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BBC Micro Elite

Market item prices and availability

The algorithms behind the generation of each system's cargo market

References: TT151, AVL, GVL
The prices and availability of the market items on display in the Buy Cargo
screen are calculated using a couple of complex formulae, which take a base
value for each item, mix in a couple of economic variables, and blend it all
with a bit of random behaviour. The result is the heart of Elite's trading
system, where canny traders can make a killing (while the rest of us can't
seem to get a break).

Let's start by looking at the formula for prices, and then availability.

Market item prices
------------------
This is the formula for an item's price, which is performed as an 8-bit
calculation:

  price = (base_price + (random AND mask) + economy * economic_factor) * 4

The resulting price is 10 times the displayed price, so we can show it to one
decimal place. The individual items in the calculation are as follows:

  * The item's base_price is byte #0 in the market prices table at QQ23, so
    it's 19 for food, 20 for textiles, 235 for narcotics and so on.

  * Each time we arrive in a new system, a random number is generated and
    stored in location QQ26, and this is shown as "random" in the calculation
    above.

  * The item's mask is byte #3 in the market prices table at QQ23, so
    it's &01 for food, &03 for textiles, &78 for narcotics and so on. The
    more set bits there are in this mask, and the higher their position in
    this byte, the larger the price fluctuations for this commodity, as the
    random number is AND'd with the mask. So narcotics will vary wildly in
    price, while food and textiles will be relatively stable.

  * The economy for a system is given in a 3-bit value, from 0 to 7, that is
    stored in QQ28. This value is described in more detail in routine TT24,
    but this is the range of values:

      0 = Rich Industrial
      1 = Average Industrial
      2 = Poor Industrial
      3 = Mainly Industrial
      4 = Mainly Agricultural
      5 = Rich Agricultural
      6 = Average Agricultural
      7 = Poor Agricultural

  * The economic_factor is stored in bits 0-4 of byte #1 in the market prices
    table at QQ23, and its sign is in bit 7, so it's -2 for food, -1 for
    textiles, +8 for narcotics and so on. Negative factors show products that
    tend to be cheaper than average in agricultural economies but closer to
    average in rich industrial ones, while positive factors are more
    expensive in poor agricultural systems than rich industrial ones - so
    food is cheaper in poor agricultural systems while narcotics are very
    expensive, and it's the other way round in rich industrial systems,
    where narcotics are closer to the average price, but food is pricier.

  * The units for this item (i.e. tonnes, grams or kilograms) are given by
    bits 5-6 of byte #1 in the market prices table at QQ23.

Market item availability
------------------------
The availability of each item is also calculated using a formula, which is
again performed as an 8-bit calculation:

  quantity = (base_quantity + (random AND mask) - economy * economic_factor)
             mod 64

If the result of the above is less than 0, then the available quantity is set
to 0.

The item's base_availability is byte #2 in the market prices table at QQ23, so
it's 6 for food, 10 for textiles, 8 for narcotics and so on. The other
variables are described above.