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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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27 changes: 17 additions & 10 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 @@ -266,13 +266,15 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
arg match {
case Wrap(seq) =>
if (count == 1) {
lhs.append(seq(0))
lhs.append(seq.head)
} else {
// count > 1
val taken = seq.take(count)
// we may have not takeped all of count
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hmm, out of curiosity, "takeped" – is it a typo or some slang I'm just not aware of? I also see "takeping" down below, so I guess it is intentional...

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no, I did drop first and I did find and replace to take and then changed the code. So, this was dropped and dropping with s/drop/take/.

val newCount = count - taken.length
val newLhs = lhs.concat(Wrap(taken))
val wrapped = Wrap(taken)
// this is more efficient than using concat
val newLhs = if (lhs.isEmpty) wrapped else Append(lhs, wrapped)
if (newCount > 0) {
// we have to keep takeping on the rhs
go(newLhs, newCount, rhs, Chain.nil)
Expand All @@ -282,7 +284,7 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
}
}
case Append(l, r) =>
go(lhs, count, l, r.concat(rhs))
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)
Expand All @@ -308,13 +310,14 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
arg match {
case Wrap(seq) =>
if (count == 1) {
lhs.append(seq.last)
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 = seq.takeRight(count)
// we may have not takeped all of count
val newCount = count - taken.length
val newRhs = Wrap(taken).concat(rhs)
val wrapped = Wrap(taken)
val newRhs = if (rhs.isEmpty) wrapped else Append(wrapped, rhs)
if (newCount > 0) {
// we have to keep takeping on the rhs
go(Chain.nil, newCount, lhs, newRhs)
Expand All @@ -324,7 +327,7 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
}
}
case Append(l, r) =>
go(lhs.concat(l), count, r, rhs)
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)
Expand Down Expand Up @@ -381,11 +384,13 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
rhs
}
} else {
// dropped is not empty
val wrapped = Wrap(dropped)
// we must be done
Chain.fromSeq(dropped).concat(rhs)
if (rhs.isEmpty) wrapped else Append(wrapped, rhs)
}
case Append(l, r) =>
go(count, l, r.concat(rhs))
go(count, l, if (rhs.isEmpty) r else Append(r, rhs))
case Singleton(_) =>
// due to the invariant count >= 1
if (count > 1) go(count - 1, rhs, Chain.nil)
Expand Down Expand Up @@ -422,10 +427,12 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
}
} else {
// we must be done
lhs.concat(Chain.fromSeq(dropped))
// note: dropped.nonEmpty
val wrapped = Wrap(dropped)
if (lhs.isEmpty) wrapped else Append(lhs, wrapped)
}
case Append(l, r) =>
go(lhs.concat(l), count, r)
go(if (lhs.isEmpty) l else Append(lhs, l), count, r)
case Singleton(_) =>
// due to the invariant count >= 1
if (count > 1) go(Chain.nil, count - 1, lhs)
Expand Down
19 changes: 15 additions & 4 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 @@ -449,25 +449,36 @@ class ChainSuite extends CatsSuite {
}
}

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

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

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