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Revise the chance for pulling players deeper into the Abyss

The new formula removes the XL15 cutoff for the chance to pull the
player deeper into the Abyss upon shift/teleport. The chance formula is
now:

  (XL + 4 - Depth)^2 / (27^2 * (Depth + 5))

Some of the new chances:
* Characters at XL 1 have about a 0.1% chance of getting pulled from
  A:1.
* Characters at XL 13 have chances of 5.9%/4.4%/3.3%/2.6% for getting
  pulled from A:1/A:2/A:3/A:4.
* Characters at XL 27 have chances of 20.6%/16.5%/13.4%/11.1% over the
  same levels.

It's relatively easy to escape from the abyss at XL13, and a small
chance of getting pulled a level deeper where exits are more common
won't have much impact on players at these XLs.

This commit also updates comments and spacing and properly declares a
function as static. Closes #757.
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gammafunk committed Jul 4, 2018
1 parent ae4dde8 commit c01a7f0fb5e15ba2e5ff9f6307a38c4273d420b6
Showing with 21 additions and 12 deletions.
  1. +21 −12 crawl-ref/source/abyss.cc
@@ -1662,35 +1662,44 @@ void abyss_morph()
los_changed();
}
bool _abyss_force_descent()
// Force the player one level deeper in the abyss during an abyss teleport with
// probability:
// (XL + 4 - Depth)^2 / (27^2 * (Depth + 5))
//
// Consequences of this formula:
// - Chance to be pulled deeper increases with XL and decreases with current
// abyss depth.
// - Characters at XL 1 have about a 0.1% chance of getting pulled from A:1.
// - Characters at XL 13 have chances for getting pulled from A:1/A:2/A:3/A:4
// of about 5.9%/4.4%/3.3%/2.6%.
// - Characters at XL 27 have chances for getting pulled from A:1/A:2/A:3/A:4
// of about 20.6%/16.5%/13.4%/11.1%.
static bool _abyss_force_descent()
{
// Force the player deeper in the abyss during an abyss
// teleport with probability
// ( max(0, XL - Depth - 13) / 27 )^2
// Consequences of this formula:
// - Characters at xl 13 + depth or below do not go deeper
// - Characters at xl 27 have a 169/729 ~ .23 chance of being pulled
// down from A:1, and a 100/729 ~ .13 chance of A:4 -> A:5
int fraction = max(0, you.experience_level
- level_id::current().depth - 13);
return x_chance_in_y(fraction * fraction, 729);
const int depth = level_id::current().depth;
const int xl_factor = you.experience_level + 4 - depth;
return x_chance_in_y(xl_factor * xl_factor, 729 * (5 + depth));
}
void abyss_teleport()
{
xom_abyss_feature_amusement_check xomcheck;
dprf(DIAG_ABYSS, "New area Abyss teleport.");
if (level_id::current().depth < brdepth[BRANCH_ABYSS]
&& _abyss_force_descent())
{
down_stairs(DNGN_ABYSSAL_STAIR);
more();
return;
}
mprf(MSGCH_BANISHMENT, "You are suddenly pulled into a different region of the Abyss!");
mprf(MSGCH_BANISHMENT, "You are suddenly pulled into a different region of"
" the Abyss!");
_abyss_generate_new_area();
_write_abyssal_features();
grd(you.pos()) = _veto_dangerous_terrain(grd(you.pos()));
stop_delay(true);
forget_map(false);
clear_excludes();

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