interface.zig

const std = @import("std");
const mem = std.mem;
const trait = std.meta.trait;

const assert = std.debug.assert;
const expect = std.testing.expect;
const expectEqual = std.testing.expectEqual;

pub const SelfType = opaque{};

fn makeSelfPtr(ptr: anytype) *SelfType {
    if (comptime !trait.isSingleItemPtr(@TypeOf(ptr))) {
        @compileError("SelfType pointer initialization expects pointer parameter.");
    }

    const T = std.meta.Child(@TypeOf(ptr));

    if (@sizeOf(T) > 0) {
        return @ptrCast(*SelfType, ptr);
    } else {
        return undefined;
    }
}

fn selfPtrAs(self: *SelfType, comptime T: type) *T {
    if (@sizeOf(T) > 0) {
        return @alignCast(@alignOf(T), @ptrCast(*align(1) T, self));
    } else {
        return undefined;
    }
}

fn constSelfPtrAs(self: *const SelfType, comptime T: type) *const T {
    if (@sizeOf(T) > 0) {
        return @alignCast(@alignOf(T), @ptrCast(*align(1) const T, self));
    } else {
        return undefined;
    }
}

pub const Storage = struct {
    pub const Comptime = struct {
        erased_ptr: *SelfType,
        ImplType: type,

        fn makeInit(comptime TInterface: type) type {
            return struct {
                fn init(obj: anytype) !TInterface {
                    const ImplType = PtrChildOrSelf(@TypeOf(obj));

                    comptime var obj_holder = obj;

                    return TInterface{
                        .vtable_ptr = &comptime makeVTable(TInterface.VTable, ImplType),
                        .storage = Comptime{
                            .erased_ptr = makeSelfPtr(&obj_holder),
                            .ImplType = @TypeOf(obj),
                        },
                    };
                }
            };
        }

        pub fn getSelfPtr(comptime self: *Comptime) *SelfType {
            return self.erased_ptr;
        }

        pub fn deinit(comptime self: Comptime) void {}
    };

    pub const NonOwning = struct {
        erased_ptr: *SelfType,

        fn makeInit(comptime TInterface: type) type {
            return struct {
                fn init(ptr: anytype) !TInterface {
                    return TInterface{
                        .vtable_ptr = &comptime makeVTable(TInterface.VTable, PtrChildOrSelf(@TypeOf(ptr))),
                        .storage = NonOwning{
                            .erased_ptr = makeSelfPtr(ptr),
                        },
                    };
                }
            };
        }

        pub fn getSelfPtr(self: NonOwning) *SelfType {
            return self.erased_ptr;
        }

        pub fn deinit(self: NonOwning) void {}
    };

    pub const Owning = struct {
        allocator: *mem.Allocator,
        mem: []u8,

        fn makeInit(comptime TInterface: type) type {
            return struct {
                fn init(obj: anytype, allocator: *std.mem.Allocator) !TInterface {
                    const AllocT = @TypeOf(obj);

                    var ptr = try allocator.create(AllocT);
                    ptr.* = obj;

                    return TInterface{
                        .vtable_ptr = &comptime makeVTable(TInterface.VTable, PtrChildOrSelf(AllocT)),
                        .storage = Owning{
                            .allocator = allocator,
                            .mem = std.mem.asBytes(ptr)[0..],
                        },
                    };
                }
            };
        }

        pub fn getSelfPtr(self: Owning) *SelfType {
            return makeSelfPtr(&self.mem[0]);
        }

        pub fn deinit(self: Owning) void {
            const result = self.allocator.shrinkBytes(self.mem, 0, 0, 0, 0);
            assert(result == 0);
        }
    };

    pub fn Inline(comptime size: usize) type {
        return struct {
            const Self = @This();

            mem: [size]u8,

            fn makeInit(comptime TInterface: type) type {
                return struct {
                    fn init(value: anytype) !TInterface {
                        const ImplSize = @sizeOf(@TypeOf(value));

                        if (ImplSize > size) {
                            @compileError("Type does not fit in inline storage.");
                        }

                        var self = Self{
                            .mem = undefined,
                        };
                        if (ImplSize > 0) {
                            std.mem.copy(u8, self.mem[0..], @ptrCast([*]const u8, &args[0])[0..ImplSize]);
                        }

                        return TInterface{
                            .vtable_ptr = &comptime makeVTable(TInterface.VTable, PtrChildOrSelf(@TypeOf(value))),
                            .storage = self,
                        };
                    }
                };
            }

            pub fn getSelfPtr(self: *Self) *SelfType {
                return makeSelfPtr(&self.mem[0]);
            }

            pub fn deinit(self: Self) void {}
        };
    }

    pub fn InlineOrOwning(comptime size: usize) type {
        return struct {
            const Self = @This();

            data: union(enum) {
                Inline: Inline(size),
                Owning: Owning,
            },

            pub fn init(args: anytype) !Self {
                if (args.len != 2) {
                    @compileError("InlineOrOwning storage expected a 2-tuple in initialization.");
                }

                const ImplSize = @sizeOf(@TypeOf(args[0]));

                if (ImplSize > size) {
                    return Self{
                        .data = .{
                            .Owning = try Owning.init(args),
                        },
                    };
                } else {
                    return Self{
                        .data = .{
                            .Inline = try Inline(size).init(.{args[0]}),
                        },
                    };
                }
            }

            pub fn getSelfPtr(self: *Self) *SelfType {
                return switch (self.data) {
                    .Inline => |*i| i.getSelfPtr(),
                    .Owning => |*o| o.getSelfPtr(),
                };
            }

            pub fn deinit(self: Self) void {
                switch (self.data) {
                    .Inline => |i| i.deinit(),
                    .Owning => |o| o.deinit(),
                }
            }
        };
    }
};

fn PtrChildOrSelf(comptime T: type) type {
    if (comptime trait.isSingleItemPtr(T)) {
        return std.meta.Child(T);
    }

    return T;
}

const GenCallType = enum {
    BothAsync,
    BothBlocking,
    AsyncCallsBlocking,
    BlockingCallsAsync,
};

fn makeCall(
    comptime name: []const u8,
    comptime CurrSelfType: type,
    comptime Return: type,
    comptime ImplT: type,
    comptime call_type: GenCallType,
    self_ptr: CurrSelfType,
    args: anytype,
) Return {
    const is_const = CurrSelfType == *const SelfType;
    const self = if (is_const) constSelfPtrAs(self_ptr, ImplT) else selfPtrAs(self_ptr, ImplT);
    const fptr = @field(ImplT, name);
    const first_arg_ptr = comptime std.meta.trait.is(.Pointer)(@typeInfo(@TypeOf(fptr)).Fn.args[0].arg_type.?);
    const self_arg = if (first_arg_ptr) .{self} else .{self.*};

    return switch (call_type) {
        .BothBlocking => @call(.{ .modifier = .always_inline }, fptr, self_arg ++ args),
        .AsyncCallsBlocking, .BothAsync => await @call(.{ .modifier = .async_kw }, fptr, self_arg ++ args),
        .BlockingCallsAsync => @compileError("Trying to implement blocking virtual function " ++ name ++ " with async implementation."),
    };
}

fn getFunctionFromImpl(comptime name: []const u8, comptime FnT: type, comptime ImplT: type) ?FnT {
    const our_cc = @typeInfo(FnT).Fn.calling_convention;

    // Find the candidate in the implementation type.
    for (std.meta.declarations(ImplT)) |decl| {
        if (std.mem.eql(u8, name, decl.name)) {
            switch (decl.data) {
                .Fn => |fn_decl| {
                    const args = @typeInfo(fn_decl.fn_type).Fn.args;

                    if (args.len == 0) {
                        return @field(ImplT, name);
                    }

                    if (args.len > 0) {
                        const arg0_type = args[0].arg_type.?;
                        const is_method = arg0_type == ImplT or arg0_type == *ImplT or arg0_type == *const ImplT;

                        const candidate_cc = @typeInfo(fn_decl.fn_type).Fn.calling_convention;
                        switch (candidate_cc) {
                            .Async, .Unspecified => {},
                            else => return null,
                        }

                        const Return = @typeInfo(FnT).Fn.return_type orelse noreturn;
                        const CurrSelfType = @typeInfo(FnT).Fn.args[0].arg_type.?;

                        const call_type: GenCallType = switch (our_cc) {
                            .Async => if (candidate_cc == .Async) .BothAsync else .AsyncCallsBlocking,
                            .Unspecified => if (candidate_cc == .Unspecified) .BothBlocking else .BlockingCallsAsync,
                            else => unreachable,
                        };

                        if (!is_method) {
                            return @field(ImplT, name);
                        }

                        // TODO: Make this less hacky somehow?
                        // We need some new feature to do so unfortunately.
                        return switch (args.len) {
                            1 => struct {
                                fn impl(self_ptr: CurrSelfType) callconv(our_cc) Return {
                                    return @call(.{ .modifier = .always_inline }, makeCall, .{ name, CurrSelfType, Return, ImplT, call_type, self_ptr, .{} });
                                }
                            }.impl,
                            2 => struct {
                                fn impl(self_ptr: CurrSelfType, arg: args[1].arg_type.?) callconv(our_cc) Return {
                                    return @call(.{ .modifier = .always_inline }, makeCall, .{ name, CurrSelfType, Return, ImplT, call_type, self_ptr, .{arg} });
                                }
                            }.impl,
                            3 => struct {
                                fn impl(self_ptr: CurrSelfType, arg1: args[1].arg_type.?, arg2: args[2].arg_type.?) callconv(our_cc) Return {
                                    return @call(.{ .modifier = .always_inline }, makeCall, .{ name, CurrSelfType, Return, ImplT, call_type, self_ptr, .{ arg1, arg2 } });
                                }
                            }.impl,
                            4 => struct {
                                fn impl(self_ptr: CurrSelfType, arg1: args[1].arg_type.?, arg2: args[2].arg_type.?, arg3: args[3].arg_type.?) callconv(our_cc) Return {
                                    return @call(.{ .modifier = .always_inline }, makeCall, .{ name, CurrSelfType, Return, ImplT, call_type, self_ptr, .{ arg1, arg2, arg3 } });
                                }
                            }.impl,
                            5 => struct {
                                fn impl(self_ptr: CurrSelfType, arg1: args[1].arg_type.?, arg2: args[2].arg_type.?, arg3: args[3].arg_type.?, arg4: args[4].arg_type.?) callconv(our_cc) Return {
                                    return @call(.{ .modifier = .always_inline }, makeCall, .{ name, CurrSelfType, Return, ImplT, call_type, self_ptr, .{ arg1, arg2, arg3, arg4 } });
                                }
                            }.impl,
                            6 => struct {
                                fn impl(self_ptr: CurrSelfType, arg1: args[1].arg_type.?, arg2: args[2].arg_type.?, arg3: args[3].arg_type.?, arg4: args[4].arg_type.?, arg5: args[5].arg_type.?) callconv(our_cc) Return {
                                    return @call(.{ .modifier = .always_inline }, makeCall, .{ name, CurrSelfType, Return, ImplT, call_type, self_ptr, .{ arg1, arg2, arg3, arg4, arg5 } });
                                }
                            }.impl,
                            else => @compileError("Unsupported number of arguments, please provide a manually written vtable."),
                        };
                    }
                },
                else => return null,
            }
        }
    }

    return null;
}

fn makeVTable(comptime VTableT: type, comptime ImplT: type) VTableT {
    if (comptime !trait.isContainer(ImplT)) {
        @compileError("Type '" ++ @typeName(ImplT) ++ "' must be a container to implement interface.");
    }
    var vtable: VTableT = undefined;

    for (std.meta.fields(VTableT)) |field| {
        var fn_type = field.field_type;
        const is_optional = trait.is(.Optional)(fn_type);
        if (is_optional) {
            fn_type = std.meta.Child(fn_type);
        }

        const candidate = comptime getFunctionFromImpl(field.name, fn_type, ImplT);
        if (candidate == null and !is_optional) {
            @compileError("Type '" ++ @typeName(ImplT) ++ "' does not implement non optional function '" ++ field.name ++ "'.");
        } else if (!is_optional) {
            @field(vtable, field.name) = candidate.?;
        } else {
            @field(vtable, field.name) = candidate;
        }
    }

    return vtable;
}

fn checkVtableType(comptime VTableT: type) void {
    if (comptime !trait.is(.Struct)(VTableT)) {
        @compileError("VTable type " ++ @typeName(VTableT) ++ " must be a struct.");
    }

    for (std.meta.declarations(VTableT)) |decl| {
        switch (decl.data) {
            .Fn => @compileError("VTable type defines method '" ++ decl.name ++ "'."),
            .Type, .Var => {},
        }
    }

    for (std.meta.fields(VTableT)) |field| {
        var field_type = field.field_type;

        if (trait.is(.Optional)(field_type)) {
            field_type = std.meta.Child(field_type);
        }

        if (!trait.is(.Fn)(field_type)) {
            @compileError("VTable type defines non function field '" ++ field.name ++ "'.");
        }

        const type_info = @typeInfo(field_type);

        if (type_info.Fn.is_generic) {
            @compileError("Virtual function '" ++ field.name ++ "' cannot be generic.");
        }

        switch (type_info.Fn.calling_convention) {
            .Unspecified, .Async => {},
            else => @compileError("Virtual function's  '" ++ field.name ++ "' calling convention is not default or async."),
        }
    }
}

fn vtableHasMethod(comptime VTableT: type, comptime name: []const u8, is_optional: *bool, is_async: *bool, is_method: *bool) bool {
    for (std.meta.fields(VTableT)) |field| {
        if (std.mem.eql(u8, name, field.name)) {
            is_optional.* = trait.is(.Optional)(field.field_type);
            const fn_typeinfo = @typeInfo(if (is_optional.*) std.meta.Child(field.field_type) else field.field_type).Fn;
            is_async.* = fn_typeinfo.calling_convention == .Async;
            is_method.* = fn_typeinfo.args.len > 0 and blk: {
                const first_arg_type = fn_typeinfo.args[0].arg_type.?;
                break :blk first_arg_type == *SelfType or first_arg_type == *const SelfType;
            };
            return true;
        }
    }

    return false;
}

fn VTableReturnType(comptime VTableT: type, comptime name: []const u8) type {
    for (std.meta.fields(VTableT)) |field| {
        if (std.mem.eql(u8, name, field.name)) {
            const is_optional = trait.is(.Optional)(field.field_type);

            var fn_ret_type = (if (is_optional)
                @typeInfo(std.meta.Child(field.field_type)).Fn.return_type
            else
                @typeInfo(field.field_type).Fn.return_type) orelse noreturn;

            if (is_optional) {
                return ?fn_ret_type;
            }

            return fn_ret_type;
        }
    }

    @compileError("VTable type '" ++ @typeName(VTableT) ++ "' has no virtual function '" ++ name ++ "'.");
}

pub fn Interface(comptime VTableT: type, comptime StorageT: type) type {
    comptime checkVtableType(VTableT);

    const stack_size: usize = if (@hasDecl(VTableT, "async_call_stack_size"))
        VTableT.async_call_stack_size
    else
        1 * 1024 * 1024;

    return struct {
        vtable_ptr: *const VTableT,
        storage: StorageT,

        const Self = @This();
        const VTable = VTableT;
        const Storage = StorageT;

        pub const init = StorageT.makeInit(Self).init;

        pub fn initWithVTable(vtable_ptr: *const VTableT, args: anytype) !Self {
            return .{
                .vtable_ptr = vtable_ptr,
                .storage = try init(args),
            };
        }

        pub fn call(self: anytype, comptime name: []const u8, args: anytype) VTableReturnType(VTableT, name) {
            comptime var is_optional = true;
            comptime var is_async = true;
            comptime var is_method = true;
            comptime assert(vtableHasMethod(VTableT, name, &is_optional, &is_async, &is_method));

            const fn_ptr = if (is_optional) blk: {
                const val = @field(self.vtable_ptr, name);
                if (val) |v| break :blk v;
                return null;
            } else @field(self.vtable_ptr, name);

            if (is_method) {
                const self_ptr = self.storage.getSelfPtr();
                const new_args = .{self_ptr};

                if (!is_async) {
                    return @call(.{}, fn_ptr, new_args ++ args);
                } else {
                    var stack_frame: [stack_size]u8 align(std.Target.stack_align) = undefined;
                    return await @asyncCall(&stack_frame, {}, fn_ptr, new_args ++ args);
                }
            } else {
                if (!is_async) {
                    return @call(.{}, fn_ptr, args);
                } else {
                    var stack_frame: [stack_size]u8 align(std.Target.stack_align) = undefined;
                    return await @asyncCall(&stack_frame, {}, fn_ptr, args);
                }
            }
        }

        pub fn deinit(self: Self) void {
            self.storage.deinit();
        }
    };
}