Optimize reading from iterable of coordinate values

[navispatial]

For example Position has:

  /// Creates a position of [R] from [coords] given in order: x, y, z, m.
  ///
  /// The [coords] must contain at least two coordinate values (x and y)
  /// starting from [offset]. If [coords] contains three values, then 3rd item
  /// is z. If [coords] contains four values, then 4th item is m.
  ///
  /// A position instance is created using the factory function [to].
  static R createFromCoords<R extends Position>(
    Iterable<num> coords, {
    required CreatePosition<R> to,
    int offset = 0,
  }) {
    final len = coords.length - offset;
    if (len < 2) {
      throw const FormatException('Coords must contain at least two items');
    }
    return to.call(
      x: coords.elementAt(offset),
      y: coords.elementAt(offset + 1),
      z: len >= 3 ? coords.elementAt(offset + 2) : null,
      m: len >= 4 ? coords.elementAt(offset + 3) : null,
    );
  }

This could be optimized (in case iterable is not a list) by taking Iterator from coords and iterating available coordinate values.

[navispatial]

Optimized

  /// Creates a position of [R] from [coords] given in order: x, y, z, m.
  ///
  /// The [coords] must contain at least two coordinate values (x and y)
  /// starting from [offset]. If [coords] contains three values, then 3rd item
  /// is z. If [coords] contains four values, then 4th item is m.
  ///
  /// A position instance is created using the factory function [to].
  static R createFromCoords<R extends Position>(
    Iterable<num> coords, {
    required CreatePosition<R> to,
    int offset = 0,
  }) {
    // resolve iterator for source coordinates
    final Iterator<num> iter;
    if (offset == 0) {
      iter = coords.iterator;
    } else if (coords.length >= offset + 2) {
      iter = coords.skip(offset).iterator;
    } else {
      throw invalidCoordinates;
    }

    // iterate at least to x and y + optionally z + m => then create position
    if (iter.moveNext()) {
      final x = iter.current;
      if (iter.moveNext()) {
        final y = iter.current;
        final optZ = iter.moveNext() ? iter.current : null;
        final optM = iter.moveNext() ? iter.current : null;
        return to.call(x: x, y: y, z: optZ, m: optM);
      }
    }
    throw invalidCoordinates;
  }

[navispatial]

Other coordinate factory methods taking iterable input with similar impls.

[navispatial]

Still other optimizations as commented above.

Implemented in geobase 0.3.0-dev.1.