Add bytesNeeded

dingosky · dingosky · commit 851f837b667c · 2017-08-18T13:31:15.000-07:00
diff --git a/lib/charSet.js b/lib/charSet.js
@@ -2,7 +2,8 @@ import lcm from './lcm'
 import WeakMap from 'weak-map'
 
 const propMap = new WeakMap()
-const bitsPerByte = 8
+
+const BITS_PER_BYTE = 8
 
 export default class CharSet {
   constructor(chars) {
@@ -28,7 +29,7 @@ export default class CharSet {
       bitsPerChar,
       length,
       ndxFn: _ndxFn(bitsPerChar),
-      charsPerChunk: lcm(bitsPerChar, bitsPerByte) / bitsPerChar
+      charsPerChunk: lcm(bitsPerChar, BITS_PER_BYTE) / bitsPerChar
     }
     propMap.set(this, privProps)
   }
@@ -57,38 +58,44 @@ export default class CharSet {
     return propMap.get(this).ndxFn
   }
 
+  bytesNeeded(entropyBits) {
+    const count = Math.ceil(entropyBits / this.bitsPerChar())
+    return Math.ceil(count * this.bitsPerChar() / BITS_PER_BYTE)
+  }
+
   // Aliases
   chars() { return this.getChars() }
   ndxFn() { return this.getNdxFn() }
+  bitsPerChar() { return this.getBitsPerChar() }
 }
 
 const _ndxFn = (bitsPerChar) => {
-  // If bitsPerBytes is a multiple of bitsPerChar, we can slice off an integer number
+  // If BITS_PER_BYTEs is a multiple of bitsPerChar, we can slice off an integer number
   // of chars per byte.
-  if (lcm(bitsPerChar, bitsPerByte) === bitsPerByte) {
+  if (lcm(bitsPerChar, BITS_PER_BYTE) === BITS_PER_BYTE) {
     return function(chunk, slice, bytes) {
       let lShift = bitsPerChar
-      let rShift = bitsPerByte - bitsPerChar
+      let rShift = BITS_PER_BYTE - bitsPerChar
       return ((bytes[chunk]<<(lShift*slice))&0xff)>>rShift
     }
   }
   // Otherwise, while slicing off bits per char, we will possibly straddle a couple
   // of bytes, so a bit more work is involved
   else {
     return function(chunk, slice, bytes) {
-      let slicesPerChunk = lcm(bitsPerChar, bitsPerByte) / bitsPerByte
+      let slicesPerChunk = lcm(bitsPerChar, BITS_PER_BYTE) / BITS_PER_BYTE
       let bNum = chunk * slicesPerChunk
 
-      let offset = (slice*bitsPerChar)/bitsPerByte
+      let offset = (slice*bitsPerChar)/BITS_PER_BYTE
       let lOffset = Math.floor(offset)
       let rOffset = Math.ceil(offset)
 
-      let rShift = bitsPerByte - bitsPerChar
-      let lShift = (slice*bitsPerChar) % bitsPerByte
+      let rShift = BITS_PER_BYTE - bitsPerChar
+      let lShift = (slice*bitsPerChar) % BITS_PER_BYTE
 
       let ndx = ((bytes[bNum+lOffset]<<lShift)&0xff)>>rShift
 
-      let rShiftIt = ((rOffset+1)*bitsPerByte - (slice+1)*bitsPerChar) % bitsPerByte
+      let rShiftIt = ((rOffset+1)*BITS_PER_BYTE - (slice+1)*bitsPerChar) % BITS_PER_BYTE
       if (rShift < rShiftIt) {
         ndx += bytes[bNum+rOffset]>>rShiftIt
       }
