Nim - This repository contains the Nim compiler, Nim's stdlib, tools, and documentation. (mirror)

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# # # 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 + z[1].uint64 + 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) for test in 0u64..300u64: let wrLen = writeVu64(dest, test) let rdLen = readVu64(dest, got) assert wrLen == rdLen if got != test: echo "BUG! expected: ", test, " got: ", got, " z0: ", dest[0] # 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")


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