const std = @import("std"); const Allocator = std.mem.Allocator; pub const Result = enum {    accepted,    refused,    unavailable,}; pub const ValidateFn = *const fn (?*anyopaque, Allocator, []const u8) Result; pub const Provider = struct {    /// When set, context must remain valid until every in-flight validation returns.    context: ?*anyopaque = null,    validate_fn: ValidateFn,     pub fn validate(self: Provider, alloc: Allocator, api_key: []const u8) Result {        return self.validate_fn(self.context, alloc, api_key);    }}; fn validateUnavailable(_: ?*anyopaque, _: Allocator, _: []const u8) Result {    return .unavailable;} pub const unavailable_provider = Provider{    .validate_fn = validateUnavailable,}; test "api key validator dispatches through the injected provider" {    const Fake = struct {        calls: usize = 0,         fn validate(raw_ctx: ?*anyopaque, _: Allocator, api_key: []const u8) Result {            const self: *@This() = @ptrCast(@alignCast(raw_ctx.?));            self.calls += 1;            return if (std.mem.eql(u8, api_key, "accepted-key")) .accepted else .refused;        }    };     var fake: Fake = .{};    const provider = Provider{        .context = &fake,        .validate_fn = Fake.validate,    };     try std.testing.expectEqual(Result.accepted, provider.validate(std.testing.allocator, "accepted-key"));    try std.testing.expectEqual(Result.refused, provider.validate(std.testing.allocator, "other-key"));    try std.testing.expectEqual(@as(usize, 2), fake.calls);}