From 73c9e177dd18e885ebb230b0eea5c4330d406b9a Mon Sep 17 00:00:00 2001
From: Dimitri Lozeve <dimitri@lozeve.com>
Date: Tue, 19 May 2020 18:58:46 +0200
Subject: [PATCH] Solve problem 7 task 3

---
 Contest2020/Contest2020.dyalog | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/Contest2020/Contest2020.dyalog b/Contest2020/Contest2020.dyalog
index ff76227..e917adf 100644
--- a/Contest2020/Contest2020.dyalog
+++ b/Contest2020/Contest2020.dyalog
@@ -141,8 +141,21 @@
 
                 ∇ digits←ReadUPC bits
                   ⍝ 2020 APL Problem Solving Competition Phase II
-                  ⍝ Stub function for Problem 7, Task 3 - ReadUPC
-                  ⍝ Put your code and comments below here
+                  ⍝ Problem 7, Task 3 - ReadUPC
+                  :If 95≠⍴bits ⍝ incorrect number of bits
+                          digits←¯1
+                  :Else
+                          ⍝ Test if the barcode was scanned right-to-left.
+                          :If 0=2|+/bits[3+⍳7]
+                                  bits←⌽bits
+                          :EndIf
+                          digits←({¯1+lrepr⍳⍵}¨(7/⍳6)⊆42↑3↓bits),{¯1+rrepr⍳⍵}¨(7/⍳6)⊆¯42↑¯3↓bits
+                          :If ~∧/digits∊0,⍳9 ⍝ incorrect parity
+                                  digits←¯1
+                          :ElseIf (⊃⌽digits)≠CheckDigit ¯1↓digits ⍝ incorrect check digit
+                                  digits←¯1
+                          :EndIf
+                  :EndIf
                 ∇
 
                 ∇ parts←Balance nums;subsets;partitions