Add support for the slice operator.

The slice operator extracts a section of the input `string` or `Array`,
and it follows the semantics of Array.prototype.slice.

Signed-off-by: Roberto Raggi <roberto.raggi@here.com>

@here/harp-datasource-protocol/StyleExpressions.md

@@ -488,6 +488,15 @@ Returns the element of the array at the given position.
 ["at", number, array]
 ```
 
+## slice
+
+Extracts a section of the input `string` or `Array`.
+
+```javascript
+["slice", input, start, end]
+["slice", input, start]
+```
+
 ## concat
 
 Concatenates the given string values.

@here/harp-datasource-protocol/lib/operators/ArrayOperators.ts

@@ -90,6 +90,30 @@ const operators = {
             }
             return index >= 0 && index < value.length ? value[index] : null;
         }
+    },
+
+    slice: {
+        call: (context: ExprEvaluatorContext, call: CallExpr) => {
+            if (call.args.length < 2) {
+                throw new Error("not enough arguments");
+            }
+            const input = context.evaluate(call.args[0]);
+            if (!(typeof input === "string" || Array.isArray(input))) {
+                throw new Error("input must be a string or an array");
+            }
+            const start = context.evaluate(call.args[1]);
+            if (typeof start !== "number") {
+                throw new Error("expected an index");
+            }
+            let end: number | undefined;
+            if (call.args.length > 2) {
+                end = context.evaluate(call.args[2]) as any;
+                if (typeof end !== "number") {
+                    throw new Error("expected an index");
+                }
+            }
+            return input.slice(start, end);
+        }
     }
 };
 

@here/harp-datasource-protocol/test/ExprEvaluatorTest.ts

@@ -2002,4 +2002,76 @@ describe("ExprEvaluator", function() {
             );
         });
     });
+
+    describe("Operator 'slice'", () => {
+        it("parse", () => {
+            assert.throw(() => evaluate(["slice"]), "not enough arguments");
+            assert.throw(() => evaluate(["slice", "abc"]), "not enough arguments");
+            assert.throw(() => evaluate(["slice", 123, 0]), "input must be a string or an array");
+        });
+
+        it("slice of strings", () => {
+            assert.deepStrictEqual(evaluate(["slice", "abc", 0]), "abc");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 1]), "bc");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 2]), "c");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 3]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 4]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", -1]), "c");
+            assert.deepStrictEqual(evaluate(["slice", "abc", -2]), "bc");
+            assert.deepStrictEqual(evaluate(["slice", "abc", -3]), "abc");
+            assert.deepStrictEqual(evaluate(["slice", "abc", -4]), "abc");
+        });
+
+        it("empty string slices", () => {
+            assert.deepStrictEqual(evaluate(["slice", "abc", 0, 0]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 1, 1]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 2, 2]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 3, 3]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 4, 4]), "");
+        });
+
+        it("extracts slices of one character from strings", () => {
+            assert.deepStrictEqual(evaluate(["slice", "abc", 0, 1]), "a");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 1, 2]), "b");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 2, 3]), "c");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 3, 4]), "");
+            assert.deepStrictEqual(evaluate(["slice", "abc", 4, 5]), "");
+        });
+
+        it("slice of arrays", () => {
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 0]), [10, 20, 30]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 1]), [20, 30]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 2]), [30]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 3]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 4]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], -1]), [30]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], -2]), [20, 30]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], -3]), [
+                10,
+                20,
+                30
+            ]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], -4]), [
+                10,
+                20,
+                30
+            ]);
+        });
+
+        it("extracts empty slices", () => {
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 0, 0]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 1, 1]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 2, 2]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 3, 3]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 4, 4]), []);
+        });
+
+        it("extracts slices of one element from arrays", () => {
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 0, 1]), [10]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 1, 2]), [20]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 2, 3]), [30]);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 3, 4]), []);
+            assert.deepStrictEqual(evaluate(["slice", ["literal", [10, 20, 30]], 4, 5]), []);
+        });
+    });
 });