fix(simulation): a river course was one D8 hop, not a river (T-1237)
D-261 asked for contiguous river strokes, and the client got them by chaining hops back together after the fact. That could not work: each hop was warped independently, so a shared confluence point arrived as two points that no longer coincided -- 375 hops rejoined into 260 pieces, and Ferrath's Global map showed scratches rather than watercourses. The join belongs before invention, so it now happens on the server. river_course::build_paths walks the D8 cell graph into whole rivers from headwater to mouth, edge-drain, or junction with an already-walked river (including the joint cell, so a tributary visibly meets its trunk). step_canvas emits one course per river instead of one per cell, which also drops the per-hop warp and resampling -- a path's shape is the terrain's, so there is nothing left to invent. It is cheaper too: one point per river cell rather than three. The client's _chain_runs() and its endpoint index are deleted. Runs survive only for the reason D-261 gives them -- water splits a course, and a river crossing a lake is genuinely two strokes that must not be rejoined. Pinned by a real-terrain test on GJ380c rather than a synthetic graph, because the bug was caught by eye on real terrain: a lake must have an outflow that runs to sea level, and no such line existed. It asserts the property the eye was checking -- rivers are long, at least one reaches the sea, and every path is a contiguous walk. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Claude Opus 5
@@ -241,6 +241,127 @@ pub fn build_edges(rn: &RiverNetwork) -> Vec<RiverEdge> {
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/// step is only used to identify the neighbor cell for `Interior` edges,
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/// where the offset is by construction in-bounds — it came from the same D8
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/// walk `extract_river_network` already validated).
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/// A whole river: the ordered cell path from a headwater down to a mouth, an
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/// edge-drain, or a junction with an already-walked river.
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pub struct RiverPath {
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pub cells: Vec<RiverCell>,
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pub class: u8,
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pub terminus: EdgeTerminusKind,
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}
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/// Assemble the D8 cell graph into whole rivers (D-261, 2026-07-28).
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///
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/// WHY THIS EXISTS. [`build_edges`] emits one edge PER RIVER CELL — a single
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/// D8 step — so "a course" was never a river, it was one hop. Rendering each
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/// hop independently produced exactly what you would expect: on Ferrath's
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/// Global canvas, 375 courses of 1-4 points each (median 3), drawn as ~70 km
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/// straight stubs, with no continuous line anywhere. Jeroen spotted it from
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/// the map alone: a lake must have an outflow that runs to sea level, and no
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/// such line existed.
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///
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/// Chaining the hops back together on the CLIENT does not work, because
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/// `invent_course` warps each hop independently — the shared confluence cell
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/// receives a different offset in each, so the endpoints that ought to be one
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/// point are two. Measured: 375 hops chained into only 260 pieces. The join
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/// has to happen before invention, which is here.
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///
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/// A tributary stops at the first cell already claimed by another river, but
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/// INCLUDES that cell, so it visibly meets the trunk instead of stopping one
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/// cell short of it.
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pub fn build_paths(rn: &RiverNetwork) -> Vec<RiverPath> {
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// BTreeMap, not HashMap — D-030 bans the hashed containers in simulation
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// code. This one is pure lookup, never iterated, so order would not have
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// bitten us; the ban is blanket and the walk runs once per canvas.
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use std::collections::BTreeMap;
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let mut index: BTreeMap<RiverCell, usize> = BTreeMap::new();
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for (i, &cell) in rn.river_cells.iter().enumerate() {
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index.insert(cell, i);
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}
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// A headwater is a cell nothing flows into.
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let mut has_upstream = vec![false; rn.river_cells.len()];
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for (i, &cell) in rn.river_cells.iter().enumerate() {
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let Some(&sentinel) = rn.river_downstream.get(i) else {
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continue;
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};
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if sentinel < 8 {
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let (dr, dc) = d8_offset(sentinel);
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if let Some(&j) = index.get(&step_cell(cell, dr, dc)) {
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has_upstream[j] = true;
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}
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}
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}
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let mut claimed = vec![false; rn.river_cells.len()];
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let mut paths: Vec<RiverPath> = Vec::new();
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// Headwaters first so main stems are walked before the leftovers pass.
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let order = (0..rn.river_cells.len())
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.filter(|&i| !has_upstream[i])
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.chain(0..rn.river_cells.len());
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for start in order {
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if claimed[start] {
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continue;
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}
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let mut cells: Vec<RiverCell> = Vec::new();
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let mut terminus = EdgeTerminusKind::Interior;
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let mut i = start;
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loop {
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claimed[i] = true;
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cells.push(rn.river_cells[i]);
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let Some(&sentinel) = rn.river_downstream.get(i) else {
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break;
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};
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if sentinel == RIVER_DOWNSTREAM_TERMINAL {
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break;
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}
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if sentinel == RIVER_DOWNSTREAM_MOUTH {
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// The seaward cell is off-network; append it so the line
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// actually reaches the water rather than stopping inland.
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let seaward = rn.river_seaward.get(i).copied().unwrap_or((0, 0));
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if seaward != (0, 0) {
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cells.push(seaward);
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}
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terminus = EdgeTerminusKind::Mouth;
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break;
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}
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if sentinel == RIVER_DOWNSTREAM_EDGE_DRAIN {
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terminus = EdgeTerminusKind::EdgeDrain;
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break;
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}
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let (dr, dc) = d8_offset(sentinel);
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let Some(&next) = index.get(&step_cell(rn.river_cells[i], dr, dc)) else {
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break;
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};
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if claimed[next] {
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// Junction with a river already walked — include the joint so
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// the tributary visibly meets it, then stop.
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cells.push(rn.river_cells[next]);
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break;
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}
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i = next;
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}
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if cells.len() < 2 {
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continue;
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}
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// Class of the DOWNSTREAM end: a river reads by what it becomes, not
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// by the trickle it starts as.
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let class = rn
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.river_class
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.get(i)
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.copied()
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.unwrap_or_else(|| rn.river_class.get(start).copied().unwrap_or(0));
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paths.push(RiverPath {
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cells,
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class,
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terminus,
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});
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}
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paths
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}
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fn step_cell(cell: RiverCell, dr: i32, dc: i32) -> RiverCell {
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let r = (cell.0 as i32 + dr).max(0) as u16;
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let c = (cell.1 as i32 + dc).max(0) as u16;
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@@ -1243,4 +1364,62 @@ mod tests {
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0.0
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));
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}
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/// The bug Jeroen caught by eye, pinned on the terrain he caught it on.
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///
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/// His tell was precise: *a lake should have a run in (which may be too
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/// short to show) and a run out towards sealevel that should definitely be
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/// long enough.* No such line existed on Ferrath's Global map, because
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/// every "course" was one D8 hop — `build_edges` emits one edge per river
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/// cell. 375 courses, median 3 points, all of them stubs.
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///
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/// So this asserts the property the eye was checking, not the mechanism:
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/// real terrain must yield rivers that are LONG, and at least one of them
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/// must actually reach the sea. A per-hop regression fails both — its
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/// longest path would be 2 cells and no path would be a Mouth.
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#[test]
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fn real_terrain_yields_long_rivers_that_reach_the_sea() {
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let src = std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR"))
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.join("../wiki/star-systems/GJ-380/bodies/GJ380c/heightmap.png");
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let heightmap = crate::atlas::heightmap::load_heightmap_png(&src, "GJ380c", 0.3)
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.expect("decode GJ380c heightmap");
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let small = heightmap.downsample(512, 256);
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let dr = drainage::analyze(&small.data, small.width, small.height, small.sea_level);
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let paths = build_paths(&dr.river_network);
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assert!(!paths.is_empty(), "GJ380c must have rivers at all");
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let longest = paths.iter().map(|p| p.cells.len()).max().unwrap_or(0);
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assert!(
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longest >= 20,
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"longest river is only {longest} cells — rivers are being cut into hops again \
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(the whole-body map then shows scratches, not watercourses)"
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);
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let to_sea = paths
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.iter()
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.filter(|p| p.terminus == EdgeTerminusKind::Mouth)
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.count();
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assert!(
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to_sea > 0,
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"no river on GJ380c reaches the sea — an outflow to sea level must exist"
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);
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// Every path is a walk of adjacent cells, so a gap means the chain
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// broke and the stroke would jump across the map.
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for p in &paths {
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for w in p.cells.windows(2) {
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let (dr_, dc_) = (
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(w[1].0 as i64 - w[0].0 as i64).abs(),
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(w[1].1 as i64 - w[0].1 as i64).abs(),
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);
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assert!(
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dr_ <= 1 && dc_ <= 1,
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"river path jumps from {:?} to {:?} — not a contiguous walk",
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w[0],
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w[1]
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);
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}
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}
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}
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}
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@@ -649,14 +649,19 @@ fn fixed_canvas_world_rect(
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/// return.
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#[allow(clippy::too_many_arguments)]
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fn invent_courses_for_canvas(
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seed: SeedChain,
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// Unused since D-261 moved rivers from per-hop invention to whole cell
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// paths: a path's shape IS the terrain's, so there is no warp to seed and
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// no resampling wavelength to cut off. Kept in the signature because the
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// deferred polish (T-1238) reintroduces a per-river warp, and threading
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// them back through later is worse than leaving the plumbing in place.
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_seed: SeedChain,
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params: &BodyParams,
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ta: &TerrainAnalysis,
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river_network: &RiverNetwork,
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canvas_rect: (f64, f64, f64, f64),
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rung: StepCanvasRung,
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step_m: f64,
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min_wavelength_m: f64,
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_step_m: f64,
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_min_wavelength_m: f64,
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) -> Vec<InventedCourse> {
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// D-261: courses are invented at EVERY rung, Global included. They used to
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// be withheld from the orbital rung, which is why the whole-body map had no
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@@ -676,52 +681,63 @@ fn invent_courses_for_canvas(
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// direction — a District canvas spans ~3.6 km post-inversion, so a 2,048 m
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// station pitch would place two stations across the whole view and render
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// every river as a straight line.
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let station_spacing_m = step_m;
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let (win_x0, win_y0, win_x1, win_y1) = canvas_rect;
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let edges = river_course::build_edges(river_network);
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// D-261: one course per RIVER, not per D8 hop.
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//
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// build_edges() emits one edge per river cell — a single step — so every
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// "course" used to be one hop, inflated by invent_course into a ~70 km
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// straight stub. Measured on Ferrath's Global canvas: 375 courses of 1-4
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// points each, median 3. Nothing on that map read as a watercourse, and a
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// lake's outflow to the sea — which must exist and must be long — was
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// nowhere to be seen.
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//
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// A path IS the river, so its points are the river's own cells in order:
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// no resampling, no per-hop warp, and the shape is a function of the
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// terrain rather than of the zoom level (which is what D-261 asks for).
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// It is also CHEAPER — the whole network becomes one point per river cell
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// instead of three per cell.
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let paths = river_course::build_paths(river_network);
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let mut courses = Vec::new();
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for edge in &edges {
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let anchor_a = crate::atlas::district_profile::pixel_to_world_m(
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edge.upstream.1 as f64,
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edge.upstream.0 as f64,
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ta.w,
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ta.h,
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params.body_radius_km,
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);
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let anchor_b = crate::atlas::district_profile::pixel_to_world_m(
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edge.downstream.1 as f64,
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edge.downstream.0 as f64,
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ta.w,
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ta.h,
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params.body_radius_km,
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);
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let chord_m =
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((anchor_a.0 - anchor_b.0).powi(2) + (anchor_a.1 - anchor_b.1).powi(2)).sqrt();
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// Same inflation fraction layer_proxy::COURSE_BBOX_INFLATION_FRACTION
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// uses — mirrors the Stage-B peak-amplitude bound exactly (0.08 =
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// STAGE_B_PEAK_FRACTION_OF_CHORD, pinned equal by a const assert in
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// layer_proxy.rs).
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let inflate_m = chord_m * 0.08;
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let (bx0, bx1) = (
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anchor_a.0.min(anchor_b.0) - inflate_m,
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anchor_a.0.max(anchor_b.0) + inflate_m,
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);
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let (by0, by1) = (
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anchor_a.1.min(anchor_b.1) - inflate_m,
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anchor_a.1.max(anchor_b.1) + inflate_m,
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);
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for (path_idx, path) in paths.iter().enumerate() {
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let points: Vec<river_course::CoursePoint> = path
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.cells
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.iter()
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.map(|&cell| {
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crate::atlas::district_profile::pixel_to_world_m(
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cell.1 as f64,
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cell.0 as f64,
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ta.w,
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ta.h,
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params.body_radius_km,
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)
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})
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.collect();
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if points.len() < 2 {
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continue;
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}
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// Cull whole rivers that miss this canvas entirely, cheaply, before
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// any per-point work downstream.
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let mut bx0 = f64::MAX;
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let mut bx1 = f64::MIN;
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let mut by0 = f64::MAX;
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let mut by1 = f64::MIN;
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for p in &points {
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bx0 = bx0.min(p.0);
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bx1 = bx1.max(p.0);
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by0 = by0.min(p.1);
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by1 = by1.max(p.1);
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}
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if bx1 < win_x0 || bx0 > win_x1 || by1 < win_y0 || by0 > win_y1 {
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continue;
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}
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courses.push(river_course::invent_course(
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seed,
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edge,
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ta,
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params,
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station_spacing_m,
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min_wavelength_m,
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));
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courses.push(InventedCourse {
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edge_id: path_idx as u32,
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class: path.class,
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terminus: path.terminus,
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bbox: (bx0, by0, bx1, by1),
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points,
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});
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}
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courses
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Reference in New Issue
Block a user