naismith.ts

// Naismith's Rule time estimate. See WIREFRAMES.md's Load-bearing values:
// 5 km/h base pace + 1 hour per 600m of ascent, rounded to 5-minute steps,
// always prefixed ≈, never shown as an arrival clock, no descent credit (a
// known weakness of the rule - don't silently "improve" it by adding one).
//
// Deliberately no `descentFt` parameter at all - not just "ignored if
// passed" but structurally absent, so a future call site can't accidentally
// wire descent in without touching this function's signature first.

const KM_PER_HOUR = 5
const MILES_TO_KM = 1.609344
const FEET_TO_METERS = 0.3048
const MINUTES_PER_HOUR_OF_ASCENT = 60 // 1h per 600m
const METERS_PER_ASCENT_HOUR = 600
const ROUND_TO_MINUTES = 5

export interface NaismithInput {
  distanceMi: number
  ascentFt: number
}

export function naismithTime({ distanceMi, ascentFt }: NaismithInput): string {
  const distanceKm = distanceMi * MILES_TO_KM
  const ascentM = ascentFt * FEET_TO_METERS

  const distanceMinutes = (distanceKm / KM_PER_HOUR) * 60
  const ascentMinutes = (ascentM / METERS_PER_ASCENT_HOUR) * MINUTES_PER_HOUR_OF_ASCENT

  const totalMinutes = distanceMinutes + ascentMinutes
  const rounded = Math.round(totalMinutes / ROUND_TO_MINUTES) * ROUND_TO_MINUTES

  const hours = Math.floor(rounded / 60)
  const minutes = rounded % 60

  if (hours === 0) return `≈${minutes}m`
  if (minutes === 0) return `≈${hours}h`
  return `≈${hours}h ${minutes}m`
}