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/*
* Unlike a traditional sorting algorithm, which attempts to sort with the fewest
* comparisons possible, the goal of pancake sort is to sort the sequence in as few reversals as
* possible. The idea is to do something similar to Selection Sort. We one by one place
* maximum element at the end and reduce the size of current array by one.
*
* Source: https://www.geeksforgeeks.org/pancake-sorting/
*
* This sorting algorithm is inspired by the pancake problem (hence the name),
* where a spatula can be placed anywhere between two pancakes and flip all pancakes
* above.
*
* The interesting about this algorithm (besides its name) is that instead of comparisons,
* the algorithm relies on flipping an array.
*
* Source: https://en.wikipedia.org/wiki/Pancake_sorting#The_original_pancake_problem
*
*/
/**
* Unlike Array.prototype.reverse, flipArray reverses only a subarray of the given
* array, determined by the parameters startIndex and endIndex
*
*
@param
{
number[]
} array The array to flip
*
@param
{
number
} startIndex The start of the subarray
*
@param
{
number
} endIndex The end of the subarray
*
@returns
The flipped array
*/
export
function
flipArray
(
array
,
startIndex
,
endIndex
)
{
while
(
startIndex
<
endIndex
)
{
// swap front and back of the subarray
const
temp
=
array
[
startIndex
]
array
[
startIndex
]
=
array
[
endIndex
]
array
[
endIndex
]
=
temp
// essentially reducing the problem to a smaller subarray
startIndex
++
endIndex
--
}
return
array
}
/**
* Returns the index of the maximum number of a subarray in a given array
*
*
@param
{
number[]
} array The array to found the maximum number's index
*
@param
{
*
} startIndex The start of the subarray
*
@param
{
*
} endIndex The end of the subarray
*
@returns
The index of the maximum number
*/
export
function
findMax
(
array
,
startIndex
,
endIndex
)
{
let
maxIndex
=
0
for
(
let
i
=
startIndex
;
i
<=
endIndex
;
i
++
)
{
if
(
array
[
i
]
>
array
[
maxIndex
]
)
maxIndex
=
i
}
return
maxIndex
}
/**
* The Pancake Sort algorithm.
*
* Note that even though it's a completely different concept of sorting an
* array, it's rather simple!
*
*
@param
{
number[]
} array The array to sort
*
@returns
The sorted array
*/
export
function
pancakeSort
(
array
)
{
for
(
let
subarraySize
=
array
.
length
;
subarraySize
>
1
;
subarraySize
--
)
{
const
maximumIndex
=
findMax
(
array
,
0
,
subarraySize
-
1
)
if
(
maximumIndex
!==
subarraySize
-
1
)
{
flipArray
(
array
,
0
,
maximumIndex
)
flipArray
(
array
,
0
,
subarraySize
-
1
)
}
}
return
array
}
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