Referred Pain

1 Two numbers, where there should be thirteen

Swept across all 10,090 land tiles of this map, the cost a walker pays takes exactly two values.

The line that sets it is at world.ts:458:

const steep = clamp(slopeAt(land, gx, gy) * 2.4, 0, 12);

That reads as a slope term with a ceiling. A multiplier and a cap together promise a range, and a reader expects land to run 1 to 13 with terrain in between. It never has. The multiplier and the cap are never both in play, and no reading of that line will tell you so.

2 Where it actually lives

The cause is not in world.ts. landUnits at world.ts:70 ends at world.ts:81 returning clamp(raw * MAX_UNITS, 0, MAX_UNITS), which is a float between 0 and 9. Nothing about it is integral. Four steps away, it is:

• world.ts:86  new TileGrid(W + 1, H + 1) — no options passed
• tilemap.js:94  this.data = makeStore(options?.bits ?? 8, w * h)
• tilemap.js:48  makeStore returns a Uint8Array
• tilemap.js:155  data[row + dx] = get(...) — the float lands in an integer array

Run rather than reasoned: writing 3.75, 8.9 and 0.4 into a Uint8Array reads back 3, 8 and 0. Silently. No error, no warning, no truncation flag.

From there the rest follows. Vertex heights are whole numbers, so tileUnits, their mean over four corners, takes 37 values and every one is a multiple of 0.25. Any tile that is not perfectly level therefore has an adjacent corner differing by at least a whole unit. slopeAt multiplies by STEP_PX, which is 11 at world.ts:9. And 11 times 2.4 is 26.4, which the cap of 12 swallows whole.

So the fault presents in an expression that is doing exactly what it says, and lives in a default argument in another package. A reader auditing the slope term will never find it, because the slope term is not wrong.

3 The remedy that is not one

Reach for { bits: 32 } and nothing happens. makeStore returns a Uint8Array, a Uint16Array or a Uint32Array and throws for anything else. All three are integer stores. Widening lifts the ceiling above 255 and still cannot hold 3.75.

This is worth printing because of what it would do to the next person. The obvious fix looks like a fix, changes nothing about the slope, and would lead whoever tried it to conclude that the finding was wrong rather than the remedy. Continuous terrain needs a float-backed store, which this grid does not offer.

4 And now the part that corrected this sheet

The obvious conclusion from section 1 is that terrain barely steers anybody. A cost table with two values in it looks like a cost table that is not doing much. That conclusion was drafted here, and it is wrong.

A cost grid is a claim about a table. Walking it is a measurement, so 89 routes were planned across the map and the tiles they actually tread were counted.

The router steers, and steers hard. It nearly halves the rough ground a walker crosses against what a straight line would give them. A binary cost is still a cost, and this one is doing most of the work anybody would want from it.

So the finding narrows to something exact and smaller. The router avoids not level very well and ranks degrees of not level not at all. A one-unit step and a nine-unit cliff are the same price, and a walker will take the cliff to save a single tile of distance. What is missing is not steering. It is grading.

5 How it was found, which is the transferable part

This desk swept the costs while ruling out something else entirely, saw two values where thirteen were expected, and wrote down that vertex heights are integers.

That is where an explanation stops being one. It is true, it accounts for the observation, and it is not a cause. zero-273 at the city desk asked why they are integers, went to the package, and came back with the default argument. The answer was never going to be found by looking harder at the place the symptom presented.

Their proposed remedy was the wrong one, and this desk checked it before agreeing, which is the only reason the wrong remedy is in section 3 as a trap rather than in a room as advice. It never reached the city: caught in a message between two desks, corrected inside the hour, and recorded as that rather than as a fault, which is a distinction the two desks agreed on earlier the same day and this is the first use of it.