mobile-nixos/lib/image-builder/makeFAT32.nix

{ lib, imageBuilder, dosfstools, mtools, libfaketime}:

/*  */ let scope = { "fileSystem.makeFAT32" =

let
  inherit (lib.strings) splitString;
  inherit (imageBuilder) makeFilesystem;
  # The default from `mkfs.fat`.
  reservedSectors = 32;
  # The default from `mkfs.fat`.
  hiddenSectors = 0;

  # The default from `mkfs.fat`.
  numberOfFats = 2;
  # Extra padding per FAT, a constant in code
  fatPadding = 4;

  # I have not been able to validate that it could be different from 1 for FAT32.
  # It seems the different values (e.g. 4) are for FAT12 and FAT16.
  # This is the only "bad" assumption here.
  clusterSize = 1;

  # Bash doesn't do floating point representations. Multiplications and divisions
  # are handled with enough precision that we can multiply and divide to get a precision.
  precision = 1000;

  first = list: lib.lists.last (lib.lists.reverseList list);
  chopDecimal = f: first (splitString "." (toString f));
in
{ partitionID
# These defaults are assuming small~ish FAT32 filesystems are generated.
, blockSize ? 512
, sectorSize ? 512
, ... } @ args:
makeFilesystem (args // {
  # FAT32 can be used for ESP. Let's make this obvious.
  filesystemType = if args ? filesystemType then args.filesystemType else "FAT32";

  inherit blockSize sectorSize;
  minimumSize = imageBuilder.size.KiB 500;

  nativeBuildInputs = [
    libfaketime
    dosfstools
    mtools
  ];

  computeMinimalSize = ''
    # `local size` is in bytes.

    # This amount is a static amount of reserved space.
    local static_reserved=${toString ( (reservedSectors + hiddenSectors) * sectorSize )}

    # This is a constant representing the relative reserved space ratio.
    local relative_reserved=${
      chopDecimal (
        precision - (
          1.0 * sectorSize / ((clusterSize * sectorSize) + (numberOfFats * fatPadding))
          # ^ forces floating point
        ) * precision
      )
    }
    # Rounds up the likely truncated result. At worst it's a bit more space.
    (( relative_reserved++ ))

    echo "static_reserved=$static_reserved" 1>&2
    echo "relative_reserved=$relative_reserved" 1>&2

    local reservedSize=$(( (static_reserved + size) * relative_reserved / ${toString precision} + static_reserved ))

    echo -n "Adding reservedSize: $size + $reservedSize = " 1>&2
    size=$((size + reservedSize))
    echo "$size" 1>&2
  '';

  filesystemPhase = ''
    faketime -f "1970-01-01 00:00:00" mkfs.vfat \
      -R ${toString reservedSectors} \
      -h ${toString hiddenSectors} \
      -s ${toString (blockSize / sectorSize)} \
      -S ${toString sectorSize} \
      -i $partitionID \
      -n $partName \
      "$img"
  '';

  copyPhase = ''
    for f in ./* ./.*; do
      if [[ "$f" != "./." && "$f" != "./.." ]]; then
        faketime -f "1970-01-01 00:00:00" \
          mcopy -psv -i "$img" "$f" ::
      fi
    done
  '';

  checkPhase = ''
    # Always verify FS
    fsck.vfat -vn "$img"
  '';
})

/*  */ ;}; in scope."fileSystem.makeFAT32"