// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package testing

import (
	"flag"
	"fmt"
	"os"
	"time"
)

var matchBenchmarks = flag.String("benchmarks", "", "regular expression to select benchmarks to run")

// An internal type but exported because it is cross-package; part of the implementation
// of gotest.
type Benchmark struct {
	Name string
	F    func(b *B)
}

// B is a type passed to Benchmark functions to manage benchmark
// timing and to specify the number of iterations to run.
type B struct {
	N         int
	benchmark Benchmark
	ns        int64
	bytes     int64
	start     int64
}

// StartTimer starts timing a test.  This function is called automatically
// before a benchmark starts, but it can also used to resume timing after
// a call to StopTimer.
func (b *B) StartTimer() { b.start = time.Nanoseconds() }

// StopTimer stops timing a test.  This can be used to pause the timer
// while performing complex initialization that you don't
// want to measure.
func (b *B) StopTimer() {
	if b.start > 0 {
		b.ns += time.Nanoseconds() - b.start
	}
	b.start = 0
}

// ResetTimer stops the timer and sets the elapsed benchmark time to zero.
func (b *B) ResetTimer() {
	b.start = 0
	b.ns = 0
}

// SetBytes records the number of bytes processed in a single operation.
// If this is called, the benchmark will report ns/op and MB/s.
func (b *B) SetBytes(n int64) { b.bytes = n }

func (b *B) nsPerOp() int64 {
	if b.N <= 0 {
		return 0
	}
	return b.ns / int64(b.N)
}

// runN runs a single benchmark for the specified number of iterations.
func (b *B) runN(n int) {
	b.N = n
	b.ResetTimer()
	b.StartTimer()
	b.benchmark.F(b)
	b.StopTimer()
}

func min(x, y int) int {
	if x > y {
		return y
	}
	return x
}

// roundDown10 rounds a number down to the nearest power of 10.
func roundDown10(n int) int {
	var tens = 0
	// tens = floor(log_10(n))
	for n > 10 {
		n = n / 10
		tens++
	}
	// result = 10^tens
	result := 1
	for i := 0; i < tens; i++ {
		result *= 10
	}
	return result
}

// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX].
func roundUp(n int) int {
	base := roundDown10(n)
	if n < (2 * base) {
		return 2 * base
	}
	if n < (5 * base) {
		return 5 * base
	}
	return 10 * base
}

// run times the benchmark function.  It gradually increases the number
// of benchmark iterations until the benchmark runs for a second in order
// to get a reasonable measurement.  It prints timing information in this form
//		testing.BenchmarkHello	100000		19 ns/op
func (b *B) run() {
	// Run the benchmark for a single iteration in case it's expensive.
	n := 1
	b.runN(n)
	// Run the benchmark for at least a second.
	for b.ns < 1e9 && n < 1e9 {
		last := n
		// Predict iterations/sec.
		if b.nsPerOp() == 0 {
			n = 1e9
		} else {
			n = 1e9 / int(b.nsPerOp())
		}
		// Run more iterations than we think we'll need for a second (1.5x).
		// Don't grow too fast in case we had timing errors previously.
		n = min(int(1.5*float(n)), 100*last)
		// Round up to something easy to read.
		n = roundUp(n)
		b.runN(n)
	}
	ns := b.nsPerOp()
	mb := ""
	if ns > 0 && b.bytes > 0 {
		mb = fmt.Sprintf("\t%7.2f MB/s", (float64(b.bytes)/1e6)/(float64(ns)/1e9))
	}
	fmt.Printf("%s\t%8d\t%10d ns/op%s\n", b.benchmark.Name, b.N, b.nsPerOp(), mb)
}

// An internal function but exported because it is cross-package; part of the implementation
// of gotest.
func RunBenchmarks(benchmarks []Benchmark) {
	// If no flag was specified, don't run benchmarks.
	if len(*matchBenchmarks) == 0 {
		return
	}
	re, err := CompileRegexp(*matchBenchmarks)
	if err != "" {
		println("invalid regexp for -benchmarks:", err)
		os.Exit(1)
	}
	for _, Benchmark := range benchmarks {
		if !re.MatchString(Benchmark.Name) {
			continue
		}
		b := &B{benchmark: Benchmark}
		b.run()
	}
}