Add support for +1234 command line option

Fixes #155.

m/pager.go

@@ -2,6 +2,7 @@ package m
 
 import (
 	"fmt"
+	"math"
 	"regexp"
 	"time"
 
@@ -59,7 +60,9 @@ type Pager struct {
 	searchString   string
 	searchPattern  *regexp.Regexp
 	gotoLineString string
-	Following      bool
+
+	// We used to have a "Following" field here. If you want to follow, set
+	// TargetLineNumberOneBased to math.MaxInt instead, see below.
 
 	isShowingHelp bool
 	preHelpState  *_PreHelpState
@@ -83,16 +86,20 @@ type Pager struct {
 
 	SideScrollAmount int // Should be positive
 
+	// If non-zero, scroll to this line number as soon as possible. Set to
+	// math.MaxInt to follow the end of the input (tail).
+	TargetLineNumberOneBased int
+
 	// If true, pager will clear the screen on return. If false, pager will
 	// clear the last line, and show the cursor.
 	DeInit bool
 }
 
 type _PreHelpState struct {
-	reader              *Reader
-	scrollPosition      scrollPosition
-	leftColumnZeroBased int
-	following           bool
+	reader                   *Reader
+	scrollPosition           scrollPosition
+	leftColumnZeroBased      int
+	targetLineNumberOneBased int
 }
 
 const _EofMarkerFormat = "\x1b[7m" // Reverse video
@@ -210,7 +217,7 @@ func (p *Pager) Quit() {
 	p.reader = p.preHelpState.reader
 	p.scrollPosition = p.preHelpState.scrollPosition
 	p.leftColumnZeroBased = p.preHelpState.leftColumnZeroBased
-	p.Following = p.preHelpState.following
+	p.TargetLineNumberOneBased = p.preHelpState.targetLineNumberOneBased
 	p.preHelpState = nil
 }
 
@@ -235,11 +242,15 @@ func (p *Pager) moveRight(delta int) {
 }
 
 func (p *Pager) handleScrolledUp() {
-	p.Following = false
+	p.TargetLineNumberOneBased = 0
 }
 
 func (p *Pager) handleScrolledDown() {
-	p.Following = p.isScrolledToEnd()
+	if p.isScrolledToEnd() {
+		p.TargetLineNumberOneBased = math.MaxInt
+	} else {
+		p.TargetLineNumberOneBased = 0
+	}
 }
 
 func (p *Pager) onKey(keyCode twin.KeyCode) {
@@ -326,15 +337,15 @@ func (p *Pager) onRune(char rune) {
 	case '?':
 		if !p.isShowingHelp {
 			p.preHelpState = &_PreHelpState{
-				reader:              p.reader,
-				scrollPosition:      p.scrollPosition,
-				leftColumnZeroBased: p.leftColumnZeroBased,
-				following:           p.Following,
+				reader:                   p.reader,
+				scrollPosition:           p.scrollPosition,
+				leftColumnZeroBased:      p.leftColumnZeroBased,
+				targetLineNumberOneBased: p.TargetLineNumberOneBased,
 			}
 			p.reader = _HelpReader
 			p.scrollPosition = newScrollPosition("Pager scroll position")
 			p.leftColumnZeroBased = 0
-			p.Following = false
+			p.TargetLineNumberOneBased = 0
 			p.isShowingHelp = true
 		}
 
@@ -539,8 +550,15 @@ func (p *Pager) StartPaging(screen twin.Screen) {
 			return
 
 		case eventMoreLinesAvailable:
-			if p.mode.isViewing() && p.Following {
-				p.scrollToEnd()
+			if p.mode.isViewing() && p.TargetLineNumberOneBased > 0 {
+				// The user wants to scroll down to a specific line number
+				if p.reader.GetLineCount() >= p.TargetLineNumberOneBased {
+					p.scrollPosition = NewScrollPositionFromLineNumberOneBased(p.TargetLineNumberOneBased, "goToTargetLineNumber")
+					p.TargetLineNumberOneBased = 0
+				} else {
+					// Not there yet, keep scrolling
+					p.scrollToEnd()
+				}
 			}
 
 		case eventMaybeDone:

m/scrollPosition.go

@@ -1,6 +1,9 @@
 package m
 
-import "fmt"
+import (
+	"fmt"
+	"math"
+)
 
 // Please create using newScrollPosition(name)
 type scrollPosition struct {
@@ -305,7 +308,13 @@ func (p *Pager) scrollToEnd() {
 	// lines than the number of characters it contains.
 	p.scrollPosition.internalDontTouch.deltaScreenLines = len(lastInputLine.raw)
 
-	p.Following = true
+	if p.TargetLineNumberOneBased == 0 {
+		// Start following the end of the file
+		//
+		// Otherwise, if we're already aiming for some place, don't overwrite
+		// that.
+		p.TargetLineNumberOneBased = math.MaxInt
+	}
 }
 
 // Can be either because Pager.scrollToEnd() was just called or because the user

m/search.go

@@ -117,7 +117,7 @@ func (p *Pager) scrollToNextSearchHit() {
 	p.scrollPosition = *firstHitPosition
 
 	// Don't let any search hit scroll out of sight
-	p.Following = false
+	p.TargetLineNumberOneBased = 0
 }
 
 func (p *Pager) scrollToPreviousSearchHit() {
@@ -155,7 +155,7 @@ func (p *Pager) scrollToPreviousSearchHit() {
 	p.scrollPosition = *firstHitPosition
 
 	// Don't let any search hit scroll out of sight
-	p.Following = false
+	p.TargetLineNumberOneBased = 0
 }
 
 func (p *Pager) updateSearchPattern() {

moar.1

@@ -95,6 +95,10 @@ Print trace logs after exiting, more verbose than
 \fB\-\-wrap\fR
 Wrap long lines, toggle with
 .B w
+.TP
+\fB\+\1234\fR
+Immediately scroll to line
+.B 1234
 .SH ENVIRONMENT
 Having
 .B PAGER=moar

moar.go

@@ -4,6 +4,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"math"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -80,6 +81,9 @@ func printUsage(output io.Writer, flagSet *flag.FlagSet, printCommandline bool)
 	_, _ = fmt.Fprintln(output, "Options:")
 
 	flagSet.PrintDefaults()
+
+	_, _ = fmt.Fprintln(output, "  +1234")
+	_, _ = fmt.Fprintln(output, "    \tImmediately scroll to line 1234")
 }
 
 // "moar" if we're in the $PATH, otherwise an absolute path
@@ -271,6 +275,35 @@ func tryOpen(filename string) error {
 	return err
 }
 
+// Parses an argument like "+123" anywhere on the command line into a one-based
+// line number, and returns the remaining args.
+//
+// Returns 0 on no target line number specified.
+func getTargetLineNumberOneBased(flagSet *flag.FlagSet) (int, []string) {
+	args := flagSet.Args()
+	for i, arg := range args {
+		if !strings.HasPrefix(arg, "+") {
+			continue
+		}
+
+		lineNumber, err := strconv.ParseInt(arg[1:], 10, 32)
+		if err != nil {
+			continue
+		}
+
+		// Remove the target line number from the args
+		//
+		// Ref: https://stackoverflow.com/a/57213476/473672
+		remainingArgs := make([]string, 0)
+		remainingArgs = append(remainingArgs, args[:i]...)
+		remainingArgs = append(remainingArgs, args[i+1:]...)
+
+		return int(lineNumber), remainingArgs
+	}
+
+	return 0, args
+}
+
 func main() {
 	// FIXME: If we get a CTRL-C, get terminal back into a useful state before terminating
 
@@ -370,8 +403,10 @@ func main() {
 		TimestampFormat: time.StampMicro,
 	})
 
-	if len(flagSet.Args()) > 1 {
-		fmt.Fprintln(os.Stderr, "ERROR: Expected exactly one filename, or data piped from stdin, got:", flagSet.Args())
+	targetLineNumberOneBased, remainingArgs := getTargetLineNumberOneBased(flagSet)
+
+	if len(remainingArgs) > 1 {
+		fmt.Fprintln(os.Stderr, "ERROR: Expected exactly one filename, or data piped from stdin, got:", remainingArgs)
 		fmt.Fprintln(os.Stderr)
 		printUsage(os.Stderr, flagSet, true)
 
@@ -381,7 +416,7 @@ func main() {
 	stdinIsRedirected := !term.IsTerminal(int(os.Stdin.Fd()))
 	stdoutIsRedirected := !term.IsTerminal(int(os.Stdout.Fd()))
 	var inputFilename *string
-	if len(flagSet.Args()) == 1 {
+	if len(remainingArgs) == 1 {
 		word := flagSet.Arg(0)
 		inputFilename = &word
 
@@ -444,7 +479,6 @@ func main() {
 
 	pager := m.NewPager(reader)
 	pager.WrapLongLines = *wrap
-	pager.Following = *follow
 	pager.ShowLineNumbers = !*noLineNumbers
 	pager.ShowStatusBar = !*noStatusBar
 	pager.DeInit = !*noClearOnExit
@@ -454,6 +488,12 @@ func main() {
 	pager.ScrollLeftHint = *scrollLeftHint
 	pager.ScrollRightHint = *scrollRightHint
 	pager.SideScrollAmount = int(*shift)
+
+	pager.TargetLineNumberOneBased = targetLineNumberOneBased
+	if *follow && pager.TargetLineNumberOneBased == 0 {
+		pager.TargetLineNumberOneBased = math.MaxInt
+	}
+
 	startPaging(pager, screen)
 }