循环遍历2个数组并稍稍发现差异

Question

Ok so this is kind of a hard one for me to wrap my head around so im hoping that one of you guys might be able to help. A little context, this is for an updater im writing that grabs an xml list of files from a cdn, then compares that vs an older list to look for file differences so i know which files are out of date and need to be redownloaded. Right now i am unable to find a proper solution for this.

Currently i have 3 arrays. Array1, Array2, and DiffArray. Array1 stores the xml entries from the CDN..ie the master Array2 stores the old entries of what we currently have..ie the slave Array3 stores the differences of what has changed between the 2.

Heres a sample bit of info thats in each array. Note that each new line is parsed into a separate index of their corresponding array

Array1:

cbt/ar/816.mp3
2019-06-05T16:40:33.212Z
cbt/ar/817.mp3
2019-06-05T16:40:31.509Z
cbt/ar/818.mp3
2019-04-05T16:40:30.978Z
cbt/ar/819.mp3
2019-04-05T16:40:29.807Z

Array2:

cbt/ar/816.mp3
2019-04-05T16:40:33.212Z
cbt/ar/817.mp3
2019-04-05T16:40:31.509Z
cbt/ar/818.mp3
2019-04-05T16:40:30.978Z
cbt/ar/819.mp3
2019-04-05T16:40:29.807Z

a couple things to note: 1.) this is a list of a file name, and its last modified date 2.) As you can see, array1 has a new file for both 816.mp3 and 817.mp3

The idea is to see that, note which files are different, then redownload those files with the more current version.

This is what i currently have, but as you can see its not the right solution for the job:

var a = [];
      for (var x = 0; x < remoteArray.length; x++) {
        a[remoteArray[x]] = true;

      }

      for (var y = 0; y < localArray.length; y++) {
        if (a[localArray[y]]) {
          delete a[localArray[y]];
        } else {
          a[localArray[y]] = true;
        }
      }

      for (var z in a) {
        diffArray.push(z);
        log.info("::DIFFERENCES::" + z);
      }
    }

This current code only outputs the actual literal differences and doesn't really help me know which file is different so that i can update it

Answer 1

Not sure if this is the format you want the result in, but it identifies which files need to be updated:

# Make a dictionary matching each cdn file to its timestamp
cdn = {}
for i in range(0,len(Array1),2):
    cdn[Array1[i]] = Array1[i+1]

# Make an array of files needing to be updated
update = []
for i in range(0,len(Array2),2):
    path = Array2[i]
    # If file is in CDN and the one there is newer, add it to update
    if path in cdn and cdn[path] > Array2[i+1]:
        update.append( path )

Answer 2

It might be easier to first transform your data into a list of objects representing each file. It is not the most performant approach but will make things clearer and easier to maintain.

function transformFilesList(array) {
    var files = [];
    for (var i = 0; i < array.length; i += 2) {
        files.push({
            name: array[i],
            modified: array[i + 1]
        });
    }
    return files;
}

var remote = transformFilesList(remoteArray);
var local = transformFilesList(localArray);
var needsDownload = remote.filter(file => {
    let match = local.find(localFile => localFile.name === file.name);
    // We need to download if there is no local file with this name or its modification date is older than the remote one
    return !match || match.modified < file.modified;
});

console.log('List of files to (re)download', needsDownload);
// Each file in this list will be an object { name, modified }

If you cannot use features like Array.prototype.filter or arrow functions (old browsers or Node versions), an old alternative to get needsDownload would be:

var needsDownload = [];
for (var i = 0; i < remote.length; i++) {
    var found;
    for (var j = 0; j < local.length; j++) {
        if (remote[i].name === local[j].length) {
            found = local[j];
            break;
        }
    }
    if (!found || found.modified < remote[i].modified) {
        needsDownload.push(found);
    }
}