Since the topic comes up periodically, and actual use of splitline
will require recognising discrete shapes, I thought I would have
another go at a definition.

A Shape is a simple polygon and its enclosed area.

A Block consists of one or more non-intersecting Shapes with an
associated population.

Blocks should ideally consist of a single convex shape.

A Region consists of one or more non-intersecting Blocks. Any holes
in a Region should be filled with a zero population Block.

Each contiguous group of Blocks within a Region shall be considered a
Sub-Region.

The Region shall be split into two Semi-Regions.

D is defined as the number of districts.

M is equal to the floor of D/2.

N is equal to D - M.

The target population is equal to the total population divided by D.

A Cut of a Region shall be defined by two points.

A Cut shall be considered a Valid Cut if

- the two points are on the boundary of a Sub-Region

- the line segment connecting the two points does not leave the Sub-Region

- if all Blocks that intercept the line segment are removed, the
remaining Blocks of the Sub-Region form 2 contiguous Areas

- the total population of Blocks on one side of the line is less than
N time the target population and less than M times the target
population on the other side

When determining which side of the line a Block falls, the following
rules shall be used:

- All Blocks in Sub-Regions that fall entirely on one side of the line
shall be considered to fall on that side of the line

- All Blocks in Sub-Regions that intercept the line shall be undefined

- All Blocks in the Sub-Region containing the line segment shall be
considered to fall on whichever side of the line segment the
contiguous area that contains the block falls on.

- All Blocks that intercept the line segment shall be undefined

To split a Region, find the shortest Valid Cut for that Region.

All Blocks which fall on a defined side of the line shall be placed in
that Semi-Region.

A placement of the undefined Blocks between the Semi-Regions shall be
a Valid Placement if

- The Blocks in the Sub-Region containing the line segment shall be
split into 2 contiguous areas
- For each of the other Sub-Regions, all its Blocks are placed in one
or other of the Semi-Regions

The Valid Placement that has the lowest population imbalance[**] shall
be the placement selected. Where there is a tie, find the most
Southern* undefined block that is placed differently between the
placements. The placement that places that block in the Easternmost*
Semi-Region shall be considered to win the tie.

[*] requires more detail for absolute tie breaking
[**] something like imbalance = [population(N_Semi-Region) / N] -
[population(M_Semi-Region) / M]

Recurse use D = N and D = M for the 2 Semi-Regions, until Regions of 1
district are reached.