidth,
        int srcHeight,
        int srcStride,
        uint8_t* dst,
        int dstStride,
        const CameraParam& cam,
        const IPMParam& ipmParam)
    {
        for (int by = 0; by < ipmParam.outHeight; ++by)
        {
            uint8_t* dstRow = dst + by * dstStride;

            for (int bx = 0; bx < ipmParam.outWidth; ++bx)
            {
                // 1) Output top view pixel -> World ground point
                double X, Y;
                birdPixelToGround(static_cast<double>(bx),
                                  static_cast<double>(by),
                                  ipmParam, X, Y);

                // 2) World ground point -> Original image pixel
                double u, v;
                if (!groundToImagePixel(X, Y, cam, u, v))
                {
                    dstRow[bx] = 0;
                    continue;
                }

                // 3) Bilinear sampling
                dstRow[bx] = bilinearSampleGray(src, srcWidth, srcHeight, srcStride, u, v);
            }
        }
    }

    // =========================
    // RGB Image IPM
    // =========================
    //
    // src: input RGB image, arranged as RGBRGB...
    // dst: output RGB image, arranged as RGBRGB...
    //
    inline void warpIPMRGB(
        const uint8_t* src,
        int srcWidth,
        int srcHeight,
        int srcStride,
        uint8_t* dst,
        int dstStride,
        const CameraParam& cam,
        const IPMParam& ipmParam)
    {
        for (int by = 0; by < ipmParam.outHeight; ++by)
        {
            uint8_t* dstRow = dst + by * dstStride;

            for (int bx = 0; bx < ipmParam.outWidth; ++bx)
            {
                double X, Y;
                birdPixelToGround(static_cast<double>(bx),
                                  static_cast<double>(by),
                                  ipmParam, X, Y);

                double u, v;
                if (!groundToImagePixel(X, Y, cam, u, v))
                {
                    uint8_t* p = dstRow + bx * 3;
                    p[0] = p[1] = p[2] = 0;
                    continue;
                }

                uint8_t rgb[3];
                bilinearSampleRGB(src, srcWidth, srcHeight, srcStride, u, v, rgb);

                uint8_t* p = dstRow + bx * 3;
                p[0] = rgb[0];
                p[1] = rgb[1];
                p[2] = rgb[2];
            }
        }
    }

}