sequenceDiagram

    title: ListService `listIds()` Sequence Flow

    participant l1 as ListService
    participant l2 as ListService
    participant ddb as DynamoDB

    l1->>+l2: listIds(tags,paginationKey,count)

    par async task per tag
        note right of l2: Retrieve the first page of task ids that match each tag
        loop each tag
            l2->>+l2: listIdsFromDbUsingTags<(tagKey,tagValue,tagPaginationKey,count)
            note right of l2: Refer to `listIdsFromDbUsingTags()` section below
            l2-->>-l2: task ids
        end
    end

    note right of l1: if any of the initial results are empty, then we can exit<br/>immediately as there are no common matches across all requested tags
    loop each tag
        opt no task ids
            l2-->>l1: [undefined, undefined]
        end
    end

    note right of l2: initialize a set of pointers that tracks the current position of each tags page of results
    l2->>l2: initialize pointers
    note right of l2: loop through each page of results per tag looking for task ids that are found across
    loop more item ids still to process and found items < requested count
        loop each tag index
            note right of l2: retrieve the next task id for the current tag to process
            l2->>+l2: currentTagTaskItemId = getNextItemIdFromResults(tagIndex)
            note right of l2: Refer to `getNextPageOfResults(tagIndex)` section below
            l2-->>-l2: task id

            alt tag index === last tag index
                note right of l2: if we reach here it means we found a task id that was matched across all tags
                l2->>l2: add currentTagTaskItemId to results
                l2->>l2: increment all the pointers to reference the next result for each tag

            else tag index < last tag index
                note right of l2: check for matching task ids between this and the next tag to be compared
                l2->>+l2: nextTagTaskItemId = getNextItemIdFromResults(tagIndex)
                note right of l2: Refer to `getNextPageOfResults(tagIndex)` section below
                l2-->>-l2: task id

                alt currentTagTaskItemId === nextTagTaskItemId
                    note right of l2: we have a match across the tag pair being checked, so lets move onto checking the next tag pair
                    l2-->>l2: continue loop

                else currentTagTaskItemId < nextTagTaskItemId
                    note right of l2: this tag has a lower task id, therefore increment the pointer for the current tag and restart the matching flow
                    l2-->>l2: increment pointer of current tag
                    l2-->>l2: break loop

                else currentTagTaskItemId > nextTagTaskItemId
                    note right of l2: this tag has a higher task id, therefore increment the pointer for the next tag and restart the matching flow
                    l2-->>l2: increment pointer of next tag
                    l2-->>l2: break loop

                end

            end
        end
    end

    l2-->>-l1: [taskIds, paginationKey]

    note right of l1: ~~~<br/>`getNextPageOfResults(tagIndex)` sequence flow<br/>~~~
    rect rgb(227, 211, 175)
        l1->>+l2: getNextItemIdFromResults(tagIndex)
        opt no more task ids in current page of results
            l2->>+l2: getNextPageOfResults(tagIndex)
            l2->>+l2: listIdsFromDbUsingTags(tagKey,tagValue,paginationKey,count)
            note right of l2: Refer to `listIdsFromDbUsingTags()` section below
            l2-->>-l2: task ids
            l2-->>-l2: has results?
        end
        l2-->>-l1: task id
        opt no next task id for current tag
            note right of l1: stop processing
        end
    end 

    note right of l1: ~~~<br/>`listIdsFromDbUsingTags(tagName,tagValue,exclusiveStart?,count?)` sequence flow<br/>~~~
    rect rgb(175, 227, 178)

        l1->>+l2: listIdsFromDbUsingTags<br/>(tagName,tagValue,exclusiveStart?,count?)

        l2->>+ddb:query(params)
        ddb-->>-l2: items

        l2->>l2: extract task ids

        l2-->>-l1: [taskIds, paginationKey]
    end