//! Golden-seed regression for the Layer 0→1 generation cascade (#952, D-200, D-224).
//!
//! Pins two artifacts for one fixed input heightmap, in a single diffable JSON
//! golden (matching the `golden_suite.rs` convention):
//!   - **Layer 0** — SHA-256 of the source `heightmap.png` bytes. Flips if the
//!     Python heightmap generator (or the committed file) changes.
//!   - **Layer 1** — the serialized `Layer1Output` of the cascade run on a
//!     downsampled copy. Flips if the Rust drainage / feature / sub-biome code
//!     changes.
//!
//! The input is a real committed body heightmap (the #963 bake output), so this
//! is an end-to-end determinism guard. The cascade is downsampled to keep the
//! golden small while still exercising the full Layer-1 pipeline.
//!
//! Regenerate after an intended change:
//!   UPDATE_GOLDEN=1 cargo test --test cascade_golden
//!
//! Golden captured on x86_64. The downsample and sub-biome cost use f32, so a
//! different architecture could in principle round differently — regenerate
//! per-arch if CI ever moves off x86_64.

use std::path::PathBuf;

use serde_json::{json, Value};
use settled_reach_server::atlas::cascade::{run_cascade_from_heightmap, CascadeLayer};
use settled_reach_server::atlas::heightmap::load_heightmap_png;
use settled_reach_server::seed::SeedChain;
use sha2::{Digest, Sha256};

/// Source heightmap — a real committed body, relative to the server manifest dir.
const SOURCE_HEIGHTMAP: &str = "../wiki/star-systems/GJ-1/bodies/GJ1c/heightmap.png";
/// Downsample target: large enough that GJ1c's drainage produces a real river
/// network (not just attractors), small enough for a compact golden.
const DOWNSAMPLE: (u32, u32) = (256, 128);
/// World seed for the run. Cosmetic here — Layers 0–1 are RNG-free, so changing
/// it does not change the golden; it is carried only to exercise the SeedChain
/// contract end-to-end (D-224). Seed-sensitivity gets pinned once Layer 3+ lands.
const WORLD_SEED: u64 = 42;
const GOLDEN: &str = "tests/golden/cascade_layer1.json";

#[test]
fn cascade_layer0_to_1_matches_golden() {
    let manifest = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
    let src = manifest.join(SOURCE_HEIGHTMAP);

    // ── Layer 0 — load + hash the source heightmap.png. ─────────────────────
    let png_bytes = std::fs::read(&src)
        .unwrap_or_else(|e| panic!("read source heightmap {}: {e}", src.display()));
    let mut hasher = Sha256::new();
    hasher.update(&png_bytes);
    let sha = format!("{:x}", hasher.finalize());

    let heightmap = load_heightmap_png(&src, "GJ1c", 0.3).expect("decode source heightmap");

    // ── Layer 1 — downsample, then run the cascade to topography. ───────────
    let small = heightmap.downsample(DOWNSAMPLE.0, DOWNSAMPLE.1);
    let body_seed = SeedChain::for_body(WORLD_SEED, "GJ1c");
    let snapshot =
        run_cascade_from_heightmap(body_seed, small, &[], None, None, CascadeLayer::Topography);
    let layer1 = snapshot.layer1.expect("Layer 1 ran");

    let actual = json!({
        "source_heightmap": SOURCE_HEIGHTMAP,
        "source_sha256": sha,
        "downsample": [DOWNSAMPLE.0, DOWNSAMPLE.1],
        "layer1": serde_json::to_value(&layer1).expect("serialize Layer1Output"),
    });
    let actual_json = serde_json::to_string_pretty(&actual).expect("format JSON") + "\n";

    let golden_path = manifest.join(GOLDEN);

    if std::env::var("UPDATE_GOLDEN").is_ok() {
        std::fs::create_dir_all(golden_path.parent().unwrap()).expect("mkdir golden");
        std::fs::write(&golden_path, &actual_json).expect("write golden");
        eprintln!(
            "Golden written: {} ({} bytes)",
            golden_path.display(),
            actual_json.len()
        );
        return;
    }

    let golden_json = std::fs::read_to_string(&golden_path).unwrap_or_else(|e| {
        panic!(
            "Golden not found: {}. Run: UPDATE_GOLDEN=1 cargo test --test cascade_golden\n{e}",
            golden_path.display()
        )
    });

    // Compare as parsed JSON so whitespace/formatting never causes spurious diffs.
    let actual_v: Value = serde_json::from_str(&actual_json).expect("reparse actual JSON");
    let golden_v: Value = serde_json::from_str(&golden_json).expect("parse golden JSON");

    if actual_v != golden_v {
        let count =
            |v: &Value, ptr: &str| v.pointer(ptr).and_then(Value::as_array).map_or(0, Vec::len);
        panic!(
            "Cascade golden mismatch.\n  \
             source_sha256: golden={} actual={}\n  \
             river_cells:   golden={} actual={}\n  \
             attractors:    golden={} actual={}\n\
             To update: UPDATE_GOLDEN=1 cargo test --test cascade_golden\n  Golden: {}",
            golden_v["source_sha256"],
            actual_v["source_sha256"],
            count(&golden_v, "/layer1/river_network/river_cells"),
            count(&actual_v, "/layer1/river_network/river_cells"),
            count(&golden_v, "/layer1/attractors"),
            count(&actual_v, "/layer1/attractors"),
            golden_path.display(),
        );
    }
}