urbit-ob

commit 475bd441aa2778795e4c51276805edce245cfb40
parent 04ae6635b4bca0624743fba97119969be846dc3c
Author: Jared Tobin <jared@jtobin.io>
Date:   Thu, 14 Mar 2019 16:03:00 +1300

Use alternate solution to @p problem.

@max19 recommended a solution to the @p problem in urbit/arvo#1105 that
allows us to preserve most, if not all, of the existing @p values, while
also removing colliding cases.

Diffstat:
M
src/internal/ob.js
 | 
32
++++++++++++++++++++++++++------
M
test/ob.test.js
 | 
16
++++++++--------

2 files changed, 34 insertions(+), 14 deletions(-)
diff --git a/src/internal/ob.js b/src/internal/ob.js
@@ -123,6 +123,9 @@ const fend = (arg) => {
  *
  * See: Black and Rogaway (2002), "Ciphers with arbitrary finite domains."
  *
+ * Note that this has been adjusted from the reference paper in order to
+ * support some legacy behaviour.
+ *
  * @param  {String, Number, BN}
  * @return  {BN}
  */
@@ -143,8 +146,10 @@ const fe = (r, a, b, f, m) => {
     if (j > r) {
       return (
           r % 2 !== 0
-        ? a.mul(ell).add(arr)
-        : a.mul(arr).add(ell)
+        ? a.mul(arr).add(ell)
+        : arr.eq(a)
+        ? a.mul(arr).add(ell)
+        : a.mul(ell).add(arr)
       )
     } else {
       const eff = f(j - 1, arr)
@@ -186,6 +191,11 @@ const fice = (arg) => {
 /**
  * Reverse 'feis'.
  *
+ * See: Black and Rogaway (2002), "Ciphers with arbitrary finite domains."
+ *
+ * Note that this has been adjusted from the reference paper in order to
+ * support some legacy behaviour.
+ *
  * @param {Number, String, BN}  arg
  * @return  {BN}
  */
@@ -223,15 +233,25 @@ const fen = (r, a, b, f, m) => {
     }
   }
 
-  const R =
+  const ahh =
+      r % 2 !== 0
+    ? m.div(a)
+    : m.mod(a)
+
+  const ale =
       r % 2 !== 0
     ? m.mod(a)
     : m.div(a)
 
   const L =
-      r % 2 !== 0
-    ? m.div(a)
-    : m.mod(a)
+      ale.eq(a)
+    ? ahh
+    : ale
+
+  const R =
+      ale.eq(a)
+    ? ale
+    : ahh
 
   return loop(r, L, R)
 }
diff --git a/test/ob.test.js b/test/ob.test.js
@@ -65,20 +65,20 @@ describe('fein/fynd', () => {
 
   it('fein matches expected reference values', () => {
     let input = new BN('123456789')
-    let output = new BN('1645384789')
+    let output = new BN('4133907371')
     expect(fein(input).eq(output)).to.equal(true)
 
     input = new BN('15663360')
-    output = new BN('1032628475')
+    output = new BN('3867563781')
     expect(fein(input).eq(output)).to.equal(true)
   })
 
   it('fynd matches expected reference values', () => {
-    let input = new BN('1645384789')
+    let input = new BN('4133907371')
     let output = new BN('123456789')
     expect(fynd(input).eq(output)).to.equal(true)
 
-    input = new BN('1032628475')
+    input = new BN('3867563781')
     output = new BN('15663360')
     expect(fynd(input).eq(output)).to.equal(true)
   })
@@ -124,20 +124,20 @@ describe('feis/tail', () => {
 
   it('feis matches expected reference values', () => {
     let input = new BN ('123456789')
-    let output = new BN('3869445208')
+    let output = new BN('4076532648')
     expect(feis(input).eq(output)).to.equal(true)
 
     input = new BN('15663360')
-    output = new BN('2746999232')
+    output = new BN('2239503936')
     expect(feis(input).eq(output)).to.equal(true)
   })
 
   it('tail matches expected reference values', () => {
-    let input = new BN('3869445208')
+    let input = new BN('4076532648')
     let output = new BN ('123456789')
     expect(tail(input).eq(output)).to.equal(true)
 
-    input = new BN('2746999232')
+    input = new BN('2239503936')
     output = new BN('15663360')
     expect(tail(input).eq(output)).to.equal(true)
   })