Add support for canvas and multipart

Quite incomplete canvas implementation. Crucially, the layout engine
can't do much with whatever is drawn because it doesn't support images
yet.

I've re-introduced multipart as well, with the FormData API. For the
append function I've also introduced a hack to the JS binding generator
that allows requesting the JSContext pointer in nim procs. Really I
should just fix the union generator thing and add support for overloading.

In conclusion, for now the only thing canvas can be used for is exporting
it as PNG and uploading it somewhere. Also, we now have PNG encoding and
decoding too. (Now if only we had sixels as well...)

1 files changed, 186 insertions, 0 deletions

diff --git a/lib/endians2.nim b/lib/endians2.nim
new file mode 100644
index 00000000..c9a39144
--- /dev/null
+++ b/lib/endians2.nim
@@ -0,0 +1,186 @@
+# Copyright (c) 2018-2019 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+# Endian conversion operations for unsigned integers, suitable for serializing
+# and deserializing data. The operations are only defined for unsigned
+# integers - if you wish to encode signed integers, convert / cast them to
+# unsigned first!
+#
+# Although it would be possible to enforce correctness with endians in the type
+# (`BigEndian[uin64]`) this seems like overkill. That said, some
+# static analysis tools allow you to annotate fields with endianness - perhaps
+# an idea for the future, akin to `TaintedString`?
+#
+# Keeping the above in mind, it's generally safer to use `array[N, byte]` to
+# hold values of specific endianness and read them out with `fromBytes` when the
+# integer interpretation of the bytes is needed.
+
+{.push raises: [].}
+
+type
+  SomeEndianInt* = uint8|uint16|uint32|uint64
+    ## types that we support endian conversions for - uint8 is there for
+    ## for syntactic / generic convenience. Other candidates:
+    ## * int/uint - uncertain size, thus less suitable for binary interop
+    ## * intX - over and underflow protection in nim might easily cause issues -
+    ##          need to consider before adding here
+
+const
+  useBuiltins = not defined(noIntrinsicsEndians)
+
+when (defined(gcc) or defined(llvm_gcc) or defined(clang)) and useBuiltins:
+  func swapBytesBuiltin(x: uint8): uint8 = x
+  func swapBytesBuiltin(x: uint16): uint16 {.
+      importc: "__builtin_bswap16", nodecl.}
+
+  func swapBytesBuiltin(x: uint32): uint32 {.
+      importc: "__builtin_bswap32", nodecl.}
+
+  func swapBytesBuiltin(x: uint64): uint64 {.
+      importc: "__builtin_bswap64", nodecl.}
+
+elif defined(icc) and useBuiltins:
+  func swapBytesBuiltin(x: uint8): uint8 = x
+  func swapBytesBuiltin(a: uint16): uint16 {.importc: "_bswap16", nodecl.}
+  func swapBytesBuiltin(a: uint32): uint32 {.importc: "_bswap", nodec.}
+  func swapBytesBuiltin(a: uint64): uint64 {.importc: "_bswap64", nodecl.}
+
+elif defined(vcc) and useBuiltins:
+  func swapBytesBuiltin(x: uint8): uint8 = x
+  func swapBytesBuiltin(a: uint16): uint16 {.
+      importc: "_byteswap_ushort", cdecl, header: "<intrin.h>".}
+
+  func swapBytesBuiltin(a: uint32): uint32 {.
+      importc: "_byteswap_ulong", cdecl, header: "<intrin.h>".}
+
+  func swapBytesBuiltin(a: uint64): uint64 {.
+      importc: "_byteswap_uint64", cdecl, header: "<intrin.h>".}
+
+func swapBytesNim(x: uint8): uint8 = x
+func swapBytesNim(x: uint16): uint16 = (x shl 8) or (x shr 8)
+
+func swapBytesNim(x: uint32): uint32 =
+  let v = (x shl 16) or (x shr 16)
+
+  ((v shl 8) and 0xff00ff00'u32) or ((v shr 8) and 0x00ff00ff'u32)
+
+func swapBytesNim(x: uint64): uint64 =
+  var v = (x shl 32) or (x shr 32)
+  v =
+    ((v and 0x0000ffff0000ffff'u64) shl 16) or
+    ((v and 0xffff0000ffff0000'u64) shr 16)
+
+  ((v and 0x00ff00ff00ff00ff'u64) shl 8) or
+    ((v and 0xff00ff00ff00ff00'u64) shr 8)
+
+func swapBytes*[T: SomeEndianInt](x: T): T {.inline.} =
+  ## Reverse the bytes within an integer, such that the most significant byte
+  ## changes place with the least significant one, etc
+  ##
+  ## Example:
+  ## doAssert swapBytes(0x01234567'u32) == 0x67452301
+  when nimvm:
+    swapBytesNim(x)
+  else:
+    when declared(swapBytesBuiltin):
+      swapBytesBuiltin(x)
+    else:
+      swapBytesNim(x)
+
+func toBytes*(x: SomeEndianInt, endian: Endianness = system.cpuEndian):
+    array[sizeof(x), byte] {.noinit, inline.} =
+  ## Convert integer to its corresponding byte sequence using the chosen
+  ## endianness. By default, native endianness is used which is not portable!
+  let v =
+    if endian == system.cpuEndian: x
+    else: swapBytes(x)
+
+  when nimvm: # No copyMem in vm
+    for i in 0..<sizeof(result):
+      result[i] = byte((v shr (i * 8)) and 0xff)
+  else:
+    copyMem(addr result, unsafeAddr v, sizeof(result))
+
+func toBytesLE*(x: SomeEndianInt):
+    array[sizeof(x), byte] {.inline.} =
+  ## Convert a native endian integer to a little endian byte sequence
+  toBytes(x, littleEndian)
+
+func toBytesBE*(x: SomeEndianInt):
+    array[sizeof(x), byte] {.inline.} =
+  ## Convert a native endian integer to a native endian byte sequence
+  toBytes(x, bigEndian)
+
+func fromBytes*(
+    T: typedesc[SomeEndianInt],
+    x: openArray[byte],
+    endian: Endianness = system.cpuEndian): T {.inline.} =
+  ## Read bytes and convert to an integer according to the given endianness.
+  ##
+  ## Note: The default value of `system.cpuEndian` is not portable across
+  ## machines.
+  ##
+  ## Panics when `x.len < sizeof(T)` - for shorter buffers, copy the data to
+  ## an `array` first using `arrayops.initCopyFrom`, taking care to zero-fill
+  ## at the right end - usually the beginning for big endian and the end for
+  ## little endian, but this depends on the serialization of the bytes.
+
+  # This check gets optimized away when the compiler can prove that the length
+  # is large enough - passing in an `array` or using a construct like
+  # ` toOpenArray(pos, pos + sizeof(T) - 1)` are two ways that this happens
+  doAssert x.len >= sizeof(T), "Not enough bytes for endian conversion"
+
+  when nimvm: # No copyMem in vm
+    for i in 0..<sizeof(result):
+      result = result or (T(x[i]) shl (i * 8))
+  else:
+    # `copyMem` helps compilers optimize the copy into a single instruction, when
+    # alignment etc permits
+    copyMem(addr result, unsafeAddr x[0], sizeof(result))
+
+  if endian != system.cpuEndian:
+    # The swap is turned into a CPU-specific instruction and/or combined with
+    # the copy above, again when conditions permit it - for example, on X86
+    # fromBytesBE gets compiled into a single `MOVBE` instruction
+    result = swapBytes(result)
+
+func fromBytesBE*(
+    T: typedesc[SomeEndianInt],
+    x: openArray[byte]): T {.inline.} =
+  ## Read big endian bytes and convert to an integer. At runtime, v must contain
+  ## at least sizeof(T) bytes. By default, native endianness is used which is
+  ## not portable!
+  fromBytes(T, x, bigEndian)
+
+func toBE*[T: SomeEndianInt](x: T): T {.inline.} =
+  ## Convert a native endian value to big endian. Consider toBytesBE instead
+  ## which may prevent some confusion.
+  if cpuEndian == bigEndian: x
+  else: x.swapBytes
+
+func fromBE*[T: SomeEndianInt](x: T): T {.inline.} =
+  ## Read a big endian value and return the corresponding native endian
+  # there's no difference between this and toBE, except when reading the code
+  toBE(x)
+
+func fromBytesLE*(
+    T: typedesc[SomeEndianInt],
+    x: openArray[byte]): T {.inline.} =
+  ## Read little endian bytes and convert to an integer. At runtime, v must
+  ## contain at least sizeof(T) bytes. By default, native endianness is used
+  ## which is not portable!
+  fromBytes(T, x, littleEndian)
+
+func toLE*[T: SomeEndianInt](x: T): T {.inline.} =
+  ## Convert a native endian value to little endian. Consider toBytesLE instead
+  ## which may prevent some confusion.
+  if cpuEndian == littleEndian: x
+  else: x.swapBytes
+
+func fromLE*[T: SomeEndianInt](x: T): T {.inline.} =
+  ## Read a little endian value and return the corresponding native endian
+  # there's no difference between this and toLE, except when reading the code
+  toLE(x)