//! 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)).
//!
//! Note that we throw away all information that we don't care about. Since we need only the
//! coordinates of a track, we simply use a `Vec<Coordinates>` to represent a track.
use std::{
    f64::consts::PI,
    ffi::OsStr,
    fs::{self, File},
    io::{BufReader, Read},
    path::Path,
};

use color_eyre::eyre::{eyre, Result};
use flate2::bufread::GzDecoder;
use roxmltree::{Document, Node, NodeType};

/// World coordinates.
#[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)` projection, where both are in the range `[0, 256 * 2^zoom)`.
    pub fn web_mercator(self, zoom: u32) -> (u64, u64) {
        const WIDTH: f64 = super::layer::TILE_WIDTH as f64;
        const HEIGHT: f64 = super::layer::TILE_HEIGHT as f64;

        let lambda = self.longitude.to_radians();
        let phi = self.latitude.to_radians();
        let x = 2u64.pow(zoom) as f64 / (2.0 * PI) * WIDTH * (lambda + PI);
        let y =
            2u64.pow(zoom) as f64 / (2.0 * PI) * HEIGHT * (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"
}

/// Extracts a track from the given string.
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)
}

/// Compression format of the data.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum Compression {
    /// Indicates that no compression is applied, and the file is plain GPX.
    None,
    /// Indicates that the file is gzip compressed.
    Gzip,
    /// Indicates that the file is brotli compressed.
    Brotli,
}

impl Compression {
    /// Suggests a [`Compression`] from the given path name.
    ///
    /// This will suggest [`Compression::Brotli`] for files ending in `.br`, [`Compression::Gzip`]
    /// for files ending with `.gz` or `.gzip`, and [`Compression::None`] for files ending with
    /// `.gpx`.
    ///
    /// If the file does not end with any of the aforementioned extensions, an error is returned
    /// instead.
    pub fn suggest_from_path<P: AsRef<Path>>(path: P) -> Option<Compression> {
        let Some(ext) = path.as_ref().extension() else { return None };
        if OsStr::new("br") == ext {
            Some(Compression::Brotli)
        } else if [OsStr::new("gz"), OsStr::new("gzip")].contains(&ext) {
            Some(Compression::Gzip)
        } else if OsStr::new("gpx") == ext {
            Some(Compression::None)
        } else {
            None
        }
    }
}

/// Extracts the relevant GPX data from the given file.
///
/// Note that the content must be valid UTF-8, as that is what our parser expects.
pub fn extract_from_file<P: AsRef<Path>>(
    path: P,
    compression: Compression,
) -> Result<Vec<Coordinates>> {
    let content = match compression {
        Compression::None => fs::read_to_string(path)?,
        Compression::Gzip => {
            let mut result = String::new();
            GzDecoder::new(BufReader::new(File::open(path)?)).read_to_string(&mut result)?;
            result
        }
        Compression::Brotli => {
            let mut result = Vec::new();
            brotli::BrotliDecompress(&mut BufReader::new(File::open(path)?), &mut result)?;
            String::from_utf8(result)?
        }
    };
    extract_from_str(&content)
}