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

function load() {
    while (loadQueue.length > 0 && pendingRequests.size < batchSize) {
        let tile = loadQueue.shift()
        let [col, row, url] = tile
        let xhr = new XMLHttpRequest()
        xhr.responseType = 'arraybuffer'
        xhr.onload = event => {
            pendingRequests.delete(url)
            let buffer = xhr.response
            postMessage({ success: true, url, col, row, buffer }, [buffer])
        }
        xhr.onerror = event => {
            pendingRequests.delete(url)
            let buffer = null
            postMessage({ success: false, url, col, row, buffer })
        }
        xhr.open('GET', url, true)
        xhr.send()
        pendingRequests.set(url, xhr)
    }
    if (loadQueue.length > 0) setTimeout(load, 1000 / 120)
    else {
        if (debug) console.log('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 = []
            for (let xhr of pendingRequests.values()) {
                xhr.abort()
            }
            if (debug) console.log('Abort')
            break
        default:
            console.warn('Unknown worker command: ' + msg.command)
    }
}