fix allocation bug and add test for json file to sim config

src/io.zig

@@ -20,18 +20,19 @@ const JsonSim = struct {
 };
 
 pub fn jsonFileToSimConfig(
-    file_path: []u8,
+    file_path: []const u8,
     allocator: std.mem.Allocator,
     io: std.Io,
 ) !drone.QuadCopterSim {
     const cwd = std.Io.Dir.cwd();
-    const file = try cwd.openFile(io, file_path, .{ .mode = .read });
+    const file = try cwd.openFile(io, file_path, .{ .mode = .read_only });
     defer file.close(io);
 
     var file_reader = file.reader(io, &.{});
     const json = try file_reader.interface.allocRemaining(allocator, .unlimited);
+    defer allocator.free(json);
 
-    return jsonStringToSimConfig(json, allocator, io);
+    return jsonStringToSimConfig(json, allocator);
 }
 
 pub fn jsonStringToSimConfig(
@@ -110,8 +111,10 @@ test "Verify parse from Json String works" {
         \\}
     ;
     const gpa = std.testing.allocator;
-    const sim_config = try jsonStringToSimConfig(TEST_JSON_STRING, gpa);
+    const string_sim_config = try jsonStringToSimConfig(TEST_JSON_STRING, gpa);
 
+    const validate_sim_config = struct {
+        pub fn validate(sim_config: drone.QuadCopterSim) !void {
             try std.testing.expectApproxEqAbs(0.5, sim_config.drone.mass_kg, 1e-12);
             try std.testing.expectApproxEqAbs(0.25, sim_config.drone.drag_coeff, 1e-12);
             try std.testing.expect(vec3.mat3ApproxEq(sim_config.drone.moment_of_inertia, vec3.Mat3F64.identity(), 1e-12));
@@ -147,3 +150,20 @@ test "Verify parse from Json String works" {
                 try std.testing.expect(sim_config.propellers[i].direction == expected_direction[i]);
             }
         }
+    };
+
+    try validate_sim_config.validate(string_sim_config);
+
+    {
+        const cwd = std.Io.Dir.cwd();
+        const file = try cwd.createFile(std.testing.io, "/tmp/zig_json_config_test.json", .{});
+        // Don't forget to close the file at the end.
+        defer file.close(std.testing.io);
+        // Do things with the file ...
+        _ = try file.writePositionalAll(std.testing.io, TEST_JSON_STRING, 0);
+    }
+
+    const file_sim_config = try jsonFileToSimConfig("/tmp/zig_json_config_test.json", gpa, std.testing.io);
+
+    try validate_sim_config.validate(file_sim_config);
+}