# frozen_string_literal: true

require "zlib"

module MoonModel
  class DataSet
    MAGIC = "MOONDATA1\n".b
    DATUM_RADIUS_M = 1_737_400.0

    attr_reader :width, :height, :metadata

    def self.default_path
      File.expand_path("../../data/moon_surface.bin.gz", __dir__)
    end

    def initialize(path = self.class.default_path)
      raise ArgumentError, "lunar data asset not found: #{path}" unless File.file?(path)

      raw = Zlib::GzipReader.open(path, &:read)
      raise ArgumentError, "unrecognized lunar data asset" unless raw.start_with?(MAGIC)

      offset = MAGIC.bytesize
      header_length = raw.byteslice(offset, 4).unpack1("L<")
      offset += 4
      @metadata = JSON.parse(raw.byteslice(offset, header_length))
      offset += header_length
      @width = metadata.fetch("width")
      @height = metadata.fetch("height")
      elevation_bytes = width * height * 2
      @elevation = raw.byteslice(offset, elevation_bytes).unpack("s<*")
      @reflectance = raw.byteslice(offset + elevation_bytes, width * height).bytes
    end

    def elevation_m(latitude, longitude)
      DATUM_RADIUS_M + sample(@elevation, latitude, longitude)
    end

    def reflectance(latitude, longitude)
      value = sample(@reflectance, latitude, longitude)
      low, high = metadata.fetch("reflectance_percentiles", [0.0, 255.0])
      [[(value - low) / (high - low), 0.0].max, 1.0].min
    end

    def relief_range_m
      minmax = metadata["elevation_offset_range_m"]
      minmax ? minmax[1] - minmax[0] : @elevation.minmax.then { |a, b| b - a }
    end

    private

    def sample(values, latitude, longitude)
      x = (longitude % 360.0) / 360.0 * width
      y = (90.0 - [[latitude, 90.0].min, -90.0].max) / 180.0 * (height - 1)
      x0 = x.floor % width
      x1 = (x0 + 1) % width
      y0 = y.floor
      y1 = [y0 + 1, height - 1].min
      tx = x - x.floor
      ty = y - y.floor
      a = values[y0 * width + x0] * (1.0 - tx) + values[y0 * width + x1] * tx
      b = values[y1 * width + x0] * (1.0 - tx) + values[y1 * width + x1] * tx
      a * (1.0 - ty) + b * ty
    end
  end
end