#!/usr/bin/env python3
"""Navigation computer of the SS Probably Fine.

Usage: python3 nav.py DESTINATION
       python3 nav.py --list
"""

import sys

# Distances between neighboring waypoints, in million km.
DISTANCES = {
    ("earth", "moon"): 0.4,
    ("earth", "gateway"): 0.4,
    ("gateway", "mars"): 225.0,
    ("gateway", "venus"): 41.0,
    ("mars", "phobos"): 0.01,
}

# Every route starts at Earth.
ROUTES = {
    "moon": ["earth", "moon"],
    "mars": ["earth", "gateway", "mars"],
    "venus": ["earth", "gateway", "venus"],
    "phobos": ["earth", "gateway", "mars", "phobos"],
}

NAMES = {
    "earth": "Earth",
    "moon": "Moon",
    "gateway": "Lunar Gateway",
    "mars": "Mars",
    "venus": "Venus",
    "phobos": "Phobos",
}

CRUISE_SPEED = 0.87  # million km per day
FUEL_PER_MKM = 0.142  # tonnes of fuel per million km
LAUNCH_FUEL = 380.0  # tonnes of fuel to escape Earth's gravity


def distance(a, b):
    """Return the distance between two neighboring waypoints."""
    if (a, b) in DISTANCES:
        return DISTANCES[(a, b)]
    return DISTANCES[(b, a)]


def legs(route):
    """Return the legs (pairs of consecutive waypoints) of a route."""
    return [(route[i], route[i + 1]) for i in range(len(route) - 1)]


def plan(destination):
    """Return route, distance, travel time (days) and fuel (t)."""
    route = ROUTES[destination]
    total = 0.0
    fuel = 0.0
    for start, end in legs(route):
        d = distance(start, end)
        total += d
        fuel += d * FUEL_PER_MKM
        if start == "earth":
            fuel += LAUNCH_FUEL
    return route, total, total / CRUISE_SPEED, fuel


def print_plan(destination):
    """Print the flight plan for a destination."""
    route, total, days, fuel = plan(destination)
    print("Route: " + " -> ".join(NAMES[w] for w in route))
    print(f"Distance: {total:.1f} million km")
    print(f"Travel time: {round(days)} days")
    print(f"Fuel required: {round(fuel)} t")


def main():
    if len(sys.argv) != 2:
        sys.exit(__doc__)
    if sys.argv[1] == "--list":
        for key in sorted(ROUTES):
            print(f"{key:12} {NAMES[key]}")
        return
    destination = sys.argv[1].lower()
    if destination not in ROUTES:
        known = ", ".join(sorted(ROUTES))
        sys.exit(f"Unknown destination: {destination} (known: {known})")
    print_plan(destination)


if __name__ == "__main__":
    main()