src/terrain_gen.rs

@@ -1,5 +1,15 @@
 use crate::terrain::{HeightGrid, TerrainError};
 
+const OCTAVE_COUNT: usize = 4;
+const BASE_FREQUENCY: f32 = 2.0;
+const LACUNARITY: f32 = 2.0;
+const GAIN: f32 = 0.5;
+const LATTICE_SEED_STEP: u64 = 0x9E37_79B9_7F4A_7C15;
+const LATTICE_X_MUL: u64 = 0x9E37_79B9_7F4A_7C15;
+const LATTICE_Y_MUL: u64 = 0xC2B2_AE3D_27D4_EB4F;
+const FINALIZER_MUL1: u64 = 0xBF58_476D_1CE4_E5B9;
+const FINALIZER_MUL2: u64 = 0x94D0_49BB_1331_11EB;
+
 /// Describes a terrain generation request.
 ///
 /// Specs are immutable once constructed; identical specs always yield
@@ -44,9 +54,9 @@ pub trait TerrainGenerator {
 
 /// A deterministic, in-memory reference generator.
 ///
-/// Heights are derived purely from the spec's seed and each sample's
-/// coordinates, so identical specs always produce identical grids. No noise
-/// algorithms are used yet; this is a stable baseline for CLI and rendering.
+/// Heights are derived from a small clean-room seeded value-noise fBm stack so
+/// identical specs always produce identical grids. The output stays in
+/// `[0.0, 1.0]`, which keeps later renderer and palette integration simple.
 #[derive(Debug, Clone, Copy, Default)]
 pub struct DeterministicTerrainGenerator;
 
@@ -56,29 +66,83 @@ impl DeterministicTerrainGenerator {
     }
 }
 
-/// Hashes seed and coordinates into a height in `[0.0, 1.0)`.
-fn sample_height(seed: u64, x: u32, y: u32) -> f32 {
-    // SplitMix64-style finalizer over a coordinate-mixed seed.
-    let mut h = seed
-        ^ (u64::from(x).wrapping_mul(0x9E37_79B9_7F4A_7C15))
-        ^ (u64::from(y).wrapping_mul(0xC2B2_AE3D_27D4_EB4F));
+#[inline]
+fn fade(t: f32) -> f32 {
+    t * t * (3.0 - 2.0 * t)
+}
+
+#[inline]
+fn normalized_coord(index: u32, size: u32) -> f32 {
+    if size <= 1 {
+        0.0
+    } else {
+        index as f32 / (size - 1) as f32
+    }
+}
+
+/// Hashes seed and lattice coordinates into a height in `[0.0, 1.0)`.
+#[inline]
+fn lattice_value(seed: u64, x: i64, y: i64) -> f32 {
+    let mut h =
+        seed ^ (x as u64).wrapping_mul(LATTICE_X_MUL) ^ (y as u64).wrapping_mul(LATTICE_Y_MUL);
     h ^= h >> 30;
-    h = h.wrapping_mul(0xBF58_476D_1CE4_E5B9);
+    h = h.wrapping_mul(FINALIZER_MUL1);
     h ^= h >> 27;
-    h = h.wrapping_mul(0x94D0_49BB_1331_11EB);
+    h = h.wrapping_mul(FINALIZER_MUL2);
     h ^= h >> 31;
     // Take the top 24 bits for an exact f32 fraction in [0, 1).
     (h >> 40) as f32 / (1u64 << 24) as f32
 }
 
+#[inline]
+fn value_noise(seed: u64, x: f32, y: f32) -> f32 {
+    let x0 = x.floor() as i64;
+    let y0 = y.floor() as i64;
+    let x1 = x0 + 1;
+    let y1 = y0 + 1;
+    let tx = x - x0 as f32;
+    let ty = y - y0 as f32;
+    let sx = fade(tx);
+    let sy = fade(ty);
+
+    let n00 = lattice_value(seed, x0, y0);
+    let n10 = lattice_value(seed, x1, y0);
+    let n01 = lattice_value(seed, x0, y1);
+    let n11 = lattice_value(seed, x1, y1);
+
+    let ix0 = n00 * (1.0 - sx) + n10 * sx;
+    let ix1 = n01 * (1.0 - sx) + n11 * sx;
+    ix0 * (1.0 - sy) + ix1 * sy
+}
+
+#[inline]
+fn fbm_height(seed: u64, x: f32, y: f32) -> f32 {
+    let mut total = 0.0;
+    let mut amplitude = 1.0;
+    let mut frequency = BASE_FREQUENCY;
+    let mut amplitude_sum = 0.0;
+
+    for octave in 0..OCTAVE_COUNT {
+        let octave_seed = seed.wrapping_add((octave as u64).wrapping_mul(LATTICE_SEED_STEP));
+        total += amplitude * value_noise(octave_seed, x * frequency, y * frequency);
+        amplitude_sum += amplitude;
+        amplitude *= GAIN;
+        frequency *= LACUNARITY;
+    }
+
+    total / amplitude_sum
+}
+
 impl TerrainGenerator for DeterministicTerrainGenerator {
     fn generate(&self, spec: &TerrainGenerationSpec) -> Result<HeightGrid, TerrainError> {
         let width = spec.width();
         let height = spec.height();
         let mut samples = Vec::with_capacity((width as usize) * (height as usize));
         for y in 0..height {
+            let ny = normalized_coord(y, height);
             for x in 0..width {
-                samples.push(sample_height(spec.seed(), x, y));
+                let nx = normalized_coord(x, width);
+                samples.push(fbm_height(spec.seed(), nx, ny));
             }
         }
         HeightGrid::new(width, height, samples)

tests/terrain_gen.rs

@@ -47,3 +47,32 @@ fn terrain_gen_deterministic_generator_rejects_zero_dimensions() {
     let err = TerrainGenerationSpec::new(7, 4, 0).unwrap_err();
     assert_eq!(err, TerrainError::ZeroDimension);
 }
+
+#[test]
+fn terrain_gen_deterministic_generator_produces_value_noise_fbm_sample() {
+    let spec = TerrainGenerationSpec::new(1337, 4, 4).expect("valid spec");
+    let generator = DeterministicTerrainGenerator::new();
+
+    let grid = generator.generate(&spec).expect("generation succeeds");
+
+    let expected = [
+        [0.706_783_f32, 0.390_681, 0.571_295, 0.454_375],
+        [0.667_478, 0.549_327, 0.476_506, 0.335_594],
+        [0.603_516, 0.451_503, 0.516_397, 0.357_081],
+        [0.308_838, 0.295_558, 0.570_510, 0.666_683],
+    ];
+
+    for (y, row) in expected.iter().enumerate() {
+        for (x, expected_sample) in row.iter().enumerate() {
+            let actual = grid.sample(x as u32, y as u32).expect("sample in bounds");
+            assert!(
+                (actual - expected_sample).abs() < 1e-6,
+                "mismatch at ({x}, {y}): expected {expected_sample}, got {actual}"
+            );
+        }
+    }
+
+    let (min, max) = grid.min_max().expect("non-empty grid has min/max");
+    assert!(min >= 0.0);
+    assert!(max <= 1.0);
+}