init.lua

local SEXTANT_USES = 128
local CALIBRATED_SEXTANT_USES = 256

local RANDOM_POSITION_SKEW = 64
local LOOK_SKY_ANGLE_TOLERANCE_RADIANS = math.rad(6)
local LOOK_HORIZON_ANGLE_TOLERANCE_RADIANS = math.rad(8)
local LOOK_TIME_TOLERANCE_SECONDS = 10

local TIME_NOON = 0.5
local TIME_SUN_APPEARS = (4 + (30 / 60)) / 24
local TIME_SUNRISE = 5 / 24
local TIME_MOON_DISAPPEARS = (5 + (30 / 60)) / 24
local TIME_MOON_APPEARS = (18 + (30 / 60)) / 24
local TIME_SUNSET = 19 / 24
local TIME_SUN_DISAPPEARS = (19 + (30 / 60)) / 24

local ANGLE_WEST = math.pi / 2
local ANGLE_EAST = 3 * math.pi / 2

-- Return whether two directions (v0, h0) and (v1, h1) are within the given
-- angular tolerance of each other.
local function is_within_angle(v0, h0, v1, h1, tolerance)
  local dot = math.cos(v0) * math.cos(v1) * math.cos(h0 - h1) + math.sin(v0) * math.sin(v1)

  if dot > 1 then
    dot = 1
  end
  if dot < -1 then
    dot = -1
  end

  return dot >= math.cos(tolerance)
end

local function get_sun_yaw(time)
  if time <= TIME_NOON then
    return ANGLE_EAST
  else
    return ANGLE_WEST
  end
end

local function get_moon_yaw(time)
  if time > TIME_NOON then
    return ANGLE_EAST
  else
    return ANGLE_WEST
  end
end

local function get_sun_angle(time)
  local day_half = (TIME_SUNSET - TIME_SUNRISE) / 2
  return -math.asin(math.cos((time - TIME_NOON) * ((math.pi / 2) / day_half)))
end

local function get_moon_angle(time)
  local day_half = (1 - (TIME_SUNSET - TIME_SUNRISE)) / 2
  return -math.asin(math.cos((((time + 0.5) % 1) - TIME_NOON) * ((math.pi / 2) / day_half)))
end

local function sun_is_visible(time)
  return time > TIME_SUN_APPEARS and time < TIME_SUN_DISAPPEARS
end

local function moon_is_visible(time)
  return time > TIME_MOON_APPEARS or time < TIME_MOON_DISAPPEARS
end

local function looking_at_sun_or_moon(v, h, time, tolerance)
  if sun_is_visible(time) then
    local target_angle = get_sun_angle(time)
    local target_yaw = get_sun_yaw(time)

    if is_within_angle(v, h, target_angle, target_yaw, tolerance) then
      return true, "sun"
    end
  end

  if moon_is_visible(time) then
    local target_angle = get_moon_angle(time)
    local target_yaw = get_moon_yaw(time)

    if is_within_angle(v, h, target_angle, target_yaw, tolerance) then
      return true, "moon"
    end
  end

  return false
end

local function looking_at_horizon(v, tolerance)
  return math.abs(v) < tolerance
end

local function skew_position(x, z)
  local rng = PcgRandom(core.get_us_time())
  local skewed_x = rng:next(x - RANDOM_POSITION_SKEW, x + RANDOM_POSITION_SKEW)
  local skewed_z = rng:next(z - RANDOM_POSITION_SKEW, z + RANDOM_POSITION_SKEW)
  return skewed_x, skewed_z
end

local function say_player_pos(player, is_approx)
  local pos = player:get_pos()

  local x
  local z
  if is_approx then
    x, z = skew_position(pos.x, pos.z)
  else
    x = pos.x
    z = pos.z
  end

  local north_south_direction
  if z >= 0 then
    north_south_direction = "N"
  else
    north_south_direction = "S"
  end

  local east_west_direction
  if x >= 0 then
    east_west_direction = "E"
  else
    east_west_direction = "W"
  end

  local approx_suffix
  if is_approx then
    approx_suffix = "(approximate)"
  else
    approx_suffix = "(exact)"
  end

  local message = string.format(
    "%d %s, %d %s %s",
    math.abs(z),
    north_south_direction,
    math.abs(x),
    east_west_direction,
    approx_suffix
  )

  core.chat_send_player(player:get_player_name(), message)
end

local function take_durability(item, max_uses)
  item:add_wear_by_uses(max_uses)
end

local player_sightings = {}

local function take_reading(itemstack, player, tool_uses, is_approx)
  local player_name = player:get_player_name()
  local prev_state = player_sightings[player_name]
    or {
      time = 0,
      sighted_sky = false,
      sighted_horizon = false,
    }

  local now = core.get_gametime()
  local is_fresh = now <= prev_state.time + LOOK_TIME_TOLERANCE_SECONDS

  local next_state
  if is_fresh then
    next_state = {
      time = now,
      sighted_sky = prev_state.sighted_sky,
      sighted_horizon = prev_state.sighted_horizon,
    }
  else
    next_state = {
      time = now,
      sighted_sky = false,
      sighted_horizon = false,
    }
  end

  local sighted_sky, body_name = looking_at_sun_or_moon(
    player:get_look_vertical(),
    player:get_look_horizontal(),
    core.get_timeofday(),
    LOOK_SKY_ANGLE_TOLERANCE_RADIANS
  )
  local sighted_horizon = looking_at_horizon(player:get_look_vertical(), LOOK_HORIZON_ANGLE_TOLERANCE_RADIANS)

  -- If the player is looking at both the sky and the horizon at the same time
  -- (possible around sunrise or sunset), they should still have to click
  -- twice. The first click should register as looking at the sun/moon, and the
  -- second click should register as looking at the horizon.
  if (sighted_sky and not sighted_horizon) or (sighted_sky and not next_state.sighted_sky) then
    if body_name == "sun" then
      core.chat_send_player(player_name, "Sun sighted")
    else
      core.chat_send_player(player_name, "Moon sighted")
    end

    next_state.sighted_sky = true
  elseif sighted_horizon then
    core.chat_send_player(player_name, "Horizon sighted")

    next_state.sighted_horizon = true
  end

  if next_state.sighted_sky and next_state.sighted_horizon then
    say_player_pos(player, is_approx)
    next_state.sighted_sky = false
    next_state.sighted_horizon = false
  end

  player_sightings[player_name] = next_state

  core.sound_play({
    name = "celestial_sextant",
    to_player = player_name,
    gain = 0.2,
  })

  take_durability(itemstack, tool_uses)

  return itemstack
end

core.register_tool("celestial:sextant", {
  description = "Crude Sextant",
  _doc_items_longdesc = "Find your approximate coordinates using the sun or moon.",
  _doc_items_usagehelp = "Right-click on the sun or moon, then right-click on the horizon to take a reading. A cruse sextant will only give you approximate coordinates. Craft a calibrated sextant for exact coordinates.",
  inventory_image = "sextant.png",
  on_secondary_use = function(itemstack, player, _)
    return take_reading(itemstack, player, SEXTANT_USES, true)
  end,
})

if
  core.get_modpath("mcl_core")
  and core.get_modpath("xpanes")
  and core.get_modpath("mcl_copper")
  and core.get_modpath("mcl_mobitems")
then
  core.register_craft({
    type = "shaped",
    output = "celestial:sextant 1",
    recipe = {
      { "mcl_core:stick", "xpanes:pane_natural_flat", "mcl_core:stick" },
      { "mcl_copper:copper_ingot", "mcl_mobitems:string", "mcl_copper:copper_ingot" },
      { "", "mcl_copper:copper_ingot", "" },
    },
  })
end

core.register_tool("celestial:calibrated_sextant", {
  description = "Calibrated Sextant",
  _doc_items_longdesc = "Find your exact coordinates using the sun or moon.",
  _doc_items_usagehelp = "Right-click on the sun or moon, then right-click on the horizon to take a reading. A calibrated sextant will give you exact coordinates.",
  inventory_image = "calibrated_sextant.png",
  on_secondary_use = function(itemstack, player, _)
    return take_reading(itemstack, player, CALIBRATED_SEXTANT_USES, false)
  end,
})

if core.get_modpath("mcl_spyglass") and core.get_modpath("mcl_nether") and core.get_modpath("mcl_clock") then
  core.register_craft({
    type = "shaped",
    output = "celestial:calibrated_sextant 1",
    recipe = {
      { "", "mcl_spyglass:spyglass", "" },
      { "mcl_nether:quartz", "celestial:sextant", "mcl_nether:quartz" },
      { "", "group:clock", "" },
    },
  })
end