//! wgrib2 subprocess wrapper. 1:1 port of the hot paths in
//! `lib/microwaveprop/weather/grib2/wgrib2.ex` used by f01..f48:
//!   * `extract_grid` (write GRIB2 to tmp → `wgrib2 -lola … bin` → parse)
//!   * inventory parsing from stdout
//!   * `parse_lola_binary` (Fortran-unformatted IEEE 754 little-endian f32)
//!
//! wgrib2 is called as a child process exactly as the Elixir version
//! does — same CLI flags, same temp-file lifecycle. Output binary is
//! stride-addressed: each message = 4-byte LE record length + N*4 bytes
//! of data + 4-byte duplicate record length.

use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::process::{Command, Output};
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

use crate::field_grid::FieldGrid;
use crate::grid::GridSpec;

const UNDEFINED_VALUE: f32 = 9.999e20;
static UNIQUE: AtomicU64 = AtomicU64::new(1);

#[derive(Debug, thiserror::Error)]
pub enum DecodeError {
    #[error("wgrib2 not available on PATH")]
    Wgrib2NotAvailable,
    #[error("wgrib2 failed (exit {code}): {stderr}")]
    Wgrib2Failed { code: i32, stderr: String },
    #[error("io: {0}")]
    Io(#[from] std::io::Error),
}

#[derive(Debug, Clone)]
pub struct Message {
    pub var: String,
    pub level: String,
}

// The former `CellValues` / `PointGrid` aliases (a HashMap of ~95 k
// per-cell HashMaps) are gone — see `field_grid::FieldGrid`, which
// stores the same data as dense per-message planes. Decoded grids are
// `FieldGrid` everywhere now.

pub fn wgrib2_available() -> bool {
    which_wgrib2().is_some()
}

fn which_wgrib2() -> Option<PathBuf> {
    if let Ok(val) = std::env::var("WGRIB2") {
        return Some(PathBuf::from(val));
    }
    let path_var = std::env::var_os("PATH")?;
    for dir in std::env::split_paths(&path_var) {
        let candidate = dir.join("wgrib2");
        if candidate.is_file() {
            return Some(candidate);
        }
    }
    None
}

pub fn normalize_lon(lon: f64) -> f64 {
    if lon < 0.0 {
        lon + 360.0
    } else {
        lon
    }
}

pub fn denormalize_lon(lon: f64) -> f64 {
    if lon > 180.0 {
        lon - 360.0
    } else {
        lon
    }
}

/// Per-cell HRDPS extraction via `wgrib2 -lon LON LAT`.
///
/// `extract_grid` (`-lola`) interpolates the entire source grid onto a
/// target rectangle, which on HRDPS's rotated lat/lon source runs at
/// 10+ minutes per chain step (production observation 2026-04-29). The
/// `-lon` per-point path that the probe used is far cheaper because it
/// only computes the rotation math at the points we ask for. For
/// 57k Canadian-only cells this drops chain-step wall time from "never
/// completes" to roughly 30-90 seconds.
///
/// Batched at `POINT_BATCH` points per wgrib2 invocation to keep the
/// argv list comfortably under Linux's `ARG_MAX` (about 2 MiB) and keep
/// stdout parseable in chunks.
pub fn extract_points(
    grib: &[u8],
    match_pattern: &str,
    points: &[(f64, f64)],
    grid_spec: GridSpec,
) -> Result<FieldGrid, DecodeError> {
    let wgrib2 = which_wgrib2().ok_or(DecodeError::Wgrib2NotAvailable)?;
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_nanos())
        .unwrap_or(0);
    let uniq = UNIQUE.fetch_add(1, Ordering::Relaxed);
    let grib_path: PathBuf = std::env::temp_dir().join(format!("hrdps_{nanos}_{uniq}.grib2"));

    std::fs::write(&grib_path, grib)?;
    let result = run_wgrib2_lon_batched(&wgrib2, &grib_path, match_pattern, points, grid_spec);
    let _ = std::fs::remove_file(&grib_path);
    result
}

// 1000 points per wgrib2 invocation. With ~57k Canadian cells that's
// 57 invocations per chain step — enough to amortize process-spawn
// overhead, small enough that any future wgrib2 regression around
// large -lon arg lists trips on a manageable failure mode.
//
// Note: wgrib2 3.6.0 had a memory-corruption bug surfaced by HRDPS
// rotated lat/lon files at *any* batch size (verified at N=200 in
// production on 2026-04-30 — `free(): invalid size`). The fix was
// bumping the Dockerfile to wgrib2 3.8.0; this batch size is just
// a normal performance tuning choice, not a workaround.
const POINT_BATCH: usize = 1_000;

fn run_wgrib2_lon_batched(
    wgrib2: &Path,
    grib_path: &Path,
    match_pattern: &str,
    points: &[(f64, f64)],
    grid_spec: GridSpec,
) -> Result<FieldGrid, DecodeError> {
    let mut out = FieldGrid::new(grid_spec);

    for batch in points.chunks(POINT_BATCH) {
        let batch_values = run_wgrib2_lon_one(wgrib2, grib_path, match_pattern, batch, &grid_spec)?;
        for (name, cells) in batch_values {
            let (_, plane) = out.plane_mut_or_insert(&name);
            for (cell, value) in cells {
                plane[cell] = value;
            }
        }
    }
    Ok(out)
}

/// One wgrib2 `-lon` invocation, keyed `"VAR:LEVEL" -> [(cell_index, value)]`.
/// Values are gathered per-name so the caller can scatter each name into a
/// single plane instead of touching a per-cell map.
type LonBatchValues = HashMap<String, Vec<(usize, f32)>>;

fn run_wgrib2_lon_one(
    wgrib2: &Path,
    grib_path: &Path,
    match_pattern: &str,
    batch: &[(f64, f64)],
    grid_spec: &GridSpec,
) -> Result<LonBatchValues, DecodeError> {
    let mut cmd = Command::new(wgrib2);
    cmd.arg(grib_path)
        .arg("-s")
        .arg("-match")
        .arg(match_pattern);
    for &(lat, lon) in batch {
        cmd.arg("-lon").arg(format!("{lon}")).arg(format!("{lat}"));
    }

    let Output {
        status,
        stdout,
        stderr,
    } = cmd.output()?;

    if !status.success() {
        // Surface the FIRST chars of stderr (the actual diagnostic)
        // rather than stdout (which is mostly per-cell numeric output
        // that drowned the interesting failure context). status.code()
        // returns None on signal-kill — flag that with -1 so the
        // failure mode is distinguishable from a real exit code.
        let stderr_text = String::from_utf8_lossy(&stderr).to_string();
        let stderr_snippet: String = stderr_text.chars().take(400).collect();
        let signaled = status.code().is_none();
        let suffix = if signaled {
            format!(" (killed by signal, batch={})", batch.len())
        } else {
            String::new()
        };
        return Err(DecodeError::Wgrib2Failed {
            code: status.code().unwrap_or(-1),
            stderr: format!("{stderr_snippet}{suffix}"),
        });
    }

    let text = String::from_utf8_lossy(&stdout);
    Ok(parse_lon_output(&text, batch, grid_spec))
}

/// Parse `wgrib2 -s -lon` text output. Each record line is a colon-
/// separated header followed by per-point `lon=…,lat=…,val=…` segments,
/// one per `-lon` arg.
///
/// Example line:
/// `1:0:d=...:TMP:2 m above ground:anl:lon=280.369,lat=43.670,val=280.97:lon=...`
fn parse_lon_output(text: &str, batch: &[(f64, f64)], grid_spec: &GridSpec) -> LonBatchValues {
    let mut out: LonBatchValues = HashMap::new();

    for line in text.lines() {
        if line.is_empty() {
            continue;
        }
        let mut parts = line.split(':');
        let _n = parts.next();
        let _offset = parts.next();
        let _date = parts.next();
        let var = match parts.next() {
            Some(v) => v,
            None => continue,
        };
        let level = match parts.next() {
            Some(l) => l,
            None => continue,
        };
        // Skip the record-type/forecast-time field (anl, "240 min fcst", …).
        let _kind = parts.next();
        let key = format!("{var}:{level}");
        let slot = out.entry(key).or_default();

        for segment in parts {
            if let Some((lat, lon, val)) = parse_lon_segment(segment, batch) {
                if let Some(cell) = crate::field_grid::cell_index_for(grid_spec, lat, lon) {
                    slot.push((cell, val));
                }
            }
        }
    }

    out
}

/// `lon=242.958,lat=32.938,val=306.5` → (lat, lon, val) snapped to the
/// nearest point in `batch` (handles wgrib2's lon-180/360 convention by
/// denormalising before matching).
fn parse_lon_segment(segment: &str, batch: &[(f64, f64)]) -> Option<(f64, f64, f32)> {
    let mut lon_part = None;
    let mut lat_part = None;
    let mut val_part = None;
    for kv in segment.split(',') {
        let (k, v) = kv.split_once('=')?;
        let parsed: f64 = v.trim().parse().ok()?;
        match k.trim() {
            "lon" => lon_part = Some(parsed),
            "lat" => lat_part = Some(parsed),
            "val" => val_part = Some(parsed),
            _ => {}
        }
    }
    let raw_lon = lon_part?;
    let raw_lat = lat_part?;
    let val = val_part?;

    if val > UNDEFINED_VALUE as f64 / 2.0 {
        return None;
    }

    let target_lon = denormalize_lon(raw_lon);
    let (lat, lon) = snap_to_batch(raw_lat, target_lon, batch)?;
    Some((lat, lon, val as f32))
}

fn snap_to_batch(lat: f64, lon: f64, batch: &[(f64, f64)]) -> Option<(f64, f64)> {
    // wgrib2 -lon snaps to the nearest source-grid cell, so the returned
    // lat/lon is close to but not exactly equal to what we asked for.
    // Match it back to our request (which is on the 0.125° propagation
    // grid) via the closest batch entry.
    let mut best: Option<((f64, f64), f64)> = None;
    for &(p_lat, p_lon) in batch {
        let dlat = p_lat - lat;
        let dlon = p_lon - lon;
        let d2 = dlat * dlat + dlon * dlon;
        match best {
            None => best = Some(((p_lat, p_lon), d2)),
            Some((_, b)) if d2 < b => best = Some(((p_lat, p_lon), d2)),
            _ => {}
        }
    }
    let (winner, d2) = best?;
    // Reject anything more than 0.5° away — wgrib2 returned a cell that
    // doesn't correspond to a point we asked for (probably a rounding
    // edge case at the bbox boundary).
    if d2 > 0.25 {
        return None;
    }
    Some(winner)
}

/// Extract values at the given grid spec from an in-memory GRIB2 blob.
/// Writes the blob to a temp file, runs wgrib2, parses the output.
pub fn extract_grid(
    grib: &[u8],
    match_pattern: &str,
    grid_spec: GridSpec,
) -> Result<FieldGrid, DecodeError> {
    let wgrib2 = which_wgrib2().ok_or(DecodeError::Wgrib2NotAvailable)?;
    let tmp_dir = std::env::temp_dir();
    let uniq = UNIQUE.fetch_add(1, Ordering::Relaxed);
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_nanos())
        .unwrap_or(0);
    let grib_path = tmp_dir.join(format!("hrrr_{nanos}_{uniq}.grib2"));
    let bin_path = tmp_dir.join(format!("hrrr_{nanos}_{uniq}.grib2.lola.bin"));

    let result: Result<FieldGrid, DecodeError> = (|| {
        std::fs::write(&grib_path, grib)?;
        run_wgrib2_lola(&wgrib2, &grib_path, match_pattern, &grid_spec, &bin_path)
    })();

    let _ = std::fs::remove_file(&grib_path);
    let _ = std::fs::remove_file(&bin_path);
    result
}

/// File-path variant — avoids the intermediate `write(tmp, binary)` copy.
pub fn extract_grid_from_file(
    grib_path: &Path,
    match_pattern: &str,
    grid_spec: GridSpec,
) -> Result<FieldGrid, DecodeError> {
    let wgrib2 = which_wgrib2().ok_or(DecodeError::Wgrib2NotAvailable)?;
    let bin_path = sibling_tmp_path(grib_path, "lola");
    let result = run_wgrib2_lola(&wgrib2, grib_path, match_pattern, &grid_spec, &bin_path);
    let _ = std::fs::remove_file(&bin_path);
    result
}

fn sibling_tmp_path(path: &Path, suffix: &str) -> PathBuf {
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_nanos())
        .unwrap_or(0);
    let uniq = UNIQUE.fetch_add(1, Ordering::Relaxed);
    let mut s = path.as_os_str().to_owned();
    s.push(format!(".{suffix}.{nanos}.{uniq}.bin"));
    PathBuf::from(s)
}

fn run_wgrib2_lola(
    wgrib2: &Path,
    grib_path: &Path,
    match_pattern: &str,
    grid_spec: &GridSpec,
    bin_path: &Path,
) -> Result<FieldGrid, DecodeError> {
    let lon_spec = format!(
        "{}:{}:{}",
        normalize_lon(grid_spec.lon_start),
        grid_spec.lon_count,
        grid_spec.lon_step
    );
    let lat_spec = format!(
        "{}:{}:{}",
        grid_spec.lat_start, grid_spec.lat_count, grid_spec.lat_step
    );

    let Output {
        status,
        stdout,
        stderr,
    } = Command::new(wgrib2)
        .arg(grib_path)
        .arg("-match")
        .arg(match_pattern)
        .arg("-lola")
        .arg(&lon_spec)
        .arg(&lat_spec)
        .arg(bin_path)
        .arg("bin")
        .output()?;

    if !status.success() {
        let mut combined = stdout;
        combined.extend_from_slice(&stderr);
        let text = String::from_utf8_lossy(&combined).to_string();
        let snippet: String = text.chars().take(200).collect();
        return Err(DecodeError::Wgrib2Failed {
            code: status.code().unwrap_or(-1),
            stderr: snippet,
        });
    }

    let inventory = String::from_utf8_lossy(&stdout);
    let messages = parse_inventory(&inventory);

    match std::fs::read(bin_path) {
        Ok(bin) => Ok(parse_lola_binary(&bin, &messages, grid_spec)),
        // wgrib2 matched nothing, so it never created the output file.
        // An empty grid (right spec, zero planes) is the correct result.
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(FieldGrid::new(*grid_spec)),
        Err(e) => Err(e.into()),
    }
}

/// Parse `wgrib2`'s stdout inventory (`"n:offset:date:var:level:..."`).
/// Malformed lines are dropped.
pub fn parse_inventory(out: &str) -> Vec<Message> {
    out.lines()
        .filter_map(|line| {
            let mut parts = line.splitn(8, ':');
            let _n = parts.next()?;
            let _offset = parts.next()?;
            let _date = parts.next()?;
            let var = parts.next()?;
            let level = parts.next()?;
            Some(Message {
                var: var.to_string(),
                level: level.to_string(),
            })
        })
        .collect()
}

/// Parse the Fortran-unformatted output file from `wgrib2 -lola`.
/// Layout: per message = 4-byte LE u32 record length + N·4 bytes f32 LE
/// data + 4-byte LE u32 record length. `N = nx * ny`. Cells whose value
/// exceeds `UNDEFINED_VALUE / 2` are replaced with `NaN` (the sentinel
/// wgrib2 writes for "no data"; `FieldGrid::at` maps NaN back to `None`,
/// matching the old behaviour of omitting the key entirely).
///
/// wgrib2 writes each record row-major over (lat, lon) in exactly the
/// order `FieldGrid` indexes cells, so the body is a bulk
/// little-endian `f32` decode per message — no per-cell coordinate
/// arithmetic, no hashing, no allocation beyond the plane itself.
pub fn parse_lola_binary(bin: &[u8], messages: &[Message], grid_spec: &GridSpec) -> FieldGrid {
    let nx = grid_spec.lon_count;
    let ny = grid_spec.lat_count;
    let n_cells = nx * ny;
    let bytes_per_msg = n_cells * 4;
    let record_overhead = 8;
    let stride = bytes_per_msg + record_overhead;

    let mut out = FieldGrid::new(*grid_spec);

    for (msg_idx, msg) in messages.iter().enumerate() {
        let data_offset = msg_idx * stride + 4;
        if data_offset + bytes_per_msg > bin.len() {
            continue;
        }
        let chunk = &bin[data_offset..data_offset + bytes_per_msg];
        let mut plane = Vec::with_capacity(n_cells);
        plane.extend(chunk.chunks_exact(4).map(|b| {
            // try_into on a chunks_exact(4) window is statically length 4.
            let v = f32::from_le_bytes(b.try_into().unwrap());
            if v > UNDEFINED_VALUE / 2.0 {
                f32::NAN
            } else {
                v
            }
        }));
        out.push_plane(&format!("{}:{}", msg.var, msg.level), plane);
    }

    out
}

/// Decode an HRDPS rotated-pole GRIB2 blob using decode-once +
/// lookup-many. Replaces the per-point `-lon` extraction that used to
/// brute-force the rotation math once per output cell.
///
/// 1. Runs `wgrib2 -no_header -order we:sn -bin` to dump the native
///    grid as raw f32 blocks (~0.32 s per message).
/// 2. Parses the inventory from stdout to get variable+level names.
/// 3. For each message, builds a dense `FieldGrid` plane by indexing
///    into the native grid via the precomputed `lookup` table.
///
/// `native_nx` × `native_ny` is the native grid size (2540×1290 for
/// HRDPS 0.0225°). `lookup` maps each cell in `target_spec` to a
/// native grid index, or `None` if the target cell is outside the
/// native domain.
pub fn decode_hrdps_native(
    grib: &[u8],
    match_pattern: &str,
    target_spec: &GridSpec,
    lookup: &[Option<u32>],
    native_nx: u32,
    native_ny: u32,
) -> Result<FieldGrid, DecodeError> {
    let wgrib2 = which_wgrib2().ok_or(DecodeError::Wgrib2NotAvailable)?;
    let tmp_dir = std::env::temp_dir();
    let uniq = UNIQUE.fetch_add(1, Ordering::Relaxed);
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_nanos())
        .unwrap_or(0);
    let grib_path = tmp_dir.join(format!("hrdps_native_{nanos}_{uniq}.grib2"));
    let bin_path = tmp_dir.join(format!("hrdps_native_{nanos}_{uniq}.bin"));

    std::fs::write(&grib_path, grib)?;

    let result = run_wgrib2_native_bin(
        &wgrib2,
        &grib_path,
        match_pattern,
        &bin_path,
        target_spec,
        lookup,
        (native_nx, native_ny),
    );

    let _ = std::fs::remove_file(&grib_path);
    let _ = std::fs::remove_file(&bin_path);
    result
}

fn run_wgrib2_native_bin(
    wgrib2: &Path,
    grib_path: &Path,
    match_pattern: &str,
    bin_path: &Path,
    target_spec: &GridSpec,
    lookup: &[Option<u32>],
    native_dims: (u32, u32),
) -> Result<FieldGrid, DecodeError> {
    let Output {
        status,
        stdout,
        stderr,
    } = Command::new(wgrib2)
        .arg(grib_path)
        .arg("-match")
        .arg(match_pattern)
        .arg("-no_header")
        .arg("-order")
        .arg("we:sn")
        .arg("-bin")
        .arg(bin_path)
        .output()?;

    if !status.success() {
        let stderr_text = String::from_utf8_lossy(&stderr).to_string();
        let snippet: String = stderr_text.chars().take(400).collect();
        return Err(DecodeError::Wgrib2Failed {
            code: status.code().unwrap_or(-1),
            stderr: snippet,
        });
    }

    let inventory = String::from_utf8_lossy(&stdout);
    let messages = parse_inventory(&inventory);

    match std::fs::read(bin_path) {
        Ok(bin) => Ok(parse_native_binary(
            &bin,
            &messages,
            target_spec,
            lookup,
            native_dims,
        )),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(FieldGrid::new(*target_spec)),
        Err(e) => Err(e.into()),
    }
}

/// Parse the raw `-no_header -order we:sn -bin` output: each message is
/// `nx * ny * 4` bytes of f32 LE, one block per message in the same
/// order as the inventory. No record markers, no headers.
fn parse_native_binary(
    bin: &[u8],
    messages: &[Message],
    target_spec: &GridSpec,
    lookup: &[Option<u32>],
    (native_nx, native_ny): (u32, u32),
) -> FieldGrid {
    let n_cells = target_spec.lat_count * target_spec.lon_count;
    let bytes_per_msg = (native_nx as usize) * (native_ny as usize) * 4;
    let n_native = (native_nx as usize) * (native_ny as usize);

    let mut out = FieldGrid::new(*target_spec);

    for (msg_idx, msg) in messages.iter().enumerate() {
        let data_offset = msg_idx * bytes_per_msg;
        if data_offset + bytes_per_msg > bin.len() {
            continue;
        }
        let chunk = &bin[data_offset..data_offset + bytes_per_msg];

        // Decode the entire native grid into f32 values once per message.
        // For the 2540×1290 (= 3.28M cell) native grid, this is ~13 MB
        // per message, which is acceptable for the ~41 messages HRDPS
        // publishes. The alternative would be indexing into raw bytes per
        // target cell, which adds per-cell bounds checks and offset math.
        let native: Vec<f32> = chunk
            .chunks_exact(4)
            .map(|b| {
                let v = f32::from_le_bytes(b.try_into().unwrap());
                if v > UNDEFINED_VALUE / 2.0 {
                    f32::NAN
                } else {
                    v
                }
            })
            .collect();
        debug_assert_eq!(native.len(), n_native);

        // Build the target plane by indexing into the native grid.
        let name = format!("{}:{}", msg.var, msg.level);
        let mut plane = vec![f32::NAN; n_cells];
        for (cell, slot) in lookup.iter().enumerate() {
            if let Some(native_cell) = slot {
                if (*native_cell as usize) < n_native {
                    plane[cell] = native[*native_cell as usize];
                }
            }
        }
        out.push_plane(&name, plane);
    }

    out
}

pub fn key_to_latlon(key: (i32, i32)) -> (f64, f64) {
    (key.0 as f64 / 1000.0, key.1 as f64 / 1000.0)
}

/// Run `wgrib2 -grid` on a GRIB2 file and return the stdout text.
/// Used to extract rotated-pole grid definition parameters at startup
/// so the lookup table can be validated against the actual GDS.
pub fn extract_grid_metadata(grib_path: &Path) -> Result<String, DecodeError> {
    let wgrib2 = which_wgrib2().ok_or(DecodeError::Wgrib2NotAvailable)?;
    let Output {
        status,
        stdout,
        stderr,
    } = Command::new(&wgrib2).arg(grib_path).arg("-grid").output()?;

    if !status.success() {
        let stderr_text = String::from_utf8_lossy(&stderr).to_string();
        let snippet: String = stderr_text.chars().take(400).collect();
        return Err(DecodeError::Wgrib2Failed {
            code: status.code().unwrap_or(-1),
            stderr: snippet,
        });
    }

    Ok(String::from_utf8_lossy(&stdout).to_string())
}

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

    #[test]
    fn lon_round_trip() {
        for lon in [-125.0, -97.0, -66.0, 0.0, 179.5] {
            assert!((denormalize_lon(normalize_lon(lon)) - lon).abs() < 1e-9);
        }
    }

    #[test]
    fn inventory_parses_var_level() {
        let text = "1:0:d=2026041915:TMP:2 m above ground:anl:\n\
                    2:1234:d=2026041915:DPT:2 m above ground:anl:\n";
        let msgs = parse_inventory(text);
        assert_eq!(msgs.len(), 2);
        assert_eq!(msgs[0].var, "TMP");
        assert_eq!(msgs[1].level, "2 m above ground");
    }

    #[test]
    fn inventory_drops_short_lines() {
        let text = "garbage\n1:0:d:TMP:surface:anl:\n";
        let msgs = parse_inventory(text);
        assert_eq!(msgs.len(), 1);
        assert_eq!(msgs[0].var, "TMP");
    }

    #[test]
    fn parse_lon_output_pulls_var_level_and_snaps_to_batch() {
        // Match the shape `wgrib2 -s -lon` produces. Two records, two
        // points each. Note: lon=280.369 is wgrib2's 0-360 form for
        // -79.631; parse_lon_segment denormalizes it.
        let text = "1:0:d=2026042912:TMP:2 m above ground:anl:\
                    lon=280.369,lat=43.670,val=280.97:\
                    lon=235.000,lat=49.190,val=281.62\n\
                    2:3527800:d=2026042912:DPT:2 m above ground:anl:\
                    lon=280.369,lat=43.670,val=275.00:\
                    lon=235.000,lat=49.190,val=270.00\n";
        let batch = vec![(43.670, -79.631), (49.190, -125.000)];
        // Spec chosen so both batch points land exactly on a cell.
        let spec = GridSpec {
            lon_start: -125.0,
            lon_count: 400,
            lon_step: 0.125,
            lat_start: 43.0,
            lat_count: 60,
            lat_step: 0.125,
        };

        let values = parse_lon_output(text, &batch, &spec);

        let toronto = crate::field_grid::cell_index_for(&spec, 43.670, -79.631)
            .expect("toronto lands on the grid");
        let tmp = values["TMP:2 m above ground"]
            .iter()
            .find(|(c, _)| *c == toronto)
            .expect("tmp present")
            .1;
        assert!((tmp - 280.97).abs() < 0.01);
        let dpt = values["DPT:2 m above ground"]
            .iter()
            .find(|(c, _)| *c == toronto)
            .expect("dpt present")
            .1;
        assert!((dpt - 275.00).abs() < 0.01);
    }

    #[test]
    fn parse_lon_segment_drops_undefined_values() {
        let batch = vec![(43.670, -79.631)];
        let segment = "lon=280.369,lat=43.670,val=9.999e20";
        assert!(parse_lon_segment(segment, &batch).is_none());
    }

    #[test]
    fn parse_lon_segment_rejects_far_off_points() {
        let batch = vec![(43.670, -79.631)];
        // wgrib2 returned a cell for somewhere completely unrelated.
        let segment = "lon=240.0,lat=10.0,val=290.0";
        assert!(parse_lon_segment(segment, &batch).is_none());
    }

    #[test]
    fn parse_lola_binary_reconstructs_grid() {
        // Build a synthetic 2-message, 3×2 grid. Layout matches wgrib2:
        //   [4 bytes len][6 f32 values][4 bytes len] per message.
        let spec = GridSpec {
            lon_start: -100.0,
            lon_count: 3,
            lon_step: 0.5,
            lat_start: 30.0,
            lat_count: 2,
            lat_step: 0.5,
        };
        let msgs = vec![
            Message {
                var: "TMP".into(),
                level: "surface".into(),
            },
            Message {
                var: "DPT".into(),
                level: "surface".into(),
            },
        ];
        let mut buf = Vec::new();
        for (msg_idx, _msg) in msgs.iter().enumerate() {
            let len: u32 = 3 * 2 * 4;
            buf.extend_from_slice(&len.to_le_bytes());
            for j in 0..2 {
                for i in 0..3 {
                    let v = (msg_idx as f32) * 100.0 + j as f32 * 10.0 + i as f32;
                    buf.extend_from_slice(&v.to_le_bytes());
                }
            }
            buf.extend_from_slice(&len.to_le_bytes());
        }

        let grid = parse_lola_binary(&buf, &msgs, &spec);
        assert_eq!(grid.n_cells(), 6);
        assert_eq!(grid.n_planes(), 2);
        let tmp = grid.plane_id("TMP:surface").unwrap();
        let dpt = grid.plane_id("DPT:surface").unwrap();

        // Cell (30.0, -100.0) → i=0, j=0, msg0 value = 0.0, msg1 = 100.0
        let k = grid.cell_index(30.0, -100.0).unwrap();
        assert_eq!(grid.at(tmp, k), Some(0.0));
        assert_eq!(grid.at(dpt, k), Some(100.0));

        // Cell (30.5, -99.0) → i=2, j=1, msg0 = 12.0
        let k2 = grid.cell_index(30.5, -99.0).unwrap();
        assert_eq!(grid.at(tmp, k2), Some(12.0));
        assert_eq!(grid.at(dpt, k2), Some(112.0));
    }

    #[test]
    fn parse_lola_skips_undefined_sentinel() {
        let spec = GridSpec {
            lon_start: -100.0,
            lon_count: 2,
            lon_step: 1.0,
            lat_start: 30.0,
            lat_count: 1,
            lat_step: 1.0,
        };
        let msgs = vec![Message {
            var: "TMP".into(),
            level: "surface".into(),
        }];
        let mut buf = Vec::new();
        let len: u32 = 2 * 4;
        buf.extend_from_slice(&len.to_le_bytes());
        buf.extend_from_slice(&UNDEFINED_VALUE.to_le_bytes()); // i=0
        buf.extend_from_slice(&42.0f32.to_le_bytes()); // i=1
        buf.extend_from_slice(&len.to_le_bytes());

        let grid = parse_lola_binary(&buf, &msgs, &spec);
        let tmp = grid.plane_id("TMP:surface").unwrap();
        // i=0 carried the sentinel and must read back as missing; only
        // i=1 survived. The old representation expressed this by never
        // inserting the key for i=0.
        assert_eq!(grid.at(tmp, grid.cell_index(30.0, -100.0).unwrap()), None);
        assert_eq!(
            grid.at(tmp, grid.cell_index(30.0, -99.0).unwrap()),
            Some(42.0)
        );
    }

    #[test]
    fn key_to_latlon_roundtrip() {
        let (lat, lon) = key_to_latlon((32_500, -97_250));
        assert_eq!(lat, 32.5);
        assert_eq!(lon, -97.25);
    }
}