initial commit

bash/luhn/luhn.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+#
+# External tools: none.
+#
+# V1: initial version
+
+# set to mask to enable logs. 0: none, 255: all
+(( debug=2#00000000 ))
+#(( debug=2#00001111 ))
+# $1: log level (mask), then strings to display.
+debug () {
+    (( debug & $1 )) && shift && echo Line ${BASH_LINENO[0]}: "${@}" >&2
+}
+
+shopt -s extglob
+
+# basic args usage. We will accept the args to be splitted (out of scope
+# for exercise): "luhn.sh '0 0'" is considered equivalent to "luhn.sh 0 0"
+IFS="" number="${*// /}"                # merge args, remove all <spaces>
+debug 1 number="${number}"
+
+((${#number} < 2)) ||                   # args check: only [0-9], at least 2
+    [[ "$number" != +([0-9]) ]] &&
+    echo false && exit 0
+
+((len=${#number}))
+((sum=0))                               # final sum
+
+for ((i=1; i<=len; ++i)); do            # we loop on each digit, from end
+    digit=${number:((-i)):1}
+    case $((i % 2)) in
+        0)                              # even position (relative to end)
+            ((digit *= 2))
+            ((digit >= 10)) && ((digit-=9))
+            debug 3 even ${digit}
+            ;;
+        1)                              # odd position (relative to end)
+            debug 3 odd ${digit}
+            ;;
+
+    esac
+
+    ((sum+=digit))
+    debug 2 sum=${sum}
+done
+((sum % 10)) && echo false || echo true
+
+exit 0
+
+# emacs/vim settings.
+# Local Variables:
+# sh-basic-offset: 4
+# indent-tabs-mode: nil
+# comment-column: 40
+# fill-column: 80
+# End:
+# vim: set tabstop=4 expandtab: