Reduce complexity

- doesn't run all combinations, only those that make most differences
- also adds checks for python-six

test-checkdeps.sh

@@ -1,25 +1,28 @@
 #!/bin/bash
 
-# tests checkdeps.py with all possible subsets of dependency packages installed
+# Tests checkdeps.py with each package missing individually, and then present
+# individually. It's not a full test of all combinations, but these are
+# unnecessarily long. This approach has a complexity of n * 2, whereas
+# a complete test has a complexity of n ** 2
 
 distro=$(grep ^VERSION_CODENAME= /etc/os-release | cut -d= -f2)
 
 case $distro in
     trusty)
         method="apt-get"
-        packages=(python-qt4 python-msgpack python-pyopencl python-setuptools build-essential libssl-dev libcap-dev python-prctl)
+        packages=(python-qt4 python-msgpack python-pyopencl python-setuptools build-essential libssl-dev libcap-dev python-prctl python-six)
         ;;
     xenial)
         method="apt-get"
-        packages=(python-qt4 python-msgpack python-pyopencl python-setuptools build-essential libssl-dev libcap-dev python-prctl)
+        packages=(python-qt4 python-msgpack python-pyopencl python-setuptools build-essential libssl-dev libcap-dev python-prctl python-six)
         ;;
     bionic)
         method="apt-get"
-        packages=(python-qt4 python-msgpack python-pyopencl python-setuptools build-essential libssl-dev libcap-dev python-prctl)
+        packages=(python-qt4 python-msgpack python-pyopencl python-setuptools build-essential libssl-dev libcap-dev python-prctl python-six)
         ;;
     focal)
         method="apt-get"
-        packages=(python-setuptools build-essential libssl-dev libcap-dev)
+        packages=(python-setuptools build-essential libssl-dev libcap-dev python-six)
         ;;
     *)
         echo "Unknown distro"
@@ -31,9 +34,10 @@ echo "Using $method on $distro"
 output=$(mktemp)
 succeeded=0
 failed=0
+n=${#packages[@]}
 
-# 0 - 2^n -1
-for i in $(seq 0 $((2**${#packages[@]}-1))); do
+# 0   -    2 * n - 1
+for i in $(seq 0 $((2 * n - 1))); do
     declare -a missing
     missing=()
     case $method in
@@ -44,9 +48,10 @@ for i in $(seq 0 $((2**${#packages[@]}-1))); do
      esac
 
     # iterate packages
-    for j in $(seq 0 $((${#packages[@]}-1))); do
+    for j in $(seq 0 $((n - 1))); do
         # should j-th item be included in i-th iteration?
-        if [ $(((i>>j)%2)) -eq 1 ]; then
+        # present in first half, or absent in second half 
+        if ((i < n ? i % n != j : i % n == j)); then
             case $method in
                 apt-get)
                     cmd="$cmd ${packages[j]}+"