43 lines
1023 B
Markdown
43 lines
1023 B
Markdown
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# Best First
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- Uniform cost uses an evaluation function
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- Doesn't necessarily direct towards goal
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- Doesn't know how far to goal
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- Only cost so far
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- Still blind
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- Look forward
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- Lot of techniques aiming to define best node
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1. Expand node closest to goal
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- Greedy
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2. Expand node on least cost solution path
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- A*
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# Greedy
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- **Estimate** distance to goal
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- Often can't determine exactly
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- This cost is a heuristic
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- Could be crow-flies distance
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- Expand nodes in increasing order of heuristic cost
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- **Fast**
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- **Not optimal**
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- Simple heuristic
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# A*
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- Combines advantages of uniform cost and greedy search
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- ***Optimal & Complete***
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- $h(n)$
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- Estimate of distance to goal
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- Greedy
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- $g(n)$
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- Cost of cumulative path
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- *Uniform cost*
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- $f(n)=g(n)+h(n)$
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- Heuristic must be ***admissible***
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- Underestimates the distance to the goal
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- Never overestimates
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- Monotonic
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- Expand other low cost paths
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- Pathmax
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- For heuristics which might not be monotonic
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- Select last option if going in wrong direction
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