Implement string indexing, closes #12

Author: sparkprime

test_suite/arith_string.jsonnet

@@ -0,0 +1,33 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+std.assertEqual("foo", "foo") &&
+std.assertEqual("foo" + "bar", "foobar") &&
+
+std.assertEqual("foo" + true, "footrue") &&
+std.assertEqual("foo" + false, "foofalse") &&
+std.assertEqual("foo" + 0, "foo0") &&
+std.assertEqual("foo" + null, "foonull") &&
+
+std.assertEqual(true + "foo", "truefoo") &&
+std.assertEqual(false + "foo", "falsefoo") &&
+std.assertEqual(0 + "foo", "0foo") &&
+std.assertEqual(null + "foo", "nullfoo") &&
+
+std.assertEqual("alphabet"[7], "t") &&
+std.assertEqual("alphabet"[0], "a") &&
+
+true

vm.cpp

@@ -1617,7 +1617,7 @@ namespace {
                             if (i < 0 || i >= sz) {
                                 std::stringstream ss;
                                 ss << "Array bounds error: " << i
-                                   << " not within [0, " << sz << "]";
+                                   << " not within [0, " << sz << ")";
                                 throw makeError(ast.location, ss.str());
                             }
                             auto *thunk = array->elements[i];
@@ -1644,18 +1644,39 @@ namespace {
                             stack.pop();
                             ast_ = objectIndex(ast.location, obj, fid);
                             goto recurse;
+                        } else if (target.t == Value::STRING) {
+                            auto *obj = static_cast<HeapString*>(target.v.h);
+                            assert(obj != nullptr);
+                            if (scratch.t != Value::DOUBLE) {
+                                throw makeError(ast.location,
+                                                "String index must be a number, got "
+                                                + type_str(scratch) + ".");
+                            }
+                            // TODO(dcunnin):  UTF-8 support goes here.
+                            long sz = obj->value.length();
+                            long i = (long)scratch.v.d;
+                            if (i < 0 || i >= sz) {
+                                std::stringstream ss;
+                                ss << "String bounds error: " << i
+                                   << " not within [0, " << sz << ")";
+                                throw makeError(ast.location, ss.str());
+                            }
+                            char ch[] = {obj->value[i], '\0'};
+                            scratch = makeString(ch);
                         } else {
-                            std::cerr << "INTERNAL ERROR: Neither an object nor an array."
+                            std::cerr << "INTERNAL ERROR: Not object / array / string."
                                       << std::endl;
                             abort();
                         }
                     } break;
 
                     case FRAME_INDEX_TARGET: {
                         const auto &ast = *static_cast<const Index*>(f.ast);
-                        if (scratch.t != Value::ARRAY && scratch.t != Value::OBJECT) {
+                        if (scratch.t != Value::ARRAY
+                            && scratch.t != Value::OBJECT
+                            && scratch.t != Value::STRING) {
                             throw makeError(ast.location,
-                                            "Can only index objects and arrays, got "
+                                            "Can only index objects, strings, and arrays, got "
                                             + type_str(scratch) + ".");
                         }
                         ast_ = ast.index;