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charSet.js

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const BITS_PER_BYTE = 8

const {

abs, ceil, floor, log2

} = Math

const gcd = (a, b) => {

let la = a

let lb = b

while (lb !== 0) {

[la, lb] = [lb, la % lb]

}

return abs(la)

}

const lcm = (a, b) => (a / gcd(a, b)) * b

const genNdxFn = (bitsPerChar) => {

// If BITS_PER_BYTEs is a multiple of bitsPerChar, we can slice off an integer number

// of chars per byte.

if (lcm(bitsPerChar, BITS_PER_BYTE) === BITS_PER_BYTE) {

return (chunk, slice, bytes) => {

const lShift = bitsPerChar

const rShift = BITS_PER_BYTE - bitsPerChar

return ((bytes[chunk] << (lShift * slice)) & 0xff) >> rShift

}

// Otherwise, while slicing off bits per char, we can possibly straddle two

// of bytes, so a more work is involved

const slicesPerChunk = lcm(bitsPerChar, BITS_PER_BYTE) / BITS_PER_BYTE

return (chunk, slice, bytes) => {

const bNum = chunk * slicesPerChunk

const offset = (slice * bitsPerChar) / BITS_PER_BYTE

const lOffset = floor(offset)

const rOffset = ceil(offset)

const rShift = BITS_PER_BYTE - bitsPerChar

const lShift = (slice * bitsPerChar) % BITS_PER_BYTE

let ndx = ((bytes[bNum + lOffset] << lShift) & 0xff) >> rShift

const r1Bits = (rOffset + 1) * BITS_PER_BYTE

const s1Bits = (slice + 1) * bitsPerChar

const rShiftIt = (r1Bits - s1Bits) % BITS_PER_BYTE

if (rShift < rShiftIt) {

ndx += bytes[bNum + rOffset] >> rShiftIt

}

return ndx

}

class CharSet {

constructor(chars) {

if (!(typeof chars === 'string' || chars instanceof String)) {

throw new Error('Invalid chars: Must be string')

}

const { length } = chars

if (![2, 4, 8, 16, 32, 64].includes(length)) {

throw new Error('Invalid char count: must be one of 2,4,8,16,32,64')

}

// Ensure no repeated characters

for (let i = 0; i < length; i += 1) {

const c = chars.charAt(i)

for (let j = i + 1; j < length; j += 1) {

if (c === chars.charAt(j)) {

throw new Error('Characters not unique')

}

this.chars = chars

this.bitsPerChar = floor(log2(length))

this.length = length

this.ndxFn = genNdxFn(this.bitsPerChar)

this.charsPerChunk = lcm(this.bitsPerChar, BITS_PER_BYTE) / this.bitsPerChar

}

bytesNeeded(bitLen) {

const count = ceil(bitLen / this.bitsPerChar)

return ceil((count * this.bitsPerChar) / BITS_PER_BYTE)

}

module.exports = {

CharSet

}