type
  GltfNode* = object
    children*: seq[int]
    mesh*: int = -1
    transform*: Mat4 = Unit4
  GltfData*[TMesh, TMaterial] = object
    scenes*: seq[seq[int]] # each scene has a seq of node indices
    nodes*: seq[GltfNode]  # each node has a seq of mesh indices
    meshes*: seq[seq[(TMesh, VkPrimitiveTopology)]]
    materials*: seq[TMaterial]
    textures*: seq[Image[BGRA]]
  glTFHeader = object
    magic: uint32
    version: uint32
    length: uint32
  glTFData = object
    structuredContent: JsonNode
    binaryBufferData: seq[uint8]

  MaterialAttributeNames* = object
    # pbr
    baseColorTexture*: string
    baseColorTextureUv*: string
    baseColorFactor*: string
    metallicRoughnessTexture*: string
    metallicRoughnessTextureUv*: string
    metallicFactor*: string
    roughnessFactor*: string

    # other
    normalTexture*: string
    normalTextureUv*: string
    occlusionTexture*: string
    occlusionTextureUv*: string
    emissiveTexture*: string
    emissiveTextureUv*: string
    emissiveFactor*: string

  MeshAttributeNames* = object
    POSITION*: string
    NORMAL*: string
    TANGENT*: string
    TEXCOORD*: seq[string]
    COLOR*: seq[string]
    JOINTS*: seq[string]
    WEIGHTS*: seq[string]
    indices*: string
    material*: string

const
  HEADER_MAGIC = 0x46546C67
  JSON_CHUNK = 0x4E4F534A
  BINARY_CHUNK = 0x004E4942
  SAMPLER_FILTER_MODE_MAP = {
    9728: VK_FILTER_NEAREST,
    9729: VK_FILTER_LINEAR,
    9984: VK_FILTER_NEAREST,
    9985: VK_FILTER_LINEAR,
    9986: VK_FILTER_NEAREST,
    9987: VK_FILTER_LINEAR,
  }.toTable
  SAMPLER_WRAP_MODE_MAP = {
    33071: VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE,
    33648: VK_SAMPLER_ADDRESS_MODE_MIRRORED_REPEAT,
    10497: VK_SAMPLER_ADDRESS_MODE_REPEAT
  }.toTable
  PRIMITIVE_MODE_MAP = [
    0: VK_PRIMITIVE_TOPOLOGY_POINT_LIST,
    1: VK_PRIMITIVE_TOPOLOGY_LINE_LIST,
    2: VK_PRIMITIVE_TOPOLOGY_LINE_STRIP, # not correct, as mode 2 would be a loo, but vulkan has no concept of this
    3: VK_PRIMITIVE_TOPOLOGY_LINE_STRIP,
    4: VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
    5: VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP,
    6: VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN,
  ]

proc getBufferViewData(bufferView: JsonNode, mainBuffer: seq[uint8],
    baseBufferOffset = 0): seq[uint8] =
  assert bufferView["buffer"].getInt() == 0, "Currently no external buffers supported"

  result = newSeq[uint8](bufferView["byteLength"].getInt())
  let bufferOffset = bufferView["byteOffset"].getInt() + baseBufferOffset
  var dstPointer = addr result[0]

  if bufferView.hasKey("byteStride"):
    raise newException(Exception, "Unsupported feature: byteStride in buffer view")
  copyMem(dstPointer, addr mainBuffer[bufferOffset], result.len)

proc componentTypeId(t: typedesc): int =
  if t is int8: return 5120
  elif t is uint8: return 5121
  elif t is int16: return 5122
  elif t is uint16: return 5123
  elif t is uint32: return 5125
  elif t is float32: return 5126

proc componentTypeName(id: int): string =
  if id == 5120: return int8.name
  elif id == 5121: return uint8.name
  elif id == 5122: return int16.name
  elif id == 5123: return uint16.name
  elif id == 5125: return uint32.name
  elif id == 5126: return float32.name

proc getAccessorData[T](root: JsonNode, accessor: JsonNode, mainBuffer: seq[uint8]): seq[T] =
  if accessor.hasKey("sparse"):
    raise newException(Exception, "Sparce accessors are currently not supported")

  let componentType = accessor["componentType"].getInt()
  let itemType = accessor["type"].getStr()

  when T is TVec or T is TMat:
    assert componentTypeId(elementType(default(T))) == componentType, "Requested type '" & name(elementType(default(T))) & $componentTypeId(elementType(default(T))) & "' but actual type is '" & componentTypeName(componentType) & "'"
  else:
    assert componentTypeId(T) == componentType, "Requested type '" & name(T) & "' but actual type is '" & componentTypeName(componentType) & "'"

  when T is TVec:
    when len(default(T)) == 2: assert itemType == "VEC2"
    elif len(default(T)) == 3: assert itemType == "VEC3"
    elif len(default(T)) == 4: assert itemType == "VEC4"
  elif T is TMat:
    when T is Mat2: assert itemType == "MAT2"
    elif T is Mat3: assert itemType == "MAT3"
    elif T is Mat4: assert itemType == "MAT4"
  else:
    assert itemType == "SCALAR"

  result.setLen(accessor["count"].getInt())

  let bufferView = root["bufferViews"][accessor["bufferView"].getInt()]
  assert bufferView["buffer"].getInt() == 0, "Currently no external buffers supported"
  let accessorOffset = if accessor.hasKey("byteOffset"): accessor["byteOffset"].getInt() else: 0
  let bufferOffset = (if "byteOffset" in bufferView: bufferView["byteOffset"].getInt() else: 0) + accessorOffset
  var dstPointer = result.ToCPointer()

  if bufferView.hasKey("byteStride"):
    warn "Congratulations, you try to test a feature (loading buffer data with stride attributes) that we have no idea where it is used and how it can be tested (need a coresponding *.glb file)."
    # we don't support stride, have to convert stuff here... does this even work?
    for i in 0 ..< result.len:
      copyMem(dstPointer, addr(mainBuffer[bufferOffset + i * bufferView["byteStride"].getInt()]), sizeof(T))
      dstPointer = cast[typeof(dstPointer)](cast[uint](dstPointer) + sizeof(T).uint)
  else:
    copyMem(dstPointer, addr(mainBuffer[bufferOffset]), result.len * sizeof(T))

proc loadTexture(root: JsonNode, textureNode: JsonNode, mainBuffer: seq[
    uint8]): Image[BGRA] =

  let imageIndex = textureNode["source"].getInt()

  if root["images"][imageIndex].hasKey("uri"):
    raise newException(Exception, "Unsupported feature: Cannot load images from external files")
  let imageType = root["images"][imageIndex]["mimeType"].getStr()
  assert imageType == "image/png", "glTF loader currently only supports PNG, but found '" & imageType & "'"

  let bufferView = root["bufferViews"][root["images"][imageIndex][
      "bufferView"].getInt()]
  result = LoadImageData[BGRA](getBufferViewData(bufferView, mainBuffer))

  if textureNode.hasKey("sampler"):
    let sampler = root["samplers"][textureNode["sampler"].getInt()]
    if sampler.hasKey("magFilter"):
      result.magInterpolation = SAMPLER_FILTER_MODE_MAP[sampler[
          "magFilter"].getInt()]
    if sampler.hasKey("minFilter"):
      result.minInterpolation = SAMPLER_FILTER_MODE_MAP[sampler[
          "minFilter"].getInt()]
    if sampler.hasKey("wrapS"):
      result.wrapU = SAMPLER_WRAP_MODE_MAP[sampler["wrapS"].getInt()]
    if sampler.hasKey("wrapT"):
      result.wrapV = SAMPLER_WRAP_MODE_MAP[sampler["wrapT"].getInt()]

proc getVec4f(node: JsonNode): Vec4f =
  NewVec4f(node[0].getFloat(), node[1].getFloat(), node[2].getFloat(), node[
      3].getFloat())

proc loadMaterial[TMaterial](
  root: JsonNode,
  materialNode: JsonNode,
  mapping: static MaterialAttributeNames,
  mainBuffer: seq[uint8],
): TMaterial =
  result = TMaterial()

  let pbr = materialNode["pbrMetallicRoughness"]
  for name, value in fieldPairs(result):
    for gltfAttribute, mappedName in fieldPairs(mapping):
      when gltfAttribute != "" and name == mappedName:
        if pbr.hasKey(gltfAttribute):
          when gltfAttribute.endsWith("Texture"):
            value = typeof(value)(pbr[gltfAttribute]["index"].getInt())
          elif gltfAttribute.endsWith("TextureUv"):
            value = typeof(pbr[gltfAttribute[0 ..< ^2]]["index"].getInt())
          elif gltfAttribute in ["baseColorFactor", "emissiveFactor"]:
            value = pbr[gltfAttribute].getVec4f()
          elif gltfAttribute in ["metallicFactor", "roughnessFactor"]:
            value = pbr[gltfAttribute].getFloat()
          else:
            {.error: "Unsupported gltf material attribute".}

proc loadPrimitive[TMesh](
  root: JsonNode,
  primitive: JsonNode,
  mapping: static MeshAttributeNames,
  mainBuffer: seq[uint8]
): (TMesh, VkPrimitiveTopology) =
  result[0] = TMesh()
  result[1] = VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST
  if primitive.hasKey("mode"):
    result[1] = PRIMITIVE_MODE_MAP[primitive["mode"].getInt()]

  if primitive.hasKey("indices"):
    assert mapping.indices != "", "Mesh requires indices"

  for resultFieldName, resultValue in fieldPairs(result[0]):
    for gltfAttribute, mappedName in fieldPairs(mapping):
      when typeof(mappedName) is seq:
        when resultFieldName in mappedName:
          var i = 0
          for mappedIndexName in mappedName:
            if gltfAttribute != "" and resultFieldName == mappedIndexName:
              assert resultValue is GPUData, "Attribute " & resultFieldName & " must be of type GPUData"
              let gltfAttributeIndexed = gltfAttribute & "_" & $i
              if primitive["attributes"].hasKey(gltfAttributeIndexed):
                let accessor = primitive["attributes"][gltfAttributeIndexed].getInt()
                resultValue.data = getAccessorData[elementType(resultValue.data)](root, root["accessors"][accessor], mainBuffer)
          inc i
      elif typeof(mappedName) is string:
        when resultFieldName == mappedName:
          assert resultValue is GPUData or gltfAttribute == "material", "Attribute " & resultFieldName & " must be of type GPUData"
          when gltfAttribute == "indices":
            if primitive.hasKey(gltfAttribute):
              let accessor = primitive[gltfAttribute].getInt()
              resultValue.data = getAccessorData[elementType(resultValue.data)](root, root["accessors"][accessor], mainBuffer)
          elif gltfAttribute == "material":
            if primitive.hasKey(gltfAttribute): # assuming here that materials IDs are a normal field on the mesh, not GPUData
              resultValue = typeof(resultValue)(primitive[gltfAttribute].getInt())
          else:
            if primitive["attributes"].hasKey(gltfAttribute):
              let accessor = primitive["attributes"][gltfAttribute].getInt()
              resultValue.data = getAccessorData[elementType(resultValue.data)](root, root["accessors"][accessor], mainBuffer)

proc loadNode(node: JsonNode): GltfNode =
  result = GltfNode()
  if "mesh" in node:
    result.mesh = node["mesh"].getInt()
  if "children" in node:
    for child in items(node["children"]):
      result.children.add child.getInt()
  if "matrix" in node:
    for i in 0 ..< node["matrix"].len:
      result.transform[i] = node["matrix"][i].getFloat()
    result.transform = result.transform.Transposed()
  else:
    var (t, r, s) = (Unit4, Unit4, Unit4)
    if "translation" in node:
      t = Translate(
        float32(node["translation"][0].getFloat()),
        float32(node["translation"][1].getFloat()),
        float32(node["translation"][2].getFloat())
      )
    if "rotation" in node:
      t = Rotate(
        float32(node["rotation"][3].getFloat()),
        NewVec3f(
          float32(node["rotation"][0].getFloat()),
          float32(node["rotation"][1].getFloat()),
          float32(node["rotation"][2].getFloat())
        )
      )
    if "scale" in node:
      t = Scale(
        float32(node["scale"][0].getFloat()),
        float32(node["scale"][1].getFloat()),
        float32(node["scale"][2].getFloat())
      )

    result.transform = t * r * s

proc ReadglTF*[TMesh, TMaterial](
  stream: Stream,
  meshAttributesMapping: static MeshAttributeNames,
  materialAttributesMapping: static MaterialAttributeNames,
): GltfData[TMesh, TMaterial] =
  var
    header: glTFHeader
    data: glTFData

  for name, value in fieldPairs(header):
    stream.read(value)

  assert header.magic == HEADER_MAGIC
  assert header.version == 2

  var chunkLength = stream.readUint32()
  assert stream.readUint32() == JSON_CHUNK
  data.structuredContent = parseJson(stream.readStr(int(chunkLength)))

  chunkLength = stream.readUint32()
  assert stream.readUint32() == BINARY_CHUNK
  data.binaryBufferData.setLen(chunkLength)
  assert stream.readData(addr data.binaryBufferData[0], int(chunkLength)) ==
      int(chunkLength)

  # check that the refered buffer is the same as the binary chunk
  # external binary buffers are not supported
  assert data.structuredContent["buffers"].len == 1
  assert not data.structuredContent["buffers"][0].hasKey("uri")
  let bufferLenDiff = int(chunkLength) - data.structuredContent["buffers"][0]["byteLength"].getInt()
  assert 0 <= bufferLenDiff and bufferLenDiff <= 3 # binary buffer may be aligned to 4 bytes

  debug "Loading mesh: ", data.structuredContent.pretty

  if "materials" in data.structuredContent:
    for materialnode in items(data.structuredContent["materials"]):
      result.materials.add loadMaterial[TMaterial](data.structuredContent,
          materialnode, materialAttributesMapping, data.binaryBufferData)

  if "textures" in data.structuredContent:
    for texturenode in items(data.structuredContent["textures"]):
      result.textures.add loadTexture(data.structuredContent, texturenode,
          data.binaryBufferData)

  if "meshes" in data.structuredContent:
    for mesh in items(data.structuredContent["meshes"]):
      var primitives: seq[(TMesh, VkPrimitiveTopology)]
      for primitive in items(mesh["primitives"]):
        primitives.add loadPrimitive[TMesh](data.structuredContent, primitive,
            meshAttributesMapping, data.binaryBufferData)
      result.meshes.add primitives

  if "nodes" in data.structuredContent:
    for node in items(data.structuredContent["nodes"]):
      result.nodes.add loadNode(node)

  if "scenes" in data.structuredContent:
    for scene in items(data.structuredContent["scenes"]):
      if "nodes" in scene:
        var nodes: seq[int]
        for nodeId in items(scene["nodes"]):
          nodes.add nodeId.getInt()
        result.scenes.add nodes

proc LoadMeshes*[TMesh, TMaterial](
  path: string,
  meshAttributesMapping: static MeshAttributeNames,
  materialAttributesMapping: static MaterialAttributeNames,
  package = DEFAULT_PACKAGE
): GltfData[TMesh, TMaterial] =
  ReadglTF[TMesh, TMaterial](
    stream = loadResource_intern(path, package = package),
    meshAttributesMapping = meshAttributesMapping,
    materialAttributesMapping = materialAttributesMapping,
  )