use std::f64::consts::PI;

use crate::LngLat;

static TO_RADIANS: f64 = PI / 180.0;
static EARTH_RADIUS: f64 = 6371.0088;

/// Computes the distance in kilometers between two coordinates using the Haversine formula
pub fn distance(p1: LngLat, p2: LngLat) -> f64 {
    let lat1 = p1.lat * TO_RADIANS;
    let lat2 = p2.lat * TO_RADIANS;
    let delta_lat = lat2 - lat1;
    let delta_lng = (p2.lng - p1.lng) * TO_RADIANS;

    let a =
        (delta_lat / 2.0).sin().powi(2) + lat1.cos() * lat2.cos() * (delta_lng / 2.0).sin().powi(2);
    let c = 2.0 * a.min(1.0).sqrt().asin();
    EARTH_RADIUS * c
}

pub fn time_diff(a: &Option<i64>, b: &Option<i64>) -> Option<i32> {
    match (a, b) {
        (Some(t1), Some(t2)) => Some((t1 - t2) as i32),
        _ => None,
    }
}

/// Computes the speed for a given distance in kilometers and a time in milliseconds
pub fn speed(distance: f64, time: i32) -> f64 {
    distance / (time as f64 / 3600_000.0)
}

pub fn slope(ele: f64, distance: f64) -> f64 {
    if distance == 0.0 {
        100.0
    } else {
        0.1 * ele / distance
    }
}

static METERS_PER_LATITUDE_DEGREE: f64 = 111320.0;

fn get_meters_per_longitude_degree(latitude: f64) -> f64 {
    ((latitude * PI) / 180.0).cos() * METERS_PER_LATITUDE_DEGREE
}

// Calculates the point on the line segment defined by p1 and p2
// that is closest to the third point, p3.
// Uses simple planar geometry (ignores earth curvature).
fn projected(p1: LngLat, p2: LngLat, p3: LngLat) -> LngLat {
    // Convert to meters using approximate scaling
    let meters_per_longitude_degree = get_meters_per_longitude_degree(p1.lat);

    let x1 = p1.lng * meters_per_longitude_degree;
    let y1 = p1.lat * METERS_PER_LATITUDE_DEGREE;
    let x2 = p2.lng * meters_per_longitude_degree;
    let y2 = p2.lat * METERS_PER_LATITUDE_DEGREE;
    let x3 = p3.lng * meters_per_longitude_degree;
    let y3 = p3.lat * METERS_PER_LATITUDE_DEGREE;

    let dx = x2 - x1;
    let dy = y2 - y1;
    let segment_length_squared = dx * dx + dy * dy;

    if segment_length_squared == 0.0 {
        // p1 and p2 are the same point
        p1
    } else {
        // Project p3 onto the line defined by p1-p2
        let t =
            0.0_f64.max(1.0_f64.min(((x3 - x1) * dx + (y3 - y1) * dy) / segment_length_squared));

        // Find the closest point on the segment
        let proj_x = x1 + t * dx;
        let proj_y = y1 + t * dy;

        // Convert back to degrees
        LngLat {
            lng: proj_x / meters_per_longitude_degree,
            lat: proj_y / METERS_PER_LATITUDE_DEGREE,
        }
    }
}

/// Calculates the perpendicular distance in meters
/// between a line segment (defined by p1 and p2) and a third point, p3.
/// Uses simple planar geometry (ignores earth curvature).
fn crossarc_lnglat(p1: LngLat, p2: LngLat, p3: LngLat) -> f64 {
    // Convert to meters using approximate scaling
    let meters_per_longitude_degree = get_meters_per_longitude_degree(p1.lat);
    crossarc(
        p1.lng * meters_per_longitude_degree,
        p1.lat * METERS_PER_LATITUDE_DEGREE,
        p2.lng * meters_per_longitude_degree,
        p2.lat * METERS_PER_LATITUDE_DEGREE,
        p3.lng * meters_per_longitude_degree,
        p3.lat * METERS_PER_LATITUDE_DEGREE,
    )
}

pub fn crossarc(x1: f64, y1: f64, x2: f64, y2: f64, x3: f64, y3: f64) -> f64 {
    let dx = x2 - x1;
    let dy = y2 - y1;
    let segment_length_squared = dx * dx + dy * dy;

    if segment_length_squared == 0.0 {
        // p1 and p2 are the same point
        ((x3 - x1) * (x3 - x1) + (y3 - y1) * (y3 - y1)).sqrt()
    } else {
        // Project p3 onto the line defined by p1 - p2
        let t =
            0.0_f64.max(1.0_f64.min(((x3 - x1) * dx + (y3 - y1) * dy) / segment_length_squared));

        // Find the closest point on the segment
        let proj_x = x1 + t * dx;
        let proj_y = y1 + t * dy;

        // Return distance from p3 to the projected point
        ((x3 - proj_x) * (x3 - proj_x) + (y3 - proj_y) * (y3 - proj_y)).sqrt()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn p(lng: f64, lat: f64) -> LngLat {
        LngLat { lng, lat }
    }

    #[test]
    fn test_distance() {
        assert_eq!(distance(p(4.0, 50.0), p(4.0, 50.0)), 0.0);
        // one degree of latitude
        let d = distance(p(0.0, 0.0), p(0.0, 1.0));
        assert!((d - 111.195).abs() < 0.01, "{d}");
        // symmetric
        let a = p(4.40, 50.79);
        let b = p(6.13, 45.90);
        assert!((distance(a, b) - distance(b, a)).abs() < 1e-9);
        // half the Earth's circumference, without NaN from rounding
        let antipodal = distance(p(0.0, 0.0), p(180.0, 0.0));
        assert!((antipodal - PI * EARTH_RADIUS).abs() < 1e-6);
    }

    #[test]
    fn test_time_diff() {
        assert_eq!(time_diff(&Some(5000), &Some(2000)), Some(3000));
        assert_eq!(time_diff(&Some(2000), &Some(5000)), Some(-3000));
        assert_eq!(time_diff(&None, &Some(1)), None);
        assert_eq!(time_diff(&Some(1), &None), None);
    }

    #[test]
    fn test_speed() {
        // 1 km in 1 h
        assert!((speed(1.0, 3_600_000) - 1.0).abs() < 1e-12);
        // 10 km in 30 min
        assert!((speed(10.0, 1_800_000) - 20.0).abs() < 1e-12);
    }

    #[test]
    fn test_slope() {
        // 10 m of elevation over 100 m (distance is expressed in km, hence the 0.1 factor)
        assert!((slope(10.0, 0.1) - 10.0).abs() < 1e-12);
        assert!((slope(-5.0, 0.1) + 5.0).abs() < 1e-12);
        assert_eq!(slope(0.0, 1.0), 0.0);
        assert_eq!(slope(10.0, 0.0), 100.0);
    }

    #[test]
    fn test_crossarc() {
        // perpendicular distance to the segment (0,0)-(10,0)
        assert!((crossarc(0.0, 0.0, 10.0, 0.0, 5.0, 3.0) - 3.0).abs() < 1e-12);
        // beyond the ends: distance to the closest endpoint
        assert!((crossarc(0.0, 0.0, 10.0, 0.0, 13.0, 4.0) - 5.0).abs() < 1e-12);
        assert!((crossarc(0.0, 0.0, 10.0, 0.0, -3.0, 4.0) - 5.0).abs() < 1e-12);
        // point on the segment
        assert_eq!(crossarc(0.0, 0.0, 10.0, 0.0, 4.0, 0.0), 0.0);
        // degenerate segment
        assert!((crossarc(1.0, 1.0, 1.0, 1.0, 4.0, 5.0) - 5.0).abs() < 1e-12);
    }

    #[test]
    fn test_projected() {
        let proj = projected(p(0.0, 0.0), p(1.0, 0.0), p(0.5, 1.0));
        assert!((proj.lng - 0.5).abs() < 1e-9);
        assert!(proj.lat.abs() < 1e-9);
        // clamped to the segment
        let proj = projected(p(0.0, 0.0), p(1.0, 0.0), p(2.0, 1.0));
        assert!((proj.lng - 1.0).abs() < 1e-9);
        // degenerate segment
        let proj = projected(p(3.0, 4.0), p(3.0, 4.0), p(5.0, 6.0));
        assert_eq!((proj.lng, proj.lat), (3.0, 4.0));
    }

    #[test]
    fn test_crossarc_lnglat() {
        // about 1 degree of latitude away from an east-west segment on the equator
        let d = crossarc_lnglat(p(0.0, 0.0), p(1.0, 0.0), p(0.5, 1.0));
        assert!((d - METERS_PER_LATITUDE_DEGREE).abs() < 1e-6);
    }
}