author  haftmann 
Fri, 13 Mar 2009 08:16:18 +0100  
changeset 30500  072daf3914c0 
parent 30495  a5f1e4f46d14 
child 31180  dae7be64d614 
permissions  rwrr 
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(* Author: Florian Haftmann, TU Muenchen *) 
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header {* A HOL random engine *} 
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theory Random 

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imports Code_Index 
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begin 
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notation fcomp (infixl "o>" 60) 
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notation scomp (infixl "o\<rightarrow>" 60) 
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subsection {* Auxiliary functions *} 
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definition inc_shift :: "index \<Rightarrow> index \<Rightarrow> index" where 
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"inc_shift v k = (if v = k then 1 else k + 1)" 
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definition minus_shift :: "index \<Rightarrow> index \<Rightarrow> index \<Rightarrow> index" where 
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"minus_shift r k l = (if k < l then r + k  l else k  l)" 
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fun log :: "index \<Rightarrow> index \<Rightarrow> index" where 
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"log b i = (if b \<le> 1 \<or> i < b then 1 else 1 + log b (i div b))" 
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subsection {* Random seeds *} 
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types seed = "index \<times> index" 

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primrec "next" :: "seed \<Rightarrow> index \<times> seed" where 
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"next (v, w) = (let 
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k = v div 53668; 

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v' = minus_shift 2147483563 (40014 * (v mod 53668)) (k * 12211); 

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l = w div 52774; 

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w' = minus_shift 2147483399 (40692 * (w mod 52774)) (l * 3791); 

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z = minus_shift 2147483562 v' (w' + 1) + 1 

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in (z, (v', w')))" 

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lemma next_not_0: 

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"fst (next s) \<noteq> 0" 

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by (cases s) (auto simp add: minus_shift_def Let_def) 
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primrec seed_invariant :: "seed \<Rightarrow> bool" where 
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"seed_invariant (v, w) \<longleftrightarrow> 0 < v \<and> v < 9438322952 \<and> 0 < w \<and> True" 
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lemma if_same: "(if b then f x else f y) = f (if b then x else y)" 
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by (cases b) simp_all 
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definition split_seed :: "seed \<Rightarrow> seed \<times> seed" where 
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"split_seed s = (let 
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(v, w) = s; 

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(v', w') = snd (next s); 

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v'' = inc_shift 2147483562 v; 
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s'' = (v'', w'); 
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w'' = inc_shift 2147483398 w; 
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s''' = (v', w'') 
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in (s'', s'''))" 

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subsection {* Base selectors *} 
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fun iterate :: "index \<Rightarrow> ('b \<Rightarrow> 'a \<Rightarrow> 'b \<times> 'a) \<Rightarrow> 'b \<Rightarrow> 'a \<Rightarrow> 'b \<times> 'a" where 
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"iterate k f x = (if k = 0 then Pair x else f x o\<rightarrow> iterate (k  1) f)" 
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definition range :: "index \<Rightarrow> seed \<Rightarrow> index \<times> seed" where 
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"range k = iterate (log 2147483561 k) 
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(\<lambda>l. next o\<rightarrow> (\<lambda>v. Pair (v + l * 2147483561))) 1 

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o\<rightarrow> (\<lambda>v. Pair (v mod k))" 
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lemma range: 

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"k > 0 \<Longrightarrow> fst (range k s) < k" 
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by (simp add: range_def scomp_apply split_def del: log.simps iterate.simps) 

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definition select :: "'a list \<Rightarrow> seed \<Rightarrow> 'a \<times> seed" where 
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"select xs = range (Code_Index.of_nat (length xs)) 
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o\<rightarrow> (\<lambda>k. Pair (nth xs (Code_Index.nat_of k)))" 
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lemma select: 
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assumes "xs \<noteq> []" 

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shows "fst (select xs s) \<in> set xs" 

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proof  

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from assms have "Code_Index.of_nat (length xs) > 0" by simp 
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with range have 
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"fst (range (Code_Index.of_nat (length xs)) s) < Code_Index.of_nat (length xs)" by best 
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then have 
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"Code_Index.nat_of (fst (range (Code_Index.of_nat (length xs)) s)) < length xs" by simp 
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then show ?thesis 
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by (simp add: scomp_apply split_beta select_def) 
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qed 
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definition select_default :: "index \<Rightarrow> 'a \<Rightarrow> 'a \<Rightarrow> seed \<Rightarrow> 'a \<times> seed" where 
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[code del]: "select_default k x y = range k 
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o\<rightarrow> (\<lambda>l. Pair (if l + 1 < k then x else y))" 
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lemma select_default_zero: 

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"fst (select_default 0 x y s) = y" 

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by (simp add: scomp_apply split_beta select_default_def) 
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lemma select_default_code [code]: 
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"select_default k x y = (if k = 0 
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then range 1 o\<rightarrow> (\<lambda>_. Pair y) 
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else range k o\<rightarrow> (\<lambda>l. Pair (if l + 1 < k then x else y)))" 
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proof 
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fix s 
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have "snd (range (Code_Index.of_nat 0) s) = snd (range (Code_Index.of_nat 1) s)" 
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by (simp add: range_def scomp_Pair scomp_apply split_beta) 
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then show "select_default k x y s = (if k = 0 
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then range 1 o\<rightarrow> (\<lambda>_. Pair y) 
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else range k o\<rightarrow> (\<lambda>l. Pair (if l + 1 < k then x else y))) s" 
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by (cases "k = 0") (simp_all add: select_default_def scomp_apply split_beta) 
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qed 
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subsection {* @{text ML} interface *} 

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ML {* 

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structure Random_Engine = 
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struct 
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type seed = int * int; 
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local 

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val seed = ref 
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(let 

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val now = Time.toMilliseconds (Time.now ()); 

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val (q, s1) = IntInf.divMod (now, 2147483562); 
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val s2 = q mod 2147483398; 

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in (s1 + 1, s2 + 1) end); 
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in 
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fun run f = 

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let 

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val (x, seed') = f (! seed); 
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val _ = seed := seed' 
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in x end; 

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end; 
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end; 

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*} 

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no_notation fcomp (infixl "o>" 60) 
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no_notation scomp (infixl "o\<rightarrow>" 60) 
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end 
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