const std = @import("std");

pub const Error = error {};

vtable: *const VTable,
w: *std.Io.Writer,

const Self = @This();

pub const VTable = struct {
    printField: *const fn (
        ptr: *Self,
        comptime field: []const u8,
        comptime fmt: []const u8,
        args: anytype,
    ) Error!void,
};

pub fn printField(
    self: *const Self,
    comptime field: []const u8,
    value: anytype,
) Error!void {
    self.vtable.field(self, field);
    self.vtable.value(self, value);
}

pub const IndentedFormatter = struct {
    indent: u8 = 0,
    depth: usize = 0,

    interface: Self,

    pub fn init(w: *std.Io.Writer) IndentedFormatter {
        return .{
            .interface = .{
                .vtable = &.{
                    .printField = IndentedFormatter.printField,
                },
                .w = w,
            },
        };
    }

    fn printField(
        f: *Self,
        comptime field: []const u8,
        comptime fmt: []const u8,
        args: anytype,
    ) Error!void {
        const self: *IndentedFormatter = @alignCast(@fieldParentPtr("interface", f));

        _ = try self.interface.w.splatByteAll(' ', self.indent * self.depth);
        try self.interface.w.print(field ++ ": " ++ fmt ++ "\n", args);
    }
};