Validate BotMaker collection function inputs

botmaker/src/java/com/twitter/botmaker/function/collection/First.java

@@ -18,7 +18,7 @@
     arguments = {"the list"},
     returnType = "T",
     deprecated = false,
-    description = "The First element of the list",
+    description = "Returns the first element of a non-empty list.",
     actionLevel = ActionLevel.NO_ACTION,
     examples = {
         "First(List(1))"
@@ -46,6 +46,9 @@ protected CacheLevel getCacheLevel() {
 
   @Override
   public Object apply(Context<Runtime> context, List<Object> list) {
+    if (list.isEmpty()) {
+      throw new IllegalArgumentException("First() requires a non-empty list");
+    }
     return list.get(0);
   }
 }

botmaker/src/java/com/twitter/botmaker/function/collection/FirstN.java

@@ -24,7 +24,10 @@
     },
     returnType = "List<T>",
     deprecated = false,
-    description = "Returns the first n values of the list. if n > len(list) return list.",
+    description =
+        "Returns the first n values of the list."
+            + " Returns an empty list if n is negative."
+            + " Returns the whole list if n is greater than the length of the list.",
     actionLevel = ActionLevel.NO_ACTION,
     examples = {
         "FirstN(List(\"asdf\", \"bas\", \"biz\"), 2)"
@@ -54,6 +57,7 @@ protected CacheLevel getCacheLevel() {
 
   @Override
   public Object apply(Context<Runtime> context, List<Object> list, Long n) {
-    return list.subList(0, Math.min(n.intValue(), list.size()));
+    long itemCount = Math.max(0L, Math.min(n.longValue(), (long) list.size()));
+    return list.subList(0, (int) itemCount);
   }
 }

botmaker/src/java/com/twitter/botmaker/function/collection/Slice.java

@@ -28,7 +28,9 @@
     },
     name = {"Slice", "Substring"},
     returnType = "String",
-    description = "Returns a substring or a sublist",
+    description =
+        "Returns a substring or a sublist."
+            + " Throws an exception if the requested range is invalid.",
     actionLevel = ActionLevel.NO_ACTION,
     examples = {
         "Slice(\"teststringteststring\", 1)",
@@ -74,10 +76,14 @@ private static ImmutableList<ASTNode> validateType(ImmutableList<ASTNode> childr
   protected Object apply(
       Context<Runtime> context, Object input, Long beginIndex, Long endIndex) {
     if (input instanceof String) {
-      return ((String) input).substring(beginIndex.intValue(), endIndex.intValue());
+      String string = (String) input;
+      validateRange(beginIndex, endIndex, string.length());
+      return string.substring(beginIndex.intValue(), endIndex.intValue());
     } else if (input instanceof List) {
+      List list = (List) input;
+      validateRange(beginIndex, endIndex, list.size());
       return Collections.unmodifiableList(new ArrayList<>(
-          ((List) input).subList(beginIndex.intValue(), endIndex.intValue())));
+          list.subList(beginIndex.intValue(), endIndex.intValue())));
     } else {
       throw new IllegalArgumentException(mkErrorMessage(input.getClass()));
     }
@@ -96,6 +102,16 @@ protected Object apply(Context<Runtime> context, Object input, Long beginIndex)
     }
   }
 
+  private static void validateRange(Long beginIndex, Long endIndex, int inputLength) {
+    if (beginIndex < 0 || endIndex < beginIndex || endIndex > inputLength) {
+      throw new IllegalArgumentException(String.format(
+          "invalid Slice() range [%d, %d) for input of length %d",
+          beginIndex,
+          endIndex,
+          inputLength));
+    }
+  }
+
   private static String mkErrorMessage(Class<?> input) {
     return String.format(
         "expecting the input to be of String or List type but received: %s for %s",