//! GPX data extraction functions.
//!
//! We *could* use the [gpx](https://github.com/georust/gpx) crate, but we don't care about much
//! other than the coordinates of the tracks. By implementing the little functionality ourselves,
//! we can use a fast XML parser ([roxmltree](https://github.com/RazrFalcon/roxmltree)).
use std::{f64::consts::PI, fs, path::Path};

use color_eyre::eyre::{eyre, Result};
use roxmltree::{Document, Node, NodeType};

#[derive(Debug, Copy, Clone, PartialEq)]
pub struct Coordinates {
    longitude: f64,
    latitude: f64,
}

impl Coordinates {
    /// Calculates the [Web Mercator
    /// projection](https://en.wikipedia.org/wiki/Web_Mercator_projection) of the coordinates.
    /// Returns the `(x, y)` coordinates.
    pub fn web_mercator(self, zoom: u32) -> (u64, u64) {
        let lambda = self.longitude.to_radians();
        let phi = self.latitude.to_radians();
        let x = 2u64.pow(zoom) as f64 / (2.0 * PI) * 256.0 * (lambda + PI);
        let y = 2u64.pow(zoom) as f64 / (2.0 * PI) * 256.0 * (PI - (PI / 4.0 + phi / 2.0).tan().ln());
        (x.floor() as u64, y.floor() as u64)
    }
}

fn is_track_node(node: &Node) -> bool {
    node.node_type() == NodeType::Element && node.tag_name().name() == "trk"
}

fn is_track_segment(node: &Node) -> bool {
    node.node_type() == NodeType::Element && node.tag_name().name() == "trkseg"
}

fn is_track_point(node: &Node) -> bool {
    node.node_type() == NodeType::Element && node.tag_name().name() == "trkpt"
}

pub fn extract_from_str(input: &str) -> Result<Vec<Coordinates>> {
    let mut result = Vec::new();
    let document = Document::parse(input)?;
    for node in document.root_element().children().filter(is_track_node) {
        for segment in node.children().filter(is_track_segment) {
            for point in segment.children().filter(is_track_point) {
                let latitude = point.attribute("lat")
                    .and_then(|l| l.parse::<f64>().ok())
                    .ok_or_else(|| eyre!("Invalid latitude"))?;
                let longitude = point.attribute("lon")
                    .and_then(|l| l.parse::<f64>().ok())
                    .ok_or_else(|| eyre!("Invalid longitude"))?;
                result.push(Coordinates { latitude, longitude });
            }
        }
    }
    Ok(result)
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum Compression {
    None,
}

pub fn extract_from_file<P: AsRef<Path>>(path: P, _compression: Compression) -> Result<Vec<Coordinates>> {
    let content = fs::read_to_string(path)?;
    extract_from_str(&content)
}