Hitpoints, Firepower, and Attack.

[Hans_Lemurson]

The system of having a "Hitpoints" and "Firepower" statistic on units is a blind carry-over from Civ2.

HP and FP both serve to change how many combat rounds a battle lasts, and can end up cancelling each other out. 2 Firepower vs 20HP means you'll still last 10 combat rounds.

Furthermore, if you do an analysis of the combat effects of increasing the Attack of a unit, compared to the HP of a unit, or the FP of a unit, you will find that they are virtually identical. All that these serve to do is linearly increase the Combat Power of a unit. Try it out with the war-calculator.

Civ2's system of doubling the HP of gunpowder era unit never made any sense. If anything, it should be their Firepower, but more importantly, the overall effect is that a Musketeer fights as though it had an Attack/Defense of 6. Why not just do that, I wonder?

Fiddling with HP can be used as a means of fine-tuning the combat powers of a unit when you'd otherwise be stuck choosing between an strength of "1" or "2". However, this just suggests that no units should ever had combat stats that low.

Increasing Firepower or decreasing HP can also serve to add more "luck" to battles, since they'll last fewer rounds. Consider that in a 5v4 battle, with 10HP on each side, the attacker has a 68% chance to win, but with 100HP on each side it jumps to 94%. HP/FP ratios can be used to control how much the dice control the outcome of battles, changing the "flavor" of warfare.

There is one case however where FP and HP have a unique effect in combat: Bombardment. In bombardment attacks, a hit is either delivered, or it misses, no "battle to the end". This means that the ability of bombarders to do damage is limited by their firepower, since although their attack strength could be high enough to guarantee hits, they only get a finite number of shots. Similarly, units on the receiving end, if their HP is high enough can show great resilience to damage that one might not expect from their direct-combat statistics.

But overall, the effect of a 4-Def. unit with 20HP is indistinguishable from an 8-Def. unit with 10HP that has an ability that says "Takes half damage from bombardment".

My opinion is this: HP and FP should be used as tools to change the style and properties of a battle, but NOT to grant units "Extra Power" that is hidden from their Attack/Defense stats.

• If you want to make a unit more "all or nothing" in it's attacks, of raise the damage-ceiling of bombardment, then increase its Firepower.
• If you want to make a unit resistant to bombardment, or make its combat less prone to randomness, then increase it's Hit Points.
• If you want to make a unit stronger, increase it's Attack/Defense.

[Wahazar]

One correction: in case of bombardment attack FP is set to 1.

Beside this, HP/FP give similar effect only if both units has similar A/D stats.
It was discussed at the end of this thread: viewtopic.php?id=1296

[soundnfury]

There is one case however where FP and HP have a unique effect in combat: Bombardment.

In freeciv-3.0 there's another similar case, Combat_Rounds. It brings the "no battle to the end" factor without all the other stuff Bombardment involves (no retaliation, damage all units on a tile, FP=1 etc.). I made quite a lot of use of that in my Aviation ruleset, because it means you can wear a unit down with attacks despite not having anything powerful enough to one-shot it — and doing so leaves your units weakened (and their move used up) but not (typically) destroyed.

In this setup, the attacking power of a unit is something like CR*FP*A/(A+enemyD), while the defending power is FP*HP*D/(D+enemyA). The only sense in which HP matters for attacking (assuming CRs are balanced so that units rarely or never lose their full HP in an attack) is how much of it you have left after the attack (and thus how vulnerable you are to an enemy unit taking you out). So I have for example light bombers with 8CR 1FP, medium bombers with 12CR 1FP, and heavy bombers with 10CR 2FP.

It gets complicated further in the case of fighters, which (unlike bombers in my ruleset) don't have OneAttack flag. So the amount of damage a fighter can do is limited by two things: running out of HP, and running out of fuel. Thus at short range (e.g. attacking bomber that just bombed their city so is right next to it) the former limits it, and power is proportional to A*FP*HP, whereas at long range (e.g. rushing to the aid of another city, or covering troops out in the field) CR and the A/(A+eD) ratio matter more than HP.

Another situation where FP isn't purely multiplicative with attack is if it doesn't divide the enemy HP evenly. E.g. consider attacking a 1D 10HP unit with either a 3A 2FP or a 2A 3FP. The former needs to win 5 rounds, so on average will take 6⅔ rounds and sustain 1⅔HP of damage; the latter needs to win 4 rounds (because after the third the enemy still has 1HP), will on average take 6 rounds and sustain 2HP of damage. (I've used a slightly naïve calculation here; strictly speaking I should faff around with negative binomials but this is close enough and illustrates the idea.)

This probably doesn't come up much in the normal ruleset (I think most HPs are multiples of 10 and most FPs are 1 or 2), but in my Aviation ruleset (which has lots of small upgrades to units, since its entire tech tree ranges over only about sixty years of history) there are plenty of units with HPs like 8 or 13, and the FPs go as high as 5. So you can get all kinds of effects from the leftover FP being "wasted" on the hit that kills a unit. If the target has 6 HP left, then 5FP is no better than 3FP; conversely, if the enemy has 5FP, then 6HP is twice as good as 5HP.