Rules for Gas Switching in the Deco Calculation
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The deco calculation switches gases on the following rules and conditions.


A new gas to eventually switch to is searched for:
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a) Whenever arriving or staying on a deco stop.

   If a new gas is found, it will be taken.

or

b) On each calculation step, given:

   1) Cave mode is activated.

      If a new gas is found, it will be taken.

   or

   2) - extended stops are activated, or
      - the dive is in bailout mode.

      If a new gas is found, it will only be taken if

      + the change depth of the new gas has not been passed yet (i.e. the
        diver has not passed the change depth of this gas without actually
        switching to it, so on purpose is breathing another gas),

     and

      + the dive is not on the final ascent. (i.e. no gas change will be
        considered any more after leaving the last deco stop for surfacing)


When searching for a new gas, in order to qualify a gas must:
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a) Be available, i.e. neither be disabled, lost, nor staged,

b) not be of type 'deco gas' unless:

   - extended stops are activated, or
   - the ascent calculation has arrived on a deco stop, or
   - the dive is in bailout mode,

c) have a change depth that is not shallower than the current depth,
   (i.e. it must be breathable by configured change depth)

d) have the shallowest change depth among all new gas candidates that
   have qualified so far,

e) not yet be fully used up in case contingency mode* is activated.

For a gas to become elected as the new gas, it must match all of the above criteria.

If none of the configured gases qualifies to become a new gas, the deco calculation
will continue on the current (last selected) gas.

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* contingency mode is auto-activated in case the dive started in cave mode