const std = @import("std");
const Allocator = std.mem.Allocator;

const Entity = @import("./entity.zig").Entity;

// TODO: Do something better than just `std.debug.assert`.
// TODO: Offer a way to validate paths, like checking for empty parts.

/// Represents the path of an `Entity`, describing its place in the world's
/// hierarchy which is constructed using `ChildOf` relationships.
pub const Path = struct {
    /// Whether the path is specified to be absolute.
    /// Note that a relative path can still be interpreted as absolute.
    absolute: bool,
    /// The string parts that make up the path.
    parts: []const EntityPart,
    /// The allocator that was used to allocate `parts`.
    alloc: ?Allocator = null,
    /// Whether the parts outer array itself is owned by this `Path`.
    owns_array: bool = false,
    /// Whether the parts inner strings are owned by this `Path`.
    owns_parts: bool = false,

    /// Represents an `Entity` in a `Path`, either by name or its numeric id.
    pub const EntityPart = union(enum) {
        id: u32,
        name: []const u8,
    };

    /// Format used to parse and stringify `Path`s.
    ///
    /// When this type is formatted, you may use any of the `FormatOption`
    /// constants in this type by name as `fmt` specifier, such as `"{unix}"`.
    /// An empty `fmt` results in the `default` format being used, which can be
    /// changed at runtime.
    pub const FormatOptions = struct {
        /// The root separator used for an absolute `Path`, if any.
        ///
        /// If set to `null`, absolute paths can't be represented using strings.
        /// In this case, an absolue path and relative path with identical parts
        /// will be indistinguishable.
        root_sep: ?[]const u8,
        /// The separator used between parts that make up a `Path`.
        sep: []const u8,

        /// The format used by Flecs' C API. For example `flecs.core`.
        pub const flecs_c = FormatOptions{ .root_sep = null, .sep = "." };
        /// The format used by Flecs' C++ API. For example `::flecs::core`.
        pub const flecs_cpp = FormatOptions{ .root_sep = "::", .sep = "::" };
        /// Unix-like format. For example `/flecs/core`.
        pub const unix = FormatOptions{ .root_sep = "/", .sep = "/" };

        /// The default format used when none is specified. Can be changed at runtime.
        pub var default = flecs_c;
    };

    /// Creates an array of `EntityPath`s with known size equal to the number of
    /// elements in the specified tuple argument. Each element of the tuple is
    /// either converted to a `.name` part, or an `.id` part.
    ///
    /// Keep in mind the mutability and lifetime of the string elements passed
    /// to this function, as they aren't cloned and ownership stays the same.
    /// In many cases, the lifetime of `Path`s is relatively short. When this
    /// is not the case, it's recommended to `.clone()` the path after creation.
    pub fn buildParts(parts: anytype) t: {
        if (!std.meta.trait.isTuple(@TypeOf(parts)))
            @compileError("Expected tuple, got '" ++ @typeName(@TypeOf(parts)) ++ "'");
        const len = @typeInfo(@TypeOf(parts)).Struct.fields.len;
        break :t [len]EntityPart;
    } {
        const len = @typeInfo(@TypeOf(parts)).Struct.fields.len;
        var result: [len]EntityPart = undefined;
        inline for (&result, parts) |*res, part| {
            const msg = "Expected '[]const u8' or 'u32', got '" ++ @typeName(@TypeOf(part)) ++ "'";
            res.* = switch (@typeInfo(@TypeOf(part))) {
                .Pointer => |p| switch (p.size) {
                    .One => switch (@typeInfo(p.child)) {
                        .Array => |a| if (a.child == u8) .{ .name = part } else @compileError(msg),
                        else => @compileError(msg),
                    },
                    .Slice => if (p.child == u8) .{ .name = part } else @compileError(msg),
                    else => @compileError(msg),
                },
                .Int, .ComptimeInt => .{ .id = part },
                else => @compileError(msg),
            };
        }
        return result;
    }

    /// Creates a new `Path` from the specified parts.
    ///
    /// The resulting path does not own any of the given slices.
    pub fn fromParts(absolute: bool, parts: []const EntityPart) Path {
        std.debug.assert(parts.len > 0);
        return .{ .absolute = absolute, .parts = parts };
    }

    /// Parses a string as a `Path` using the `FormatOptions` specified,
    /// or `FormatOptions.default` if the argument is `null`.
    ///
    /// If the string starts with the specified root separator (if any), the
    /// resulting path will be absolute. The rest of the string will be split
    /// by the specified seperator, becoming its parts.
    ///
    /// The parts array will be allocated with the specified `Allocator` and is
    /// owned by the resulting path. `deinit()` must be called to free it.
    pub fn fromString(path: []const u8, options: ?FormatOptions, alloc: Allocator) !Path {
        if (path.len == 0) return error.MustNotBeEmpty;
        const opt = options orelse FormatOptions.default;

        var remaining = path;
        var absolute = false;

        // If `root_sep` is set and path starts with it, the path is absolute.
        if (opt.root_sep) |p| {
            if (std.mem.startsWith(u8, remaining, p)) {
                remaining = remaining[p.len..];
                absolute = true;
            }
        }

        const parts_len = std.mem.count(u8, remaining, opt.sep) + 1;
        const parts = try alloc.alloc(EntityPart, parts_len);

        var i: usize = 0;
        var it = std.mem.splitSequence(u8, remaining, opt.sep);
        while (it.next()) |str| : (i += 1)
            parts[i] = if (parseNumericId(str)) |id|
                .{ .id = id }
            else
                .{ .name = str };

        return .{
            .absolute = absolute,
            .parts = parts,
            .alloc = alloc,
            .owns_array = true,
        };
    }

    /// Creates a `Path` for the specified `child` entity, optionally in
    /// relation to the specified `parent` entity. If `parent` is not `null`,
    /// the resulting path is relative. Otherwise it will be absolute.
    ///
    /// This function allocates an array for the parts that make up the entity's
    /// path, however each part itself is owned by Flecs and could change or be
    /// invalidated ay any time, such as when an entity is renamed or removed.
    pub fn fromEntity(comptime ctx: type, parent: ?Entity(ctx), child: Entity(ctx), alloc: Allocator) !Path {
        std.debug.assert(child.raw != 0);
        if (parent) |p| {
            std.debug.assert(p.raw != 0);
            std.debug.assert(p.raw != child.raw);
        }

        // TODO: Use a threadlocal field with reasonable size, then clone the result.
        const starting_capacity: usize = 12;
        var parts = try alloc.alloc(EntityPart, starting_capacity);
        errdefer alloc.free(parts);
        var num_parts: usize = 0;

        // Traverse up the entity hierarchy starting from the specified child
        // entity up until either the specified parent or root of the hierarchy.
        var current = child;
        while (true) {
            std.debug.assert(num_parts == 0 or current.raw != child.raw); // Cycle detected.
            parts[num_parts] = if (current.getName()) |name|
                .{ .name = name }
            else
                .{ .id = current.getEntityId() };
            num_parts += 1;

            // Move to the parent entity, if any.
            current = current.getParent() orelse
                // If `parent` wasn't specified, we reached the root. Done.
                // Otherwise, if the parent wasn't found, return an error.
                if (parent == null) break else return error.ParentNotFound;

            // If we reached the specified `parent`, we're done here!
            if (parent != null and current.raw == parent.?.raw) break;
        }

        parts = try alloc.realloc(parts, num_parts);
        std.mem.reverse(EntityPart, parts);

        return .{
            .absolute = parent != null,
            .parts = parts,
            .alloc = alloc,
            .owns_array = true,
        };
    }

    /// Creates a deep clone of this `Path` using the specified `Allocator`.
    pub fn clone(orig: Path, alloc: Allocator) !Path {
        var parts = try alloc.dupe(EntityPart, orig.parts);
        errdefer alloc.free(parts);

        var num_allocated: usize = 0;
        errdefer for (parts[0..num_allocated]) |part|
            if (part == .name) alloc.free(part.name);

        for (parts) |*part| {
            if (part.* == .name)
                part.* = .{ .name = try alloc.dupe(u8, part.name) };
            num_allocated += 1;
        }

        return .{
            .absolute = orig.absolute,
            .parts = parts,
            .alloc = alloc,
            .owns_array = true,
            .owns_parts = true,
        };
    }

    /// Destroys any memory owned by this `Path`, if any.
    pub fn deinit(self: Path) void {
        if (self.owns_parts)
            for (self.parts) |part|
                if (part == .name)
                    self.alloc.?.free(part.name);
        if (self.owns_array)
            self.alloc.?.free(self.parts);
    }

    pub fn toString(
        self: Path,
        options: ?FormatOptions,
        alloc: Allocator,
    ) ![:0]const u8 {
        const opt = options orelse FormatOptions.default;
        const length = self.calculateLength(opt);
        var result = try alloc.allocSentinel(u8, length, 0);
        var stream = std.io.fixedBufferStream(result);
        try write(self, opt, stream.writer());
        return result;
    }

    pub fn format(
        self: Path,
        comptime fmt: []const u8,
        options: std.fmt.FormatOptions,
        writer: anytype,
    ) !void {
        _ = options; // TODO: Actually make use of this.

        const opt = if (fmt.len == 0)
            FormatOptions.default
        else if (@hasDecl(FormatOptions, fmt) and @TypeOf(@field(FormatOptions, fmt)) == FormatOptions)
            @field(FormatOptions, fmt)
        else
            std.fmt.invalidFmtError(fmt, Path);

        try self.write(opt, writer);
    }

    /// Returns whether the contents of the specified `Path`s are equivalent.
    ///
    /// Path equivalency does not imply these paths are or are not referring to
    /// the same `Entity`. For example, an entity that is referred to using its
    /// entity id has a different path from the same entity referred to by name.
    pub fn equals(first: Path, second: Path) bool {
        if (first.absolute != second.absolute) return false;
        if (first.parts.len != second.parts.len) return false;
        for (first.parts, second.parts) |a, b| switch (a) {
            .id => |a_id| if (b != .id or a_id != b.id) return false,
            .name => |a_name| if (b != .name or !std.mem.eql(u8, a_name, b.name)) return false,
        };
        return true;
    }

    fn calculateLength(self: Path, opt: FormatOptions) usize {
        // Separators.
        var result = opt.sep.len * (self.parts.len - 1);
        // Root separator.
        if (self.absolute) {
            if (opt.root_sep) |p|
                result += p.len;
        }
        // Parts themselves.
        for (self.parts) |part|
            result += switch (part) {
                .id => |id| numDigits(id),
                .name => |name| name.len,
            };
        return result;
    }

    fn write(self: Path, opt: FormatOptions, writer: anytype) !void {
        // Write root separator (if applicable).
        if (self.absolute) {
            if (opt.root_sep) |p|
                try writer.writeAll(p);
        }
        // Write the first part.
        switch (self.parts[0]) {
            .id => |id| try writer.writeIntNative(u32, id),
            .name => |name| try writer.writeAll(name),
        }
        // Write the remaining parts, each preceeded bu separator.
        for (self.parts[1..]) |part| {
            try writer.writeAll(opt.sep);
            switch (part) {
                .id => |id| try writer.writeIntNative(u32, id),
                .name => |name| try writer.writeAll(name),
            }
        }
    }

    /// Attempts to parse the specified string as a numeric entity id.
    fn parseNumericId(str: []const u8) ?u32 {
        if (str.len == 0 or str.len > 10) return null;
        var result: u32 = 0;
        var place: u32 = 1;
        var it = std.mem.reverseIterator(str);
        while (it.next()) |chr| {
            const d = std.fmt.charToDigit(chr, 10) catch return null;
            result = std.math.add(u32, result, d * place) catch return null;
            place *%= 10; // Wrapping to avoid overflow error.
        }
        return result;
    }

    /// Calculates the numbers of digits of an entity id when formatted as a
    /// string. Since entity ids tend to be small this is technically optimized
    /// for smaller numbers but performance likely doesn't matter much.
    fn numDigits(n: u32) usize {
        // zig fmt: off
        return if (n < 10) 1
          else if (n < 100) 2
          else if (n < 1000) 3
          else if (n < 10000) 4
          else if (n < 100000) 5
          else if (n < 1000000) 6
          else if (n < 10000000) 7
          else if (n < 100000000) 8
          else if (n < 1000000000) 9
          else 10;
        // zig fmt: on
    }
};

test Path {
    const alloc = std.testing.allocator;
    const expect = std.testing.expect;
    const expectFmt = std.testing.expectFmt;
    const expectEqual = std.testing.expectEqual;
    const expectEqualStrings = std.testing.expectEqualStrings;

    // Paths may be constructed by parsing strings.
    const relative = try Path.fromString("some.relative.path", null, alloc);
    defer relative.deinit();

    // Alternatively they can be made by specifying the individual component
    // parts they're made of, as well as an argument specifying whether it's
    // an absolute path.
    // To do this you can use the `buildParts` helper function, which is less
    // wordy than building `EntityPart` structs manually.
    const absolute1_parts = Path.buildParts(.{ "I'm", "absolute!" });
    const absolute1 = Path.fromParts(true, &absolute1_parts);
    // No need to call `deinit()`. The path does not own the `absolute1_parts`
    // array nor the string parts, which in this case are comptime constants.

    // When handling paths, always be aware of the lifetime of its array and
    // any string parts contained within it. This API allows you to completely
    // avoid allocation if you know what you are doing.
    // In the above example, `absolute_parts` is an array allocated onto the
    // stack, and as such will only be valid until the end of the scope. This
    // means no allocation is necessary, but it also means `absolute1` is only
    // valid for as long as its parts.
    // If a path instance is not immediately consumed and you're uncertain
    // about the lifetime of its parts, consider using `.clone(alloc)`.

    // With `options` unspecified, it's not possible to represent an absolute
    // path using a string. Pass your own `FormatOptions` to be able to.
    const absolute2 = try Path.fromString("/home/copygirl", Path.FormatOptions.unix, alloc);
    defer absolute2.deinit();

    // Use `.absolute` to test if the path is absolute.
    // Relative paths can be treated as absolute in the absence of a context.
    // Using absolute paths where a relative one is expected may cause an error.
    try expectEqual(false, relative.absolute);
    try expectEqual(true, absolute1.absolute);
    try expectEqual(true, absolute2.absolute);

    // The internal component parts of the path can be accessed with `.parts`.
    try expectEqual(@as(usize, 3), relative.parts.len);
    try expectEqualStrings("some", relative.parts[0].name);
    try expectEqualStrings("relative", relative.parts[1].name);
    try expectEqualStrings("path", relative.parts[2].name);

    // Parts can also be numeric ids, used for entities that don't have a name.
    const numeric1 = try Path.fromString("100.101.bar", null, alloc);
    defer numeric1.deinit();
    try expectEqual(@as(usize, 3), numeric1.parts.len);
    try expectEqual(@as(u32, 100), numeric1.parts[0].id);
    try expectEqual(@as(u32, 101), numeric1.parts[1].id);
    try expectEqualStrings("bar", numeric1.parts[2].name);

    // Numeric ids can also be passed to `buildParts`.
    const numeric2_parts = Path.buildParts(.{ 100, 101, "bar" });
    const numeric2 = Path.fromParts(false, &numeric2_parts);
    try expect(numeric1.equals(numeric2));

    // Paths are formattable. As format specifier you can use options defined
    // on the `FormatOptions` type, or an empty string to use the default.
    try expectFmt("some.relative.path", "{}", .{relative});
    try expectFmt("::I'm::absolute!", "{flecs_cpp}", .{absolute1});
    try expectFmt("/home/copygirl", "{unix}", .{absolute2});

    // They can also be turned directly into strings, which
    // allows you to use entirely custom `FormatOptions`s:
    const absolute1_str = try absolute1.toString(.{ .root_sep = "= ", .sep = " + " }, alloc);
    defer alloc.free(absolute1_str);
    try expectEqualStrings("= I'm + absolute!", absolute1_str);

    // The default `FormatOptions` may be changed.
    Path.FormatOptions.default = Path.FormatOptions.unix;

    // This affects functions that use them to parse or format strings.
    try expectFmt("some/relative/path", "{}", .{relative});
    const another_relative = try Path.fromString("mom/sister/child", null, alloc);
    defer another_relative.deinit();
    try expectFmt("mom/sister/child", "{}", .{another_relative});

    // A deep clone of a path can be allocated using `.clone()`.
    // This clone owns the outer array and its inner strings.
    const twin = try another_relative.clone(alloc);
    try expectFmt("mom/sister/child", "{}", .{twin});
    twin.deinit();
}