// Package testlog creates a *log.Logger backed by *testing.T to ease logging
// in tests. This allows logs from components being tested to only be printed
// if the test fails (or the verbose flag is specified).
package testlog

import (
	"io"
	"log"
)

// LogPrinter is the methods of testing.T (or testing.B) needed by the test
// logger.
type LogPrinter interface {
	Logf(format string, args ...interface{})
}

// writer implements io.Writer on top of a Logger.
type writer struct {
	t LogPrinter
}

// Write to an underlying Logger. Never returns an error.
func (w *writer) Write(p []byte) (n int, err error) {
	w.t.Logf(string(p))
	return len(p), nil
}

// NewWriter creates a new io.Writer backed by a Logger.
func NewWriter(t LogPrinter) io.Writer {
	return &writer{t}
}

// New returns a new test logger. See https://golang.org/pkg/log/#New
func New(t LogPrinter, prefix string, flag int) *log.Logger {
	return log.New(&writer{t}, prefix, flag)
}

// WithPrefix returns a new test logger with the Lmicroseconds flag set.
func WithPrefix(t LogPrinter, prefix string) *log.Logger {
	return New(t, prefix, log.Lmicroseconds)
}

// NewLog logger with "TEST" prefix and the Lmicroseconds flag.
func Logger(t LogPrinter) *log.Logger {
	return WithPrefix(t, "")
}