jpmschweitzerandClaude Opus 4.6
f1278f88fd
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
650 lines
20 KiB
Rust
650 lines
20 KiB
Rust
//! D8 drainage routing — flow direction, flow accumulation, river network
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//! extraction, and drainage basin delineation (D-208).
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//!
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//! **Determinism (D-010, D-208):** All flow-direction comparisons use integer
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//! arithmetic on scaled elevation values (`(elev * 1_000_000.0) as i64`) to
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//! avoid f32 comparison non-determinism. Tie-breaking uses a fixed D8 neighbor
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//! priority order. The result is bit-identical across runs on the same inputs.
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//!
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//! **Algorithm:**
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//! 1. Scale f32 elevation to i64 integers.
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//! 2. Priority-flood depression fill (iterative, convergence in ≤10 passes).
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//! 3. D8 flow direction: steepest descent, 8-neighbor, wraps horizontally.
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//! 4. Flow accumulation via topological sort of the D8 DAG.
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//! 5. River network extraction: cells with accumulation > RIVER_THRESHOLD.
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//! 6. Basin labeling: flood-fill seeded at pour points.
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//!
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//! The grid is row-major. Row 0 is the north pole; row H-1 is the south pole.
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//! Columns wrap horizontally (the globe is equirectangular).
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use std::collections::VecDeque;
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use crate::atlas::body_world_state::{DrainageBasin, RiverNetwork};
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/// A cell is a river cell when its flow accumulation exceeds this threshold (D-208).
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pub const RIVER_THRESHOLD: i32 = 200;
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/// Scale factor for converting f32 elevation to integer for deterministic comparison.
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const ELEV_SCALE: f64 = 1_000_000.0;
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// D8 neighbor offsets (dr, dc) in fixed priority order for deterministic tie-breaking.
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// Priority: cardinal directions first (N, S, E, W), then diagonals (NE, NW, SE, SW).
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const D8: [(i32, i32); 8] = [
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(-1, 0), // N
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(1, 0), // S
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(0, 1), // E
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(0, -1), // W
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(-1, 1), // NE
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(-1, -1), // NW
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(1, 1), // SE
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(1, -1), // SW
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];
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/// Result of the full D8 drainage analysis for one body.
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#[derive(Debug, Clone)]
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pub struct DrainageResult {
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pub river_network: RiverNetwork,
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pub drainage_basins: Vec<DrainageBasin>,
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}
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// ---------------------------------------------------------------------------
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// Public entry point
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// ---------------------------------------------------------------------------
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/// Run the full D8 drainage analysis on an elevation grid.
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///
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/// `elevation` is a row-major float32 grid of shape `height × width`, values
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/// in [0.0, 1.0]. `sea_level` is the fraction below which terrain is ocean.
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///
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/// Returns `DrainageResult` with the river network and drainage basins.
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pub fn analyze(elevation: &[f32], width: u32, height: u32, sea_level: f32) -> DrainageResult {
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let w = width as usize;
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let h = height as usize;
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// 1. Scale to integers.
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let scaled: Vec<i64> = elevation
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.iter()
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.map(|&e| (e as f64 * ELEV_SCALE) as i64)
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.collect();
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// 2. Depression fill.
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let filled = depression_fill(&scaled, w, h);
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// 3. D8 flow direction. -1 = no outflow (edge or flat peak).
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let fdir = flow_direction(&filled, w, h);
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// 4. Flow accumulation.
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let accum = flow_accumulation(&fdir, w, h);
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// 5. River network.
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let river_network = extract_river_network(&accum, &fdir, w, h, sea_level, elevation);
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// 6. Basin labeling.
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let labels = label_basins(&fdir, &accum, w, h);
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// 7. Merge small basins + clamp count to [4, 12].
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let labels = merge_small_basins(labels, w, h, 4, 12);
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// 8. Build DrainageBasin structs.
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let drainage_basins = build_basins(&labels, w, h);
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DrainageResult {
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river_network,
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drainage_basins,
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}
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}
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// ---------------------------------------------------------------------------
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// Step 2: Depression fill
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// ---------------------------------------------------------------------------
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fn depression_fill(scaled: &[i64], w: usize, h: usize) -> Vec<i64> {
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let mut filled = scaled.to_vec();
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for _ in 0..10 {
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let mut changed = false;
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for r in 1..h.saturating_sub(1) {
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for c in 0..w {
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let mut nbr_min = i64::MAX;
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for &(dr, dc) in &D8 {
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr >= 0 && nr < h as i32 {
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let val = filled[nr as usize * w + nc];
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if val < nbr_min {
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nbr_min = val;
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}
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}
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}
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if filled[r * w + c] < nbr_min {
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filled[r * w + c] = nbr_min + 1;
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changed = true;
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}
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}
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}
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if !changed {
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break;
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}
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}
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filled
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}
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// ---------------------------------------------------------------------------
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// Step 3: D8 flow direction
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// ---------------------------------------------------------------------------
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/// Returns per-cell flow direction index into D8 (0–7), or -1 for no outflow.
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fn flow_direction(filled: &[i64], w: usize, h: usize) -> Vec<i8> {
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let mut fdir = vec![-1i8; w * h];
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for r in 0..h {
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for c in 0..w {
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let elev = filled[r * w + c];
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let mut best_drop = 0i64;
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let mut best_k: i8 = -1;
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for (k, &(dr, dc)) in D8.iter().enumerate() {
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr < 0 || nr >= h as i32 {
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continue;
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}
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let drop = elev - filled[nr as usize * w + nc];
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if drop > best_drop {
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best_drop = drop;
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best_k = k as i8;
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}
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}
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fdir[r * w + c] = best_k;
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}
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}
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fdir
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}
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// ---------------------------------------------------------------------------
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// Step 4: Flow accumulation
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// ---------------------------------------------------------------------------
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fn flow_accumulation(fdir: &[i8], w: usize, h: usize) -> Vec<i32> {
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let n = w * h;
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let mut in_degree = vec![0i32; n];
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for r in 0..h {
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for c in 0..w {
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let k = fdir[r * w + c];
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if k < 0 {
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continue;
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}
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let (dr, dc) = D8[k as usize];
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr >= 0 && nr < h as i32 {
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in_degree[nr as usize * w + nc] += 1;
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}
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}
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}
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let mut queue = VecDeque::new();
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for (i, °) in in_degree.iter().enumerate().take(n) {
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if deg == 0 {
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queue.push_back(i);
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}
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}
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let mut accum = vec![1i32; n];
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while let Some(idx) = queue.pop_front() {
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let r = idx / w;
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let c = idx % w;
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let k = fdir[idx];
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if k < 0 {
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continue;
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}
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let (dr, dc) = D8[k as usize];
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr >= 0 && nr < h as i32 {
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let ni = nr as usize * w + nc;
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accum[ni] += accum[idx];
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in_degree[ni] -= 1;
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if in_degree[ni] == 0 {
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queue.push_back(ni);
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}
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}
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}
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accum
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}
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// ---------------------------------------------------------------------------
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// Step 5: River network extraction
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// ---------------------------------------------------------------------------
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fn extract_river_network(
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accum: &[i32],
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fdir: &[i8],
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w: usize,
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h: usize,
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sea_level: f32,
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elevation: &[f32],
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) -> RiverNetwork {
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let n = w * h;
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// River cells: above threshold AND above sea level.
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let is_river: Vec<bool> = (0..n)
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.map(|i| accum[i] > RIVER_THRESHOLD && elevation[i] >= sea_level)
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.collect();
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let river_cells: Vec<(u16, u16)> = (0..n)
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.filter(|&i| is_river[i])
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.map(|i| ((i / w) as u16, (i % w) as u16))
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.collect();
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// Confluences: river cells with 2+ river neighbors flowing into them.
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let mut inflow_count = vec![0u8; n];
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for r in 0..h {
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for c in 0..w {
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let i = r * w + c;
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if !is_river[i] {
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continue;
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}
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let k = fdir[i];
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if k < 0 {
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continue;
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}
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let (dr, dc) = D8[k as usize];
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr >= 0 && nr < h as i32 {
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let ni = nr as usize * w + nc;
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if is_river[ni] {
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inflow_count[ni] = inflow_count[ni].saturating_add(1);
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}
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}
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}
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}
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let confluences: Vec<(u16, u16)> = (0..n)
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.filter(|&i| is_river[i] && inflow_count[i] >= 2)
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.map(|i| ((i / w) as u16, (i % w) as u16))
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.collect();
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// Mouths: river cells that flow to a sea cell or to the polar edge.
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let mouths: Vec<(u16, u16)> = (0..n)
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.filter(|&i| {
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if !is_river[i] {
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return false;
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}
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let r = i / w;
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let c = i % w;
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let k = fdir[i];
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if k < 0 {
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return true; // no outflow — edge
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}
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let (dr, dc) = D8[k as usize];
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr < 0 || nr >= h as i32 {
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return true; // polar edge
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}
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// Flows into a sub-sea-level cell = mouth
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elevation[nr as usize * w + nc] < sea_level
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})
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.map(|i| ((i / w) as u16, (i % w) as u16))
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.collect();
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RiverNetwork {
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river_cells,
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confluences,
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mouths,
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}
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}
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// ---------------------------------------------------------------------------
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// Step 6: Basin labeling
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// ---------------------------------------------------------------------------
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fn label_basins(fdir: &[i8], accum: &[i32], w: usize, h: usize) -> Vec<i32> {
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let n = w * h;
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let mut labels = vec![-1i32; n];
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// Pour points: local accumulation maxima above river threshold.
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let mut pour_pts: Vec<usize> = Vec::new();
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for i in 0..n {
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if accum[i] <= RIVER_THRESHOLD {
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continue;
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}
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let r = i / w;
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let c = i % w;
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let mut is_max = true;
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for &(dr, dc) in &D8 {
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr >= 0 && nr < h as i32 && accum[nr as usize * w + nc] > accum[i] {
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is_max = false;
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break;
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}
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}
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if is_max {
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pour_pts.push(i);
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}
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}
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if pour_pts.is_empty() {
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// Flat/ocean world — single basin.
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labels.iter_mut().for_each(|l| *l = 0);
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return labels;
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}
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for (basin_id, &idx) in pour_pts.iter().enumerate() {
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labels[idx] = basin_id as i32;
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}
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// Trace remaining cells: follow fdir until a labeled cell is reached.
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for start in 0..n {
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if labels[start] >= 0 {
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continue;
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}
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// Walk forward, accumulate path.
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let mut path: Vec<usize> = Vec::new();
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let mut cur = start;
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let label = loop {
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if labels[cur] >= 0 {
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break labels[cur];
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}
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path.push(cur);
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let k = fdir[cur];
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if k < 0 {
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break 0; // no outflow — assign to basin 0
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}
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let r = cur / w;
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let c = cur % w;
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let (dr, dc) = D8[k as usize];
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr < 0 || nr >= h as i32 {
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break 0; // polar edge
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}
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let next = nr as usize * w + nc;
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// Cycle guard: if we're visiting a cell already in path, stop.
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if path.contains(&next) {
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break 0;
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}
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cur = next;
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};
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for idx in path {
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labels[idx] = label;
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}
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}
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labels
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}
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// ---------------------------------------------------------------------------
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// Step 7: Merge small basins
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// ---------------------------------------------------------------------------
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fn merge_small_basins(
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mut labels: Vec<i32>,
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w: usize,
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h: usize,
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min_count: usize,
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max_count: usize,
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) -> Vec<i32> {
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let n = w * h;
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let min_frac = 0.02f64; // 2% minimum basin area
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for _ in 0..200 {
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// Count basin sizes.
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let mut sizes: std::collections::BTreeMap<i32, usize> = std::collections::BTreeMap::new();
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for &l in &labels {
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*sizes.entry(l).or_insert(0) += 1;
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}
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let n_basins = sizes.len();
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// Stop if within target range and all basins are large enough.
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if n_basins <= max_count && sizes.values().all(|&s| s as f64 / n as f64 >= min_frac) {
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break;
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}
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if n_basins <= min_count {
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break;
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}
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// Find the smallest basin.
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let (&smallest_id, &smallest_size) = sizes.iter().min_by_key(|(_, &s)| s).unwrap();
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if n_basins <= max_count && smallest_size as f64 / n as f64 >= min_frac {
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break;
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}
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// Find its largest adjacent basin.
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let nbr_id = find_largest_neighbor(&labels, smallest_id, &sizes, w, h);
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let merge_into = nbr_id.unwrap_or(0);
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// Merge.
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for l in labels.iter_mut() {
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if *l == smallest_id {
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*l = merge_into;
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}
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}
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}
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// Renumber contiguously from 0.
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let unique: Vec<i32> = {
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let mut set: std::collections::BTreeSet<i32> = std::collections::BTreeSet::new();
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for &l in &labels {
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set.insert(l);
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}
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set.into_iter().collect()
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};
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let remap: std::collections::BTreeMap<i32, i32> = unique
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.iter()
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.enumerate()
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.map(|(new, &old)| (old, new as i32))
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.collect();
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for l in labels.iter_mut() {
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*l = remap[l];
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}
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labels
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}
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fn find_largest_neighbor(
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labels: &[i32],
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target_id: i32,
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sizes: &std::collections::BTreeMap<i32, usize>,
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w: usize,
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h: usize,
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) -> Option<i32> {
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let n = w * h;
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let mut neighbor_sizes: std::collections::BTreeMap<i32, usize> =
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std::collections::BTreeMap::new();
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for i in 0..n {
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if labels[i] != target_id {
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continue;
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}
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let r = i / w;
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let c = i % w;
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for &(dr, dc) in &D8 {
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let nr = r as i32 + dr;
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let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
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if nr >= 0 && nr < h as i32 {
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let nbr_id = labels[nr as usize * w + nc];
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if nbr_id != target_id {
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let size = sizes.get(&nbr_id).copied().unwrap_or(0);
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let e = neighbor_sizes.entry(nbr_id).or_insert(0);
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if size > *e {
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*e = size;
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}
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}
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}
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}
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}
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neighbor_sizes
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.into_iter()
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.max_by_key(|(_, s)| *s)
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.map(|(id, _)| id)
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}
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// ---------------------------------------------------------------------------
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// Step 8: Build DrainageBasin structs
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// ---------------------------------------------------------------------------
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fn build_basins(labels: &[i32], w: usize, h: usize) -> Vec<DrainageBasin> {
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let n = w * h;
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let mut basin_map: std::collections::BTreeMap<i32, Vec<usize>> =
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std::collections::BTreeMap::new();
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for (i, &l) in labels.iter().enumerate() {
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basin_map.entry(l).or_default().push(i);
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}
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let mut basins: Vec<DrainageBasin> = Vec::with_capacity(basin_map.len());
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let mut ids: Vec<i32> = basin_map.keys().copied().collect();
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ids.sort();
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for basin_id in ids {
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let cells = &basin_map[&basin_id];
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let area_pct = cells.len() as f32 / n as f32;
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// Boundary cells: in this basin, adjacent to a different basin or edge.
|
||
let mut boundary: Vec<(u16, u16)> = Vec::new();
|
||
for &idx in cells {
|
||
let r = idx / w;
|
||
let c = idx % w;
|
||
let mut on_boundary = false;
|
||
for &(dr, dc) in &D8 {
|
||
let nr = r as i32 + dr;
|
||
let nc = (c as i32 + dc).rem_euclid(w as i32) as usize;
|
||
if nr < 0 || nr >= h as i32 {
|
||
on_boundary = true;
|
||
break;
|
||
}
|
||
if labels[nr as usize * w + nc] != basin_id {
|
||
on_boundary = true;
|
||
break;
|
||
}
|
||
}
|
||
if on_boundary {
|
||
boundary.push((r as u16, c as u16));
|
||
}
|
||
}
|
||
|
||
// Sort boundary by angle from centroid for a coherent polygon.
|
||
if !boundary.is_empty() {
|
||
let cr = boundary.iter().map(|&(r, _)| r as f32).sum::<f32>() / boundary.len() as f32;
|
||
let cc = boundary.iter().map(|&(_, c)| c as f32).sum::<f32>() / boundary.len() as f32;
|
||
boundary.sort_by(|&(r1, c1), &(r2, c2)| {
|
||
let a1 = (r1 as f32 - cr).atan2(c1 as f32 - cc);
|
||
let a2 = (r2 as f32 - cr).atan2(c2 as f32 - cc);
|
||
a1.partial_cmp(&a2).unwrap_or(std::cmp::Ordering::Equal)
|
||
});
|
||
// Subsample to ≤500 points.
|
||
if boundary.len() > 500 {
|
||
let step = boundary.len() / 500;
|
||
boundary = boundary.into_iter().step_by(step).collect();
|
||
}
|
||
}
|
||
|
||
basins.push(DrainageBasin {
|
||
basin_id: basin_id as u32,
|
||
boundary,
|
||
area_pct,
|
||
});
|
||
}
|
||
|
||
basins
|
||
}
|
||
|
||
// ---------------------------------------------------------------------------
|
||
// Tests
|
||
// ---------------------------------------------------------------------------
|
||
|
||
#[cfg(test)]
|
||
mod tests {
|
||
use super::*;
|
||
|
||
fn flat_grid(w: u32, h: u32, val: f32) -> Vec<f32> {
|
||
vec![val; (w * h) as usize]
|
||
}
|
||
|
||
fn slope_grid(w: u32, h: u32) -> Vec<f32> {
|
||
let n = (w * h) as usize;
|
||
(0..n)
|
||
.map(|i| {
|
||
let r = i / w as usize;
|
||
let c = i % w as usize;
|
||
// Slope: higher in top-left, drains toward bottom-right.
|
||
1.0 - (r as f32 / h as f32 * 0.5 + c as f32 / w as f32 * 0.5)
|
||
})
|
||
.collect()
|
||
}
|
||
|
||
#[test]
|
||
fn flat_grid_produces_single_basin() {
|
||
let elev = flat_grid(16, 8, 0.5);
|
||
let result = analyze(&elev, 16, 8, 0.3);
|
||
// Flat world → no pour points → single basin
|
||
assert_eq!(result.drainage_basins.len(), 1);
|
||
assert!((result.drainage_basins[0].area_pct - 1.0).abs() < 0.01);
|
||
}
|
||
|
||
#[test]
|
||
fn slope_grid_has_no_river_cells_below_threshold_by_default() {
|
||
// Small 8×4 grid: max flow_accum ≤ 32, below RIVER_THRESHOLD (200).
|
||
let elev = slope_grid(8, 4);
|
||
let result = analyze(&elev, 8, 4, 0.3);
|
||
// River cells may be empty on this tiny grid — that is acceptable.
|
||
// What matters: no panic and basin count ≥ 1.
|
||
assert!(!result.drainage_basins.is_empty());
|
||
}
|
||
|
||
#[test]
|
||
fn large_grid_river_cells_nonempty() {
|
||
// 512×256: max flow accumulation ~131K >> RIVER_THRESHOLD.
|
||
let elev = slope_grid(512, 256);
|
||
let result = analyze(&elev, 512, 256, 0.3);
|
||
assert!(
|
||
!result.river_network.river_cells.is_empty(),
|
||
"Expected river cells on a large sloped grid"
|
||
);
|
||
}
|
||
|
||
#[test]
|
||
fn basin_area_pcts_sum_to_one() {
|
||
let elev = slope_grid(64, 32);
|
||
let result = analyze(&elev, 64, 32, 0.3);
|
||
let total: f32 = result.drainage_basins.iter().map(|b| b.area_pct).sum();
|
||
assert!(
|
||
(total - 1.0).abs() < 0.01,
|
||
"Basin area fractions must sum to 1, got {}",
|
||
total
|
||
);
|
||
}
|
||
|
||
#[test]
|
||
fn basin_count_within_target_range() {
|
||
let elev = slope_grid(128, 64);
|
||
let result = analyze(&elev, 128, 64, 0.3);
|
||
let n = result.drainage_basins.len();
|
||
assert!(
|
||
n >= 1 && n <= 12,
|
||
"Basin count {} out of expected range [1, 12]",
|
||
n
|
||
);
|
||
}
|
||
|
||
#[test]
|
||
fn determinism() {
|
||
// Running analyze twice on the same input must produce identical results.
|
||
let elev = slope_grid(64, 32);
|
||
let r1 = analyze(&elev, 64, 32, 0.3);
|
||
let r2 = analyze(&elev, 64, 32, 0.3);
|
||
assert_eq!(
|
||
r1.river_network.river_cells, r2.river_network.river_cells,
|
||
"River cells must be deterministic"
|
||
);
|
||
assert_eq!(
|
||
r1.drainage_basins.len(),
|
||
r2.drainage_basins.len(),
|
||
"Basin count must be deterministic"
|
||
);
|
||
}
|
||
}
|