# Poker Effective Hand Strength (EHS) algorithm

Effective Hand Strength (EHS) is a poker algorithm conceived by computer scientists Darse Billings, Denis Papp, Jonathan Schaeffer and Duane Szafron that has been published for the first time in a research paper (1998). "Opponent Modeling in Poker" (PDF). AAAI-98 Proceedings.

It has since then been considered as a reference in the realm of poker artificial intelligence and has been the basis of further research such as:

• "Computer poker: a review" (PDF). Artificial Intelligence, Department of Computer Science, University of Auckland, New Zealand: 962–963. 2010.
• "Application of AI in poker" (PDF). VU University Amsterdam Faculty of Sciences: 12–13. 2011.
• "Advances in artificial intelligence – SBIA 2008 : 19th Brazilian Symposium on Artificial Intelligence, Salvador, Brazil, October 26–30, 2008 ; proceedings". 2008: 85–86. Cite journal requires |journal= (help)

## Algorithm

The algorithm is a numerical approach to quantify the strength of a poker hand where its result expresses the strength of a particular hand in percentile (i.e. ranging from 0 to 1), compared to all other possible hands. The underlying assumption is that an Effective Hand Strength (EHS) is composed of the current Hand Strength (HS) and its potential to improve or deteriorate (PPOT and NPOT):

${\displaystyle EHS=HS\times (1-NPOT)+(1-HS)\times PPOT}$

where:

• ${\displaystyle EHS}$ is the Effective Hand Strength
• ${\displaystyle HS}$ is the current Hand Strength (i.e. not taking into account potential to improve or deteriorate, depending on upcoming table cards
• ${\displaystyle NPOT}$ is the Negative POTential (i.e. the probability that our current hand, if the strongest, deteriorates and becomes a losing hand)
• ${\displaystyle PPOT}$ is the Positive POTential (i.e. the probability that our current hand, if losing, improves and becomes the winning hand)

## Pseudocode

Hand Strength (HS) will enumerate all possible opponent hand cards and count the occurrences where our hand is strongest (+50% of the cases where we are tied):

HandStrength(ourcards, boardcards) {
ahead = tied = behind = 0
ourrank = Rank(ourcards, boardcards)
for each case(oppcards) {
opprank = Rank(oppcards, boardcards)
if (ourrank > opprank) ahead += 1
else if (ourrank == opprank) tied += 1
else behind += 1
}
handstrength = (ahead + tied / 2) / (ahead + tied + behind)
return handstrength
}

In addition, EHS will consider the hand potential (i.e. its probabilities to improve or deteriorate):

HandPotential(ourcards,boardcards) {
// Hand potential array, each index represents ahead, tied, and behind
integer array HP[3][3] // initialize to 0
integer array HPTotal[3] // initialize to 0
ourrank = Rank(ourcards, boardcards)
// Consider all two card combinations of the remaining cards for the opponent
for each case(oppcards) {
opprank = Rank(oppcards, boardcards)
if (ourrank > opprank) index = ahead
else if (ourrank == opprank) index = tied
else index = behind
HPTotal[index] += 1
// All possible board cards to come
for each case(turn, river) {
// Final 5-card board
board = [boardcards, turn, river]
ourbest = Rank(ourcards, board)
oppbest = Rank(oppcards, board)
if (ourbest > oppbest) HP[index][ahead] += 1
else if (ourbest == oppbest) HP[index][tied] += 1
else HP[index][behind] += 1
}
}
// Ppot: were behind but moved ahead
Ppot = (HP[behind][ahead] + HP[behind][tied] / 2 + HP[tied][ahead] / 2) / (HPTotal[behind] + HPTotal[tied])
// Npot: were ahead but fell behind
}