module BomRadarPng

# File lib/bom_radar_png.rb, line 80
def apply_filter(filter, row, previous)
  case filter
  when 0
    row
  when 1
    row.each_with_index.map { |value, index| (value + (index.positive? ? row[index - 1] : 0)) % 256 }
  when 2
    return row unless previous

    row.each_with_index.map { |value, index| (value + previous[index]) % 256 }
  else
    row
  end
end

# File lib/bom_radar_png.rb, line 12
def decode_indices(bytes)
  data = bytes.to_s.b
  raise ArgumentError, "not a PNG" unless data.start_with?("\x89PNG\r\n\x1a\n".b)

  width = nil
  height = nil
  bit_depth = nil
  color_type = nil
  palette = []
  idat = +"".b
  pos = 8

  while pos < data.bytesize
    length = data[pos, 4].unpack1("N")
    type = data[pos + 4, 4]
    chunk = data[pos + 8, length]
    pos += 12 + length

    case type
    when "IHDR"
      width, height, bit_depth, color_type = chunk.unpack("NNCC")
    when "PLTE"
      palette = chunk.bytes.each_slice(3).map { |rgb| rgb }
    when "IDAT"
      idat << chunk
    when "IEND"
      break
    end
  end

  raise ArgumentError, "unsupported PNG" unless width && height && color_type == 3

  inflated = Zlib::Inflate.inflate(idat)
  grid = unpack_scanlines(inflated, width: width, height: height, bit_depth: bit_depth)
  { width: width, height: height, grid: grid, palette: palette }
end

# File lib/bom_radar_png.rb, line 67
def unpack_row(row_bytes, width:, bit_depth:)
  if bit_depth == 2
    row_bytes.bytes.flat_map { |byte| [byte >> 6, (byte >> 4) & 3, (byte >> 2) & 3, byte & 3] }.first(width)
  elsif bit_depth == 4
    nibbles = row_bytes.bytes.flat_map { |byte| [byte >> 4, byte & 0x0F] }
    nibbles.first(width)
  elsif bit_depth == 8
    row_bytes.bytes.first(width)
  else
    raise ArgumentError, "unsupported bit depth #{bit_depth}"
  end
end

# File lib/bom_radar_png.rb, line 49
def unpack_scanlines(inflated, width:, height:, bit_depth:)
  bytes_per_pixel = (bit_depth * width / 8.0).ceil
  stride = 1 + bytes_per_pixel
  offset = 0
  grid = []

  height.times do
    filter = inflated.getbyte(offset)
    row_bytes = inflated.byteslice(offset + 1, bytes_per_pixel)
    offset += stride
    row = unpack_row(row_bytes, width: width, bit_depth: bit_depth)
    row = apply_filter(filter, row, grid.last) if grid.any?
    grid << row
  end

  grid
end