package stats

import (
	"os"
	"path/filepath"
	"reflect"
	"testing"
)

func TestTailer(t *testing.T) {
	dir := t.TempDir()
	path := filepath.Join(dir, "r.log")
	var got []string
	collect := func(l string) { got = append(got, l) }
	appendTo := func(p, s string) {
		f, err := os.OpenFile(p, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
		if err != nil {
			t.Fatal(err)
		}
		f.WriteString(s)
		f.Close()
	}
	tl := &Tailer{Path: path}
	// The file doesn't exist yet.
	if n, err := tl.Read(collect); n != 0 || err != nil {
		t.Fatalf("missing file: %d, %v", n, err)
	}
	appendTo(path, "one\ntwo\nthr")
	tl.Read(collect)
	appendTo(path, "ee\n")
	tl.Read(collect)
	if !reflect.DeepEqual(got, []string{"one", "two", "three"}) {
		t.Fatalf("got %q", got)
	}
	// named rotates: renames the file and starts a new one. Lines written
	// to the old file before the rename are not lost.
	appendTo(path, "four\n")
	os.Rename(path, path+".0")
	appendTo(path, "five\n")
	tl.Read(collect)
	if !reflect.DeepEqual(got[3:], []string{"four", "five"}) {
		t.Fatalf("after rotation got %q", got)
	}
	// Restart: resume from the saved position.
	pos := tl.Position()
	tl.Close()
	appendTo(path, "six\n")
	tl2 := &Tailer{Path: path}
	tl2.Resume(pos)
	got = nil
	tl2.Read(collect)
	if !reflect.DeepEqual(got, []string{"six"}) {
		t.Fatalf("after resume got %q", got)
	}
	// Rotation while jarld was down: finish the rotated file first.
	pos = tl2.Position()
	tl2.Close()
	appendTo(path, "seven\n")
	os.Rename(path, path+".0")
	appendTo(path, "eight\n")
	tl3 := &Tailer{Path: path}
	tl3.Resume(pos)
	got = nil
	tl3.Read(collect)
	if !reflect.DeepEqual(got, []string{"seven", "eight"}) {
		t.Fatalf("after offline rotation got %q", got)
	}
	// Truncation: start over.
	os.WriteFile(path, []byte("x\n"), 0o644)
	got = nil
	tl3.Read(collect)
	tl3.Read(collect)
	if !reflect.DeepEqual(got, []string{"x"}) {
		t.Fatalf("after truncation got %q", got)
	}
	tl3.Close()
}