93 lines
2.3 KiB
Python
93 lines
2.3 KiB
Python
 
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"""A seeded random generator that is the same on every Python: xoshiro256** seeded through splitmix64 from a SHA-256
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of the seed and a name. `sub(name)` gives an independent stream, so changing how many posts are drawn never reshuffles
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the follow graph."""
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import hashlib
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MASK = (1 << 64) - 1
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def _rotl(x, k):
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return ((x << k) | (x >> (64 - k))) & MASK
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def _splitmix(state):
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while True:
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state = (state + 0x9E3779B97F4A7C15) & MASK
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z = state
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z = ((z ^ (z >> 30)) * 0xBF58476D1CE4E5B9) & MASK
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z = ((z ^ (z >> 27)) * 0x94D049BB133111EB) & MASK
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yield z ^ (z >> 31)
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class Rng:
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def __init__(self, seed, name="root"):
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self.seed = seed
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self.name = name
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digest = hashlib.sha256(f"{seed}/{name}".encode()).digest()
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mix = _splitmix(int.from_bytes(digest[:8], "little"))
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self.s = [next(mix) for _ in range(4)]
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def sub(self, name):
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return Rng(self.seed, f"{self.name}/{name}")
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def next64(self):
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s = self.s
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result = (_rotl((s[1] * 5) & MASK, 7) * 9) & MASK
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t = (s[1] << 17) & MASK
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s[2] ^= s[0]
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s[3] ^= s[1]
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s[1] ^= s[2]
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s[0] ^= s[3]
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s[2] ^= t
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s[3] = _rotl(s[3], 45)
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return result
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def random(self):
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return (self.next64() >> 11) / float(1 << 53)
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def below(self, n):
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"""An integer in [0, n)."""
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if n <= 0:
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raise ValueError("below() needs n > 0")
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limit = MASK - (MASK % n)
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while True:
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x = self.next64()
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if x < limit:
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return x % n
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def between(self, lo, hi):
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"""An integer in [lo, hi]."""
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return lo + self.below(hi - lo + 1)
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def chance(self, p):
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return self.random() < p
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def choice(self, seq):
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return seq[self.below(len(seq))]
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def weighted(self, weights):
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"""A key of {key: weight}, keys taken in sorted order so the draw does not depend on dict order."""
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keys = sorted(k for k, w in weights.items() if w > 0)
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total = sum(weights[k] for k in keys)
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x = self.random() * total
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for k in keys:
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x -= weights[k]
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if x < 0:
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return k
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return keys[-1]
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def shuffle(self, items):
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items = list(items)
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for i in range(len(items) - 1, 0, -1):
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j = self.below(i + 1)
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items[i], items[j] = items[j], items[i]
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return items
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def sample(self, seq, k):
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return self.shuffle(seq)[:max(0, min(k, len(seq)))]
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def span(self, pair):
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"""A value drawn from [lo, hi] given as a two-item list, or the value itself."""
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if isinstance(pair, (list, tuple)):
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return self.between(pair[0], pair[1])
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return pair
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