{ pkgs }:

let
  firmwarePartition = {
    offset = 8;
    id = "0xfeed3425";
    name = "FIRMWARE";
    size = 512;
  };

  rootfsImage = import ./rootfs.nix { inherit pkgs; };

  configTxt = pkgs.writeText "config.txt" (builtins.readFile ./config.txt);
  populateFirmwareCommands = ''
    # Add config.txt
    cp ${configTxt} firmware/config.txt

    # Copy the firmware files
    cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/bootcode.bin firmware/
    cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/fixup*.dat firmware/
    cp ${pkgs.raspberrypifw}/share/raspberrypi/boot/start*.elf firmware/

    # Add pi3 specific files
    cp ${pkgs.ubootRaspberryPi3_64bit}/u-boot.bin firmware/u-boot-rpi3.bin
  '';
in
  pkgs.stdenv.mkDerivation {
    name = "orange";

    nativeBuildInputs = with pkgs; [ dosfstools e2fsprogs libfaketime mtools util-linux zstd ];

    buildCommand = ''
      img=./sd-card.img
      root_fs=./rootfs.img
      zstd -d --no-progress "${rootfsImage}" -o $root_fs

      # Gap in front of the first partition, in MiB
      gap=${toString firmwarePartition.offset}

      # Create the image file sized to fit /boot/firmware and /, plus slack for the gap.
      rootSizeBlocks=$(du -B 512 --apparent-size $root_fs | awk '{ print $1 }')
      firmwareSizeBlocks=$((${toString firmwarePartition.size} * 1024 * 1024 / 512))
      imageSize=$((rootSizeBlocks * 512 + firmwareSizeBlocks * 512 + gap * 1024 * 1024))
      truncate -s $imageSize $img

      # type=b is 'W95 FAT32', type=83 is 'Linux'.
      # The "bootable" partition is where u-boot will look file for the bootloader
      # information (dtbs, extlinux.conf file).
      sfdisk $img <<EOF
        label: dos
        label-id: ${firmwarePartition.id}
        start=''${gap}M, size=$firmwareSizeBlocks, type=b
        start=$((gap + ${toString firmwarePartition.size}))M, type=83, bootable
      EOF

      # Copy the rootfs into the SD image
      eval $(partx $img -o START,SECTORS --nr 2 --pairs)
      dd conv=notrunc if=$root_fs of=$img seek=$START count=$SECTORS

      # Create a FAT32 /boot/firmware partition of suitable size into firmware_part.img
      eval $(partx $img -o START,SECTORS --nr 1 --pairs)
      truncate -s $((SECTORS * 512)) firmware_part.img
      mkfs.vfat --invariant -i ${toString firmwarePartition.size} -n ${firmwarePartition.name} firmware_part.img

      # Populate the files intended for /boot/firmware
      mkdir firmware
      ${populateFirmwareCommands}
      find firmware -exec touch --date=2000-01-01 {} +

      # Copy the populated /boot/firmware into the SD image
      cd firmware
      # Force a fixed order in mcopy for better determinism, and avoid file globbing
      for d in $(find . -type d -mindepth 1 | sort); do
      faketime "2000-01-01 00:00:00" mmd -i ../firmware_part.img "::/$d"
      done
      for f in $(find . -type f | sort); do
      mcopy -pvm -i ../firmware_part.img "$f" "::/$f"
      done
      cd ..

      # Verify the FAT partition before copying it.
      fsck.vfat -vn firmware_part.img

      dd conv=notrunc if=firmware_part.img of=$img seek=$START count=$SECTORS

      mkdir $out
      zstd -T$NIX_BUILD_CORES $img -o $out/orange.img.zstd
    '';
  }