rustial-engine 0.0.1

//! Quantized-mesh elevation source.
//!
//! Implements an HTTP-backed [`ElevationSource`] for Cesium-style
//! quantized-mesh terrain tiles.  The parser decodes quantized
//! `(u, v, height)` vertex streams and rasterizes them to an
//! [`ElevationGrid`] using nearest-vertex sampling.

use crate::io::{HttpClient, HttpRequest};
use crate::terrain::elevation_source::{
    ElevationSource, ElevationSourceDiagnostics, ElevationSourceFailureDiagnostics,
};
use crate::terrain::error::TerrainError;
use rustial_math::{ElevationGrid, TileId};
use std::collections::{HashMap, VecDeque};
use std::sync::Mutex;

const DEFAULT_MAX_CONCURRENT_TERRAIN_REQUESTS: usize = 32;

/// HTTP-backed quantized-mesh elevation source.
pub struct QuantizedMeshSource {
    url_template: String,
    client: Box<dyn HttpClient>,
    pending: Mutex<HashMap<String, TileId>>,
    queued: Mutex<VecDeque<(String, TileId)>>,
    grid_size: u32,
    max_concurrent: usize,
    diagnostics: Mutex<ElevationSourceFailureDiagnostics>,
}

impl std::fmt::Debug for QuantizedMeshSource {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("QuantizedMeshSource")
            .field("url_template", &self.url_template)
            .field("grid_size", &self.grid_size)
            .field("max_concurrent", &self.max_concurrent)
            .finish()
    }
}

impl QuantizedMeshSource {
    /// Create a quantized-mesh source.
    ///
    /// `grid_size` is the output elevation grid resolution per edge
    /// (e.g. 65 for typical terrain tiles).
    pub fn new(
        url_template: impl Into<String>,
        client: Box<dyn HttpClient>,
        grid_size: u32,
    ) -> Self {
        Self {
            url_template: url_template.into(),
            client,
            pending: Mutex::new(HashMap::new()),
            queued: Mutex::new(VecDeque::new()),
            grid_size: grid_size.max(2),
            max_concurrent: DEFAULT_MAX_CONCURRENT_TERRAIN_REQUESTS,
            diagnostics: Mutex::new(ElevationSourceFailureDiagnostics::default()),
        }
    }

    /// Override the maximum number of concurrent quantized-mesh HTTP requests.
    pub fn with_max_concurrent_requests(mut self, max_concurrent: usize) -> Self {
        self.max_concurrent = max_concurrent.max(1);
        self
    }

    fn tile_url(&self, id: &TileId) -> String {
        self.url_template
            .replace("{z}", &id.zoom.to_string())
            .replace("{x}", &id.x.to_string())
            .replace("{y}", &id.y.to_string())
    }

    fn parse_quantized_mesh(
        &self,
        id: TileId,
        bytes: &[u8],
    ) -> Result<ElevationGrid, TerrainError> {
        // Cesium quantized-mesh header length in bytes:
        // 3*f64 + 2*f32 + 4*f64 + 3*f64 = 88
        const HEADER_LEN: usize = 88;
        if bytes.len() < HEADER_LEN + 4 {
            return Err(TerrainError::Decode("quantized-mesh tile too short".into()));
        }

        let mut cursor = 0usize;

        // Skip centerX/Y/Z (f64), min/max height (f32),
        // bounding sphere (4*f64), horizon occlusion point (3*f64).
        cursor += HEADER_LEN;

        let vertex_count = read_u32_le(bytes, &mut cursor)? as usize;
        if vertex_count == 0 {
            return Err(TerrainError::Decode(
                "quantized-mesh has zero vertices".into(),
            ));
        }

        let mut u = vec![0i32; vertex_count];
        let mut v = vec![0i32; vertex_count];
        let mut h = vec![0i32; vertex_count];

        decode_delta_zigzag_stream(bytes, &mut cursor, &mut u)?;
        decode_delta_zigzag_stream(bytes, &mut cursor, &mut v)?;
        decode_delta_zigzag_stream(bytes, &mut cursor, &mut h)?;

        // Read min/max height from header for de-quantization.
        // minHeight starts at byte 24 (after 3*f64).
        let min_h = read_f32_le_at(bytes, 24)?;
        let max_h = read_f32_le_at(bytes, 28)?;
        let height_span = max_h - min_h;

        let vertices: Vec<(f32, f32, f32)> = (0..vertex_count)
            .map(|i| {
                let uu = (u[i].clamp(0, 32767) as f32) / 32767.0;
                let vv = (v[i].clamp(0, 32767) as f32) / 32767.0;
                let hh = (h[i].clamp(0, 32767) as f32) / 32767.0;
                let elev = min_h + hh * height_span;
                (uu, vv, elev)
            })
            .collect();

        // Rasterize to a regular grid by nearest-vertex lookup.
        let n = self.grid_size;
        let mut data = Vec::with_capacity((n * n) as usize);
        for gy in 0..n {
            let tv = gy as f32 / (n - 1) as f32;
            for gx in 0..n {
                let tu = gx as f32 / (n - 1) as f32;
                let mut best_d2 = f32::MAX;
                let mut best_h = 0.0f32;
                for &(uu, vv, hh) in &vertices {
                    let du = uu - tu;
                    let dv = vv - tv;
                    let d2 = du * du + dv * dv;
                    if d2 < best_d2 {
                        best_d2 = d2;
                        best_h = hh;
                    }
                }
                data.push(best_h);
            }
        }

        ElevationGrid::from_data(id, n, n, data)
            .ok_or_else(|| TerrainError::Decode("failed to build elevation grid".into()))
    }

    fn flush_queued(&self) {
        let mut pending = match self.pending.lock() {
            Ok(p) => p,
            Err(_) => return,
        };
        let mut queued = match self.queued.lock() {
            Ok(q) => q,
            Err(_) => return,
        };

        while pending.len() < self.max_concurrent {
            let Some((url, id)) = queued.pop_front() else {
                break;
            };
            pending.insert(url.clone(), id);
            self.client.send(HttpRequest::get(url));
        }
    }
}

impl ElevationSource for QuantizedMeshSource {
    fn request(&self, id: TileId) {
        let url = self.tile_url(&id);
        let mut pending = match self.pending.lock() {
            Ok(p) => p,
            Err(_) => return,
        };
        if pending.values().any(|existing| *existing == id) {
            return;
        }
        let mut queued = match self.queued.lock() {
            Ok(q) => q,
            Err(_) => return,
        };
        if queued.iter().any(|(_, existing)| *existing == id) {
            return;
        }

        if pending.len() < self.max_concurrent {
            pending.insert(url.clone(), id);
            drop(queued);
            drop(pending);
            self.client.send(HttpRequest::get(url));
        } else {
            queued.push_back((url, id));
        }
    }

    fn poll(&self) -> Vec<(TileId, Result<ElevationGrid, TerrainError>)> {
        let responses = self.client.poll();
        if responses.is_empty() {
            return Vec::new();
        }

        let mut pending = match self.pending.lock() {
            Ok(p) => p,
            Err(_) => return Vec::new(),
        };

        let mut out = Vec::with_capacity(responses.len());
        for (url, result) in responses {
            let id = match pending.remove(&url) {
                Some(id) => id,
                None => {
                    if let Ok(mut diagnostics) = self.diagnostics.lock() {
                        diagnostics.ignored_completed_responses += 1;
                    }
                    continue;
                }
            };

            match result {
                Ok(resp) if resp.is_success() => {
                    let parsed = self.parse_quantized_mesh(id, &resp.body);
                    if let Err(err) = &parsed {
                        if let Ok(mut diagnostics) = self.diagnostics.lock() {
                            match err {
                                TerrainError::Decode(_) => diagnostics.decode_failures += 1,
                                TerrainError::Network(_) => diagnostics.network_failures += 1,
                                TerrainError::UnsupportedFormat(_) => {
                                    diagnostics.unsupported_format_failures += 1
                                }
                                TerrainError::Other(_) => diagnostics.other_failures += 1,
                            }
                        }
                    }
                    out.push((id, parsed));
                }
                Ok(resp) => {
                    if let Ok(mut diagnostics) = self.diagnostics.lock() {
                        diagnostics.network_failures += 1;
                    }
                    out.push((
                        id,
                        Err(TerrainError::Network(format!("HTTP {}", resp.status))),
                    ));
                }
                Err(e) => {
                    if let Ok(mut diagnostics) = self.diagnostics.lock() {
                        diagnostics.network_failures += 1;
                    }
                    out.push((id, Err(TerrainError::Network(e))))
                }
            }
        }
        drop(pending);
        self.flush_queued();
        out
    }

    fn diagnostics(&self) -> Option<ElevationSourceDiagnostics> {
        let pending = self.pending.lock().map(|p| p.len()).unwrap_or(0);
        let queued = self.queued.lock().map(|q| q.len()).unwrap_or(0);
        let failures = self
            .diagnostics
            .lock()
            .map(|d| d.clone())
            .unwrap_or_default();
        Some(ElevationSourceDiagnostics {
            queued_requests: queued,
            in_flight_requests: pending,
            max_concurrent_requests: self.max_concurrent,
            known_requests: queued + pending,
            cancelled_in_flight_requests: 0,
            failure_diagnostics: failures,
        })
    }

    fn cancel(&self, id: TileId) -> bool {
        if let Ok(mut queued) = self.queued.lock() {
            let before = queued.len();
            queued.retain(|(_, queued_id)| *queued_id != id);
            return queued.len() != before;
        }
        false
    }
}

fn read_u16_le(bytes: &[u8], cursor: &mut usize) -> Result<u16, TerrainError> {
    if *cursor + 2 > bytes.len() {
        return Err(TerrainError::Decode("unexpected EOF (u16)".into()));
    }
    let v = u16::from_le_bytes([bytes[*cursor], bytes[*cursor + 1]]);
    *cursor += 2;
    Ok(v)
}

fn read_u32_le(bytes: &[u8], cursor: &mut usize) -> Result<u32, TerrainError> {
    if *cursor + 4 > bytes.len() {
        return Err(TerrainError::Decode("unexpected EOF (u32)".into()));
    }
    let v = u32::from_le_bytes([
        bytes[*cursor],
        bytes[*cursor + 1],
        bytes[*cursor + 2],
        bytes[*cursor + 3],
    ]);
    *cursor += 4;
    Ok(v)
}

fn read_f32_le_at(bytes: &[u8], offset: usize) -> Result<f32, TerrainError> {
    if offset + 4 > bytes.len() {
        return Err(TerrainError::Decode("unexpected EOF (f32)".into()));
    }
    Ok(f32::from_le_bytes([
        bytes[offset],
        bytes[offset + 1],
        bytes[offset + 2],
        bytes[offset + 3],
    ]))
}

fn zigzag_decode(v: u16) -> i32 {
    ((v >> 1) as i32) ^ -((v & 1) as i32)
}

fn decode_delta_zigzag_stream(
    bytes: &[u8],
    cursor: &mut usize,
    out: &mut [i32],
) -> Result<(), TerrainError> {
    let mut acc = 0i32;
    for item in out.iter_mut() {
        let enc = read_u16_le(bytes, cursor)?;
        let delta = zigzag_decode(enc);
        acc += delta;
        *item = acc;
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::io::{HttpClient, HttpRequest as Req, HttpResponse};
    use std::sync::Mutex as StdMutex;

    struct MockClient {
        sent: StdMutex<Vec<String>>,
        responses: StdMutex<Vec<(String, Result<HttpResponse, String>)>>,
    }

    impl MockClient {
        fn new() -> Self {
            Self {
                sent: StdMutex::new(Vec::new()),
                responses: StdMutex::new(Vec::new()),
            }
        }
    }

    impl HttpClient for MockClient {
        fn send(&self, request: Req) {
            self.sent.lock().unwrap().push(request.url);
        }

        fn poll(&self) -> Vec<(String, Result<HttpResponse, String>)> {
            std::mem::take(&mut *self.responses.lock().unwrap())
        }
    }

    fn zigzag_encode(v: i32) -> u16 {
        ((v << 1) ^ (v >> 31)) as u16
    }

    fn make_synthetic_quantized_mesh() -> Vec<u8> {
        let mut bytes = vec![0u8; 88];
        // minHeight=0.0 at byte 24, maxHeight=100.0 at byte 28.
        bytes[24..28].copy_from_slice(&0.0f32.to_le_bytes());
        bytes[28..32].copy_from_slice(&100.0f32.to_le_bytes());

        let vertex_count = 4u32;
        bytes.extend_from_slice(&vertex_count.to_le_bytes());

        // Corners in quantized space.
        let u_abs = [0i32, 32767, 0, 32767];
        let v_abs = [0i32, 0, 32767, 32767];
        let h_abs = [0i32, 8192, 16384, 32767];

        for stream in [&u_abs, &v_abs, &h_abs] {
            let mut prev = 0i32;
            for &cur in stream {
                let delta = cur - prev;
                prev = cur;
                bytes.extend_from_slice(&zigzag_encode(delta).to_le_bytes());
            }
        }

        bytes
    }

    #[test]
    fn parse_quantized_mesh_to_grid() {
        let client = MockClient::new();
        let source = QuantizedMeshSource::new(
            "https://example.com/{z}/{x}/{y}.terrain",
            Box::new(client),
            2,
        );

        let grid = source
            .parse_quantized_mesh(TileId::new(0, 0, 0), &make_synthetic_quantized_mesh())
            .expect("parse");

        assert_eq!(grid.width, 2);
        assert_eq!(grid.height, 2);
        assert_eq!(grid.data.len(), 4);
        assert!(grid.max_elev > grid.min_elev);
    }

    #[test]
    fn poll_returns_decoded_grid() {
        let client = MockClient::new();
        client.responses.lock().unwrap().push((
            "https://example.com/0/0/0.terrain".into(),
            Ok(HttpResponse {
                status: 200,
                body: make_synthetic_quantized_mesh(),
                headers: vec![],
            }),
        ));

        let source = QuantizedMeshSource::new(
            "https://example.com/{z}/{x}/{y}.terrain",
            Box::new(client),
            2,
        );

        source.request(TileId::new(0, 0, 0));
        let out = source.poll();
        assert_eq!(out.len(), 1);
        assert!(out[0].1.is_ok());
    }

    #[test]
    fn invalid_bytes_error() {
        let client = MockClient::new();
        let source = QuantizedMeshSource::new(
            "https://example.com/{z}/{x}/{y}.terrain",
            Box::new(client),
            2,
        );
        let err = source
            .parse_quantized_mesh(TileId::new(0, 0, 0), &[1, 2, 3])
            .unwrap_err();
        assert!(matches!(err, TerrainError::Decode(_)));
    }

    #[test]
    fn respects_concurrency_cap_and_queues_excess_requests() {
        let client = MockClient::new();
        let source = QuantizedMeshSource::new(
            "https://example.com/{z}/{x}/{y}.terrain",
            Box::new(client),
            2,
        )
        .with_max_concurrent_requests(1);

        source.request(TileId::new(1, 0, 0));
        source.request(TileId::new(1, 0, 1));

        let diagnostics = source.diagnostics().expect("terrain diagnostics");
        assert_eq!(diagnostics.in_flight_requests, 1);
        assert_eq!(diagnostics.queued_requests, 1);
        assert_eq!(diagnostics.max_concurrent_requests, 1);
        assert_eq!(diagnostics.known_requests, 2);
    }

    #[test]
    fn cancel_removes_queued_request_only() {
        let client = MockClient::new();
        let source = QuantizedMeshSource::new(
            "https://example.com/{z}/{x}/{y}.terrain",
            Box::new(client),
            2,
        )
        .with_max_concurrent_requests(1);

        let a = TileId::new(1, 0, 0);
        let b = TileId::new(1, 0, 1);
        source.request(a);
        source.request(b);

        assert!(!source.cancel(a), "in-flight request should not cancel");
        assert!(source.cancel(b), "queued request should cancel");

        let diagnostics = source.diagnostics().expect("terrain diagnostics");
        assert_eq!(diagnostics.in_flight_requests, 1);
        assert_eq!(diagnostics.queued_requests, 0);
        assert_eq!(diagnostics.known_requests, 1);
    }
}