Adding partial builds to Travis

.travis.yml

@@ -50,5 +50,4 @@ script:
   - bin/fetch-configlet
   - bin/configlet lint .
   - bin/check-configlet-fmt.sh
-  - cmake -G Ninja
-  - ninja
+  - bin/check-exercises.sh

CMakeLists.txt

@@ -20,56 +20,10 @@ endfunction()
 
 option(EXERCISM_RUN_ALL_TESTS "Run all Exercism tests" On)
 
-foreach(exercise
-    hello-world
-    bob
-    word-count
-    hamming
-    anagram
-    food-chain
-    beer-song
-    nucleotide-count
-    rna-transcription
-    phone-number
-    grade-school
-    robot-name
-    leap
-    etl
-    space-age
-    grains
-    gigasecond
-    triangle
-    clock
-    raindrops
-    difference-of-squares
-    roman-numerals
-    nth-prime
-    sieve
-    binary
-    sum-of-multiples
-    series
-    prime-factors
-    pascals-triangle
-    trinary
-    crypto-square
-    scrabble-score
-    hexadecimal
-    say
-    meetup
-    queen-attack
-    allergies
-    atbash-cipher
-    bracket-push
-    all-your-base
-    pangram
-    binary-search-tree
-    robot-simulator
-    binary-search
-    isogram
-    reverse-string
-    acronym
-    luhn
-)
+file(GLOB exercise_list ${CMAKE_CURRENT_SOURCE_DIR}/exercises/*)
+
+foreach(exercise_dir ${exercise_list})
+    get_filename_component(exercise ${exercise_dir} NAME)
     travis_fixup(${exercise} ${alt_exercise_tree})
     add_subdirectory(${alt_exercise_tree}/${exercise})
 endforeach()

bin/check-exercises.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+#
+# Runs tests only if the current Pull Request has modified a test or the root
+# CMakeLists.txt, or if we are running on the master branch.
+
+# Fail if any command fails
+set -e
+
+repo=$(cd "$(dirname "$0")/.." && pwd)
+
+# Configure all the tests.
+cmake -G Ninja "$repo"
+echo ""
+
+if [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
+    echo "This is not a Pull Request, running all tests."
+    cmake --build "$repo"
+elif git diff --name-only master | grep "^CMakeLists.txt"; then
+    echo "This PR changes the root CMakeLists.txt."
+    echo "Running a single test and then all tests."
+    cmake --build "$repo" -- test_hello-world
+    cmake --build "$repo"
+else
+    tests="$(git diff --name-only master | grep "^exercises/" | cut -d '/' -f2 | sed 's/.*/test_&/' | uniq | tr '\n' ' ')"
+    if [ ${#tests} -ne 0 ]; then
+        echo "Running only tests that have changed."
+        cmake --build "$repo" -- $tests
+    else
+        echo "No tests changed."
+    fi
+fi