/*
Copyright 2018 Google LLC All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package build

import (
	"context"
	"sync"
)

// Caching wraps a builder implementation in a layer that shares build results
// for the same inputs using a simple "future" implementation.  Cached results
// may be invalidated by calling Invalidate with the same input passed to Build.
type Caching struct {
	inner Interface

	m       sync.Mutex
	results map[string]*future
}

// Caching implements Interface
var _ Interface = (*Caching)(nil)

// NewCaching wraps the provided build.Interface in an implementation that
// shares build results for a given path until the result has been invalidated.
func NewCaching(inner Interface) (*Caching, error) {
	return &Caching{
		inner:   inner,
		results: make(map[string]*future),
	}, nil
}

// Build implements Interface
func (c *Caching) Build(ctx context.Context, ip string) (Result, error) {
	f := func() *future {
		// Lock the map of futures.
		c.m.Lock()
		defer c.m.Unlock()

		// If a future for "ip" exists, then return it.
		f, ok := c.results[ip]
		if ok {
			return f
		}
		// Otherwise create and record a future for a Build of "ip".
		f = newFuture(func() (Result, error) {
			return c.inner.Build(ctx, ip)
		})
		c.results[ip] = f
		return f
	}()

	return f.Get()
}

// QualifyImport implements Interface
func (c *Caching) QualifyImport(ip string) (string, error) {
	return c.inner.QualifyImport(ip)
}

// IsSupportedReference implements Interface
func (c *Caching) IsSupportedReference(ip string) error {
	return c.inner.IsSupportedReference(ip)
}

// Invalidate removes an import path's cached results.
func (c *Caching) Invalidate(ip string) {
	c.m.Lock()
	defer c.m.Unlock()

	delete(c.results, ip)
}
