let loadQueue = []
let requestPool = []
let pendingRequests = new Map()
let requestCount = 0
const batchSize = 8
const debug = false

function recycledXMLHttpRequest() {
    // https://nullprogram.com/blog/2013/02/08/
    if (requestPool.length > 0) {
        return requestPool.pop()
    }
    requestCount += 1
    if (debug) console.log('create XMLHttpRequest', requestCount)
    let xhr = new XMLHttpRequest()
    return xhr
}

function load() {
    while (loadQueue.length > 0 && pendingRequests.size < batchSize) {
        let tile = loadQueue.shift()
        let [col, row, url] = tile
        let xhr = recycledXMLHttpRequest()
        xhr.onload = event => {
            let buffer = xhr.response
            postMessage({ success: true, url, col, row, buffer }, [buffer])
            pendingRequests.delete(url)
        }
        xhr.onerror = event => {
            let buffer = null
            postMessage({ success: false, url, col, row, buffer })
            pendingRequests.delete(url)
        }
        xhr.onreadystatechange = () => {
            // In local files, status is 0 upon success in Mozilla Firefox
            if (xhr.readyState === XMLHttpRequest.DONE) {
                pendingRequests.delete(url)
                requestPool.push(xhr)
                if (debug) console.log('resuse XMLHttpRequest')
            }
        }
        if (debug) console.log('open XMLHttpRequest')
        xhr.open('GET', url, true)
        xhr.responseType = 'arraybuffer'
        if (debug) console.log('send XMLHttpRequest')
        xhr.send()
        pendingRequests.set(url, xhr)
    }
    if (loadQueue.length > 0) setTimeout(load, 1000 / 120)
    else {
        if (debug) console.log('tileloader ready')
    }
}

self.onmessage = event => {
    let msg = event.data
    switch (msg.command) {
        case 'load':
            for (let tile of msg.tiles) {
                loadQueue.push(tile)
            }
            load()
            break
        case 'abort':
            loadQueue = []
            requestPool = []
            for (let xhr of pendingRequests.values()) {
                xhr.abort()
            }
            pendingRequests.clear()
            if (debug) console.log('tileloader aborted')
            break
        default:
            console.warn('Unknown worker command: ' + msg.command)
    }
}