# Removing duplicates

The most straightforward way to remove duplicate elements from an
array is to sort the array, then use a `filter`

to keep only those
elements that are either first in the resulting array or differ
from the following element. First we import a sorting function
from the merge sort example.

`module merge = import "merge-sort"`

Then we write a function that removes consecutive duplicate elements.

```
def neq lte x y = if x `lte` y then !(y `lte` x) else true
def pack lte xs =
-1) xs)
zip3 (indices xs) xs (rotate (0 || neq lte x y) |> map (.1)
|> filter (\(i,x,y) -> i ==
def pack_i32 = pack (i32.<=)
```

`> pack_i32 [3,0,0,1,1,1,2,2,3,1]`

`[3i32, 0i32, 1i32, 2i32, 3i32, 1i32]`

And then by sorting first, a function that removes any duplicate elements.

```
def dedup_sort lte xs = merge.sort lte xs |> pack lte
def dedup_sort_i32 = dedup_sort (i32.<=)
```

`> dedup_sort_i32 [3,0,0,1,1,1,2,2,3,1]`

`[0i32, 1i32, 2i32, 3i32]`

One downside of this definition is that the original element order is not preserved, as shown above. We can address this by associating the original indexes with the input sequence and sorting by those at the end.

```
def nub lte xs =
zip xs (indices xs)
|> merge.sort (\(a,_) (b,_) -> a `lte` b)
|> pack (\(a,_) (b,_) -> a `lte` b)
|> merge.sort (\(_,i) (_,j) -> i <= j)0)
|> map (.
def nub_i32 = nub (i32.<=)
```

`> nub_i32 [3,0,0,1,1,1,2,2,3,1]`

`[3i32, 0i32, 2i32, 1i32]`

This is a general solution, but all this sorting is quite inefficient. If we know we will be operating on numbers or number-like data, we can use a radix sort to make the sorting a bit faster. However, if we can make assumptions about the input, even more efficient approaches are possible.

For example, if we are deduplicating `k`

numbers in the range
`[0,n-1]`

, where `k`

is much larger than `n`

, we can use
scatter to figure out which numbers are
contained, followed by a filter:

```
def dedup_scatter [k] (n: i64) (xs: [k]i64) : []i64 =
scatter (replicate n false) xs (replicate k true)
|> zip (iota n)1)
|> filter (.0) |> map (.
```

`> dedup_scatter 4 [3,0,0,1,1,1,2,2,3,1]`

`[0i64, 1i64, 2i64, 3i64]`

Note that this also does not preserve original element order.

If we can *characterise* the elements to deduplicate by an integer,
but they also contain other information we wish to preserve, we
need to use a generalised histogram. The idea
is similar to the `scatter`

above, but instead of storing
true/false, we store the *lowest index of an element that hits that
bucket*, then afterwards each element checks whether it “won”.

```
def nub_hist [k] 't (n: i64) (is: [k]i64) (xs: [k]t) : []t =
let H = hist i64.min k n is (indices xs)
in map2 (\i j -> H[i] == j) is (indices xs)
|> zip xs1)
|> filter (.0)
|> map (.
def nub_hist_i32 n is xs : []i32 = nub_hist n is xs
```

`> nub_hist_i32 4 [3,0,0,1,1,1,2,2,3,1] [0,1,2,3,4,5,6,7,8,9]`

`[0i32, 1i32, 3i32, 6i32]`

This prefers the first occurence of an element. It’s
straightforward to change it to prefer the last occurrence instead.
One particularly crucial aspect of this definition is that the
operator we use for the histogram, `i64.min`

, is likely to be
directly supported in hardware, meaning the histogram will be
computed very efficiencly.