Writer cursors that precisely track to which page they belong

faststreams/output_stream.nim

@@ -12,31 +12,47 @@ type
     delayedWrites: int16
 
   OutputStream* = object
-    head, bufferEnd: ptr byte
+    cursor: WriteCursor
     pages: Deque[OutputPage]
-    firstPage: int
     endPos: int
-    outputResource: pointer
     vtable: ptr OutputStreamVTable
+    outputDevice: RootRef
 
-  DelayedWriteCursor* = object
+  WriteCursor* = object
     head, bufferEnd: ptr byte
     stream: OutputStreamVar
-    page: int
+    absPageIdx: int
 
   OutputStreamVar* = ref OutputStream
 
+  # Keep this temporary for backward-compatibility
+  DelayedWriteCursor* = WriteCursor
+
 const
   allocatorMetadata = 0 # TODO: Get this from Nim's allocator.
                         # The goal is to make perfect page-aligned allocations
   pageSize = 4096 - allocatorMetadata - 1 # 1 byte for the null terminator
 
+func remainingBytesToWrite*(c: WriteCursor): int {.inline.} =
+  distance(c.head, c.bufferEnd)
+
+template relToAbsPageIdx(s: OutputStreamVar, idx: int): int =
+  # original code: s.firstPage + idx
+  idx - s.cursor.absPageIdx - 1
+
+template absToRelPageIdx(s: OutputStreamVar, idx: int): int =
+  # original code: idx - s.firstPage
+  idx + s.cursor.absPageIdx + 1
+
+func relPage(c: WriteCursor): int {.inline.} =
+  c.stream.absToRelPageIdx c.absPageIdx
+
 proc addPage(s: OutputStreamVar) =
   s.pages.addLast OutputPage(buffer: newString(pageSize),
                              delayedWrites: 0,
                              startOffset: 0)
-  s.head = cast[ptr byte](addr s.pages[s.pages.len - 1].buffer[0])
-  s.bufferEnd = cast[ptr byte](shift(s.head, pageSize))
+  s.cursor.head = cast[ptr byte](addr s.pages[s.pages.len - 1].buffer[0])
+  s.cursor.bufferEnd = cast[ptr byte](shift(s.cursor.head, pageSize))
   s.endPos += pageSize
 
 proc init*(T: type OutputStream): ref OutputStream =
@@ -44,9 +60,11 @@ proc init*(T: type OutputStream): ref OutputStream =
   result.vtable = nil
   result.pages = initDeque[OutputPage]()
   result.addPage()
+  result.cursor.absPageIdx = -1
+  result.cursor.stream = result
 
 proc pos*(s: OutputStreamVar): int =
-  s.endPos - distance(s.head, s.bufferEnd)
+  s.endPos - s.cursor.remainingBytesToWrite
 
 proc tryFlushing(s: OutputStreamVar) =
   # TODO This is relevant when writing to files and layered streams (e.g. zip)
@@ -57,35 +75,55 @@ proc tryFlushing(s: OutputStreamVar) =
   #  * The head and bufferEnd pointers point to the new top page
   s.addPage()
 
-proc append*(s: OutputStreamVar, b: byte) =
-  if s.head == s.bufferEnd:
-    s.tryFlushing()
+template isDelayedWrite(c: WriteCursor): bool =
+  c.absPageIdx >= 0
 
-  s.head[] = b
-  s.head = shift(s.head, 1)
+proc append*(c: var WriteCursor, b: byte) =
+  if c.head == c.bufferEnd:
+    # Delayed write cursors are not allowed to reach
+    # the end of the buffer and allocate new pages:
+    doAssert(not c.isDelayedWrite)
+    c.stream.tryFlushing()
 
-template append*(s: OutputStreamVar, c: char) =
+  c.head[] = b
+  c.head = shift(c.head, 1)
+
+template append*(c: var WriteCursor, x: char) =
   bind append
-  append s, byte(c)
+  c.append byte(x)
 
-proc append*(s: OutputStreamVar, bytes: openarray[byte]) =
+proc append*(c: var WriteCursor, bytes: openarray[byte]) =
   # TODO: this can use copyMem
   for b in bytes:
-    s.append b
+    c.append b
 
-proc append*(s: OutputStreamVar, chars: openarray[char]) =
+proc append*(c: var WriteCursor, chars: openarray[char]) =
   # TODO: this can use copyMem
-  for c in chars:
-    s.append byte(c)
+  for x in chars:
+    c.append byte(x)
 
-template append*(s: OutputStreamVar, str: string) =
-  s.append str.toOpenArrayByte(0, str.len - 1)
+template appendMemCopy*(c: var WriteCursor, value: auto) =
+  bind append
+  # TODO: add a check that this is a trivial type
+  c.append makeOpenArray(cast[ptr byte](unsafeAddr(value)), sizeof(value))
+
+template append*(c: var WriteCursor, str: string) =
+  bind append
+  c.append str.toOpenArrayByte(0, str.len - 1)
+
+template append*(s: OutputStreamVar, value: auto) =
+  bind append
+  s.cursor.append value
+
+template appendMemCopy*(s: OutputStreamVar, value: auto) =
+  bind append
+  s.cursor.append value
 
 proc flush*(s: OutputStreamVar) =
   s.vtable.finish(addr s[])
 
 proc getOutput*(s: OutputStreamVar, T: type string): string =
-  s.pages[s.pages.len - 1].buffer.setLen(pageSize - distance(s.head, s.bufferEnd))
+  s.pages[s.pages.len - 1].buffer.setLen(pageSize - s.cursor.remainingBytesToWrite)
 
   if s.pages.len == 1 and s.pages[0].startOffset == 0:
     result.swap s.pages[0].buffer
@@ -110,10 +148,10 @@ proc flushDelayedPages*(s: OutputStreamVar) =
     # TODO:
     # Send to output
 
-proc delayFixedSizeWrite*(s: OutputStreamVar, size: int): DelayedWriteCursor =
+proc delayFixedSizeWrite*(s: OutputStreamVar, size: int): WriteCursor =
   doAssert size < pageSize
 
-  let remainingBytesInPage = distance(s.head, s.bufferEnd)
+  let remainingBytesInPage = s.cursor.remainingBytesToWrite
   if size > remainingBytesInPage:
     s.pages[s.pages.len - 1].buffer.setLen(pageSize - remainingBytesInPage)
     s.endPos -= remainingBytesInPage
@@ -122,12 +160,14 @@ proc delayFixedSizeWrite*(s: OutputStreamVar, size: int): DelayedWriteCursor =
   let curPageIdx = s.pages.len - 1
   inc s.pages[curPageIdx].delayedWrites
 
-  result = DelayedWriteCursor(head: s.head, bufferEnd: s.head.shift(size),
-                              page: s.firstPage + curPageIdx, stream: s)
+  result = WriteCursor(head: s.cursor.head,
+                       bufferEnd: s.cursor.head.shift(size),
+                       absPageIdx: s.relToAbsPageIdx(curPageIdx),
+                       stream: s)
 
-  s.head = result.bufferEnd
+  s.cursor.head = result.bufferEnd
 
-proc delayVarSizeWrite*(s: OutputStreamVar, maxSize: int): DelayedWriteCursor =
+proc delayVarSizeWrite*(s: OutputStreamVar, maxSize: int): WriteCursor =
   # TODO
   discard
 
@@ -136,23 +176,23 @@ proc decRef(x: var int16): int16 =
   doAssert result >= 0
   x = result
 
-proc totalBytesWrittenAfterCursor*(cursor: DelayedWriteCursor): int =
+proc totalBytesWrittenAfterCursor*(cursor: WriteCursor): int =
   template s: auto = cursor.stream
 
   let
-    spanningPagesTotal = (cursor.stream.pages.len - cursor.page) * pageSize
-    deductedFromFirstPage = distance(unsafeAddr s.pages[cursor.page].buffer[0],
+    relPageIdx = cursor.relPage
+    spanningPagesTotal = (cursor.stream.pages.len - relPageIdx) * pageSize
+    deductedFromFirstPage = distance(unsafeAddr s.pages[relPageIdx].buffer[0],
                                      cursor.bufferEnd)
-    deductedFromLastPage = distance(s.head, s.bufferEnd)
+    deductedFromLastPage = s.cursor.remainingBytesToWrite
 
   spanningPagesTotal - deductedFromFirstPage - deductedFromLastPage
 
-proc endWrite*(cursor: DelayedWriteCursor, data: openarray[byte]) =
-  if data.len != distance(cursor.head, cursor.bufferEnd):
-    doAssert false
+proc endWrite*(cursor: WriteCursor, data: openarray[byte]) =
+  doAssert data.len == cursor.remainingBytesToWrite
 
   copyMem(cursor.head, unsafeAddr data[0], data.len)
-  if cursor.stream.pages[cursor.page - cursor.stream.firstPage].delayedWrites.decRef <= 0:
+  if cursor.stream.pages[cursor.relPage].delayedWrites.decRef <= 0:
     cursor.stream.flushDelayedPages()
 
 # Any stream