Migration and Magnates - How they work
by QSI Programming

Migration, sometimes called "auto-colonization" is a feature where members of your empire will move around by themselves. It is at its most obvious when combined with a multi-race empire, especially when Magnate Civs are involved. This post is an attempt to explain how both Magnate Civs and Migration work.

Magnate Civ is a special that appears on a planet. The "effect" of the special is that it has a population of one of the unplayable races on it. The special is assigned to a planet through the normal special lottery and the species on the planet is determined by determining which race is the best fit for the planet. Originally, Magnate Civ and Splinter Colony were two separate specials. Splinter Colonies would then appear on yellow and even red planets, even when the empire did not have reasonable tech for that. So, we merged the two specials and it is a Splinter Colony if the planet is green for the discovering player's race and a Magnate otherwise. This is why certain Magnates are difficult to find when playing certain races (essentilly the Magnates will ALWAYS have a different planet preference than you, if you discover the planet first). If it is a Magnate Civ, the population is doubled. All the population is placed in a random region on the planet and no migration processing is done until a player colonizes it, to make sure there is an open region to colonize. This is also part of why the Magnates migrate so quickly (they are often over-crowded in the one region).
The Magnate Civ line in the specials.txt sheet (which is outside the table) is no longer functional, and in fact will just generate a planet that SAYS it is a Magnate Civ with no population on it. The odds of a Magnate Civ can be modded by upping the lottery balls on the Splinter Colony spreadsheet.

Migration comes in two pieces. The spreadsheet data used by migration is in the file population.txt. In that file, the table PopulationPushFactor is not used for anything (although it is read in by the code).

The first and simplest piece is growth migration, which moves population from one region to another on the same planet. This considers only the population added through normal population growth. Tables StayPercentage, PopulationGrowthPlanet and PopGrowth are used for growth migration, along with the density values in PopMisc.

The main migration processing is the "push/pull" migration, which is a vastly simplified version of the original migration design. For each empire in the game, each populated region (including outposts) is processed system by system. For each each region, the push factors are calculated and if the number exceeds a D100 roll, people will be moved.

The push factors are defined in the PushFactor table. The first two are for overpopulation, and an overcrowded planet will take both penalties. The unrest value is used for regions in unrest1, doubled for unrest2 and tripled for unrest3. The revolt value is used for regions in revolt, so using the current values, revolt is less of a push than unrest3. The final push is for the existence of migration targets, so that people will come when you give them a place to go (although they may choose to go elsewhere).
EDIT: Planets which are a migration target and regions which have more jobs than people will ignore the presence of other migration targets when calculating push factors if all other push factors are zero.

The number of people is determined using the PopulationMoving table, which specifies a percentage of the population based on the amount of population in the region. If this results in more than one population point, the group is divided into groups of one population point.

Candidate systems are determined by spreading out from the source system and continuing until stopped by a system which is either blockaded or contians a colony or outpost of another empire without also containing a colony or outpost of the source empire. All planets in these systems which are not owned by another empire are candidates for migration.

Pull factors for each planet are calculated using the PushPullPlanet table. Distance to system is a negative number to make farther away planets less attractive. Distance is calculated in jumps only. A larger negative number would make farther planets even less attractive. The habitibility and gravity values are straightforward. The potential maximum population uses the capacity of the planet for the race moving without regard to any existing population. The Existing pop values are multiplied by the number of regions currently populated by the same species or race (same race will get both values). There is no value assigned to an unpopulated planet (there is a space in the table, but any value there is ignored). If the planet has a set migration flag, they get the larger bonus.

Planets whose pull factors are less than 20% of the highest value are discarded and the planet is chosen from the remainder, weighted by their pull factors. Then a region is selected using the region pull factors from the PushPullRegion table. Some of the vlaues in this table are planet-wide values, but are not used to pick the planet. The -999 for different race, starving, unrest and revolt are designed to prevent immigration. (Yes, if you change starvation to a positive number, migrants will prefer starving regions). The unemployment value is not used or checked. The same is true for the tax rate values. The Underemployment value is a multiplier for the number of available jobs in excess of population (rather than unemployment rate). The population numbers are as advertised (and once again overcrowded gets both penalties). Values for piracy and pollution are available in the spreadsheet, but both are set to 0 in the original values.

If no region has a positive pull factor, the migration is cancelled. If the region has already reached its immigration cap as defined in the immigration cap entries in the PopMisc table, that group does not move.

PopMisc has several values that are not used. The used values are the two values for PopDensity, which define the percentages used to consider a planet full (natural limit) and crowded. These values are used for both types of migration. The immigration cap values are used to limit the immigration to .05 population points (50) or 5X the natural growth (whichever is larger), using the original numbers.

EDIT: I meant to mention that these rules apply to ALL races at present, except the New Orions. It is possible that at some time in the future, the Harvesters will also have some modifications in the area of preferences for planets with same/different race........

The original design included many more push and pull factors, generating three different types of emigrants, those that prefered to stay on established planets, those that preferred unpopulated planets and systems, and those that wanted to leave the empire to join another empire (yes, the original design allowed migration to go across empire boundaries). There were multiple persecution policies (racial, religious and several levels of slavery) which were major factors in this calculation, along with many other policies, many of which could be set on a planet by planet basis.