//! Region climate stack — the ~205 km top hard block (D-243 §3 / §4, T-1078).
//!
//! A [`RegionProfile`] holds the **region-level climate context**: latitude-driven
//! temperature baseline, weather state, seasonal clock structure. Every district and
//! tile inside the region inherits these values, then applies its own modulation
//! (elevation lapse + slope aspect at the district tier; freeze/snow scatter at
//! the chunk/voxel tier).
//!
//! ## The climate three-level stack (D-243 §3)
//!
//! ```text
//! Region  (~205 km) — climate context: baseline temperature, weather state, season
//!   ↓ modulation: elevation lapse + slope aspect
//! District (2 km)  — local temperature = baseline + lapse + aspect
//!   ↓ scatter: freeze/snow (D-239 §3)
//! Chunk/Voxel (64 m / 1 m) — cover scatter
//! ```
//!
//! ## Edge fuzz (D-243 §4)
//!
//! Climate does not change on a line. A tile's climate value is a **continuous,
//! warp-perturbed bilinear blend of surrounding region baselines** — the same
//! meta-rule as [D-239 §4]'s domain warp for terrain, but for the climate scalar
//! field. [`region_baseline_at_district`] implements this: it looks up the four
//! surrounding region baselines and blends them with a noise-displaced bilinear
//! interpolation.
//!
//! ## Q-105 deferral
//!
//! The `RegionClock` structure is scaffolded here (season + weather state) to
//! unblock Q-105's transient clock callbacks. The mean-state derivation and the
//! edge-fuzz blend are fully implemented; the **transient clock-phase tick
//! callbacks** (seasonal frost form/melt, crop-cycle cadence, recompute schedule)
//! are deferred to Q-105. `RegionClock` carries field-level Q-105 annotations.
//!
//! ## D-010 compliance
//!
//! All structural gating decisions use integer arithmetic. `f64` is used only for
//! positional computations (bilinear weights, warp displacement) that are then
//! quantised before any structural decision is made.

use std::collections::BTreeMap;

use serde::{Deserialize, Serialize};

use crate::atlas::district_profile::{BodyParams, ClimateConstants};
use crate::atlas::scale::{self, DistrictPos, RegionPos};
use crate::seed::{splitmix64, SeedChain};

// ---------------------------------------------------------------------------
// SeasonPhase — the region's current seasonal position
// ---------------------------------------------------------------------------

/// Broad seasonal phase for a region's clock (Q-105 forward contract).
///
/// The discriminants are **pinned and append-only** (D-010). The tick callbacks
/// that advance through these phases are deferred to Q-105 — today only the
/// struct is scaffolded.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize, Default)]
#[repr(u8)]
pub enum SeasonPhase {
    /// Warmest quarter of the year — long days, peak vegetation.
    #[default]
    Summer = 0,
    /// Cooling quarter — harvest, leaf-fall, falling precipitation.
    Autumn = 1,
    /// Coldest quarter of the year — short days, peak freeze extent.
    Winter = 2,
    /// Warming quarter — melt, sowing, rising temperatures.
    Spring = 3,
}

// ---------------------------------------------------------------------------
// WeatherState — the region's weather snapshot
// ---------------------------------------------------------------------------

/// Coarse weather state for a region (Q-105 forward contract).
///
/// The **mean-state** used today is `Clear` (no active weather event). Q-105
/// will provide the tick that advances through these states based on the
/// region's climate class and seasonal phase.
///
/// Integer-discriminant, append-only (D-010).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
#[repr(u8)]
pub enum WeatherState {
    /// Clear sky — no active weather event.
    #[default]
    Clear = 0,
    /// Overcast — cloud cover, diffuse light, reduced heat gain.
    Overcast = 1,
    /// Rain — precipitation in liquid form (temperature ≥ 0 °C).
    Rain = 2,
    /// Snow — precipitation in solid form (temperature < 0 °C).
    Snow = 3,
    /// Blizzard — heavy snow + high wind; severe passability impact.
    Blizzard = 4,
    /// Dust storm — driven by arid conditions + wind.
    DustStorm = 5,
}

// ---------------------------------------------------------------------------
// RegionClock — scaffolded seasonal/weather clock (Q-105 forward contract)
// ---------------------------------------------------------------------------

/// Region-level time state: the seasonal clock and active weather.
///
/// Today only the **mean-state struct is scaffolded** (season = Summer,
/// weather = Clear, all clock fields at their neutral values). The tick
/// callbacks that drive seasonal transitions and weather events are
/// **deferred to Q-105**.
///
/// Consumers needing the mean-state (static seasonal derivation) read this
/// struct directly. Q-105 will extend `RegionClock` with a tick-phase callback
/// and the `recompute_schedule` logic — the base fields here are load-bearing.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RegionClock {
    /// Current broad seasonal phase (Q-105: ticked by the region clock).
    pub season: SeasonPhase,
    /// Active weather event for the region (Q-105: ticked by the weather cycle).
    pub weather: WeatherState,
    /// Mean-annual temperature baseline at the region centre (°C), `None` for
    /// airless bodies. This is the **pre-edge-fuzz** region-level baseline
    /// derived from `planet_class` + latitude + greenhouse nudge (D-243 §3).
    ///
    /// District temperatures are derived by modulating this value with elevation
    /// lapse and slope aspect — see [`district_profile::derive_district_temperature_c`].
    pub mean_temp_c: Option<f32>,
}

impl Default for RegionClock {
    fn default() -> Self {
        Self {
            season: SeasonPhase::Summer,
            weather: WeatherState::Clear,
            mean_temp_c: None,
        }
    }
}

// ---------------------------------------------------------------------------
// RegionProfile — the region climate carrier
// ---------------------------------------------------------------------------

/// Per-region (~205 km) climate context (D-243 §3, T-1078).
///
/// Derived once per region; every district inside inherits it and applies
/// its own modulation (elevation lapse, slope aspect). The edge-fuzz blend
/// ([`region_baseline_at_district`]) ensures the ~205 km grid is invisible in
/// the output — temperatures grade smoothly and raggedly across region boundaries.
///
/// **Derivation inputs:** `(seed, body_params, region_pos)`. Pure and
/// deterministic — no I/O, no side effects (D-010 / D-227).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RegionProfile {
    /// Region position (region-units) — the key in `BodyWorldState.regions`.
    pub pos: RegionPos,

    /// Climate context for this region: baseline temperature, season, weather.
    pub clock: RegionClock,

    /// Latitude of the region centre in the body's reference frame (degrees).
    /// 0.0 = equator, ±90.0 = poles. Derived from `region_pos`.
    pub latitude_deg: f32,

    /// Region-level moisture primitive (0–100), inherited from body params.
    /// Moisture modulation is body-scale for now; per-district refinement lives
    /// in `DistrictProfile.moisture_q`.
    pub moisture_q: i32,
}

// ---------------------------------------------------------------------------
// Region baseline derivation
// ---------------------------------------------------------------------------

/// Derive the region-level mean-annual temperature baseline in °C.
///
/// This is **step (a)** of the D-239 §2 / D-243 §3 split:
/// - Latitude lerp across the maritime-moderated band
/// - Atmosphere greenhouse nudge
/// - Seed nudge (deterministic ±~3 °C per-body variety)
/// - Clamp to [cold, warm] class band (hard invariant, D-240)
///
/// **Not included here:** elevation lapse and slope aspect. Those are
/// district-level modulations applied in
/// [`crate::atlas::district_profile::derive_district_temperature_c`].
///
/// ## Inputs
///
/// - `body_params` — `planet_class`, `atmosphere`, `hydrosphere`, and the
///   **region centre latitude** in `latitude_deg` (repurposed for
///   the region's central latitude — the field name is historical).
/// - `constants` — tunable climate constants (D-240 table).
/// - `body_seed` — the body-scoped seed for the deterministic nudge (D-010).
///
/// ## Returns
///
/// `None` if the body is airless (`atmosphere == "none"`). Otherwise the
/// region baseline temperature in °C, clamped to the class band.
///
/// ## D-010 compliance
///
/// All structural gating uses integer arithmetic. f32 is used only for the
/// positional physics (latitude lerp, greenhouse fraction) — downstream
/// gating casts to `i32` before comparison.
pub fn derive_region_baseline_c(
    body_params: &BodyParams,
    constants: &ClimateConstants,
    body_seed: u64,
) -> Option<f32> {
    let atmosphere = body_params.atmosphere.as_deref().unwrap_or("none");

    // No atmosphere → airless body; baseline is None.
    if atmosphere == "none" {
        return None;
    }

    // Step 1: planet_class → (cold, warm) envelope (D-240).
    let planet_class = body_params.planet_class.as_deref().unwrap_or("temperate");
    let (cold, warm) = constants.envelope(planet_class);
    let band_width = warm - cold;

    // Step 2: latitude lerp across the maritime-moderated band (D-240).
    // Water-rich worlds compress the equator→pole gradient toward the band midpoint.
    let hydrosphere = body_params.hydrosphere.as_deref().unwrap_or("none");
    let maritime = constants.maritime_factor(hydrosphere);
    let mid = (cold + warm) * 0.5;
    let half = band_width * 0.5 * maritime;
    let lat_frac = (body_params.latitude_deg.abs() as f32 / 90.0).clamp(0.0, 1.0);
    // equator (frac 0) → mid + half; pole (frac 1) → mid − half.
    let t_lat = (mid + half) - (2.0 * half) * lat_frac;

    // Step 3: atmosphere greenhouse nudge — fraction of band_width toward warm end.
    let gh_frac = constants.greenhouse(atmosphere);
    let t_atmo = t_lat + gh_frac * band_width;

    // Step 4: seed nudge — deterministic ±~3 °C per-body variety (D-010, D-240).
    // Uses the same splitmix64 mixing as the original derive_temperature_c so the
    // body-level nudge character is preserved across the refactor.
    let nudge = {
        let h = body_seed
            .wrapping_add(0x9e37_79b9_7f4a_7c15)
            .wrapping_mul(0x6c62_272e_07bb_0142);
        let unit = (h as i64 as f64 / i64::MAX as f64) as f32;
        unit * 3.0_f32
    };
    let t_nudged = t_atmo + nudge;

    // Step 5: clamp to [cold, warm] — class band is a hard invariant (D-240).
    // Elevation lapse and slope aspect are NOT applied here — those are district
    // modulations (D-243 §3; applied in derive_district_temperature_c).
    Some(t_nudged.clamp(cold, warm))
}

// ---------------------------------------------------------------------------
// RegionProfile builder
// ---------------------------------------------------------------------------

/// Compute the latitude of a region centre from its grid position.
///
/// The region grid is equirectangular. Row 0 sits at the north pole; the
/// equator is in the middle. Returns degrees: +90.0 = north pole, −90.0 =
/// south pole.
///
/// If no body radius is available (tiny test bodies), the region lat is 0.0.
pub fn region_centre_latitude_deg(region_pos: RegionPos, body_radius_km: Option<f64>) -> f64 {
    let Some(r_km) = body_radius_km else {
        return 0.0;
    };
    if r_km <= 0.0 {
        return 0.0;
    }
    // The meridian spans πR km. Each region is REGION_M metres tall.
    // region_y = 0 maps to the north pole (lat +90), rising y → south.
    let meridian_m = std::f64::consts::PI * r_km * 1_000.0;
    let region_centre_y_m = (region_pos.1 as f64 + 0.5) * scale::REGION_M as f64;
    // Clamp: lat_frac in [0, 1]; 0 = N pole (+90°), 1 = S pole (−90°).
    let lat_frac = (region_centre_y_m / meridian_m).clamp(0.0, 1.0);
    90.0 - lat_frac * 180.0
}

/// Build a [`RegionProfile`] for the region at `region_pos`.
///
/// Pure and deterministic: `(seed, body_params, region_pos)` → `RegionProfile`.
///
/// `body_params.latitude_deg` is **overridden** to the region centre's
/// latitude internally — callers do not need to pre-set it.
pub fn build_region_profile(
    seed: SeedChain,
    body_params: &BodyParams,
    constants: &ClimateConstants,
    region_pos: RegionPos,
) -> RegionProfile {
    let lat_deg = region_centre_latitude_deg(region_pos, body_params.body_radius_km);

    // Build region-local params: override latitude to the region centre.
    let region_params = BodyParams {
        latitude_deg: lat_deg,
        // Elevation at region level is sea level (baseline only; no lapse here).
        elevation_km: 0.0,
        ..body_params.clone()
    };

    let body_seed = seed.seed();
    let baseline_temp_c = derive_region_baseline_c(&region_params, constants, body_seed);

    // Region-tier moisture = body ceiling × region-latitude gradient (T-1080). The
    // elevation + continentality components are district-scale, so at the region tier
    // elevation is sea level (0) and continentality is neutral (ocean_fraction 100);
    // the district derivation adds those on top.
    let moisture_q =
        crate::atlas::district_profile::derive_moisture_q(&region_params, 0, 100, constants);

    RegionProfile {
        pos: region_pos,
        clock: RegionClock {
            season: SeasonPhase::Summer,
            weather: WeatherState::Clear,
            mean_temp_c: baseline_temp_c,
        },
        latitude_deg: lat_deg as f32,
        moisture_q,
    }
}

// ---------------------------------------------------------------------------
// Edge-fuzz blend — D-243 §4
// ---------------------------------------------------------------------------

/// Region-baseline temperature at a **district position**, using the D-243 §4
/// edge-fuzz blend.
///
/// Climate does not change on a line. This function blends the four surrounding
/// region baselines (bilinear across region centres), displaced by a
/// noise-warp field so the blend boundary is ragged rather than a straight
/// gradient. The ~205 km region grid is therefore **invisible** in the output —
/// temperatures grade smoothly and organically across boundaries.
///
/// ## Algorithm
///
/// 1. Compute the district's fractional position within the region grid,
///    displaced by a per-district warp (keyed on `(world_seed, body_id, district_pos)`).
/// 2. Identify the four surrounding region positions.
/// 3. Derive (or look up from the optional `region_cache`) the baseline
///    temperature for each region.
/// 4. Bilinear-blend the four baselines by the displaced fractional weights.
///
/// ## D-010 compliance
///
/// All warp and blend arithmetic is f64 positional math. The blended baseline
/// returned here is f32 and is used only as input to the district modulation
/// (elevation lapse, slope aspect) in
/// [`crate::atlas::district_profile::derive_district_temperature_c`].
/// No structural gate is applied to this value — gates happen downstream on
/// integer casts of the final district temperature.
///
/// ## Parameters
///
/// - `world_seed` — master world seed (the edge-fuzz warp is keyed to it).
/// - `body_id` — body identifier for warp domain separation.
/// - `district_pos` — the district being derived.
/// - `body_params` — body physical parameters (planet_class, atmosphere, etc.).
/// - `constants` — climate constants.
/// - `seed` — body SeedChain (for per-region baseline derivation).
/// - `region_cache` — optional pre-computed region baselines; if a region is
///   missing it is derived on the fly.
pub fn region_baseline_at_district(
    world_seed: u64,
    body_id: &str,
    district_pos: DistrictPos,
    body_params: &BodyParams,
    constants: &ClimateConstants,
    seed: SeedChain,
    region_cache: Option<&BTreeMap<RegionPos, RegionProfile>>,
) -> Option<f32> {
    // Atmosphere gate: airless bodies have no temperature baseline.
    let atmosphere = body_params.atmosphere.as_deref().unwrap_or("none");
    if atmosphere == "none" {
        return None;
    }

    // ── Step 1: Fractional district position in the region grid ─────────────
    // A district at `(dx, dy)` lies at fractional position
    //   fx = (dx mod DISTRICTS_PER_REGION + 0.5) / DISTRICTS_PER_REGION
    //   fy = (dy mod DISTRICTS_PER_REGION + 0.5) / DISTRICTS_PER_REGION
    // in region space, where (0,0) is the region corner.
    let dpir = scale::DISTRICTS_PER_REGION as f64;

    // The district's offset within its region (0.0..1.0 each axis).
    let local_fx = {
        let rem = district_pos.0.rem_euclid(scale::DISTRICTS_PER_REGION) as f64;
        (rem + 0.5) / dpir
    };
    let local_fy = {
        let rem = district_pos.1.rem_euclid(scale::DISTRICTS_PER_REGION) as f64;
        (rem + 0.5) / dpir
    };

    // ── Step 2: Edge-fuzz warp displacement ─────────────────────────────────
    // A per-district noise warp displaces the sampling point so the blend
    // boundary is ragged. The warp is keyed on (world_seed, body_id,
    // district_pos) and is bounded to ±CLIMATE_WARP_FRAC of the region extent
    // (not ±8 m — climate warp is fractional region units, not metres).
    //
    // Implementation: derive two hash values from the district position and
    // map them to [-WARP_FRAC, +WARP_FRAC].
    //
    // D-010: the warp arithmetic is f64 positional math; the downstream blend
    // weights are not used in any structural comparison.
    const CLIMATE_WARP_FRAC: f64 = 0.25; // ±25% of a region's width/height.

    let (warp_dx, warp_dy) = climate_edge_warp(world_seed, body_id, district_pos);
    let warped_fx = (local_fx + warp_dx * CLIMATE_WARP_FRAC).clamp(0.0, 1.0);
    let warped_fy = (local_fy + warp_dy * CLIMATE_WARP_FRAC).clamp(0.0, 1.0);

    // ── Step 3: Identify four surrounding region positions ───────────────────
    // The district's parent region.
    let base_region = scale::district_to_region(district_pos);

    // Determine which quadrant of the region the district falls in (after warp):
    // if warped_fx > 0.5 the district is in the eastern half → blend with +X
    // neighbour; else blend with −X neighbour. Same for Y.
    let (neighbour_dx, blend_tx) = if warped_fx >= 0.5 {
        (1i32, (warped_fx - 0.5) * 2.0) // 0.0 at centre → 1.0 at +X edge
    } else {
        (-1i32, (0.5 - warped_fx) * 2.0) // 0.0 at centre → 1.0 at −X edge
    };
    let (neighbour_dy, blend_ty) = if warped_fy >= 0.5 {
        (1i32, (warped_fy - 0.5) * 2.0)
    } else {
        (-1i32, (0.5 - warped_fy) * 2.0)
    };

    // Four corner regions: (base, x-neighbour, y-neighbour, xy-neighbour).
    let r00 = base_region;
    let r10 = (base_region.0 + neighbour_dx, base_region.1);
    let r01 = (base_region.0, base_region.1 + neighbour_dy);
    let r11 = (base_region.0 + neighbour_dx, base_region.1 + neighbour_dy);

    // ── Step 4: Fetch or derive the four region baselines ───────────────────
    let baseline_for = |rpos: RegionPos| -> Option<f32> {
        // Try the cache first.
        if let Some(cache) = region_cache {
            if let Some(rp) = cache.get(&rpos) {
                return rp.clock.mean_temp_c;
            }
        }
        // Derive on the fly (test path / cache miss).
        let lat = region_centre_latitude_deg(rpos, body_params.body_radius_km);
        let r_params = BodyParams {
            latitude_deg: lat,
            elevation_km: 0.0,
            ..body_params.clone()
        };
        derive_region_baseline_c(&r_params, constants, seed.seed())
    };

    let b00 = baseline_for(r00)?;
    let b10 = baseline_for(r10)?;
    let b01 = baseline_for(r01)?;
    let b11 = baseline_for(r11)?;

    // ── Step 5: Bilinear blend ───────────────────────────────────────────────
    // Standard bilinear: tx blends X pairs, ty blends the Y result.
    let tx = blend_tx as f32;
    let ty = blend_ty as f32;
    let top = b00 + (b10 - b00) * tx;
    let bot = b01 + (b11 - b01) * tx;
    Some(top + (bot - top) * ty)
}

/// Compute the climate edge-fuzz warp displacement for a district.
///
/// Returns `(warp_dx, warp_dy)` each in `[-1.0, +1.0]`, intended to be scaled
/// by `CLIMATE_WARP_FRAC` by the caller. Keyed on
/// `(world_seed, body_id, district_pos)` — same domain-separation conventions
/// as [`crate::atlas::domain_warp`], but using a distinct hash path so the
/// climate warp is never correlated with the terrain warp.
///
/// ## D-010 compliance
///
/// Pure integer hash, f64 mapping. No structural comparison.
fn climate_edge_warp(world_seed: u64, body_id: &str, district_pos: DistrictPos) -> (f64, f64) {
    // Hash body_id into a u64 using FNV-1a (canonical per D-224).
    let body_hash = crate::seed::fnv1a_64(body_id);
    let base = world_seed
        .wrapping_add(body_hash)
        .wrapping_add(0x1234_5678_9abc_def0);

    // Per-district position hash (zigzag + Cantor pairing — same as domain_warp).
    let zz = |v: i32| -> u64 {
        let v = v as i64;
        ((v << 1) ^ (v >> 63)) as u64
    };
    let x = zz(district_pos.0);
    let y = zz(district_pos.1);
    let s = x.wrapping_add(y);
    let pos_id = s
        .wrapping_mul(s.wrapping_add(1))
        .wrapping_div(2)
        .wrapping_add(y);

    // Two independent streams: one for dx, one for dy.
    let seed_x = splitmix64(base.wrapping_add(pos_id));
    let seed_y = splitmix64(base.wrapping_add(pos_id).wrapping_add(0xdeadbeef_cafebabe));

    let u64_to_unit = |h: u64| -> f64 {
        let unit = (h >> 11) as f64 * (1.0 / (1u64 << 53) as f64);
        (unit - 0.5) * 2.0 // [-1.0, +1.0)
    };

    (u64_to_unit(seed_x), u64_to_unit(seed_y))
}

// ---------------------------------------------------------------------------
// Region cache builder (for batch derivation)
// ---------------------------------------------------------------------------

/// Derive [`RegionProfile`]s for all regions that cover the set of districts
/// supplied, returning a `BTreeMap<RegionPos, RegionProfile>`.
///
/// Used when processing a batch of districts: call this first to populate the
/// region cache, then pass the cache into [`region_baseline_at_district`].
///
/// Pure and deterministic. Does not de-duplicate within this call — the caller
/// provides the unique set of region positions.
pub fn derive_regions_for_body(
    seed: SeedChain,
    body_params: &BodyParams,
    constants: &ClimateConstants,
    region_positions: impl IntoIterator<Item = RegionPos>,
) -> BTreeMap<RegionPos, RegionProfile> {
    let mut out = BTreeMap::new();
    for rpos in region_positions {
        let profile = build_region_profile(seed, body_params, constants, rpos);
        out.insert(rpos, profile);
    }
    out
}

// ---------------------------------------------------------------------------
// SeedDomain extension: RegionClimate = 11
// ---------------------------------------------------------------------------
//
// A new SeedDomain variant `RegionClimate = 11` is registered in
// `crate::seed::SeedDomain` to enable region-scoped seed derivation without
// colliding with other domains (D-224 domain separation). Because `SeedDomain`
// is in seed.rs (not this file), the variant is added there.
//
// This module uses the `seed.seed()` output for region baseline derivation
// (the same approach as `derive_temperature_c` uses `body_seed`), which is
// equivalent to calling `.derive(SeedDomain::Body, body_id_hash)` at the
// parent level and then reading the raw seed. No additional derivation level
// is needed unless per-region RNG streams are required (deferred to Q-105).

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;
    use crate::atlas::district_profile::{BodyParams, ClimateConstants};
    use crate::seed::{SeedChain, SeedDomain};

    fn body_seed() -> SeedChain {
        SeedChain::root(42).derive(SeedDomain::Body, 1)
    }

    fn earth_params() -> BodyParams {
        BodyParams {
            hydrosphere: Some("ocean".into()),
            atmosphere: Some("breathable".into()),
            planet_class: Some("temperate".into()),
            body_radius_km: Some(6371.0),
            ..Default::default()
        }
    }

    // ── RegionProfile builder ────────────────────────────────────────────────

    #[test]
    fn build_region_profile_is_deterministic() {
        let params = earth_params();
        let constants = ClimateConstants::default();
        let a = build_region_profile(body_seed(), &params, &constants, (5, 10));
        let b = build_region_profile(body_seed(), &params, &constants, (5, 10));
        assert_eq!(
            a.clock.mean_temp_c, b.clock.mean_temp_c,
            "region baseline must be deterministic"
        );
        assert_eq!(a.latitude_deg, b.latitude_deg);
        assert_eq!(a.moisture_q, b.moisture_q);
    }

    #[test]
    fn airless_body_has_no_region_baseline() {
        let params = BodyParams {
            atmosphere: Some("none".into()),
            planet_class: Some("frozen".into()),
            body_radius_km: Some(1500.0),
            ..Default::default()
        };
        let constants = ClimateConstants::default();
        let rp = build_region_profile(body_seed(), &params, &constants, (0, 0));
        assert_eq!(
            rp.clock.mean_temp_c, None,
            "airless body must have None region baseline"
        );
    }

    #[test]
    fn equatorial_region_is_warmer_than_polar() {
        // With a body radius, north-pole region vs equatorial region.
        let params = earth_params();
        let constants = ClimateConstants::default();
        // Region (0, 0) is near the north pole; (97, 15) is roughly equatorial.
        let polar = build_region_profile(body_seed(), &params, &constants, (0, 0));
        let equatorial = build_region_profile(body_seed(), &params, &constants, (97, 15));
        match (polar.clock.mean_temp_c, equatorial.clock.mean_temp_c) {
            (Some(p), Some(e)) => assert!(
                p < e,
                "polar baseline {p}°C must be colder than equatorial {e}°C"
            ),
            _ => panic!("breathable body must have a temperature"),
        }
    }

    #[test]
    fn frozen_body_region_within_class_band() {
        let params = BodyParams {
            atmosphere: Some("thin".into()),
            planet_class: Some("frozen".into()),
            body_radius_km: Some(2000.0),
            ..Default::default()
        };
        let constants = ClimateConstants::default();
        let (cold, warm) = constants.envelope("frozen");
        for ry in [0i32, 5, 10] {
            let rp = build_region_profile(body_seed(), &params, &constants, (0, ry));
            let t = rp
                .clock
                .mean_temp_c
                .expect("non-airless body must have temperature");
            assert!(
                t >= cold && t <= warm,
                "frozen body region ({}, {ry}) baseline {t}°C outside band [{cold}, {warm}]",
                0
            );
        }
    }

    #[test]
    fn region_clock_default_state() {
        let params = earth_params();
        let constants = ClimateConstants::default();
        let rp = build_region_profile(body_seed(), &params, &constants, (10, 10));
        // Q-105 is deferred: mean state is Summer + Clear.
        assert_eq!(rp.clock.season, SeasonPhase::Summer);
        assert_eq!(rp.clock.weather, WeatherState::Clear);
    }

    // ── Baseline derivation ──────────────────────────────────────────────────

    #[test]
    fn region_baseline_no_elevation_lapse() {
        // The region baseline must NOT include elevation lapse — that is a
        // district-level modulation. Two regions at the same latitude but
        // different positions should produce the same baseline if seed is the
        // same (latitude is the only varying input here).
        let constants = ClimateConstants::default();
        let params = BodyParams {
            atmosphere: Some("breathable".into()),
            planet_class: Some("temperate".into()),
            latitude_deg: 30.0,
            elevation_km: 5.0, // This must be ignored by the region baseline
            ..Default::default()
        };
        // Explicitly pass elevation_km = 5.0; the baseline function overrides
        // it to 0.0 internally.
        let t_high_elev = derive_region_baseline_c(&params, &constants, 42);
        let params_low = BodyParams {
            elevation_km: 0.0,
            ..params.clone()
        };
        let t_low_elev = derive_region_baseline_c(&params_low, &constants, 42);
        // Both must be equal — elevation is NOT a region-level input.
        assert_eq!(
            t_high_elev, t_low_elev,
            "region baseline must be independent of elevation_km (lapse is district-level)"
        );
    }

    // ── Edge-fuzz blend ──────────────────────────────────────────────────────

    #[test]
    fn edge_fuzz_is_deterministic() {
        let params = earth_params();
        let constants = ClimateConstants::default();
        let a = region_baseline_at_district(
            42,
            "TestBody",
            (50, 75),
            &params,
            &constants,
            body_seed(),
            None,
        );
        let b = region_baseline_at_district(
            42,
            "TestBody",
            (50, 75),
            &params,
            &constants,
            body_seed(),
            None,
        );
        assert_eq!(a, b, "edge-fuzz blend must be deterministic");
    }

    #[test]
    fn edge_fuzz_within_class_band() {
        let params = earth_params();
        let constants = ClimateConstants::default();
        let (cold, warm) = constants.envelope("temperate");
        // Check several district positions.
        for dx in [0i32, 50, 99, 100, 150] {
            for dy in [0i32, 25, 50, 75] {
                let t = region_baseline_at_district(
                    42,
                    "TestBody",
                    (dx, dy),
                    &params,
                    &constants,
                    body_seed(),
                    None,
                )
                .expect("breathable body must have a baseline");
                assert!(
                    t >= cold && t <= warm,
                    "edge-fuzz baseline at ({dx},{dy}): {t}°C outside [{cold}, {warm}]"
                );
            }
        }
    }

    #[test]
    fn edge_fuzz_varies_with_body_id() {
        let params = earth_params();
        let constants = ClimateConstants::default();
        let a = region_baseline_at_district(
            42,
            "BodyA",
            (50, 50),
            &params,
            &constants,
            body_seed(),
            None,
        );
        let b = region_baseline_at_district(
            42,
            "BodyB",
            (50, 50),
            &params,
            &constants,
            body_seed(),
            None,
        );
        // Same region pos but different body_id → different warp → different blend.
        // Not guaranteed to differ (could accidentally hit same blend), but should
        // for these inputs.
        assert_ne!(
            a, b,
            "edge-fuzz blend should vary with body_id (different warp)"
        );
    }

    #[test]
    fn edge_fuzz_airless_returns_none() {
        let params = BodyParams {
            atmosphere: Some("none".into()),
            planet_class: Some("frozen".into()),
            ..Default::default()
        };
        let constants = ClimateConstants::default();
        let t = region_baseline_at_district(
            42,
            "AirlessBody",
            (10, 10),
            &params,
            &constants,
            body_seed(),
            None,
        );
        assert_eq!(t, None, "airless body must return None from edge-fuzz");
    }

    // ── SeasonPhase / WeatherState discriminant pin ──────────────────────────

    #[test]
    fn season_phase_discriminants_pinned() {
        // Append-only (D-010): renaming breaks serialised state.
        assert_eq!(SeasonPhase::Summer as u8, 0);
        assert_eq!(SeasonPhase::Autumn as u8, 1);
        assert_eq!(SeasonPhase::Winter as u8, 2);
        assert_eq!(SeasonPhase::Spring as u8, 3);
    }

    #[test]
    fn weather_state_discriminants_pinned() {
        assert_eq!(WeatherState::Clear as u8, 0);
        assert_eq!(WeatherState::Overcast as u8, 1);
        assert_eq!(WeatherState::Rain as u8, 2);
        assert_eq!(WeatherState::Snow as u8, 3);
        assert_eq!(WeatherState::Blizzard as u8, 4);
        assert_eq!(WeatherState::DustStorm as u8, 5);
    }

    // ── Region cache batch builder ───────────────────────────────────────────

    #[test]
    fn derive_regions_for_body_covers_all_positions() {
        let params = earth_params();
        let constants = ClimateConstants::default();
        let positions: Vec<RegionPos> = vec![(0, 0), (1, 0), (0, 1), (5, 5)];
        let cache = derive_regions_for_body(body_seed(), &params, &constants, positions.clone());
        assert_eq!(
            cache.len(),
            positions.len(),
            "all positions must be present"
        );
        for pos in &positions {
            assert!(
                cache.contains_key(pos),
                "region {pos:?} must be in the cache"
            );
        }
    }

    #[test]
    fn edge_fuzz_cache_hit_matches_derived() {
        // region_baseline_at_district with a cache should produce the same
        // result as without (within the bilinear blend — the same regions are
        // sampled in both paths).
        let params = earth_params();
        let constants = ClimateConstants::default();
        let district_pos = (55i32, 20i32);
        let base_region = scale::district_to_region(district_pos);
        let positions: Vec<RegionPos> = vec![
            base_region,
            (base_region.0 + 1, base_region.1),
            (base_region.0 - 1, base_region.1),
            (base_region.0, base_region.1 + 1),
            (base_region.0, base_region.1 - 1),
            (base_region.0 + 1, base_region.1 + 1),
            (base_region.0 - 1, base_region.1 + 1),
            (base_region.0 + 1, base_region.1 - 1),
            (base_region.0 - 1, base_region.1 - 1),
        ];
        let cache = derive_regions_for_body(body_seed(), &params, &constants, positions);
        let t_cached = region_baseline_at_district(
            42,
            "TestBody",
            district_pos,
            &params,
            &constants,
            body_seed(),
            Some(&cache),
        );
        let t_derived = region_baseline_at_district(
            42,
            "TestBody",
            district_pos,
            &params,
            &constants,
            body_seed(),
            None,
        );
        // Both paths should produce the same result (both use the same seed
        // for on-demand derivation when the cache derives with the same seed).
        assert_eq!(
            t_cached, t_derived,
            "cache hit and on-demand derivation must agree"
        );
    }
}