Organize Buffer's segments as a regular list

Previously, Buffer's segments were organized into a circular list.
That allowed storing only a single reference to buffer's head,
and also facilitated insertion/removal of new list nodes.
The downside of a circular list is that one has
to always compare a current node with a head when
iterating over segments.
That complicates the implementation of a public API
for segments iterations.

See #135 (comment)
for details on segment iteration API.

Author: fzhinkin

core/apple/src/AppleCore.kt

@@ -52,8 +52,7 @@ private open class OutputStreamSink(
             source.size -= bytesWritten
 
             if (head.pos == head.limit) {
-                source.head = head.pop()
-                SegmentPool.recycle(head)
+                source.recycleHead()
             }
         }
     }
@@ -101,8 +100,7 @@ private open class NSInputStreamSource(
         if (bytesRead == 0L) {
             if (tail.pos == tail.limit) {
                 // We allocated a tail segment, but didn't end up needing it. Recycle!
-                sink.head = tail.pop()
-                SegmentPool.recycle(tail)
+                sink.recycleTail()
             }
             return -1
         }

core/apple/src/BuffersApple.kt

@@ -8,7 +8,9 @@
 package kotlinx.io
 
 import kotlinx.cinterop.*
-import platform.Foundation.*
+import platform.Foundation.NSData
+import platform.Foundation.create
+import platform.Foundation.data
 import platform.darwin.NSUIntegerMax
 import platform.posix.*
 
@@ -44,8 +46,7 @@ internal fun Buffer.readAtMostTo(sink: CPointer<uint8_tVar>, maxLength: Int): In
     size -= toCopy.toLong()
 
     if (s.pos == s.limit) {
-        head = s.pop()
-        SegmentPool.recycle(s)
+        recycleHead()
     }
 
     return toCopy
@@ -70,6 +71,6 @@ internal fun Buffer.snapshotAsNSData(): NSData {
         }
         curr = curr.next
         index += length
-    } while (curr !== head)
+    } while (curr !== null)
     return NSData.create(bytesNoCopy = bytes, length = size.convert())
 }

core/common/src/Buffer.kt

@@ -42,6 +42,9 @@ public class Buffer : Source, Sink {
     @JvmField
     internal var head: Segment? = null
 
+    @JvmField
+    internal var tail: Segment? = null
+
     /**
      * The number of bytes accessible for read from this buffer.
      */
@@ -76,8 +79,7 @@ public class Buffer : Source, Sink {
         val b = data[pos++]
         size -= 1L
         if (pos == limit) {
-            head = segment.pop()
-            SegmentPool.recycle(segment)
+            recycleHead()
         } else {
             segment.pos = pos
         }
@@ -102,8 +104,7 @@ public class Buffer : Source, Sink {
         size -= 2L
 
         if (pos == limit) {
-            head = segment.pop()
-            SegmentPool.recycle(segment)
+            recycleHead()
         } else {
             segment.pos = pos
         }
@@ -138,8 +139,7 @@ public class Buffer : Source, Sink {
         size -= 4L
 
         if (pos == limit) {
-            head = segment.pop()
-            SegmentPool.recycle(segment)
+            recycleHead()
         } else {
             segment.pos = pos
         }
@@ -176,8 +176,7 @@ public class Buffer : Source, Sink {
         size -= 8L
 
         if (pos == limit) {
-            head = segment.pop()
-            SegmentPool.recycle(segment)
+            recycleHead()
         } else {
             segment.pos = pos
         }
@@ -242,11 +241,10 @@ public class Buffer : Source, Sink {
             copy.pos += currentOffset.toInt()
             copy.limit = minOf(copy.pos + remainingByteCount.toInt(), copy.limit)
             if (out.head == null) {
-                copy.prev = copy
-                copy.next = copy.prev
-                out.head = copy.next
+                out.head = copy
+                out.tail = copy
             } else {
-                out.head!!.prev!!.push(copy)
+                out.tail = out.tail!!.push(copy)
             }
             remainingByteCount -= (copy.limit - copy.pos).toLong()
             currentOffset = 0L
@@ -264,7 +262,7 @@ public class Buffer : Source, Sink {
         if (result == 0L) return 0L
 
         // Omit the tail if it's still writable.
-        val tail = head!!.prev!!
+        val tail = tail!!
         if (tail.limit < Segment.SIZE && tail.owner) {
             result -= (tail.limit - tail.pos).toLong()
         }
@@ -317,8 +315,7 @@ public class Buffer : Source, Sink {
             head.pos += toSkip
 
             if (head.pos == head.limit) {
-                this.head = head.pop()
-                SegmentPool.recycle(head)
+                recycleHead()
             }
         }
     }
@@ -336,8 +333,7 @@ public class Buffer : Source, Sink {
         size -= toCopy.toLong()
 
         if (s.pos == s.limit) {
-            head = s.pop()
-            SegmentPool.recycle(s)
+            recycleHead()
         }
 
         return toCopy
@@ -377,19 +373,20 @@ public class Buffer : Source, Sink {
     internal fun writableSegment(minimumCapacity: Int): Segment {
         require(minimumCapacity >= 1 && minimumCapacity <= Segment.SIZE) { "unexpected capacity" }
 
-        if (head == null) {
+        if (tail == null) {
             val result = SegmentPool.take() // Acquire a first segment.
             head = result
-            result.prev = result
-            result.next = result
+            tail = result
             return result
         }
 
-        var tail = head!!.prev
-        if (tail!!.limit + minimumCapacity > Segment.SIZE || !tail.owner) {
-            tail = tail.push(SegmentPool.take()) // Append a new empty segment to fill up.
+        val t = tail!!
+        if (t.limit + minimumCapacity > Segment.SIZE || !t.owner) {
+            val newTail = t.push(SegmentPool.take()) // Append a new empty segment to fill up.
+            tail = newTail
+            return newTail
         }
-        return tail
+        return t
     }
 
     override fun write(source: ByteArray, startIndex: Int, endIndex: Int) {
@@ -486,7 +483,7 @@ public class Buffer : Source, Sink {
         while (remainingByteCount > 0L) {
             // Is a prefix of the source's head segment all that we need to move?
             if (remainingByteCount < source.head!!.limit - source.head!!.pos) {
-                val tail = if (head != null) head!!.prev else null
+                val tail = tail
                 if (tail != null && tail.owner &&
                     remainingByteCount + tail.limit - (if (tail.shared) 0 else tail.pos) <= Segment.SIZE
                 ) {
@@ -498,22 +495,32 @@ public class Buffer : Source, Sink {
                 } else {
                     // We're going to need another segment. Split the source's head
                     // segment in two, then move the first of those two to this buffer.
-                    source.head = source.head!!.split(remainingByteCount.toInt())
+                    val newHead = source.head!!.split(remainingByteCount.toInt())
+                    if (source.head === source.tail) {
+                        source.tail = newHead
+                    }
+                    source.head = newHead
                 }
             }
 
             // Remove the source's head segment and append it to our tail.
             val segmentToMove = source.head
             val movedByteCount = (segmentToMove!!.limit - segmentToMove.pos).toLong()
             source.head = segmentToMove.pop()
+            if (source.head == null) {
+                source.tail = null
+            }
             if (head == null) {
                 head = segmentToMove
-                segmentToMove.prev = segmentToMove
-                segmentToMove.next = segmentToMove.prev
+                tail = segmentToMove
+                segmentToMove.prev = null
+                segmentToMove.next = null
             } else {
-                var tail = head!!.prev
-                tail = tail!!.push(segmentToMove)
-                tail.compact()
+                val newTail = tail!!.push(segmentToMove).compact()
+                tail = newTail
+                if (newTail.prev == null) {
+                    this.head = newTail
+                }
             }
             source.size -= movedByteCount
             size += movedByteCount
@@ -582,16 +589,15 @@ public class Buffer : Source, Sink {
         val result = Buffer()
         if (size == 0L) return result
 
-        val head = head!!
+        val head = this.head!!
         val headCopy = head.sharedCopy()
 
         result.head = headCopy
-        headCopy.prev = result.head
-        headCopy.next = headCopy.prev
+        result.tail = headCopy
 
         var s = head.next
-        while (s !== head) {
-            headCopy.prev!!.push(s!!.sharedCopy())
+        while (s != null) {
+            result.tail = result.tail!!.push(s.sharedCopy())
             s = s.next
         }
 
@@ -642,6 +648,48 @@ public class Buffer : Source, Sink {
 
         return "Buffer(size=$size hex=$builder)"
     }
+
+    /**
+     * Unlinks and recycles this buffer's head.
+     *
+     * If head had a successor, it'll become a new head.
+     * Otherwise, both [head] and [tail] will be set to null.
+     *
+     * It's up to a caller to ensure that the head exists.
+     */
+    internal fun recycleHead() {
+        val oldHead = head!!
+        val nextHead = oldHead.next
+        head = nextHead
+        if (nextHead == null) {
+            tail = null
+        } else {
+            nextHead.prev = null
+        }
+        oldHead.next = null
+        SegmentPool.recycle(oldHead)
+    }
+
+    /**
+     * Unlinks and recycles this buffer's tail segment.
+     *
+     * If tail had a predecessor, it'll become a new tail.
+     * Otherwise, both [head] and [tail] will be set to null.
+     *
+     * It's up to a caller to ensure that the tail exists.
+     */
+    internal fun recycleTail() {
+        val oldTail = tail!!
+        val newTail = oldTail.prev
+        tail = newTail
+        if (newTail == null) {
+            head = null
+        } else {
+            newTail.next = null
+        }
+        oldTail.prev = null
+        SegmentPool.recycle(oldTail)
+    }
 }
 
 /**
@@ -652,23 +700,26 @@ internal inline fun <T> Buffer.seek(
     fromIndex: Long,
     lambda: (Segment?, Long) -> T
 ): T {
-    var s: Segment = head ?: return lambda(null, -1L)
+    if (this.head == null) lambda(null, -1L)
 
     if (size - fromIndex < fromIndex) {
+        var s = tail
         // We're scanning in the back half of this buffer. Find the segment starting at the back.
         var offset = size
-        while (offset > fromIndex) {
-            s = s.prev!!
+        while (s != null && offset > fromIndex) {
             offset -= (s.limit - s.pos).toLong()
+            if (offset <= fromIndex) break
+            s = s.prev
         }
         return lambda(s, offset)
     } else {
+        var s = this.head
         // We're scanning in the front half of this buffer. Find the segment starting at the front.
         var offset = 0L
-        while (true) {
+        while (s != null) {
             val nextOffset = offset + (s.limit - s.pos)
             if (nextOffset > fromIndex) break
-            s = s.next!!
+            s = s.next
             offset = nextOffset
         }
         return lambda(s, offset)

core/common/src/Buffers.kt

@@ -24,7 +24,7 @@ public fun Buffer.snapshot(): ByteString {
             check(curr != null) { "Current segment is null" }
             append(curr.data, curr.pos, curr.limit)
             curr = curr.next
-        } while (curr !== head)
+        } while (curr !== null)
     }
 }
 
@@ -53,10 +53,11 @@ public fun Buffer.indexOf(byte: Byte, startIndex: Long = 0, endIndex: Long = siz
         if (o == -1L) {
             return -1L
         }
-        var segment = seg!!
+        var segment: Segment? = seg!!
         var offset = o
         do {
             check(endOffset > offset)
+            segment!!
             val idx = segment.indexOf(
                 byte,
                 // If start index within this segment, the diff will be positive and
@@ -71,8 +72,8 @@ public fun Buffer.indexOf(byte: Byte, startIndex: Long = 0, endIndex: Long = siz
                 return offset + idx.toLong()
             }
             offset += segment.size
-            segment = segment.next!!
-        } while (segment !== head && offset < endOffset)
+            segment = segment.next
+        } while (segment !== null && offset < endOffset)
         return -1L
     }
 }

core/common/src/ByteStrings.kt

@@ -129,9 +129,10 @@ public fun Buffer.indexOf(byteString: ByteString, startIndex: Long = 0): Long {
             if (o == -1L) {
                 return -1L
             }
-            var segment = seg!!
+            var segment: Segment? = seg
             var offset = o
             do {
+                segment!!
                 // If start index within this segment, the diff will be positive and
                 // we'll scan the segment starting from the corresponding offset.
                 // Otherwise, the diff will be negative and we'll scan the segment from the beginning.
@@ -147,16 +148,16 @@ public fun Buffer.indexOf(byteString: ByteString, startIndex: Long = 0): Long {
                 val firstOutboundOffset = maxOf(startOffset, segment.size - byteStringData.size + 1)
                 // Try to find a pattern in all suffixes shorter than the pattern. These suffixes start
                 // in the current segment, but ends in the following segments; thus we're using outbound function.
-                val idx1 = segment.indexOfBytesOutbound(byteStringData, firstOutboundOffset, head)
+                val idx1 = segment.indexOfBytesOutbound(byteStringData, firstOutboundOffset)
                 if (idx1 != -1) {
                     // Offset corresponds to the segment's start, idx - to offset within the segment.
                     return offset + idx1.toLong()
                 }
 
                 // We scanned the whole segment, so let's go to the next one
                 offset += segment.size
-                segment = segment.next!!
-            } while (segment !== head && offset + byteString.size <= size)
+                segment = segment.next
+            } while (segment !== null && offset + byteString.size <= size)
             return -1L
         }
     }