Difference between revisions of "User:Selinger"

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(First draft of a proposed article on bridge ladders)
(Adjusted terminology.)
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A ''bridge ladder'' sometimes happens when one player repeatedly tries to [[blocking|block]] the other from the edge, and the other player repeatedly [[bridge]]s to one side, like this:
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A ''bridge ladder'' is a sequence of moves such as the following:
<hexboard size="7x7"
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<hexboard size="7x8"
   edges="bottom right"
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   edges="bottom"
 
   coords="none"
 
   coords="none"
 
   contents="B a3 R c2 B 1:b4 R 2:d3 B 3:c5 R 4:e4 B 5:d6 R 6:f5 B 7:e7 R 8:g6"
 
   contents="B a3 R c2 B 1:b4 R 2:d3 B 3:c5 R 4:e4 B 5:d6 R 6:f5 B 7:e7 R 8:g6"
 
   />
 
   />
In the above example, Red ''wins'' the bridge ladder (i.e., Red connects to the edge). However, if the ladder starts in a slightly different place, the outcome can be different:
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Here, Red is the ''attacker'', Blue is the ''defender'', and both players play a sequence of [[bridge]]s that approach the attacker's edge at a 30 degree angle, with the defender being closer to the edge than the attacker. Bridge ladders sometimes happen when the defender repeatedly tries to [[blocking|block]] the attacker with a [[Blocking#The_near_block|near block]], and the attacker repeatedly [[bridge]]s to one side.
 +
 
 +
In the above example, Red ''wins'' the bridge ladder (i.e., Red connects to the edge). However, if the ladder starts closer to a blue edge, the outcome can be different:
 
<hexboard size="7x7"
 
<hexboard size="7x7"
 
   edges="bottom right"
 
   edges="bottom right"
Line 16: Line 18:
 
   contents="B a2 R c1 B 1:b3 R 2:d2 B 3:c4 R 4:e3 B 5:d5 R 6:f4 B 7:e6 R 8:g5 B 9:f7"
 
   contents="B a2 R c1 B 1:b3 R 2:d2 B 3:c4 R 4:e3 B 5:d5 R 6:f4 B 7:e6 R 8:g5 B 9:f7"
 
   />
 
   />
This time Blue wins the ladder. Generally, when a bridge ladder moves towards an empty acute corner, whichever player is closer to the [[board#Diagonals|long diagonal]] wins the ladder. We call the winning player the ''attacker'' and the losing player the ''defender'' of the bridge ladder. Thus, in the first example above, Red is the attacker and Blue is the defender, and in the second example, Blue is the attacker and Red is the defender.
+
This time Blue wins the ladder. Generally, when a bridge ladder moves towards an empty acute corner, whichever player is closer to the [[board#Diagonals|long diagonal]] wins the ladder. Also note that from the point of view of the red edge, Red is the attacker and Blue is the defender, but from the point of view of the blue edge, Blue is the attacker and Red is the defender. This is typical for bridge ladders approaching an acute corner.
  
 
== Bottlenecking from a bridge ladder ==
 
== Bottlenecking from a bridge ladder ==
  
In the case of an [[ladder|ordinary ladder]], if the ladder continues until the end, it is the defender who connects. Therefore, it is up to the attacker to figure out how to disrupt the ladder, usually by [[Ladder_handling#Attacking|pivoting]] or [[cornering]].
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Let us call the player who would lose a bridge ladder if it continued until the end the ''underdog''. So Blue is the underdog in the first example above, and Red is the underdog in the second example.
  
For bridge ladders, the situation is reversed. If the ladder continues until the end, it is the attacker who connects, and therefore, the onus is on the defender to do something about it. The defender usually does this by creating a [[bottleneck]].
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Since the underdog stands to lose the bridge ladder, the onus is usually on them to do something about it, typically by creating a [[bottleneck]].
  
 
=== Example ===
 
=== Example ===
  
Consider a bridge ladder starting on the 6th row, with Red as the attacker.
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Consider a bridge ladder starting on the 6th row. Blue is the underdog.
 
<hexboard size="7x8"
 
<hexboard size="7x8"
 
   edges="bottom right"
 
   edges="bottom right"
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Red gets a pair of 2nd row ladders.
 
Red gets a pair of 2nd row ladders.
  
The defender must choose carefully when to bottleneck. One might think that it is good for the defender to bottleneck as soon as possible, because this results in a ladder further from the attacker's edge. But on the other hand, especially when the bridge ladder is approaching an acute corner, bottlenecking sooner also keeps the ''defender'' further from ''their'' edge. For example, in each of the above scenarios, Red may try to pivot as follows:
+
Blue must choose carefully when to bottleneck. One might think that it is good for Blue to bottleneck as soon as possible, because this results in a ladder further from the red edge. But on the other hand, especially when the bridge ladder is approaching an acute corner, bottlenecking sooner also keeps the Blue further from the blue edge. For example, in each of the above scenarios, Red may try to pivot as follows:
  
 
'''5th row bottleneck:'''
 
'''5th row bottleneck:'''
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== Bridge ladder approaching an obtuse corner ==
 
== Bridge ladder approaching an obtuse corner ==
  
When the bridge ladder approaches an obtuse corner, the situation is similar to the examples above.
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When a bridge ladder approaches an obtuse corner, the situation is in principle similar, but there are some differences depending on who is the underdog.
  
 
For example, consider the following:
 
For example, consider the following:
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   contents="R g1 B f3 R 1:e2 B 2:d4 R 3:c3 B 4:b5 R 5:a4 B 6:a5"
 
   contents="R g1 B f3 R 1:e2 B 2:d4 R 3:c3 B 4:b5 R 5:a4 B 6:a5"
 
   />
 
   />
If the bridge ladder continues to the end, Blue connects. Rather than being able to create a bottleneck, Red can turn the ladder around, for example like this, resulting in a 2nd row ladder for Blue:
+
If the bridge ladder continues to the end, Blue connects. Red can't create a bottleneck, but Red can turn the ladder around, for example like this, resulting in a 2nd row ladder for Blue:
 
<hexboard size="5x9"
 
<hexboard size="5x9"
 
   coords="none"
 
   coords="none"
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== Application: last opportunity to pivot from a ladder ==
 
== Application: last opportunity to pivot from a ladder ==
  
Consider an (ordinary) [[ladder]] moving parallel to an edge. In the absence of a ladder escape, the attacker must at some point do something, typically [[Ladder_handling#Attacking|pivot]] or play a [[cornering]] move. One may ask when is the last possible opportunity to pivot. A useful heuristic is to consider the bridge ladder that would result if the defender yielded after the pivot. For example, consider a 4th row ladder approaching from the left. If Red pivots at 5, then Red is the ''attacker'' in the resulting bridge ladder, so Blue has to do something else (like bottlenecking).
+
Consider an (ordinary) [[ladder]] moving parallel to an edge. In the absence of a ladder escape, the attacker must at some point do something, typically [[Ladder_handling#Attacking|pivot]] or play a [[cornering]] move. One may ask when is the last possible opportunity to pivot. A useful heuristic is to consider the bridge ladder that would result if the defender yielded after the pivot. For example, consider a 4th row ladder approaching from the left. If Red pivots at 5, then Blue is the underdog in the resulting bridge ladder, so Blue has to do something else (like bottlenecking).
 
<hexboard size="5x8"
 
<hexboard size="5x8"
 
   edges="bottom right"
 
   edges="bottom right"
Line 127: Line 129:
 
   contents="R a2 B a3 R b2 B b3 R 1:c2 B 2:c3 R 3:d2 B 4:d3 R 5:f2 B 6:e4 R 7:g3 B 8:f5 R 9:h4 B 9:g6"
 
   contents="R a2 B a3 R b2 B b3 R 1:c2 B 2:c3 R 3:d2 B 4:d3 R 5:f2 B 6:e4 R 7:g3 B 8:f5 R 9:h4 B 9:g6"
 
   />
 
   />
On the other hand, if Red waits until 7 to pivot, Red ends up being the ''defender'' in the resulting bridge ladder, and cannot connect.
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On the other hand, if Red waits until 7 to pivot, ''Red'' ends up being the underdog, and cannot connect.
 
<hexboard size="5x8"
 
<hexboard size="5x8"
 
   edges="bottom right"
 
   edges="bottom right"
Line 133: Line 135:
 
   contents="R a2 B a3 R b2 B b3 R 1:c2 B 2:c3 R 3:d2 B 4:d3 R 5:e2 B 6:e3 R 7:g2 B 8:f4 R 9:h3 B 10:g5"
 
   contents="R a2 B a3 R b2 B b3 R 1:c2 B 2:c3 R 3:d2 B 4:d3 R 5:e2 B 6:e3 R 7:g2 B 8:f4 R 9:h3 B 10:g5"
 
   />
 
   />
Therefore generally speaking, the last opportunity to pivot from a ladder is before the ladder has reached the long diagonal (in case of a ladder approaching an [[Board#Corners|acute corner]]). In case the ladder approaches an [[Board#Corners|obtuse corner]], a similar heuristic applies.
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Therefore generally speaking, the last opportunity to pivot from a ladder approaching an [[Board#Corners|acute corner]] is before the ladder has reached the long diagonal. A similar analysis applies to ladders approaching an [[Board#Corners|obtuse corner]].

Revision as of 22:57, 8 September 2021

Proposed article: Bridge ladder

To do: figure out a useful way to define "attacker" and "defender". I currently define this in terms of who will "win" the ladder, but that only makes sense when the ladder approaches an acute corner. Maybe it should be defined with reference to an edge instead, or not at all.


A bridge ladder is a sequence of moves such as the following:

21436587

Here, Red is the attacker, Blue is the defender, and both players play a sequence of bridges that approach the attacker's edge at a 30 degree angle, with the defender being closer to the edge than the attacker. Bridge ladders sometimes happen when the defender repeatedly tries to block the attacker with a near block, and the attacker repeatedly bridges to one side.

In the above example, Red wins the bridge ladder (i.e., Red connects to the edge). However, if the ladder starts closer to a blue edge, the outcome can be different:

214365879

This time Blue wins the ladder. Generally, when a bridge ladder moves towards an empty acute corner, whichever player is closer to the long diagonal wins the ladder. Also note that from the point of view of the red edge, Red is the attacker and Blue is the defender, but from the point of view of the blue edge, Blue is the attacker and Red is the defender. This is typical for bridge ladders approaching an acute corner.

Bottlenecking from a bridge ladder

Let us call the player who would lose a bridge ladder if it continued until the end the underdog. So Blue is the underdog in the first example above, and Red is the underdog in the second example.

Since the underdog stands to lose the bridge ladder, the onus is usually on them to do something about it, typically by creating a bottleneck.

Example

Consider a bridge ladder starting on the 6th row. Blue is the underdog.

21436587

Instead of continuing the ladder to the end, Blue has the choice to create a bottleneck on the 5th row, 4th row, or 3rd row:

5th row bottleneck:

213

Red gets a pair of 4th row ladders.

4th row bottleneck:

21435

Red gets a pair of 3rd row ladders.

3rd row bottleneck:

2143657

Red gets a pair of 2nd row ladders.

Blue must choose carefully when to bottleneck. One might think that it is good for Blue to bottleneck as soon as possible, because this results in a ladder further from the red edge. But on the other hand, especially when the bridge ladder is approaching an acute corner, bottlenecking sooner also keeps the Blue further from the blue edge. For example, in each of the above scenarios, Red may try to pivot as follows:

5th row bottleneck:

651432

Red pivots at 3. Assuming 3 connects to the bottom edge, Red gets a 4th row ladder along the bottom edge, and Blue gets a 4th row ladder along the right edge.

4th row bottleneck:

651432

Red gets a 3th row ladder and pivots at 3. Red gets a 3rd row ladder along the bottom edge, and Blue gets a 3rd row ladder along the right edge.

3rd row bottleneck:

651432

Red gets a 3th row ladder and pivots at 3. Red gets a 2nd row ladder along the bottom edge, and Blue gets a 2nd row ladder along the right edge.

Bridge ladder approaching an obtuse corner

When a bridge ladder approaches an obtuse corner, the situation is in principle similar, but there are some differences depending on who is the underdog.

For example, consider the following:

13524

Here, Red wins the ladder, and Blue's last opportunity to bottleneck was move 2, which would have given Red a 2nd row ladder. On the other hand, when the bridge ladder starts further to the left, the situation is different:

135264

If the bridge ladder continues to the end, Blue connects. Red can't create a bottleneck, but Red can turn the ladder around, for example like this, resulting in a 2nd row ladder for Blue:

136254

or like this, resulting in a 4th row ladder for Blue:

1432

or even like this, resulting in no ladder for Blue:

312

Application: last opportunity to pivot from a ladder

Consider an (ordinary) ladder moving parallel to an edge. In the absence of a ladder escape, the attacker must at some point do something, typically pivot or play a cornering move. One may ask when is the last possible opportunity to pivot. A useful heuristic is to consider the bridge ladder that would result if the defender yielded after the pivot. For example, consider a 4th row ladder approaching from the left. If Red pivots at 5, then Blue is the underdog in the resulting bridge ladder, so Blue has to do something else (like bottlenecking).

135247698

On the other hand, if Red waits until 7 to pivot, Red ends up being the underdog, and cannot connect.

13572469810

Therefore generally speaking, the last opportunity to pivot from a ladder approaching an acute corner is before the ladder has reached the long diagonal. A similar analysis applies to ladders approaching an obtuse corner.