file_identifier "LRSD";

table Property {
    key:string (required, key);
    value:string (required);
}

enum GeometryType : byte { Geographic = 1, Schematic = 2 }

table Lrs {
    properties:[Property];

    /// In the network topology, segments are connected by nodes.
    segments:[Segment];
    /// In the network topology, a node is the end of a segment and usually the intersection of multiple segments.
    nodes:[Node];
    /// Each network has traversals, which can be thought of as roads, railway lines, tracks, paths or trips.
    traversals:[Traversal];

    anchors:[Anchor];
    linear_referencing_methods:[LinearReferencingMethod];

    /// Whether the geometry is geographic or projected.
    /// Computation of distances and length will be influenced accordingly.
    geometry_type:GeometryType = Geographic;
}

/// A continuous link between two network nodes. Segments can be located in space.
/// It could be a section of roads between intersections, a piece of railway tracks without switches, a continuous piece of sewer pipe.
/// Segments are directed: one of its ends is its begining, and the other one its end.
table Segment {
    id:string (required);
    properties:[Property];
    geometry:[Point] (required);
    start_node_index:uint64;
    end_node_index:uint64;
}

/// Nodes are connections between segment endpoints.
/// A node can have a single connection (at the end of a dead end), or multiple segment connections (at an intersection).
/// Nodes can be used to model point of interest in a network, such as road intersections, railway switches, the end of a dead ends.
table Node {
    id:string (required);
    properties:[Property];
    geometry: Point;
}

/// A traversal is a path in a network.
/// Traversals may be used to model roads, railway tracks, railway lines or trips.
/// Traversals are defined as a sequence of segment and direction pairs.
enum Direction : byte { Increasing, Decreasing }

struct SegmentOfTraversal {
    segment_index:uint64;
    direction:Direction;
}

table Traversal {
    id:string (required);
    properties:[Property];
    segments:[SegmentOfTraversal] (required);
}

/// Anchors are reference locations, used for positioning within a Linear Referencing Method.
/// There are two types of anchors:
///   * most anchors are standalone reference locations, such as milestones or kilometer markers
///   * some anchors are associated with a network node. The location of the anchor is deduced from the location of the node.
table Anchor {
    id:string (required);
    properties:[Property];

    /// Most anchors have a name, which is used to reference the location.
    /// It can often be a kilometer or mile number, but it can also be a letter or word.
    name:string;
    /// Anchors can be bound to a node, or defined independently.
    /// If the anchor is bound to a node, its location is deduced from location of the node.
    node:uint32;
    /// The anchor can also be defined by a geographical position.
    geometry: Point;
}

/// The anchors can be placed outside of the curve (a visible landmark outside, bound to an other parallel track…)
/// We can store the projected anchors to avoid the recomputation when loading the data
table ProjectedAnchor {
    /// The projected position on the curve
    geometry: Point;
    /// The distance from the start of the curve until the projected position of the anchor
    distance_along_curve:float64;
}

enum DistanceUnit : byte { Meters, MilliMeters }

/// Linear Referencing Methods (LRMs) are curves in space, along which distances can be measured.
/// Each linear referencing method has:
///   * a network traversal, which defines the path of the curve
///   * a sequence of anchors, which are projected on the curve, and used as positioning reference points
///   * distances between anchors have to be defined: even though distances can be measured on the curve,
//      it can be imprecise enough to joepardize correct ordering of object positioned relative to different reference points.
table LinearReferencingMethod {
    id:string (required);
    properties:[Property];

    traversal_index:uint32;
    anchor_indices:[uint64] (required);
    distances:[double] (required);

    /// If the anchors are projected, the all must be projected.
    /// `projected_anchors` is either null, or has the same size as `anchor_indices`
    projected_anchors:[ProjectedAnchor];
    /// The unit used to measure the distance between anchors
    distance_unit:DistanceUnit = Meters;
    /// The unit used to express measures relative to anchors (12+230).
    measure_unit:DistanceUnit = Meters;
}

struct Point {
    x:float64;
    y:float64;
}

root_type Lrs;