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122 lines
3.4 KiB
Nim
122 lines
3.4 KiB
Nim
#
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#
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# Nim's Runtime Library
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# (c) Copyright 2018 Nim contributors
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#
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# See the file "copying.txt", included in this
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# distribution, for details about the copyright.
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#
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## A variable length integer
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## encoding implementation inspired by SQLite.
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##
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## Unstable API.
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const
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maxVarIntLen* = 9 ## the maximal number of bytes a varint can take
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proc readVu64*(z: openArray[byte]; pResult: var uint64): int =
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if z[0] <= 240:
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pResult = z[0]
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return 1
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if z[0] <= 248:
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if z.len < 2: return 0
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pResult = (uint64 z[0] - 241) * 256 + z[1].uint64 + 240
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return 2
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if z.len < int(z[0]-246): return 0
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if z[0] == 249:
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pResult = 2288u64 + 256u64*z[1].uint64 + z[2].uint64
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return 3
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if z[0] == 250:
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pResult = (z[1].uint64 shl 16u64) + (z[2].uint64 shl 8u64) + z[3].uint64
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return 4
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let x = (z[1].uint64 shl 24) + (z[2].uint64 shl 16) + (z[3].uint64 shl 8) + z[4].uint64
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if z[0] == 251:
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pResult = x
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return 5
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if z[0] == 252:
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pResult = (((uint64)x) shl 8) + z[5].uint64
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return 6
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if z[0] == 253:
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pResult = (((uint64)x) shl 16) + (z[5].uint64 shl 8) + z[6].uint64
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return 7
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if z[0] == 254:
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pResult = (((uint64)x) shl 24) + (z[5].uint64 shl 16) + (z[6].uint64 shl 8) + z[7].uint64
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return 8
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pResult = (((uint64)x) shl 32) +
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(0xffffffff'u64 and ((z[5].uint64 shl 24) +
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(z[6].uint64 shl 16) + (z[7].uint64 shl 8) + z[8].uint64))
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return 9
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proc varintWrite32(z: var openArray[byte]; y: uint32) =
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z[0] = cast[uint8](y shr 24)
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z[1] = cast[uint8](y shr 16)
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z[2] = cast[uint8](y shr 8)
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z[3] = cast[uint8](y)
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proc writeVu64*(z: var openArray[byte], x: uint64): int =
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## Write a varint into z. The buffer z must be at least 9 characters
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## long to accommodate the largest possible varint. Returns the number of
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## bytes used.
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if x <= 240:
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z[0] = cast[uint8](x)
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return 1
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if x <= 2287:
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let y = cast[uint32](x - 240)
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z[0] = cast[uint8](y shr 8 + 241)
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z[1] = cast[uint8](y and 255)
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return 2
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if x <= 67823:
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let y = cast[uint32](x - 2288)
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z[0] = 249
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z[1] = cast[uint8](y shr 8)
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z[2] = cast[uint8](y and 255)
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return 3
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let y = cast[uint32](x)
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let w = cast[uint32](x shr 32)
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if w == 0:
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if y <= 16777215:
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z[0] = 250
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z[1] = cast[uint8](y shr 16)
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z[2] = cast[uint8](y shr 8)
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z[3] = cast[uint8](y)
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return 4
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z[0] = 251
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varintWrite32(toOpenArray(z, 1, z.high-1), y)
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return 5
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if w <= 255:
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z[0] = 252
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z[1] = cast[uint8](w)
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varintWrite32(toOpenArray(z, 2, z.high-2), y)
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return 6
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if w <= 65535:
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z[0] = 253
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z[1] = cast[uint8](w shr 8)
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z[2] = cast[uint8](w)
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varintWrite32(toOpenArray(z, 3, z.high-3), y)
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return 7
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if w <= 16777215:
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z[0] = 254
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z[1] = cast[uint8](w shr 16)
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z[2] = cast[uint8](w shr 8)
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z[3] = cast[uint8](w)
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varintWrite32(toOpenArray(z, 4, z.high-4), y)
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return 8
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z[0] = 255
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varintWrite32(toOpenArray(z, 1, z.high-1), w)
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varintWrite32(toOpenArray(z, 5, z.high-5), y)
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return 9
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proc sar(a, b: int64): int64 =
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{.emit: [result, " = ", a, " >> ", b, ";"].}
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proc sal(a, b: int64): int64 =
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{.emit: [result, " = ", a, " << ", b, ";"].}
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proc encodeZigzag*(x: int64): uint64 {.inline.} =
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uint64(sal(x, 1)) xor uint64(sar(x, 63))
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proc decodeZigzag*(x: uint64): int64 {.inline.} =
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let casted = cast[int64](x)
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result = (`shr`(casted, 1)) xor (-(casted and 1))
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