Theory Parallel_Example

section ‹A simple example demonstrating parallelism for code generated towards Isabelle/ML›

theory Parallel_Example
imports Complex_Main "HOL-Library.Parallel" "HOL-Library.Debug"
begin

subsection ‹Compute-intensive examples.›

subsubsection ‹Fragments of the harmonic series›

definition harmonic :: "nat ⇒ rat" where
  "harmonic n = sum_list (map (λn. 1 / of_nat n) [1..<n])"


subsubsection ‹The sieve of Erathostenes›

text ‹
  The attentive reader may relate this ad-hoc implementation to the
  arithmetic notion of prime numbers as a little exercise.
›

primrec mark :: "nat ⇒ nat ⇒ bool list ⇒ bool list" where
  "mark _ _ [] = []"
| "mark m n (p # ps) = (case n of 0 ⇒ False # mark m m ps
    | Suc n ⇒ p # mark m n ps)"

lemma length_mark [simp]:
  "length (mark m n ps) = length ps"
  by (induct ps arbitrary: n) (simp_all split: nat.split)

function sieve :: "nat ⇒ bool list ⇒ bool list" where
  "sieve m ps = (case dropWhile Not ps
   of [] ⇒ ps
    | p#ps' ⇒ let n = m - length ps' in takeWhile Not ps @ p # sieve m (mark n n ps'))"
by pat_completeness auto

termination ― ‹tuning of this proof is left as an exercise to the reader›
  apply (relation "measure (length ∘ snd)")
  apply rule
  apply (auto simp add: length_dropWhile_le)
proof -
  fix ps qs q
  assume "dropWhile Not ps = q # qs"
  then have "length (q # qs) = length (dropWhile Not ps)" by simp
  then have "length qs < length (dropWhile Not ps)" by simp
  moreover have "length (dropWhile Not ps) ≤ length ps"
    by (simp add: length_dropWhile_le)
  ultimately show "length qs < length ps" by auto
qed

primrec natify :: "nat ⇒ bool list ⇒ nat list" where
  "natify _ [] = []"
| "natify n (p#ps) = (if p then n # natify (Suc n) ps else natify (Suc n) ps)"

primrec list_primes where
  "list_primes (Suc n) = natify 1 (sieve n (False # replicate n True))"


subsubsection ‹Naive factorisation›

function factorise_from :: "nat ⇒ nat ⇒ nat list" where
  "factorise_from k n = (if 1 < k ∧ k ≤ n
    then
      let (q, r) = Divides.divmod_nat n k 
      in if r = 0 then k # factorise_from k q
        else factorise_from (Suc k) n
    else [])" 
by pat_completeness auto

termination factorise_from ― ‹tuning of this proof is left as an exercise to the reader›
  apply (relation "measure (λ(k, n). 2 * n - k)")
    apply (auto simp add: prod_eq_iff algebra_simps elim!: dvdE)
  apply (case_tac "k ≤ ka * 2")
   apply (auto intro: diff_less_mono)
  done

definition factorise :: "nat ⇒ nat list" where
  "factorise n = factorise_from 2 n"


subsection ‹Concurrent computation via futures›

definition computation_harmonic :: "unit ⇒ rat" where
  "computation_harmonic _ = Debug.timing (STR ''harmonic example'') harmonic 300"

definition computation_primes :: "unit ⇒ nat list" where
  "computation_primes _ = Debug.timing (STR ''primes example'') list_primes 4000"

definition computation_future :: "unit ⇒ nat list × rat" where
  "computation_future = Debug.timing (STR ''overall computation'')
   (λ() ⇒ let c = Parallel.fork computation_harmonic
     in (computation_primes (), Parallel.join c))"

value "computation_future ()"

definition computation_factorise :: "nat ⇒ nat list" where
  "computation_factorise = Debug.timing (STR ''factorise'') factorise"

definition computation_parallel :: "unit ⇒ nat list list" where
  "computation_parallel _ = Debug.timing (STR ''overall computation'')
     (Parallel.map computation_factorise) [20000..<20100]"

value "computation_parallel ()"

end