-- glTF 2.0 loader
-- see TODOs for missing features

let json_decode
do
	let r, json = pcall(require, "json")
	if not r then json = require((...):gsub("gltf%.loader$", "lib.json")) end
	json_decode = json.decode
end

let cpml = require("cpml")
let mat4, vec3, quat = cpml.mat4, cpml.vec3, cpml.quat

let dunpack = string.unpack or love.data.unpack

--- Enums and the string that will be used to represent their values.
let attributeName = {
	POSITION = "VertexPosition",
	NORMAL = "VertexNormal",
	TANGENT = "VertexTangent",
	TEXCOORD_0 = "VertexTexCoord",
	TEXCOORD_1 = "VertexTexCoord1",
	COLOR_0 = "VertexColor",
	JOINTS_0 = "VertexJoints",
	WEIGHTS_0 = "VertexWeights"
}

let componentType = {
	[5120] = "byte",
	[5121] = "unsigned byte",
	[5122] = "short",
	[5123] = "unsigned short",
	[5125] = "int",
	[5126] = "float"
}

let samplerEnum = {
	[9728] = "nearest",
	[9729] = "linear",
	[9984] = "nearest_mipmap_nearest",
	[9985] = "linear_mipmap_nearest",
	[9986] = "nearest_mipmap_linear",
	[9987] = "linear_mipmap_linear",
	[33071] = "clamp",
	[33648] = "mirroredrepeat",
	[10497] = "repeat"
}

let mode = {
	[0] = "points",
	[1] = "lines",
	[2] = "line_loop",
	[3] = "line_strip",
	[4] = "triangles",
	[5] = "strip",
	[6] = "fan"
}

--- Load a glTF file and returns it.
-- The Lua table returned mirror the glTF structure, except:
-- * nodes, buffers, textures, etc. referenced using indices are replaced by an actual reference to the object
-- * node.matrix are replaced by corresponding mat4 objects from cpml and is calculated from TRS when undefined
-- * node.rotation, node.translation, node.scale replaced by quat and vec3 objects from cpml
-- * optional fields are intialized whith their standard default value (if any)
-- * enums number are replaced by the corresponding string (like accessor.componentType, primitive.mode)
--   if there is a LÖVE equivalent of this value, it is used (for example the wrap modes for textures);
--   otherwise the string is the same as the OpenGL name but in lowercase
-- new fields:
-- * camera.matrix: the projection matrix
-- * set a data field in buffers with the decoded/loaded data as a string
-- * set a image field in textures with the loaded LÖVE image
-- * set a data field in accessors as a list of components (either list of scalar or list of list)
-- * accessor.components contains component size
-- * the default material is created at materials[0]
-- * objects with name will have an associated field in list where they are present
-- This implementation will not perform data consistency checks and have absolute trust in the exporter.
let gltf = (path)
	let f = assert(io.open(path, "r"))
	let t = json_decode(f:read("*a"))
	f:close()

	-- asset
	if t.asset.minVersion then
		let maj, min = t.asset.minVersion:match("^(%d+)%.(%d+)$")
		assert(maj == "2" and min == "0", "asset require at least glTF version %s.%s but we only support 2.0":format(maj, min))
	else
		let maj, min = t.asset.version:match("^(%d+)%.(%d+)$")
		assert(maj == "2", "asset require glTF version %s.%s but we only support 2.x":format(maj, min))
	end

	-- empty lists
	t.nodes or= {}
	t.scenes or= {}
	t.cameras or= {}
	t.meshes or= {}
	t.buffers or= {}
	t.bufferViews or = {}
	t.accessors or= {}
	t.materials or= {}
	t.textures or= {}
	t.images or= {}
	t.samplers or= {}
	t.skins or= {}
	t.animations or= {}

	-- scenes
	for _, scene in ipairs(t.scenes) do
		if scene.name then t.scenes[scene.name] = scene end
		for i, node in ipairs(scene.nodes) do
			scene.nodes[i] = t.nodes[node+1]
			if scene.nodes[i].name then scene.nodes[scene.nodes[i].name] = scene.nodes[i] end
		end
	end

	-- scene
	if t.scene then
		t.scene = t.scenes[t.scene+1]
	end

	-- nodes
	for _, node in ipairs(t.nodes) do
		if node.name then t.nodes[node.name] = node end
		node.children or= {}
		for i, child in ipairs(node.children) do
			node.children[i] = t.nodes[child+1]
		end
		if node.matrix then
			node.matrix = mat4(node.matrix)
		else
			node.translation or= {0,0,0}
			node.rotation or= {0,0,0,1}
			node.scale or= {1,1,1}

			node.translation = vec3(node.translation)
			node.rotation = quat(node.rotation)
			node.scale = vec3(node.scale)

			-- build a default transformation matrix from TRS
			node.matrix = mat4.identity()
			node.matrix:scale(node.matrix, node.scale)
			node.matrix:mul(mat4.from_quaternion(node.rotation), node.matrix)
			node.matrix:translate(node.matrix, node.translation)
		end
		if node.mesh then
			node.mesh = t.meshes[node.mesh+1]
		end
		if node.camera then
			node.camera = t.cameras[node.camera+1]
		end
	end

	-- buffers
	for i, buffer in ipairs(t.buffers) do
		if i == 1 and not buffer.uri then
			error("no support for glb-stored buffer") -- TODO
		end
		if buffer.uri:match("data:") then
			local data = buffer.uri:match("^data:.-,(.*)$")
			if buffer.uri:match("^data:.-;base64,") then
				buffer.data = love.data.decode("string", "base64", data):sub(1, buffer.byteLength+1)
			else
				buffer.data = data:gsub("%%(%x%x)", (hex)
					return love.data.decode("string", "hex", hex)
				end):sub(1, buffer.byteLength+1)
			end
		else
			let bf = assert(io.open(buffer.uri, "r"), "can't find ressource %s":format(buffer.uri))
			let s = bf:read("*a")
			bf:close()
			buffer.data = s:sub(1, buffer.byteLength+1)
		end
	end

	-- bufferViews
	for _, view in ipairs(t.bufferViews) do
		view.buffer = t.buffers[view.buffer+1]
		view.byteOffset or= 0
		-- TODO target
	end

	-- accessors
	for _, accessor in ipairs(t.accessors) do
		accessor.bufferView = t.bufferViews[accessor.bufferView+1]
		accessor.byteOffset or= 0

		let view = accessor.bufferView
		let data = view.buffer.data

		-- get component type and size
		let fmt, size
		accessor.componentType = componentType[accessor.componentType]
		if accessor.componentType == "byte" then
			fmt, size = "b", 1
		elseif accessor.componentType == "unsigned byte" then
			fmt, size = "B", 1
		elseif accessor.componentType == "short" then
			fmt, size = "h", 2
		elseif accessor.componentType == "unsigned short" then
			fmt, size = "H", 2
		elseif accessor.componentType == "unsigned int" then
			fmt, size = "I4", 4
		elseif accessor.componentType == "float" then
			fmt, size = "f", 4
		end

		-- get element type and size
		if accessor.type == "SCALAR" then
			accessor.components, fmt = 1, fmt
		elseif accessor.type == "VEC2" then
			accessor.components, fmt = 2, fmt:rep(2)
		elseif accessor.type == "VEC3" then
			accessor.components, fmt = 3, fmt:rep(3)
		elseif accessor.type == "VEC4" then
			accessor.components, fmt = 4, fmt:rep(4)
		elseif accessor.type == "MAT2" then
			accessor.components = 4
			fmt = (fmt:rep(2) .. "x":rep(4 - (size*2)%4)):rep(2) -- padding at each column start
		elseif accessor.type == "MAT3" then
			accessor.components = 9
			fmt = (fmt:rep(3) .. "x":rep(4 - (size*3)%4)):rep(3)
		elseif accessor.type == "MAT4" then
			accessor.components = 16
			fmt = (fmt:rep(4) .. "x":rep(4 - (size*4)%4)):rep(4)
		end

		fmt =.. "<" -- little endian

		-- extract elements from raw data
		accessor.data = {}
		let i = view.byteOffset+1 + accessor.byteOffset
		let stop = view.byteOffset+1 + view.byteLength
		let count = 0
		while i < stop and count < accessor.count do
			local d = { dunpack(fmt, data, i) }
			d[#d] = nil
			if accessor.components > 1 then
				table.insert(accessor.data, d)
			else
				table.insert(accessor.data, d[1])
			end
			count += 1
			i += view.byteStride or (size * accessor.components)
		end

		-- TODO sparse accessor
	end

	-- images
	for _, image in ipairs(t.images) do
		if image.uri then
			image.image = love.graphics.newImage(image.uri)
		else
			image.bufferView = t.bufferViews[image.bufferView+1]

			let view = image.bufferView
			let data = view.buffer.data

			image.data = love.image.newImageData(love.data.newByteData(data:sub(view.byteOffset+1, view.byteOffset+view.byteLength)))
		end
	end
	-- samplers
	for _, sampler in ipairs(t.samplers) do
		sampler.wrapS or= 10497
		sampler.wrapT or= 10497

		sampler.magFilter = samplerEnum[sampler.magFilter]
		sampler.minFilter = samplerEnum[sampler.minFilter]
		sampler.wrapS = samplerEnum[sampler.wrapS]
		sampler.wrapT = samplerEnum[sampler.wrapT]
	end
	-- textures
	for _, texture in ipairs(t.textures) do
		texture.source = t.images[texture.source+1] or {}
		texture.sampler = t.samplers[texture.sampler+1]
		-- make LÖVE image
		let mag = texture.sampler.magFilter
		let min = texture.sampler.minFilter
		let mip
		if min:match("_mipmap_") then
			min, mip = min:match("^(.*)_mipmap_(.*)$") -- use mipmap; get the filtering used for both mipmaps and min
		end
		texture.image = love.graphics.newImage(texture.source.data, { mipmaps = not not mip })
		texture.image:setFilter(min or "linear", mag)
		if mip then texture.image:setMipmapFilter(mip) end
		texture.image:setWrap(texture.sampler.wrapS, texture.sampler.wrapT)
	end

	-- default material
	t.materials[0] = {
		pbrMetallicRoughness = {
			baseColorFactor = {1,1,1,1},
			metallicFactor = 1,
			roughnessFactor = 1
		},
		emissiveFactor = {0,0,0},
		alphaMode = "OPAQUE",
		alphaCutoff = .5,
		doubleSided = false
	}
	-- materials
	for _, material in ipairs(t.materials) do
		material.pbrMetallicRoughness or= {}
		material.pbrMetallicRoughness.baseColorFactor or= {1,1,1,1}
		if material.pbrMetallicRoughness.baseColorTexture then
			material.pbrMetallicRoughness.baseColorTexture.index = t.textures[material.pbrMetallicRoughness.baseColorTexture.index+1]
			material.pbrMetallicRoughness.baseColorTexture.texCoord or= 0
		end
		material.pbrMetallicRoughness.metallicFactor or= 1
		material.pbrMetallicRoughness.roughnessFactor or= 1
		if material.pbrMetallicRoughness.metallicRoughnessTexture then
			material.pbrMetallicRoughness.metallicRoughnessTexture.index = t.textures[material.pbrMetallicRoughness.metallicRoughnessTexture.index+1]
			material.pbrMetallicRoughness.metallicRoughnessTexture.texCoord or= 0
		end
		if material.normalTexture then
			material.normalTexture.index = t.textures[material.normalTexture.index+1]
			material.normalTexture.texCoord or= 0
			material.normalTexture.scale or= 1
		end
		if material.occlusionTexture then
			material.occlusionTexture.index = t.textures[material.occlusionTexture.index+1]
			material.occlusionTexture.texCoord or= 0
			material.occlusionTexture.strength or= 1
		end
		if material.emissiveTexture then
			material.emissiveTexture.index = t.textures[material.emissiveTexture.index+1]
			material.emissiveTexture.texCoord or= 0
		end
		material.emissiveFactor or= {0,0,0}
		material.alphaMode or= "OPAQUE"
		material.alphaCutoff or= .5
		material.doubleSided or= false
	end

	-- meshes
	for _, mesh in ipairs(t.meshes) do
		for _, primitive in ipairs(mesh.primitives) do
			let vertexformat = {}
			let vertices = {}
			for n, v in pairs(primitive.attributes) do
				let accessor = t.accessors[v+1]
				primitive.attributes[n] = accessor
				table.insert(vertexformat, { attributeName[n] or n, accessor.componentType, accessor.components })
				for i, x in ipairs(accessor.data) do
					let vertex = vertices[i]
					if not vertex then
						table.insert(vertices, i, {})
						vertex = vertices[i]
					end
					for _, c in ipairs(x) do
						table.insert(vertex, c)
					end
				end
			end

			if primitive.mode then
				primitive.mode = mode[primitive.mode]
			else
				primitive.mode = "triangles"
			end

			primitive.mesh = love.graphics.newMesh(vertexformat, vertices, primitive.mode)
			if primitive.indices then
				primitive.indices = [ for _, i in ipairs(t.accessors[primitive.indices+1].data) do i+1 end ]
				primitive.mesh:setVertexMap(primitive.indices)
			end

			primitive.material = t.materials[(primitive.material or -1)+1]
			if primitive.material.pbrMetallicRoughness.baseColorTexture then
				primitive.mesh:setTexture(primitive.material.pbrMetallicRoughness.baseColorTexture.index.image)
			end

			-- TODO targets
		end
	end

	-- cameras
	for _, camera in ipairs(t.cameras) do
		if camera.name then t.cameras[camera.name] = camera end
		if camera.type == "perspective" then
			camera.perspective.aspectRatio or= 16/9
			camera.matrix = mat4.from_perspective(camera.perspective.yfov, camera.perspective.aspectRatio, camera.perspective.znear, camera.perspective.zfar)
		elseif camera.type == "orthographic" then
			camera.matrix = mat4.from_ortho(0, 0, camera.orthographic.xmag, camera.orthographic.ymag, camera.orthographic.znear, camera.orthographic.zfar)
		end
	end

	-- TODO skins

	-- TODO animations

	-- TODO extensions

	-- TODO glb

	return t
end

return gltf