-- NOTE the API of this file should probably change?

local lume = require "lib.lume"
local simplex_map = require "lib.simplex_map"

local biomes = {
  -- biomes are checked in order, with the first candidate always being selected
  [800] = {
    name = "sea ice",
    altitude = {min = -math.huge, max = 0},
    temperature = {min = -math.huge, max = -2},
    rainfall = {min = -math.huge, max = math.huge},
    color = {0.6, 0.6, 1, 1},
  },
  [900] = {
    name = "deep ocean",
    altitude = {min = -math.huge, max = -5000},
    temperature = {min = -math.huge, max = math.huge},
    rainfall = {min = -math.huge, max = math.huge},
    color = {0, 0, 0.25, 1},
  },
  [1000] = {
    name = "ocean (definition, 0 meters)",
    altitude = {min = -math.huge, max = 0},
    temperature = {min = -math.huge, max = math.huge},
    rainfall = {min = -math.huge, max = math.huge},
    color = {0, 0, 0.5, 1},
  },
  [1500] = {
    name = "frozen (definition, 0 C)",
    altitude = {min = -math.huge, max = math.huge},
    temperature = {min = -math.huge, max = 0},
    rainfall = {min = -math.huge, max = math.huge},
    color = {1, 1, 1, 1},
  },
  [2000] = {
    name = "snowline (middle estimate)",
    altitude = {min = 4000, max = math.huge},
    temperature = {min = -math.huge, max = math.huge},
    rainfall = {min = -math.huge, max = math.huge},
    color = {1, 1, 1, 1},
  },
  [2100] = {
    name = "alpine zone",
    altitude = {min = 3500, max = math.huge},
    temperature = {min = -math.huge, max = math.huge},
    rainfall = {min = -math.huge, max = math.huge},
    color = {0.8, 0.8, 0.8, 1},
  },
  [2200] = {
    name = "subalpine zone",
    altitude = {min = 3000, max = math.huge},
    temperature = {min = -math.huge, max = math.huge},
    rainfall = {min = -math.huge, max = math.huge},
    color = {0.67, 0.67, 0.67, 1},
  },
  [2300] = {
    name = "montane zone",
    altitude = {min = 2500, max = math.huge},
    temperature = {min = -math.huge, max = math.huge},
    rainfall = {min = -math.huge, max = math.huge},
    color = {0.67, 0.67, 0.5, 1},
  },
}

local generate = function()
  local map = {}
  map.size = 500
  map.tile_size = 1 -- TODO tile_size really shouldn't be the responsibility of the map generator - it's a rendering detail, not a map feature
  map.altitude = simplex_map.generate{   -- will be converted to meters
    size = map.size, scale = 0.001, min = 0, max = 1,
    octaves = 7, lacunarity = 2, gain = 0.5,
  }
  map.temperature = simplex_map.generate{   -- in celsius
    -- -20 to 45 chosen by looking at the temperature range shown on https://openclimatemap.org/
    size = map.size, scale = 0.005, min = 20, max = 65, -- approximate target extremes of -89.2 to 56.7
    octaves = 2, lacunarity = 2, gain = 0.5,
  }
  map.rainfall = simplex_map.generate{   -- in millimeters of rainfall
    size = map.size, scale = 0.005, -- min = 18, max = 3240,
    octaves = 1, lacunarity = 2, gain = 0.5,
  }

  local function earthlike_altitude_adjustment(altitude)
    -- TODO replace with a simpler function?
    return 6099 - 114909 * altitude + 830891 * altitude^2 - 2.72e6 * altitude^3 + 4.19e6 * altitude^4 - 3.02e6 * altitude^5 + 815866 * altitude^6
  end

  for x = 1, map.size do
    for y = 1, map.size do
      map.altitude[x][y] = earthlike_altitude_adjustment(map.altitude[x][y])
    end
  end
  map.altitude.min = simplex_map.min(map.altitude)
  map.altitude.max = simplex_map.max(map.altitude)

  local biome_order = {}
  for order in pairs(biomes) do
    biome_order[#biome_order + 1] = order
  end
  table.sort(biome_order)

  local radius_squared = (map.size / 2)^2
  map.biome = {}
  for x = 1, map.size do
    map.biome[x] = {}
    for y = 1, map.size do
      if lume.distance(x, y, map.size / 2, map.size / 2, true) < radius_squared then
        local altitude = map.altitude[x][y]
        local temperature = map.temperature[x][y]
        local rainfall = map.rainfall[x][y]

        -- modify temperature based on altitude
        if altitude > 0 then
          temperature = temperature - 0.0065 * altitude  -- -6.5 C / km
        else
          temperature = temperature + altitude / 1000 -- -1 C / km (deep water adjustment)
        end
        -- modify temperature by latitude
        local polar_distance = math.abs(y - (map.size / 2))^1.3 -- experimenting with slight non-linearity
        local max_temperature_delta = 10
        temperature = temperature - (polar_distance / map.size * 2 * max_temperature_delta)
        -- 'save' temperature changes
        map.temperature[x][y] = temperature

        -- choose biome
        for _, order in ipairs(biome_order) do
          local biome = biomes[order]
          if altitude >= biome.altitude.min and altitude <= biome.altitude.max then
            if temperature >= biome.temperature.min and temperature <= biome.temperature.max then
              if rainfall >= biome.rainfall.min and rainfall <= biome.rainfall.max then
                map.biome[x][y] = biome.color
                break
              end
            end
          end
        end
      else
        -- clear non-planet zones
        map.altitude[x][y] = nil
        map.temperature[x][y] = nil
        map.rainfall[x][y] = nil
      end
    end
  end

  return map
end

return {
  generate = generate,
}