> For the complete documentation index, see [llms.txt](https://docs.urbit.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.urbit.org/hoon/stdlib/2k.md).
# 2k: Queue Logic
Queue data structure operations for first-in, first-out data handling.
## `+to`
Queue operations.
Container arm for `+que` operation arms. The contained arms inherit `.a`, its sample `+que`.
#### Accepts
`.a` is a tree.
#### Source
```hoon
++ to
=| a=(tree) :: (qeu)
|@
```
#### Discussion
`+tree` is the generic tree-shape mold. Maps, sets, and `+que`s all share that shape, but they put their elements in different places and have different interfaces. `+to` is the interface core for `+qeu`; it's not generic across all the different containers that use `+tree`.
***
### `+apt:to`
Check correctness.
Test whether `+que` `.a` is correctly ordered, producing a `$flag`.
#### Accepts
`.a` is a `+que`.
#### Produces
A `$flag`.
#### Source
```hoon
++ apt
|- ^- ?
?~ a &
?& ?~(l.a & ?&((mor n.a n.l.a) $(a l.a)))
?~(r.a & ?&((mor n.a n.r.a) $(a r.a)))
==
```
#### Examples
```
> =bad `(qeu @ud)`[1 ~ 2 ~ 3 ~ ~]
> ~(apt to bad)
%.n
```
```
> =good `(qeu @ud)`(~(gas to `(qeu @ud)`~) ~[1 2 3])
> ~(apt to good)
%.y
```
***
### `+bal:to`
Balance.
Vertically rebalances `+que` `.a`.
#### Accepts
`.a` is a `+qeu`.
#### Produces
A `+qeu`.
#### Source
```hoon
++ bal
|- ^+ a
?~ a ~
?. |(?=(~ l.a) (mor n.a n.l.a))
$(a l.a(r $(a a(l r.l.a))))
?. |(?=(~ r.a) (mor n.a n.r.a))
$(a r.a(l $(a a(r l.r.a))))
a
```
#### Examples
```
> =a `(qeu @ud)`[1 ~ 2 ~ 3 ~ ~]
> a
{1 2 3}
> `*`a
[1 0 2 0 3 0 0]
> ~(apt to a)
%.n
> =z `(qeu @ud)`~(bal to a)
> z
{1 2 3}
> `*`z
[2 [1 0 0] 3 0 0]
> ~(apt to z)
%.y
```
***
### `+dep:to`
Maximum Depth.
Produces the maximum depth of leaves (`.r.a` and `.l.a`) in `+que` `.a`.
#### Accepts
`.a` is a `+que`.
#### Produces
An `$atom`.
#### Source
```hoon
++ dep
|- ^- @
?~ a 0
+((max $(a l.a) $(a r.a)))
```
#### Examples
```
> =a (~(gas to `(qeu @)`~) `(list @)`[1 2 3 4 5 6 7 ~])
> ~(dep to a)
4
> =a (~(gas to `(qeu @)`~) `(list @)`[1 2 3 4 ~])
> ~(dep to a)
3
> =a (~(gas to `(qeu @)`~) `(list @)`[1 2 ~])
> ~(dep to a)
2
> ~(dep to `(qeu tape)`["a" ~ "b" ~ "c" ~ "d" ~ "e" ~ "f" ~ "g" ~ ~])
7
> ~(dep to ~(bal to `(qeu tape)`["a" ~ "b" ~ "c" ~ "d" ~ "e" ~ "f" ~ "g" ~ ~]))
4
```
***
### `+gas:to`
Push `+list`.
Push all elements of `+list` `.b` into the `+que` `.a`.
#### Accepts
`.a` is a `+que`, and is the sample of `+to`.
`.b` is a `+list`.
#### Produces
A `+que`.
#### Source
```hoon
++ gas
|= b=(list _?>(?=(^ a) n.a))
|- ^+ a
?~(b a $(b t.b, a (put i.b)))
```
#### Examples
```
> `(qeu)`(~(gas to `(qeu @)`~) [1 2 3 ~])
{3 2 1}
```
```
> =a (~(gas to `(qeu @)`~) [1 2 3 ~])
> =b [4 5 6 ~]
> `(qeu)`(~(gas to a) b)
{6 5 4 3 2 1}
```
***
### `+get:to`
Head-tail pair.
Produces the head and tail `+que` of `.a`.
#### Accepts
`.a` is a `+que`.
#### Produces
A cell of the last element in `.a` along with the rest of `+que` `.a`.
#### Source
```hoon
++ get
|- ^+ ?>(?=(^ a) [p=n.a q=*(tree _n.a)])
?~ a
!!
?~ r.a
[n.a l.a]
=+ b=$(a r.a)
:- p.b
?: |(?=(~ q.b) (mor n.a n.q.b))
a(r q.b)
a(n n.q.b, l a(r l.q.b), r r.q.b)
```
#### Examples
```
> =s (~(gas to *(tree @)) `(list @)`~[1 2 3])
> ~(get to s)
[p=1 q={3 2}]
> ~(get to ~)
dojo: hoon expression failed
```
***
### `+nap:to`
Remove head.
Removes the head of `+que` `.a`, producing the resulting `+que`.
#### Accepts
`.a` is a `+que`.
#### Produces
A `+que`.
#### Source
```hoon
++ nap
?> ?=(^ a)
?: =(~ l.a) r.a
=+ b=get(a l.a)
bal(n.a p.b, l.a q.b)
```
#### Examples
```
> =a (~(gas to `(qeu @)`~) `(list @)`[1 2 3 4 5 6 ~])
> -.a
n=6
> =b ~(nap to a)
> -.b
n=2
> a
[n=6 l={} r={5 4 3 2 1}]
> b
[n=2 l=[n=4 l={5} r={3}] r=[n=1 l={} r={}]]
> `(qeu)`b
{5 4 3 2 1}
> `(qeu)`a
{6 5 4 3 2 1}
```
***
### `+nip:to`
Removes the root of `+que` `.a`, producing the resulting `+que`.
#### Accepts
`.a` is a `+que`.
#### Produces
A `+que`.
#### Source
```hoon
++ nip
|- ^+ a
?~ a ~
?~ l.a r.a
?~ r.a l.a
?: (mor n.l.a n.r.a)
l.a(r $(l.a r.l.a))
r.a(l $(r.a l.r.a))
```
#### Examples
```
> =a (~(gas to `(qeu @)`~) `(list @)`[1 2 3 4 5 6 ~])
> a
[n=6 l={} r={5 4 3 2 1}]
> ~(nip to a)
[n=2 l=[n=4 l={5} r={3}] r=[n=1 l={} r={}]]
```
***
### `+put:to`
Insert.
Accept `$noun` `.b` and adds to `+que` `.a` as the head, producing the resulting `+que`.
#### Accepts
`.a` is a `+que`, and is the sample of `+to`.
`.b` is a `$noun`.
#### Produces
A `+que`.
#### Source
```hoon
++ put
|* b=*
|- ^+ a
?~ a
[b ~ ~]
bal(l.a $(a l.a))
```
#### Examples
```
> =a (~(gas to `(qeu @)`~) `(list @)`[3 1 2 4 5 6 ~])
> `(qeu)`(~(put to a) 7)
{7 6 5 4 2 1 3}
```
***
### `+tap:to`
Queue to `+list`.
Produces `+que` `.a` as a `+list` from front to back.
#### Accepts
`.a` is a `+que`.
#### Produces
A `+list`.
#### Source
```hoon
++ tap
=+ b=`(list _?>(?=(^ a) n.a))`~
|- ^+ b
=+ 0 :: hack for jet match
?~ a
b
$(a r.a, b [n.a $(a l.a)])
```
#### Examples
```
> =a (~(gas to `(qeu @)`~) [3 1 2 4 5 6 ~])
> `*`a
[6 0 2 [4 [5 0 0] 0] 1 0 3 0 0]
> ~(tap to a)
~[3 1 2 4 5 6]
```
***
### `+top:to`
Produce head.
Produces the head of `+que` `.a` as a `+unit`.
#### Accepts
`.a` is a `+que`.
#### Produces
A unit.
#### Source
```hoon
++ top
|- ^- (unit _?>(?=(^ a) n.a))
?~ a ~
?~(r.a [~ n.a] $(a r.a))
```
#### Examples
```
> =a (~(gas to `(qeu @)`~) `(list @)`[1 2 3 4 5 6 ~])
> ~(top to a)
[~ u=1]
```
#### Discussion
An empty `+que` has no head.
***
---
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