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166 lines
48 KiB
Markdown
166 lines
48 KiB
Markdown
# 重定义森林火灾模拟
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在前面的例子中，我们定义了一个 `BurnableForest`，实现了一个循序渐进的生长和燃烧过程。
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假设我们现在想要定义一个立即燃烧的过程（每次着火之后燃烧到不能燃烧为止，之后再生长，而不是每次只燃烧周围的一圈树木），由于燃烧过程不同，我们需要从 `BurnableForest` 中派生出两个新的子类 `SlowBurnForest`（原来的燃烧过程） 和 `InsantBurnForest`，为此
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* 将 `BurnableForest` 中的 `burn_trees()` 方法改写，不做任何操作，直接 `pass`（因为在 `advance_one_step()` 中调用了它，所以不能直接去掉）
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* 在两个子类中定义新的 `burn_trees()` 方法。
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In [1]:
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```py
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import numpy as np
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from scipy.ndimage.measurements import label
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class Forest(object):
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""" Forest can grow trees which eventually die."""
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def __init__(self, size=(150,150), p_sapling=0.0025):
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self.size = size
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self.trees = np.zeros(self.size, dtype=bool)
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self.p_sapling = p_sapling
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def __repr__(self):
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my_repr = "{}(size={})".format(self.__class__.__name__, self.size)
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return my_repr
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def __str__(self):
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return self.__class__.__name__
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@property
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def num_cells(self):
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"""Number of cells available for growing trees"""
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return np.prod(self.size)
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@property
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def tree_fraction(self):
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"""
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Fraction of trees
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"""
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num_trees = self.trees.sum()
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return float(num_trees) / self.num_cells
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def _rand_bool(self, p):
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"""
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Random boolean distributed according to p, less than p will be True
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"""
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return np.random.uniform(size=self.trees.shape) < p
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def grow_trees(self):
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"""
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Growing trees.
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"""
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growth_sites = self._rand_bool(self.p_sapling)
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self.trees[growth_sites] = True
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def advance_one_step(self):
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"""
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Advance one step
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"""
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self.grow_trees()
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class BurnableForest(Forest):
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"""
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Burnable forest support fires
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"""
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def __init__(self, p_lightning=5.0e-6, **kwargs):
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super(BurnableForest, self).__init__(**kwargs)
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self.p_lightning = p_lightning
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self.fires = np.zeros((self.size), dtype=bool)
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def advance_one_step(self):
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"""
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Advance one step
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"""
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super(BurnableForest, self).advance_one_step()
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self.start_fires()
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self.burn_trees()
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@property
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def fire_fraction(self):
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"""
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Fraction of fires
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"""
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num_fires = self.fires.sum()
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return float(num_fires) / self.num_cells
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def start_fires(self):
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"""
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Start of fire.
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"""
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lightning_strikes = (self._rand_bool(self.p_lightning) &
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self.trees)
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self.fires[lightning_strikes] = True
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def burn_trees(self):
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pass
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class SlowBurnForest(BurnableForest):
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def burn_trees(self):
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"""
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Burn trees.
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"""
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fires = np.zeros((self.size[0] + 2, self.size[1] + 2), dtype=bool)
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fires[1:-1, 1:-1] = self.fires
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north = fires[:-2, 1:-1]
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south = fires[2:, 1:-1]
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east = fires[1:-1, :-2]
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west = fires[1:-1, 2:]
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new_fires = (north | south | east | west) & self.trees
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self.trees[self.fires] = False
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self.fires = new_fires
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class InstantBurnForest(BurnableForest):
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def burn_trees(self):
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# 起火点
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strikes = self.fires
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# 找到连通区域
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groves, num_groves = label(self.trees)
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fires = set(groves[strikes])
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self.fires.fill(False)
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# 将与着火点相连的区域都烧掉
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for fire in fires:
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self.fires[groves == fire] = True
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self.trees[self.fires] = False
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self.fires.fill(False)
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```
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测试：
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In [2]:
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```py
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forest = Forest()
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sb_forest = SlowBurnForest()
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ib_forest = InstantBurnForest()
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forests = [forest, sb_forest, ib_forest]
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tree_history = []
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for i in xrange(1500):
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for fst in forests:
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fst.advance_one_step()
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tree_history.append(tuple(fst.tree_fraction for fst in forests))
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```
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显示结果：
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In [3]:
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```py
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import matplotlib.pyplot as plt
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%matplotlib inline
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plt.figure(figsize=(10,6))
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plt.plot(tree_history)
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plt.legend([f.__str__() for f in forests])
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plt.show()
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```
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