# bit-stream

Lazy, infinite, compact stream of `Bool`

with O(1) indexing.
Most useful for memoization of predicates.

## Example 1

Consider following predicate:

```
isOdd :: Word -> Bool
isOdd 0 = False
isOdd n = not (isOdd (n - 1))
```

Its computation is expensive, so we’d like to memoize its values into
`BitStream`

using `tabulate`

and access this stream via `index`

instead of recalculation of `isOdd`

:

```
isOddBS :: BitStream
isOddBS = tabulate isOdd
isOdd' :: Word -> Bool
isOdd' = index isOddBS
```

We can do even better by replacing part of recursive calls to `isOdd`

by indexing memoized values. Write `isOddF`

such that `isOdd = fix isOddF`

:

```
isOddF :: (Word -> Bool) -> Word -> Bool
isOddF _ 0 = False
isOddF f n = not (f (n - 1))
```

and use `tabulateFix`

:

```
isOddBS :: BitStream
isOddBS = tabulateFix isOddF
isOdd' :: Word -> Bool
isOdd' = index isOddBS
```

## Example 2

Define a predicate, which checks whether its argument is
a prime number by trial division.

```
isPrime :: Word -> Bool
isPrime n
| n < 2 = False
| n < 4 = True
| even n = False
| otherwise = and [ n `rem` d /= 0 | d <- [3, 5 .. ceiling (sqrt (fromIntegral n))], isPrime d]
```

Convert it to unfixed form:

```
isPrimeF :: (Word -> Bool) -> Word -> Bool
isPrimeF f n
| n < 2 = False
| n < 4 = True
| even n = False
| otherwise = and [ n `rem` d /= 0 | d <- [3, 5 .. ceiling (sqrt (fromIntegral n))], f d]
```

Create its memoized version for faster evaluation:

```
isPrimeBS :: BitStream
isPrimeBS = tabulateFix isPrimeF
isPrime' :: Word -> Bool
isPrime' = index isPrimeBS
```