travis: Build only changed things

Build library only if the library changed

.travis.yml

@@ -16,14 +16,39 @@ before_script:
 
 script:
     - |
-        if [[ "doc" == $(git diff --name-only $TRAVIS_BRANCH..$TRAVIS_COMMIT | cut -d "/" -f 1 | uniq) ]]; then
-            echo "There are only changes in the ./doc directory... not doing anything"
-        else
+        changes_in() {
+            [[ $(git diff --name-only $TRAVIS_BRANCH..$TRAVIS_COMMIT | \
+                cut -d "/" -f 1 | \
+                grep "$n") ]] > /dev/null
+        }
+
+        travis_cargo_run_in() {
+            [[ -d "$1" ]] &&
+            cd "$1" &&
             travis-cargo build &&
             travis-cargo test &&
             travis-cargo bench &&
-            travis-cargo --only stable doc
-        fi
+            travis-cargo --only stable doc &&
+            cd -
+        }
+
+        if_changed_then_build_or_exit() {
+            if [[ $(changes_in "$1") ]]; then
+                echo "Changes in $1, building..."
+                travis_cargo_run_in "$1" || exit 1
+            else
+                echo "No changes in $1"
+            fi
+        }
+
+        [[ $(changes_in "doc") ]] && echo "Changes in ./doc are not build by CI"
+
+        if_changed_then_build_or_exit "libimagmodule"
+        if_changed_then_build_or_exit "libimagrt"
+        if_changed_then_build_or_exit "libimagstore"
+        if_changed_then_build_or_exit "libimagutil"
+
+        (changes_in "src" && travis_cargo_run_in ".") || true
 
 addons:
     apt: