feat(slice): support negative steps like [::-1]

The slice selector only handled a positive step: the check that routes
[start:end:step] to _slice allowed only digits after the second colon, so
[::-1] never reached it, and a step of 0 was silently turned into 1.

Slices now follow RFC 9535 section 2.3.4.2.2. A negative step walks
backwards, with the last item as the default start and the first item as
the default end, and a step of 0 selects nothing. Positive steps behave as
before.

Fixes #176

Author: simonkundrik

src/jsonpath.js

@@ -707,7 +707,7 @@ class JSONPathClass {
             return retObj;
         } else if (loc === '$') { // root only
             addRet(this._trace(x, val, path, null, null, callback, hasArrExpr));
-        } else if ((/^(-?\d*):(-?\d*):?(\d*)$/v).test(loc)) { // [start:end:step]  Python slice syntax
+        } else if ((/^(-?\d*):(-?\d*):?(-?\d*)$/v).test(loc)) { // [start:end:step]  Python slice syntax
             const sliceResult = this._slice(
                 loc, x, val, path, parent, parentPropName, callback
             );
@@ -984,14 +984,37 @@ class JSONPathClass {
             return undefined;
         }
         const len = val.length, parts = loc.split(':'),
-            step = (parts[2] && Number(parts[2])) || 1;
-        let start = (parts[0] && Number(parts[0])) || 0,
-            end = parts[1] ? Number(parts[1]) : len;
-        start = (start < 0) ? Math.max(0, start + len) : Math.min(len, start);
-        end = (end < 0) ? Math.max(0, end + len) : Math.min(len, end);
+            step = parts[2] ? Number(parts[2]) : 1;
         /** @type {ReturnObject[]} */
         const ret = [];
-        for (let i = start; i < end; i += step) {
+        // A step of zero selects nothing (RFC 9535 section 2.3.4.2.2)
+        if (step === 0) {
+            return ret;
+        }
+        /**
+         * @param {number} index
+         * @returns {number}
+         */
+        const normalize = (index) => (index < 0 ? index + len : index);
+        // With a negative step the defaults run backwards: from the last
+        //   item to (and including) the first
+        const start = normalize(
+                parts[0] ? Number(parts[0]) : (step > 0 ? 0 : len - 1)
+            ),
+            end = normalize(
+                parts[1] ? Number(parts[1]) : (step > 0 ? len : -len - 1)
+            );
+        // The first and last positions to visit, whichever way we are going
+        const [from, to] = step > 0
+            ? [
+                Math.min(Math.max(start, 0), len),
+                Math.min(Math.max(end, 0), len)
+            ]
+            : [
+                Math.min(Math.max(start, -1), len - 1),
+                Math.min(Math.max(end, -1), len - 1)
+            ];
+        for (let i = from; step > 0 ? i < to : i > to; i += step) {
             const tmp = this._trace(
                 unshift(i, expr),
                 val,

test/test.slice.js

@@ -64,3 +64,49 @@ describe('JSONPath - slice', function () {
         }), []);
     });
 });
+
+describe('JSONPath - slice with a negative step', function () {
+    const small = [0, 1, 2, 3];
+    const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
+
+    // From RFC 9535 section 2.3.4 and the JSONPath compliance test suite
+    /** @type {[string, number[], number[]][]} */
+    const cases = [
+        ['$[::-1]', small, [3, 2, 1, 0]],
+        ['$[:0:-1]', small, [3, 2, 1]],
+        ['$[2::-1]', small, [2, 1, 0]],
+        ['$[::-2]', small, [3, 1]],
+        ['$[-1:-3:-1]', digits, [9, 8]],
+        ['$[-1:-6:-2]', digits, [9, 7, 5]],
+        ['$[-1:-7:-2]', digits, [9, 7, 5]],
+        ['$[-1:1:-1]', digits, [9, 8, 7, 6, 5, 4, 3, 2]],
+        ['$[7:-5:-1]', digits, [7, 6]],
+        ['$[9:0:-1]', digits, [9, 8, 7, 6, 5, 4, 3, 2, 1]],
+        ['$[113667776004:0:-1]', digits, [9, 8, 7, 6, 5, 4, 3, 2, 1]],
+        ['$[3:-113667776004:-1]', digits, [3, 2, 1, 0]],
+        ['$[-1:-10:-113667776004]', digits, [9]],
+        ['$[::-1]', [], []]
+    ];
+
+    for (const [path, json, expected] of cases) {
+        it(`should evaluate ${path} on ${JSON.stringify(json)}`, function () {
+            assert.deepEqual(jsonpath({json, path}), expected);
+        });
+    }
+
+    it('should reverse objects and report their paths', function () {
+        const json = {children: [{a: 1}, {a: 2}, {a: 3}]};
+        assert.deepEqual(
+            jsonpath({json, path: '$.children[::-1].a'}), [3, 2, 1]
+        );
+        assert.deepEqual(
+            jsonpath({json, path: '$.children[::-2]', resultType: 'path'}),
+            ["$['children'][2]", "$['children'][0]"]
+        );
+    });
+
+    it('should select nothing for a step of zero', function () {
+        assert.deepEqual(jsonpath({json: digits, path: '$[1:2:0]'}), []);
+        assert.deepEqual(jsonpath({json: digits, path: '$[::0]'}), []);
+    });
+});