package main

import (
	"bufio"
	"bytes"
	"io"
	"os"
	"runtime"
	"sync"
)

const lf = byte('\n')

func main() {
	if len(os.Args) != 2 {
		print("One argument required\n")
		os.Exit(1)
	}

	f, err := os.Open("locate.txt")
	if err != nil {
		panic(err)
	}
	defer f.Close()

	var needle = os.Args[1]

	// To make sure all threads have ended when the program finishes
	var wg sync.WaitGroup

	// Make a channel and a thread for each CPU core
	var cores = runtime.NumCPU()
	var workchannel = make(chan []byte)
	for i := 0; i < cores; i++ {
		go scannerThread(workchannel, []byte(needle), &wg)
		wg.Add(1)
	}

	var br = bufio.NewReaderSize(f, 1<<25) // reader with 32 MiB buffer
	var buf = make([]byte, 1<<20)          // 1 MiB buffer for searching
	var remainder []byte
	var nread int

	for {
		nread, err = br.Read(buf)
		if err != nil && err != io.EOF {
			panic(err)
		}
		if err != io.EOF {
			// Get the remainder of the last line
			remainder, err = br.ReadBytes(byte('\n'))
			if err != nil && err != io.EOF {
				panic(err)
			}
		}

		workchannel <- append(buf[:nread], remainder...)

		if err == io.EOF {
			break
		}
	}

	close(workchannel)
	wg.Wait()
}

func scannerThread(workchannel chan []byte, needle []byte, wg *sync.WaitGroup) {
	var batch []byte
	var ok bool
	var start, end int
	var linelen int
	var i int
	defer wg.Done()
	for {
		batch, ok = <-workchannel
		if !ok {
			return // channel closed. we're done
		}

		// This loop is for every result found, when there are no more results it stops
		for i = bytes.Index(batch, needle); i != -1; i = bytes.Index(batch, needle) {
			start, end = i, i
			// needle was found, but where?
			for { // find the start
				if batch[start] == lf {
					start++ // the line feed is from the previous line, so skip it
					break
				} else if start == 0 {
					break // result is at start of file
				}
				start--
			}

			for { // find the end
				if batch[end] == lf {
					end++ // include the line feed in the line
					// https://stackoverflow.com/questions/26857582/in-a-go-slice-why-does-slohi-end-at-element-hi-1
					break
				} else if end == linelen {
					break
				}
				end++
			}

			os.Stdout.Write(batch[start:end])

			// Chop all of the bytes before the result off so it doesn't get
			// searched again
			batch = batch[end:]
			linelen = len(batch)
		}
	}
}