A whole lot of stuff

lib/services/estimated_refund.dart

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+/// The Missouri Highway Fuel Tax Refund rate per gallon — the basis for
+/// [estimatedFuelRefund]. Set by Missouri law and subject to change at any
+/// time; the UI Settings toggle that turns these estimates on says as much,
+/// and this is deliberately the highway rate only (not the separate,
+/// higher non-highway-use refund) since that's the one that applies to
+/// ordinary vehicle fill-ups this app is built around.
+const moHighwayFuelTaxRefundRatePerGallon = 0.125;
+
+/// A rough estimate of the Missouri Highway Fuel Tax Refund for [gallons]
+/// of fuel, at the current rate. Purely informational — not tax advice,
+/// and not a substitute for confirming the actual claimable amount with
+/// the Missouri Department of Revenue.
+///
+/// [gallons] is rounded to the nearest whole gallon *before* multiplying —
+/// matching how the actual refund calculation works, rather than
+/// multiplying the precise (fractional) logged amount. The result is then
+/// rounded down to the nearest cent, never up, so this never overstates
+/// what's actually claimable.
+double estimatedFuelRefund(double gallons) {
+  final roundedGallons = gallons.roundToDouble();
+  final rawRefund = roundedGallons * moHighwayFuelTaxRefundRatePerGallon;
+  // The tiny epsilon guards against binary floating-point representation
+  // error nudging an exact cent value (e.g. 2.50) just under its true
+  // value and floor()ing it down a cent it doesn't actually owe.
+  return (rawRefund * 100 + 1e-9).floorToDouble() / 100;
+}