Sorting multiple maps in marklogic 8 - xquery

Question

This is more of an XQuery than MarkLogic. I have three map:map and each map has key-value pair of "id" and score. I would like to sort all the distinct ids based on the score from each maps.
For eg:
map1 : 1:2048, 5:2000
map2 : 2:5000, 1:1000, 4:3000
map3 : 6:100, 7:5000, 2:2000
In the above example, each map is id:score for key value (did not know how to represent here :))..
I want the sorted list of id from three maps based on score..
Is there a good way or better way of doing the sorting, or do I have to union the keys of the map and iterate the sequence of keys and sort them ?

Answer 1

This seems like a great use case for folding. Its part of Xquery 3.0 spec.
Folding can go through a sequence of items and gets the result for each item as it goes through. In this example $combinedMaps is the result of the last call and $mapToMerge is the item in the sequence it is currently going through.
Here an example of what you would want to do.
declare function local:sortMaps(
$newMap as map:map,
$mapA as map:map,
$mapB as map:map
) as map:map {
let $build :=
for $key in map:keys($mapA)
let $otherMapValue :=
(map:get($mapB, $key), 0)[1]
let $value := map:get($mapA, $key)
return
if ($value gt $otherMapValue) then (
map:put($newMap, $key, $value)
) else (
map:put($newMap, $key, $otherMapValue)
)
return $newMap
};
let $map1 :=
map:new((
map:entry("1",2048),
map:entry("5",2000)
))
let $map2 :=
map:new((
map:entry("2",5000),
map:entry("1",1000),
map:entry("4",3000)
))
let $map3 :=
map:new((
map:entry("6",100),
map:entry("7",5000),
map:entry("2",2000)
))
let $maps := ($map1, $map2, $map3)
return
fn:fold-left(
function($combinedMaps, $mapToMerge) {
let $newMap := map:map()
let $newMap := local:sortMaps($newMap, $combinedMaps, $mapToMerge)
let $newMap := local:sortMaps($newMap, $mapToMerge, $combinedMaps)
return $newMap
},
$maps[1],
$maps
)