[IO] dumping 2^63 works

constantine/bigints.nim

@@ -40,6 +40,8 @@
 import ./word_types
 
 type Word* = Ct[uint64]
+type BaseType* = uint64 # Exported type for conversion in "normal integers"
+
 const WordBitSize* = sizeof(Word) * 8 - 1
   ## Limbs are 63-bit by default
 

constantine/io.nim

@@ -119,7 +119,7 @@ func parseRawUint*(
 #
 # ############################################################
 
-template bigEndian[T: uint16 or uint32 or uint64](outp: pointer, inp: ptr T) =
+template bigEndianXX[T: uint16 or uint32 or uint64](outp: pointer, inp: ptr T) =
   when T is uint64:
     bigEndian64(outp, inp)
   elif T is uint32:
@@ -127,7 +127,7 @@ template bigEndian[T: uint16 or uint32 or uint64](outp: pointer, inp: ptr T) =
   elif T is uint16:
     bigEndian16(outp, inp)
 
-template littleEndian[T: uint16 or uint32 or uint64](outp: pointer, inp: ptr T) =
+template littleEndianXX[T: uint16 or uint32 or uint64](outp: pointer, inp: ptr T) =
   when T is uint64:
     littleEndian64(outp, inp)
   elif T is uint32:
@@ -135,15 +135,55 @@ template littleEndian[T: uint16 or uint32 or uint64](outp: pointer, inp: ptr T)
   elif T is uint16:
     littleEndian16(outp, inp)
 
-func round_step_up(x: Natural, step: static Natural): int {.inline.} =
-  ## Round the input to the next multiple of "step"
-  assert (step and (step - 1)) == 0, "Step must be a power of 2"
-  result = (x + step - 1) and not(step - 1)
+func dumpRawUintLE(
+        dst: var openarray[byte],
+        src: BigInt) {.inline.}=
+  ## Serialize a bigint into its canonical big-endian representation
+  ## I.e least significant bit is aligned to buffer boundary
+
+  var
+    src_idx, dst_idx = 0
+    acc: BaseType = 0
+    acc_len = 0
+
+  var tail = dst.len
+  while tail > 0:
+    let w = if src_idx < src.limbs.len: src[src_idx].BaseType
+            else: 0
+    inc src_idx
+
+    if acc_len == 0:
+      # Edge case, we need to refill the buffer to output 64-bit
+      # as we can only read 63-bit per word
+      acc = w
+      acc_len = WordBitSize
+    else:
+      let lo = (w shl acc_len) or acc
+      dec acc_len
+      acc = w shr (WordBitSize - acc_len)
+
+      if tail >= sizeof(Word):
+        # Unrolled copy
+        # debugecho src.repr
+        littleEndianXX(dst[dst_idx].addr, lo.unsafeAddr)
+        dst_idx += sizeof(Word)
+        tail -= sizeof(Word)
+      else:
+        # Process the tail
+        when cpuEndian == littleEndian:
+          # When requesting little-endian on little-endian platform
+          # we can just copy each byte
+          for i in dst_idx ..< tail:
+            dst[dst_idx] = byte(lo shr (i-dst_idx))
+        else:
+          # We need to copy from the end
+          for i in 0 ..< tail:
+            dst[dst_idx] = byte(lo shr (tail-i))
 
 func dumpRawUint*(
         dst: var openarray[byte],
         src: BigInt,
-        endian: static Endianness) =
+        order: static Endianness) =
   ## Serialize a bigint into its canonical big-endian or little endian
   ## representation.
   ## A destination buffer of size "BigInt.bits div 8" at minimum is needed.
@@ -157,36 +197,8 @@ func dumpRawUint*(
 
   when BigInt.bits == 0:
     zeroMem(dst, dst.len)
+
+  when order == littleEndian:
+    dumpRawUintLE(dst, src)
   else:
-    var
-      src_idx = 0
-      acc = Word(0)
-      acc_len = 0
-
-    template body(){.dirty.} =
-      let w = if src_idx < src.limbs.len: src[src_idx]
-              else: Word(0)
-      inc src_idx
-
-      if acc_len == 0:
-        # Edge case to avoid shifting by 0
-        acc = w
-        acc_len = WordBitSize
-      else:
-        let lo = (w shr acc_len) or acc
-        dec acc_len
-        acc = w shr (WordBitSize - acc_len)
-        when endian == bigEndian:
-          # We're counting down
-          bigEndian(dst[dst_idx - Word.sizeof], w.unsafeAddr)
-        else:
-          littleEndian(dst[dst_idx], w.unsafeAddr)
-
-    when endian == bigEndian:
-      discard # TODO
-    else:
-      let unroll_stop = round_step_up(dst.len, Word.sizeof)
-      for dst_idx in countup(0, unroll_stop - 1, Word.sizeof):
-        body()
-
-      # Process the tail - TODO
+    {.error: "Not implemented at the moment".}

tests/test_io.nim

@@ -5,10 +5,11 @@
 #   * 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.
 
-import  unittest,
+import  unittest, random,
         ../constantine/[io, bigints]
 
-type T = uint64
+randomize(0xDEADBEEF) # Random seed for reproducibility
+type T = BaseType
 
 suite "IO":
   test "Parsing raw integers":
@@ -29,3 +30,22 @@ suite "IO":
       check:
         T(big[0]) == 0
         T(big[1]) == 1
+
+  test "Parsing and dumping round-trip":
+    block: # "Little-endian"
+      let x = 1'u64 shl 63
+      let x_bytes = cast[array[8, byte]](x)
+      let big = parseRawUint(x_bytes, 64, littleEndian) # It's fine even on big-endian platform. We only want the byte-pattern
+
+      var r_bytes: array[8, byte]
+      dumpRawUint(r_bytes, big, littleEndian)
+      check: x_bytes == r_bytes
+
+    # block: # "Little-endian"
+    #   let x = uint64 rand(0..high(int))
+    #   let x_bytes = cast[array[8, byte]](x)
+    #   let big = parseRawUint(x_bytes, 64, littleEndian) # It's fine even on big-endian platform. We only want the byte-pattern
+
+    #   var r_bytes: array[8, byte]
+    #   dumpRawUint(r_bytes, big, littleEndian)
+    #   check: x_bytes == r_bytes