/* building a model by postgrest
 *
 * a model is simply a cache layer between client
 * and the database behind an api
 *
 * options: {
 *   endpoint: 'resource location',
 *   selects: [
 *     {
 *       'label': 'column name',
 *       'alias': 'displayed name',
 *       'processor': v => process(v)
 *     }
 *   ]
 * }
 */

export default (options={}) => {
	// TODO
	// 1. sigular or prural

	// sanity checks
	if (typeof options.api == 'undefined' || typeof options.endpoint == 'undefined')
		throw 'api and endpoint are required options'

	// private model configs
	let _configs = {
		// url part
		api: options.api,
		endpoint: options.endpoint,
		endpoint_type: options.endpoint.startsWith('/rpc') ? 'function' : 'relation',
		// query part
		selects: options.selects,
		wheres: options.wheres,
		order: options.order
	}

	// private model cache & meta-info
	let _cache = {
		// upstream (data) cache
		data: null,
		count: null,
		upstream_limit: null,

		// local cache
		ambient: null,
		last_ambient: null,
	}

	// some random variables to make things work
	let _xhr = null
	let _promise = null

	// construct the model
	let _model = {
		// reflection methods
		configs() { return _configs },
		cache() { return _cache },
		data(offset=0, limit=Infinity) { return _cache.data && _cache.data.slice(offset, offset+limit) || [] },
		ambient_changed() {
			return _cache.ambient != _cache.last_ambient
		},
		reset() {
			let ambient_queries = [
				...(_configs.selects ? _configs.selects : []),
				...(_configs.wheres ? _configs.wheres.map(where => ({label: where.label, op: where.op, value: typeof where.value == 'function' ? where.value() : where.value})) : []),
				...(_configs.order ? _configs.order : [])
			]

			_cache.last_ambient = _cache.ambient
			_cache.ambient =JSON.stringify(ambient_queries)

			if (this.ambient_changed()) {
				_cache.data = []
				_cache.count = null
				_cache.upstream_limit = null
				_xhr = null
				//this.fully_loaded = false
			}
		},

		// main methods
		select(offset=0, limit=Infinity) {
			// normalize limit
			if (limit == Infinity)
				limit = _cache.upstream_limit || _cache.count || Infinity

			// be lazy 1: if the data is presented, return the value immediately
			if (this.data(offset, limit).length > 0 && !this.data(offset, limit).includes(undefined))
				return Promise.resolve(this.data(offset, limit))

			// be lazy 2: if there is a promise, return it if ambient matches or
			// cancel it
			if (_promise != null)
				return _promise

			// TODO
			// ambient = select + order + limit/offset
			// if ambient is changed, cancel currently loading xhr; otherwise return
			// current promise

			// now the hard work
			_promise = _configs.api.request({
				method: 'GET',
				url: _configs.endpoint,
				headers: _cache.count == null ? {
					Prefer: 'count=exact'
				} : {},
				config: xhr => _xhr = xhr,
				queries: [
					// transform model state to postgest queries
					// selects
					...(_configs.selects ? [{
						label: 'select',
						value: _configs.selects.map(select => select.alias ? select.alias + ':' + select.label : select.label).join(',')
					}] : []),

					...(_configs.wheres ? _configs.wheres : []),

					// order
					...(_configs.order ? [{
						label: 'order',
						value: _configs.order
							.map(o => [
								o.label,
								o.direction,
								o.nulls ? 'nulls'+o.nulls : ''
							].filter(a => a).join('.'))
							.join(',')
					}] : []),

					// limit/offset
					offset == 0 ? undefined : {label: 'offset', value: offset},
					limit == Infinity ? undefined : {label: 'limit', value: limit}
				]
			}).then(response => {
				// gather begin/end/count
				let [_range, _count] = _xhr.getResponseHeader('content-range').split('/')
				let [_begin, _end] = _range.split('-').map(v => ~~v)

				// update count if presented
				if (_count != '*') _cache.count = _count

				// see if an upstream limit is exposed
				if (_end - _begin + 1 < limit) _cache.upstream_limit = _end - _begin + 1

				// fill the data cache
				response.forEach((data, i) => {
					// process values
					_configs.selects && _configs.selects
						.filter(select => select.processor)
						.forEach(select => data[select.alias || select.label] = select.processor(data[select.alias || select.label]))

					// save the data
					_cache.data[_begin + i] = data
				})

				// assert offset/limit and returned range
				if (offset != _begin || _end - _begin + 1 > limit)
					throw 'The request and response data range mismatches!'
				if (_end - _begin + 1 < limit)
					console.warn('The response range is narrower than requested, probably due to an upstream hard limit.')

				// clean model state
				_promise = null
				this.reset()

				// return data
				return _cache.data.slice(_begin, _end + 1)
			})

			return _promise
		},

    select_all() {
			// be lazy 1: if the data is presented, return the value immediately
			if (this.data().length == _cache.count && !this.data().includes(undefined))
				return Promise.resolve(this.data())

			// be lazy 2: if there is a promise, return it if ambient matches or
			// cancel it
			if (_promise != null)
				return _promise

			_promise = _configs.api.request({
				method: 'POST',
				url: '/rpc/select_all',
				body: {
					endpoint: _configs.endpoint,
					selects: _configs.selects || [],
					wheres: _configs.wheres ? _configs.wheres.map(where => ({label: where.label, op: where.op, value: typeof where.value == 'function' ? where.value() : where.value})) : [],
					order: _configs.order
				}
			}).then(response => {
				_cache.count = response.length
				_cache.data = response.map(data => {
					// process values
					_configs.selects && _configs.selects
						.filter(select => select.processor)
						.forEach(select => data[select.alias || select.label] = select.processor(data[select.alias || select.label]))

					return data
				})

				// clean state
				_promise = null
				this.reset()
			})

			return _promise
		},

		export(options={}) {
			let _data = this.data()

			if (options.type == 'csv') {
				let headers = _configs.selects && _configs.selects.map(select => select.alias || select.label) || Object.keys(_data[0])

				let body = _data.map(row => headers.map(key => row[key]).join(',')).join('\n')

				_data = '\ufeff' + headers.join(',') + '\n' + body
				options.type = 'text/csv'
			}

			let blob = new Blob([_data], {type: options.type})
			let url = URL.createObjectURL(blob)

			options.filename = options.filename || _configs.endpoint
			if (options.filename_suffix) {
				if (options.filename_suffix == 'datetime')
					options.filename = options.filename + (new Date()).toLocaleTimeString(undefined, {
						year: "numeric",
						month: "2-digit",
						day: "2-digit",
						hour: "2-digit",
						minute: "2-digit",
						second: "2-digit",
						hour12: false
					})
				else if (options.filename_suffix == 'date')
					options.filename = options.filename + (new Date()).toLocaleDateString(undefined, {
						year: "numeric",
						month: "2-digit",
						day: "2-digit"
					})
			}

			let anchor = document.createElement('a')
			anchor.href = url
			anchor.target = '_blank'
			anchor.download = `${options.filename}.csv`

			anchor.click()

			URL.revokeObjectURL(url)
			anchor.remove()
		}
	}

	// initialize model
	_model.reset()
	return _model
}