filter_tail.go

// scriptish is a library to help you port bash scripts to Golang
//
// inspired by:
//
// - http://labix.org/pipe
// - https://github.com/bitfield/script
//
// Copyright 2019-present Ganbaro Digital Ltd
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
//
//   * Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//
//   * Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in
//     the documentation and/or other materials provided with the
//     distribution.
//
//   * Neither the names of the copyright holders nor the names of his
//     contributors may be used to endorse or promote products derived
//     from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
// FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
// COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
// INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
// BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
// ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.

package scriptish

import (
	"container/ring"
)

// Tail copies the last N lines from the pipeline's Stdin to its Stdout
func Tail(n int, opts ...*StepOption) *SequenceStep {
	// special case - deal with horrible values of n
	if n < 1 {
		return NewSequenceStep(
			func(p *Pipe) (int, error) {
				// debugging support
				Tracef("Tail(%d)", n)

				// do nothing
				return StatusOkay, nil
			},
			opts...,
		)
	}

	// general case
	return NewSequenceStep(
		func(p *Pipe) (int, error) {
			// debugging support
			Tracef("Tail(%d)", n)

			// we'll use the ring buffer for this
			buf := ring.New(n)

			// we write everything to the ring buffer, and let it throw away
			// everything but the requested number of lines :)
			for line := range p.Stdin.ReadLines() {
				buf.Value = line
				buf = buf.Next()
			}

			// at this point, the ring buffer contains (up to) the right number
			// of lines
			buf.Do(func(line interface{}) {
				// did we fill the buffer up?
				if line == nil {
					// no, we did not
					return
				}

				// if we get here, we have a line to preserve
				TracePipeStdout("%s", line.(string))
				p.Stdout.WriteString(line.(string))
				p.Stdout.WriteRune('\n')
			})

			// all done
			return StatusOkay, nil
		},
		opts...,
	)
}