added experimental undokumented std/varints module

lib/std/varints.nim

@@ -0,0 +1,145 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2018 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## Note this API is still experimental! A variable length integer
+## encoding implementation inspired by SQLite.
+
+const
+  maxVarIntLen* = 9 ## the maximal number of bytes a varint can take
+
+proc readVu64*(z: openArray[byte]; pResult: var uint64): int =
+  if z[0] <= 240:
+    pResult = z[0]
+    return 1
+  if z[0] <= 248:
+    if z.len < 2: return 0
+    pResult = (uint64 z[0] - 241) * 256 + uint64 z[1] + 240
+    return 2
+  if z.len < int(z[0]-246): return 0
+  if z[0] == 249:
+    pResult = 2288u64 + 256u64*z[1].uint64 + z[2].uint64
+    return 3
+  if z[0] == 250:
+    pResult = (z[1].uint64 shl 16u64) + (z[2].uint64 shl 8u64) + z[3].uint64
+    return 4
+  let x = (z[1].uint64 shl 24) + (z[2].uint64 shl 16) + (z[3].uint64 shl 8) + z[4].uint64
+  if z[0] == 251:
+    pResult = x
+    return 5
+  if z[0] == 252:
+    pResult = (((uint64)x) shl 8) + z[5].uint64
+    return 6
+  if z[0] == 253:
+    pResult = (((uint64)x) shl 16) + (z[5].uint64 shl 8) + z[6].uint64
+    return 7
+  if z[0] == 254:
+    pResult = (((uint64)x) shl 24) + (z[5].uint64 shl 16) + (z[6].uint64 shl 8) + z[7].uint64
+    return 8
+  pResult = (((uint64)x) shl 32) +
+              (0xffffffff'u64 and ((z[5].uint64 shl 24) +
+              (z[6].uint64 shl 16) + (z[7].uint64 shl 8) + z[8].uint64))
+  return 9
+
+proc varintWrite32(z: var openArray[byte]; y: uint32) =
+  z[0] = uint8(y shr 24)
+  z[1] = uint8(y shr 16)
+  z[2] = uint8(y shr 8)
+  z[3] = uint8(y)
+
+proc writeVu64*(z: var openArray[byte], x: uint64): int =
+  ## Write a varint into z. The buffer z must be at least 9 characters
+  ## long to accommodate the largest possible varint. Returns the number of
+  ## bytes used.
+  if x <= 240:
+    z[0] = uint8 x
+    return 1
+  if x <= 2287:
+    let y = uint32(x - 240)
+    z[0] = uint8(y shr 8 + 241)
+    z[1] = uint8(y and 255)
+    return 2
+  if x <= 67823:
+    let y = uint32(x - 2288)
+    z[0] = 249
+    z[1] = uint8(y shr 8)
+    z[2] = uint8(y and 255)
+    return 3
+  let y = uint32 x
+  let w = uint32(x shr 32)
+  if w == 0:
+    if y <= 16777215:
+      z[0] = 250
+      z[1] = uint8(y shr 16)
+      z[2] = uint8(y shr 8)
+      z[3] = uint8(y)
+      return 4
+    z[0] = 251
+    varintWrite32(toOpenArray(z, 1, z.high-1), y)
+    return 5
+  if w <= 255:
+    z[0] = 252
+    z[1] = uint8 w
+    varintWrite32(toOpenArray(z, 2, z.high-2), y)
+    return 6
+  if w <= 65535:
+    z[0] = 253
+    z[1] = uint8(w shr 8)
+    z[2] = uint8 w
+    varintWrite32(toOpenArray(z, 3, z.high-3), y)
+    return 7
+  if w <= 16777215:
+    z[0] = 254
+    z[1] = uint8(w shr 16)
+    z[2] = uint8(w shr 8)
+    z[3] = uint8 w
+    varintWrite32(toOpenArray(z, 4, z.high-4), y)
+    return 8
+  z[0] = 255
+  varintWrite32(toOpenArray(z, 1, z.high-1), w)
+  varintWrite32(toOpenArray(z, 5, z.high-5), y)
+  return 9
+
+proc sar(a, b: int64): int64 =
+  {.emit: [result, " = ", a, " >> ", b, ";"].}
+
+proc sal(a, b: int64): int64 =
+  {.emit: [result, " = ", a, " << ", b, ";"].}
+
+proc encodeZigzag*(x: int64): uint64 {.inline.} =
+  uint64(sal(x, 1)) xor uint64(sar(x, 63))
+
+proc decodeZigzag*(x: uint64): int64 {.inline.} =
+  let casted = cast[int64](x)
+  result = (`shr`(casted, 1)) xor (-(casted and 1))
+
+when isMainModule:
+  #import random
+
+  var dest: array[50, byte]
+  var got: uint64
+
+  for test in [0xFFFF_FFFF_FFFFF_FFFFu64, 77u64, 0u64, 10_000_000u64, uint64(high(int64)),
+               uint64(high(int32)),uint64(low(int32)),uint64(low(int64))]:
+    let wrLen = writeVu64(dest, test)
+    let rdLen = readVu64(dest, got)
+    assert wrLen == rdLen
+    echo(if got == test: "YES" else: "NO")
+    echo "number is ", got
+
+    if encodeZigzag(decodeZigzag(test)) != test:
+      echo "Failure for ", test, " ", encodeZigzag(decodeZigzag(test)), " ", decodeZigzag(test)
+
+  # check this also works for floats:
+  for test in [0.0, 0.1, 2.0, +Inf, Nan, NegInf]:
+    let t = cast[uint64](test)
+    let wrLenB = writeVu64(dest, t)
+    let rdLenB = readVu64(dest, got)
+    assert wrLenB == rdLenB
+    echo rdLenB
+    echo(if cast[float64](got) == test: "YES" else: "NO")