89 lines
2.0 KiB
Go
89 lines
2.0 KiB
Go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 278.
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// Package memo provides a concurrency-safe non-blocking memoization
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// of a function. Requests for different keys proceed in parallel.
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// Concurrent requests for the same key block until the first completes.
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// This implementation uses a monitor goroutine.
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package memo
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//!+Func
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// Func is the type of the function to memoize.
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type Func func(key string) (interface{}, error)
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// A result is the result of calling a Func.
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type result struct {
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value interface{}
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err error
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}
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type entry struct {
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res result
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ready chan struct{} // closed when res is ready
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}
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//!-Func
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//!+get
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// A request is a message requesting that the Func be applied to key.
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type request struct {
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key string
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response chan<- result // the client wants a single result
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}
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type Memo struct{ requests chan request }
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// New returns a memoization of f. Clients must subsequently call Close.
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func New(f Func) *Memo {
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memo := &Memo{requests: make(chan request)}
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go memo.server(f)
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return memo
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}
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func (memo *Memo) Get(key string) (interface{}, error) {
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response := make(chan result)
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memo.requests <- request{key, response}
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res := <-response
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return res.value, res.err
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}
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func (memo *Memo) Close() { close(memo.requests) }
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//!-get
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//!+monitor
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func (memo *Memo) server(f Func) {
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cache := make(map[string]*entry)
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for req := range memo.requests {
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e := cache[req.key]
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if e == nil {
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// This is the first request for this key.
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e = &entry{ready: make(chan struct{})}
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cache[req.key] = e
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go e.call(f, req.key) // call f(key)
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}
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go e.deliver(req.response)
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}
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}
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func (e *entry) call(f Func, key string) {
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// Evaluate the function.
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e.res.value, e.res.err = f(key)
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// Broadcast the ready condition.
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close(e.ready)
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}
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func (e *entry) deliver(response chan<- result) {
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// Wait for the ready condition.
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<-e.ready
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// Send the result to the client.
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response <- e.res
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}
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//!-monitor
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