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Add take/takeRight/drop/dropRight to Chain #4694

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Add take/takeRight/drop/dropRight to Chain #4694

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3b2c4bd
Add take/takeRight/drop/dropRight to Chain
johnynek Jan 3, 2025
83fe74c
fix bug, improve scalachecks
johnynek Jan 3, 2025
49625fe
respond to review comments
johnynek Jan 3, 2025
f56cad6
recurse on NonEmpty
johnynek Jan 3, 2025
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188 changes: 188 additions & 0 deletions core/src/main/scala/cats/data/Chain.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -256,6 +256,99 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
result
}

/**
* take a certain amount of items from the front of the Chain
*/
final def take(count: Long): Chain[A] = {
// invariant count >= 1
@tailrec
def go(lhs: Chain[A], count: Long, arg: NonEmpty[A], rhs: Chain[A]): Chain[A] =
arg match {
case Wrap(seq) =>
if (count == 1) {
lhs.append(seq.head)
} else {
// count > 1
val taken =
if (count < Int.MaxValue) seq.take(count.toInt)
else seq.take(Int.MaxValue)
// we may have not taken all of count
val newCount = count - taken.length
val wrapped = Wrap(taken)
// this is more efficient than using concat
val newLhs = if (lhs.isEmpty) wrapped else Append(lhs, wrapped)
rhs match {
case rhsNE: NonEmpty[A] if newCount > 0L =>
// we have to keep taking on the rhs
go(newLhs, newCount, rhsNE, Empty)
case _ =>
newLhs
}
}
case Append(l, r) =>
go(lhs, count, l, if (rhs.isEmpty) r else Append(r, rhs))
case s @ Singleton(_) =>
// due to the invariant count >= 1
val newLhs = if (lhs.isEmpty) s else Append(lhs, s)
rhs match {
case rhsNE: NonEmpty[A] if count > 1L =>
go(newLhs, count - 1L, rhsNE, Empty)
case _ => newLhs
}
}

this match {
case ne: NonEmpty[A] if count > 0L =>
go(Empty, count, ne, Empty)
case _ => Empty
}
}

/**
* take a certain amount of items from the back of the Chain
*/
final def takeRight(count: Long): Chain[A] = {
// invariant count >= 1
@tailrec
def go(lhs: Chain[A], count: Long, arg: NonEmpty[A], rhs: Chain[A]): Chain[A] =
arg match {
case Wrap(seq) =>
if (count == 1L) {
seq.last +: rhs
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this was the bug. The rest of the changes are just optimizations to the code or scalacheck improvements.

} else {
// count > 1
val taken =
if (count < Int.MaxValue) seq.takeRight(count.toInt)
else seq.takeRight(Int.MaxValue)
// we may have not taken all of count
val newCount = count - taken.length
val wrapped = Wrap(taken)
val newRhs = if (rhs.isEmpty) wrapped else Append(wrapped, rhs)
lhs match {
case lhsNE: NonEmpty[A] if newCount > 0 =>
go(Empty, newCount, lhsNE, newRhs)
case _ => newRhs
}
}
case Append(l, r) =>
go(if (lhs.isEmpty) l else Append(lhs, l), count, r, rhs)
case s @ Singleton(_) =>
// due to the invariant count >= 1
val newRhs = if (rhs.isEmpty) s else Append(s, rhs)
lhs match {
case lhsNE: NonEmpty[A] if count > 1 =>
go(Empty, count - 1, lhsNE, newRhs)
case _ => newRhs
}
}

this match {
case ne: NonEmpty[A] if count > 0L =>
go(Empty, count, ne, Empty)
case _ => Empty
}
}

/**
* Drops longest prefix of elements that satisfy a predicate.
*
Expand All @@ -275,6 +368,101 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
go(this)
}

/**
* Drop a certain amount of items from the front of the Chain
*/
final def drop(count: Long): Chain[A] = {
// invariant count >= 1
@tailrec
def go(count: Long, arg: NonEmpty[A], rhs: Chain[A]): Chain[A] =
arg match {
case Wrap(seq) =>
val dropped = if (count < Int.MaxValue) seq.drop(count.toInt) else seq.drop(Int.MaxValue)
if (dropped.isEmpty) {
// we may have not dropped all of count
val newCount = count - seq.length
rhs match {
case rhsNE: NonEmpty[A] if newCount > 0 =>
// we have to keep dropping on the rhs
go(newCount, rhsNE, Empty)
case _ =>
// we know that count >= seq.length else we wouldn't be empty
// so in this case, it is exactly count == seq.length
rhs
}
} else {
// dropped is not empty
val wrapped = Wrap(dropped)
// we must be done
if (rhs.isEmpty) wrapped else Append(wrapped, rhs)
}
case Append(l, r) =>
go(count, l, if (rhs.isEmpty) r else Append(r, rhs))
case Singleton(_) =>
// due to the invariant count >= 1
rhs match {
case rhsNE: NonEmpty[A] if count > 1L =>
go(count - 1L, rhsNE, Empty)
case _ =>
rhs
}
}

this match {
case ne: NonEmpty[A] if count > 0L =>
go(count, ne, Empty)
case _ => this
}
}

/**
* Drop a certain amount of items from the back of the Chain
*/
final def dropRight(count: Long): Chain[A] = {
// invariant count >= 1
@tailrec
def go(lhs: Chain[A], count: Long, arg: NonEmpty[A]): Chain[A] =
arg match {
case Wrap(seq) =>
val dropped = if (count < Int.MaxValue) seq.dropRight(count.toInt) else seq.dropRight(Int.MaxValue)
if (dropped.isEmpty) {
// we may have not dropped all of count
val newCount = count - seq.length
lhs match {
case lhsNE: NonEmpty[A] if newCount > 0L =>
// we have to keep dropping on the lhs
go(Empty, newCount, lhsNE)
case _ =>
// we know that count >= seq.length else we wouldn't be empty
// so in this case, it is exactly count == seq.length
lhs
}
} else {
// we must be done
// note: dropped.nonEmpty
val wrapped = Wrap(dropped)
if (lhs.isEmpty) wrapped else Append(lhs, wrapped)
}
case Append(l, r) =>
go(if (lhs.isEmpty) l else Append(lhs, l), count, r)
case Singleton(_) =>
// due to the invariant count >= 1
lhs match {
case lhsNE: NonEmpty[A] if count > 1L =>
go(Empty, count - 1L, lhsNE)
case _ =>
lhs
}
}

this match {
case ne: NonEmpty[A] if count > 0L =>
go(Empty, count, ne)
case _ =>
this
}
}

/**
* Folds over the elements from right to left using the supplied initial value and function.
*/
Expand Down
34 changes: 34 additions & 0 deletions tests/shared/src/test/scala/cats/tests/ChainSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -448,4 +448,38 @@ class ChainSuite extends CatsSuite {
assert(chain.foldRight(init)(fn) == chain.toList.foldRight(init)(fn))
}
}

private val genChainDropTakeArgs =
Arbitrary.arbitrary[Chain[Int]].flatMap { chain =>
// Bias to values close to the length
Gen
.oneOf(
Gen.choose(Int.MinValue, Int.MaxValue),
Gen.choose(-1, chain.length.toInt + 1)
)
.map((chain, _))
}

test("drop(cnt).toList == toList.drop(cnt)") {
forAll(genChainDropTakeArgs) { case (chain: Chain[Int], count: Int) =>
assertEquals(chain.drop(count).toList, chain.toList.drop(count))
}
}

test("dropRight(cnt).toList == toList.dropRight(cnt)") {
forAll(genChainDropTakeArgs) { case (chain: Chain[Int], count: Int) =>
assertEquals(chain.dropRight(count).toList, chain.toList.dropRight(count))
}
}
test("take(cnt).toList == toList.take(cnt)") {
forAll(genChainDropTakeArgs) { case (chain: Chain[Int], count: Int) =>
assertEquals(chain.take(count).toList, chain.toList.take(count))
}
}

test("takeRight(cnt).toList == toList.takeRight(cnt)") {
forAll(genChainDropTakeArgs) { case (chain: Chain[Int], count: Int) =>
assertEquals(chain.takeRight(count).toList, chain.toList.takeRight(count))
}
}
}
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