First iteration of locateErrors code.

cardano-foundation · May 16, 2019 · 2355b6f · 2355b6f
1 parent c60e0a5
commit 2355b6f
Showing 1 changed file with 45 additions and 0 deletions.
diff --git a/lib/bech32/src/Codec/Binary/Bech32/Internal.hs b/lib/bech32/src/Codec/Binary/Bech32/Internal.hs
@@ -48,6 +48,7 @@ module Codec.Binary.Bech32.Internal
     , gf_1024_exp
     , gf_1024_log
     , syndrome
+    , locateErrors
 
     ) where
 
@@ -437,3 +438,47 @@ syndrome residue = low
     `xor` (if residue `testBit` 29 then 0x265325c2 else 0)
   where
     low = residue .&. 0x1f
+
+locateErrors :: Int -> Int -> [Int]
+locateErrors residue len =
+    if residue == 0 then [] else
+    if l_s0 /= -1
+            && l_s1 /= -1
+            && l_s2 /= -1
+            && (2 * l_s1 - l_s2 - l_s0 + 2046) `mod` 1023 == 0
+        then
+            let p1 = (l_s1 - l_s0 + 1023) `mod` 1023 in
+            if (p1 >= len) then [] else
+            let l_e1 = l_s0 + (1023 - 997) * p1 in
+            if (l_e1 `mod` 33 > 0) then [] else [p1]
+        else
+            let findError p1 =
+                    let s2_s1p1 = s2 `xor` (if s1 == 0 then 0 else gf_1024_exp Arr.! ((l_s1 + p1) `mod` 1023)) in
+                    if (s2_s1p1 == 0) then [] else
+                    let s1_s0p1 = s1 `xor` (if s0 == 0 then 0 else gf_1024_exp Arr.! ((l_s0 + p1) `mod` 1023)) in
+                    if (s1_s0p1 == 0) then [] else
+                    let l_s1_s0p1 = gf_1024_log Arr.! s1_s0p1 in
+                    let p2 = ((gf_1024_log Arr.! s2_s1p1) - l_s1_s0p1 + 1023) `mod` 1023 in
+                    if (p2 >= len || p1 == p2) then [] else
+                    let s1_s0p2 = s1 `xor` (if s0 == 0 then 0 else gf_1024_exp Arr.! ((l_s0 + p2) `mod` 1023)) in
+                    if (s1_s0p2 == 0) then [] else
+                    let inv_p1_p2 = 1023 - (gf_1024_log Arr.! (gf_1024_exp Arr.! p1)) `xor` (gf_1024_exp Arr.! p2) in
+                    let l_e2 = l_s1_s0p1 + inv_p1_p2 + (1023 - 997) * p2 in
+                    if (l_e2 `mod` 33 > 0) then [] else
+                    let l_e1 = (gf_1024_log Arr.! s1_s0p2) + inv_p1_p2 + (1023 - 997) * p1 in
+                    if (l_e1 `mod` 33 > 0) then [] else
+                    if (p1 < p2)
+                        then [p1, p2]
+                        else [p2, p1] in
+            case filter (not . null) $ map findError [0 .. len - 1] of
+                []     -> []
+                es : _ -> es
+  where
+    syn = syndrome residue
+    s0 = syn .&. 0x3FF
+    s1 = (syn `unsafeShiftR` 10) .&. 0x3FF
+    s2 = syn `unsafeShiftR` 20
+    l_s0 = gf_1024_log Arr.! s0
+    l_s1 = gf_1024_log Arr.! s1
+    l_s2 = gf_1024_log Arr.! s2
+