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const { Buffer } = require('buffer')
const max = 65536
const cache = {}
// in node 6 Buffer.subarray returns a Uint8Array instead of a Buffer
// later versions return a Buffer
// alternative is Buffer.slice but that creates a new buffer
// creating new buffers takes time
// SubOk is only false on node < 8
const SubOk = Buffer.isBuffer(Buffer.from([1, 2]).subarray(0, 1))
function generateBuffer (i) {
const buffer = Buffer.allocUnsafe(2)
buffer.writeUInt8(i >> 8, 0)
buffer.writeUInt8(i & 0x00FF, 0 + 1)
return buffer
}
function generateCache () {
for (let i = 0; i < max; i++) {
cache[i] = generateBuffer(i)
}
}
function genBufVariableByteInt (num) {
const maxLength = 4 // max 4 bytes
let digit = 0
let pos = 0
const buffer = Buffer.allocUnsafe(maxLength)
do {
digit = num % 128 | 0
num = num / 128 | 0
if (num > 0) digit = digit | 0x80
buffer.writeUInt8(digit, pos++)
} while (num > 0 && pos < maxLength)
if (num > 0) {
pos = 0
}
return SubOk ? buffer.subarray(0, pos) : buffer.slice(0, pos)
}
function generate4ByteBuffer (num) {
const buffer = Buffer.allocUnsafe(4)
buffer.writeUInt32BE(num, 0)
return buffer
}
module.exports = {
cache,
generateCache,
generateNumber: generateBuffer,
genBufVariableByteInt,
generate4ByteBuffer
}
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