diff --git a/src/io.zig b/src/io.zig
index 3e74486..42a7f47 100644
--- a/src/io.zig
+++ b/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(
@@ -110,40 +111,59 @@ 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);
 
-    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));
+    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));
 
-    const expected_x: [4]vec3.Vec3F64 = .{
-        .init(1, 0, 0),
-        .init(-1, 0, 0),
-        .init(0, 1, 0),
-        .init(0, -1, 0),
+            const expected_x: [4]vec3.Vec3F64 = .{
+                .init(1, 0, 0),
+                .init(-1, 0, 0),
+                .init(0, 1, 0),
+                .init(0, -1, 0),
+            };
+
+            const expected_thrust: [4]f64 = .{ 1, 2, 1, 2 };
+            const expected_moment: [4]f64 = .{ 2, 1, 2, 1 };
+            const expected_direction: [4]constants.PropSpinDirection = .{ .cw, .ccw, .cw, .ccw };
+
+            var i: u8 = 0;
+            while (i < 4) : (i += 1) {
+                try std.testing.expectApproxEqAbs(
+                    expected_thrust[i],
+                    sim_config.propellers[i].thrust_coefficient,
+                    1e-12,
+                );
+                try std.testing.expectApproxEqAbs(
+                    expected_moment[i],
+                    sim_config.propellers[i].moment_coefficient,
+                    1e-12,
+                );
+
+                try std.testing.expect(
+                    vec3.vec3ApproxEq(sim_config.propellers[i].x_cg, expected_x[i], 1e-12),
+                );
+
+                try std.testing.expect(sim_config.propellers[i].direction == expected_direction[i]);
+            }
+        }
     };
 
-    const expected_thrust: [4]f64 = .{ 1, 2, 1, 2 };
-    const expected_moment: [4]f64 = .{ 2, 1, 2, 1 };
-    const expected_direction: [4]constants.PropSpinDirection = .{ .cw, .ccw, .cw, .ccw };
+    try validate_sim_config.validate(string_sim_config);
 
-    var i: u8 = 0;
-    while (i < 4) : (i += 1) {
-        try std.testing.expectApproxEqAbs(
-            expected_thrust[i],
-            sim_config.propellers[i].thrust_coefficient,
-            1e-12,
-        );
-        try std.testing.expectApproxEqAbs(
-            expected_moment[i],
-            sim_config.propellers[i].moment_coefficient,
-            1e-12,
-        );
-
-        try std.testing.expect(
-            vec3.vec3ApproxEq(sim_config.propellers[i].x_cg, expected_x[i], 1e-12),
-        );
-
-        try std.testing.expect(sim_config.propellers[i].direction == expected_direction[i]);
+    {
+        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);
 }