Odds rounding

const ODDS_PRECISION = 10n ** 20n;
const stepSize = 125n;                 // GET /metadata/obv3 → oddsLadderStepSize
const STEP = stepSize * 10n ** 15n;    // 1.25e17 = 0.125%

const EXTRA = new Set([10n ** 17n, 999n * 10n ** 17n]); // 0.1% and 99.9%

const onLadder = (odds) => {
  const o = BigInt(odds);
  if (o <= 0n || o >= ODDS_PRECISION) return false;
  if (EXTRA.has(o)) return true;
  return o % STEP === 0n;
};

const roundDown = (odds) => (BigInt(odds) / STEP) * STEP;
const roundUp = (odds) => {
  const o = BigInt(odds);
  return o % STEP === 0n ? o : (o / STEP + 1n) * STEP;
};

ODDS_PRECISION = 10 ** 20
step_size = 125                      # GET /metadata/obv3 -> oddsLadderStepSize
STEP = step_size * 10 ** 15          # 1.25e17 = 0.125%
EXTRA = {10 ** 17, 999 * 10 ** 17}   # 0.1% and 99.9%

def on_ladder(odds: int) -> bool:
    if odds <= 0 or odds >= ODDS_PRECISION:
        return False
    if odds in EXTRA:
        return True
    return odds % STEP == 0

def round_down(odds: int) -> int:
    return (odds // STEP) * STEP

def round_up(odds: int) -> int:
    return odds if odds % STEP == 0 else ((odds // STEP) + 1) * STEP

// Your model says 40.3%, and you want to rest on it.
const modelProbability = 0.403;

// 1. Into 1e20 space, via an integer basis — never Number * 1e20 directly.
const basisPoints = BigInt(Math.round(modelProbability * 10_000));   // 4030
const raw = (basisPoints * ODDS_PRECISION) / 10_000n;                // 40300000000000000000n

// 2. Snap. Resting order → round down.
const percentageOdds = roundDown(raw);                               // 40250000000000000000n

// 3. Submit as a string, and re-check before you do.
if (!onLadder(percentageOdds)) throw new Error("off ladder");
const field = percentageOdds.toString();                             // "40250000000000000000"

# Your model says 40.3%, and you want to rest on it.
model_probability = 0.403

# 1. Into 1e20 space via an integer basis.
basis_points = round(model_probability * 10_000)          # 4030
raw = basis_points * ODDS_PRECISION // 10_000             # 40300000000000000000

# 2. Snap. Resting order -> round down.
percentage_odds = round_down(raw)                         # 40250000000000000000

# 3. Submit as a string, and re-check before you do.
assert on_ladder(percentage_odds)
field = str(percentage_odds)                             # "40250000000000000000"