Water-fill

water_fill splits one order across several parallel routes to reduce price impact on large trades. It tries a few ways to split the order, simulates each, and returns whichever gives the most output after gas — and never less than the best single route.

It runs on the DepthAndPrice weighted graph (like Most Liquid). It has no hard derived-data requirement: it uses token gas prices when they are available but tolerates stale ones and never waits on them. With gas prices, ranking is gas-aware — it subtracts each path's activation cost and the route's gas from the output. Without them, it ranks on gross output.

What it returns

For each order, water-fill builds up to four candidate routes and returns the one with the highest output net of gas. If none beats the single path, it returns the single path.

1. Best single path — the best route that carries the whole order on its own. This is the floor: water-fill never returns less than this.

2. 20-chunk disjoint split — splits the order across paths that share no pool ("pool-disjoint"), allocating in 20 chunks. This is the safety net: it is cheap and always finishes, even under a tight timeout, so water-fill can still beat the single path when a split would win (see Never-lose floor).

3. 256-chunk disjoint split — the same pool-disjoint paths, allocated in 256 chunks for a finer split. It runs in two phases (see Two-phase activation) and then an exchange-refinement pass.

4. Fill-and-spill — a split that lets paths share a pool and branch at an intermediate token (a "tree route"). The pool-disjoint splits above cannot express these.

Incremental water-fill

Every split fills the order in small chunks. For each chunk, water-fill checks how much each path would return for that chunk, gives the chunk to the best one, and moves on.

Constant-product and tick AMMs are path-independent in cumulative input: one swap of x returns the same as two back-to-back swaps that sum to x. So instead of re-simulating each path at its full running total for every chunk (O(chunks²)), water-fill simulates only the next chunk against the pool state committed so far (O(chunks)). That saved work is what makes the 256-chunk split affordable.

Two-phase activation

Splitting into smaller chunks can backfire on large trades. A path only pays off once it carries enough volume to cover its gas cost (its "activation gate"). If the first chunk given to a path is too small, the path fails that gate and never turns on — even when it would be profitable at a larger share.

Water-fill avoids this in two phases: first decide which paths to use at coarse (20-chunk) granularity, where each path's share is large enough for the gate to be meaningful; then allocate across the chosen paths at fine (256-chunk) granularity with the gate off, since their gas is already covered.

Exchange refinement

A chunk-by-chunk split can never take back a chunk it already gave out, so the refined split is only accurate to one chunk (1/256 of the order) and can sit slightly off the ideal split. The exchange pass corrects this. Starting from the refined split, it moves a small amount of input from an over-allocated path to an under-allocated one, keeping the move only if the two paths' combined output goes up. When no move helps, it halves the amount and tries again, down to a floor below one chunk. The paths share no pool, so each trial re-simulates only the two paths it touches. The pass stops at the solve timeout or a cap on trial simulations. Because it keeps only moves that improve output, the result is never worse than the split it started from.

Candidate discovery

Water-fill finds candidate routes two ways and combines them, so no useful route is dropped:

1. Exhaustive enumeration — lists paths between the sell and buy tokens (breadth-first) and ranks them by a spot-price × depth heuristic, reusing Most Liquid on the same DepthAndPrice graph (see the Dependency note below).

2. Bounded amount-aware search — a frontier search (the discovery section of water_fill.rs) that expands from the sell token with the full order amount, preferring edges toward the buy token, the connector_tokens allowlist, or a set of anchor tokens.

Anchors are a soft ranking hint, distinct from the connector_tokens hard allowlist: when no connector_tokens allowlist is set, discovery prefers to expand through anchors but still reaches any other token. The anchor set is derived per solve from the graph itself — the most connected tokens (highest pool-edge degree, the same signal derive-connector-tokens ranks by) plus the native-ETH sentinel 0x0000000000000000000000000000000000000000. Deriving from live connectivity keeps anchoring correct on every chain with no hardcoded per-chain list; the sentinel is anchored explicitly so WETH → ETH → token routes survive on full Fynd setups where Tycho models native ETH as the zero address.

The bounded set is placed ahead of the heuristic-ranked set, so connector and anchor routes survive the spot × depth cutoff. If the bounded search fails, it is not fatal: the exhaustive set already guarantees a route.

Dependency: water-fill's discovery and single-path ranking are built on Most Liquid's find_paths, try_score_path, and simulate_path. Changing those Most Liquid internals changes water-fill's candidate set and ranking — keep both in mind when editing either algorithm.

Route assembly

Every returned candidate is assembled through the shared split primitives (build_split_route), which emit swaps in topological order, apply the tycho-execution remainder-split convention, merge shared hops (paths that share a prefix pool produce one combined swap with split downstream legs), and attach the route's token map. If a candidate's legs cannot be topologically ordered — for example two disjoint legs that form a token cycle — assembly returns nothing and that candidate is dropped, so water-fill falls back to another candidate or the single path. Assembly is the same code the router encodes on-chain, so every route water-fill returns can be encoded.

Never-lose floor

Water-fill never returns less net output than the best single path. This holds even under a tight timeout because the 20-chunk disjoint split (candidate 2) is cheap and always finishes on the same solve clock — a timeout cannot cut off the split while leaving the single path, so if a split wins, water-fill returns it.

Because water-fill always returns at least the best single path, it can be the only pool on a chain. A split router that returned nothing when it declined to split would need a single-path pool beside it to answer those orders.

Suggested configuration

[pools.water_fill_3_hops]
algorithm = "water_fill"
num_workers = 3
task_queue_capacity = 1000
max_hops = 3
timeout_ms = 5000
max_routes = 1024

• max_hops = 3 keeps discovery bounded. Water-fill simulates every candidate on every order, so each extra hop widens both the enumerated set and the per-candidate simulation cost.

• max_routes = 1024 caps the heuristic-ranked candidate set. The bounded amount-aware search is placed ahead of it, so connector and anchor routes survive the cutoff.

• timeout_ms = 5000 is generous because the exchange-refinement pass and the 256-chunk split use whatever budget remains. Cutting it short is safe — the cheap 20-chunk split always finishes on the same clock, so the never-lose floor still holds.

• Water-fill answers every order, so it can run as the only pool on a chain. It is still slower than the single-route finders, so pair it with a Bellman-Ford pool if you serve latency-sensitive traffic.

Set connector tokens. With connector_tokens unset, routes can pass through illiquid long-tail intermediates, which raises reversion risk — and a split route multiplies that exposure across every path it activates. Restrict intermediate hops to a small trusted set (a hard filter) — generate one for your chain with fynd derive-connector-tokens --chain Ethereum --top-n 10 --output toml and paste it into the pool. See Connector tokens. The soft anchor preference used when no allowlist is set is derived automatically from the graph and needs no configuration.

Source reference

File	Purpose
fynd-core/src/algorithm/water_fill.rs	WaterFillAlgorithm: candidate allocation (20-chunk disjoint split, 256-chunk disjoint split, fill-and-spill) and bounded candidate discovery
fynd-core/src/algorithm/most_liquid.rs	Most Liquid path finder reused for discovery and ranking: find_paths, try_score_path, simulate_path, and the DepthAndPrice graph weight
fynd-core/src/algorithm/split_primitives.rs	Shared-hop merging and executable route assembly
fynd-core/src/worker_pool/registry.rs	Maps "water_fill" to WaterFillAlgorithm