Add combinations/0 and combinations/1

builtin.c

@@ -1486,7 +1486,17 @@ static const char* const jq_builtins[] = {
   "def first: .[0];",
   "def last: .[-1];",
   "def nth($n): .[$n];",
-  // # transpose a possibly jagged matrix, quickly; 
+  "def combinations:"
+  "    if length == 0 then [] else"
+  "        .[0][] as $x"
+  "          | (.[1:] | combinations) as $y"
+  "          | [$x] + $y"
+  "    end;",
+  "def combinations(n):"
+  "    . as $dot"
+  "      | [range(n) | $dot]"
+  "      | combinations;",
+  // # transpose a possibly jagged matrix, quickly;
   // # rows are padded with nulls so the result is always rectangular.
   "def transpose:"
   "  if . == [] then []"

docs/content/3.manual/manual.yml

@@ -1307,6 +1307,21 @@ sections:
             input: '["foobar", "barfoo"]'
             output: ['[false, true]']
 
+      - title: "`combinations`, `combinations(n)`"
+        body: |
+
+          Outputs all combinations of the elements of the arrays in the
+          input array. If given an argument `n`, it outputs all combinations
+          of `n` repetitions of the input array.
+
+        examples:
+          - program: 'combinations'
+            input: '[[1,2], [3, 4]]'
+            output: ['[1, 3]', '[1, 4]', '[2, 3]', '[2, 4]']
+          - program: 'combinations(2)'
+            input: '[0, 1]'
+            output: ['[0, 0]', '[0, 1]', '[1, 0]', '[1, 1]']
+
       - title: "`ltrimstr(str)`"
         body: |