From b3e74de390d30b839eb37764b9be0e897d0e9acb Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sun, 21 Jan 2024 14:47:56 +0530
Subject: [PATCH] More work on pager kitten

---
 kittens/pager/file_input.go | 103 ++++++++++++++++++++++++++++--------
 kittens/pager/main.go       |  12 +++--
 kittens/pager/main.py       |   5 ++
 3 files changed, 96 insertions(+), 24 deletions(-)

diff --git a/kittens/pager/file_input.go b/kittens/pager/file_input.go
index e48742627..f438452aa 100644
--- a/kittens/pager/file_input.go
+++ b/kittens/pager/file_input.go
@@ -9,23 +9,41 @@ import (
 	"io"
 	"os"
 	"strings"
+	"time"
 
 	"golang.org/x/sys/unix"
 )
 
 var _ = fmt.Print
 
-func read_input(input_file *os.File, input_file_name string, input_channel chan<- input_line_struct) {
+func wait_for_file_to_grow(file_name string, limit int64) (err error) {
+	// TODO: Use the fsnotify package to avoid this poll
+	for {
+		time.Sleep(time.Second)
+		s, err := os.Stat(file_name)
+		if err != nil {
+			return err
+		}
+		if s.Size() > limit {
+			break
+		}
+	}
+	return
+}
+
+func read_input(input_file *os.File, input_file_name string, input_channel chan<- input_line_struct, follow bool, count_carriage_returns bool) {
 	const buf_capacity = 8192
 	var buf_array [buf_capacity]byte
 	output_buf := strings.Builder{}
 	output_buf.Grow(buf_capacity)
 	var err error
 	var n int
+	var total_read int64
+	var num_carriage_returns int
 
 	defer func() {
 		_ = input_file.Close()
-		last := input_line_struct{line: output_buf.String(), err: err}
+		last := input_line_struct{line: output_buf.String(), err: err, num_carriage_returns: num_carriage_returns}
 		if errors.Is(err, io.EOF) {
 			last.err = nil
 		}
@@ -35,30 +53,73 @@ func read_input(input_file *os.File, input_file_name string, input_channel chan<
 		close(input_channel)
 	}()
 
-	process_chunk := func(chunk []byte) {
-		for len(chunk) > 0 {
-			idx := bytes.IndexByte(chunk, '\n')
-			switch idx {
-			case -1:
-				_, _ = output_buf.Write(chunk)
-				chunk = nil
-			default:
-				_, _ = output_buf.Write(chunk[idx:])
-				chunk = chunk[idx+1:]
-				input_channel <- input_line_struct{line: output_buf.String()}
-				output_buf.Reset()
-				output_buf.Grow(buf_capacity)
+	var process_chunk func([]byte)
+
+	if count_carriage_returns {
+		process_chunk = func(chunk []byte) {
+			for _, ch := range chunk {
+				switch ch {
+				case '\r':
+					num_carriage_returns += 1
+				default:
+					_ = output_buf.WriteByte(ch)
+				case '\n':
+					input_channel <- input_line_struct{line: output_buf.String(), num_carriage_returns: num_carriage_returns, is_a_complete_line: true}
+					num_carriage_returns = 0
+					output_buf.Reset()
+					output_buf.Grow(buf_capacity)
+				}
+			}
+		}
+	} else {
+		process_chunk = func(chunk []byte) {
+			for len(chunk) > 0 {
+				idx := bytes.IndexByte(chunk, '\n')
+				switch idx {
+				case -1:
+					_, _ = output_buf.Write(chunk)
+					chunk = nil
+				default:
+					_, _ = output_buf.Write(chunk[idx:])
+					chunk = chunk[idx+1:]
+					input_channel <- input_line_struct{line: output_buf.String(), is_a_complete_line: true}
+					output_buf.Reset()
+					output_buf.Grow(buf_capacity)
+				}
 			}
 		}
 	}
 
-	for err != nil {
-		n, err = input_file.Read(buf_array[:])
-		if n > 0 {
-			process_chunk(buf_array[:n])
+	for {
+		for err != nil {
+			n, err = input_file.Read(buf_array[:])
+			if n > 0 {
+				total_read += int64(n)
+				process_chunk(buf_array[:n])
+			}
+			if err == unix.EAGAIN || err == unix.EINTR {
+				err = nil
+			}
 		}
-		if err == unix.EAGAIN || err == unix.EINTR {
-			err = nil
+		if !follow {
+			break
+		}
+		if errors.Is(err, io.EOF) {
+			input_file.Close()
+			if err = wait_for_file_to_grow(input_file_name, total_read); err != nil {
+				break
+			}
+			if input_file, err = os.Open(input_file_name); err != nil {
+				break
+			}
+			var off int64
+			if off, err = input_file.Seek(total_read, io.SeekStart); err != nil {
+				break
+			}
+			if off != total_read {
+				err = fmt.Errorf("Failed to seek in %s to: %d", input_file_name, off)
+				break
+			}
 		}
 	}
 }
diff --git a/kittens/pager/main.go b/kittens/pager/main.go
index 504a43c25..fcac9190a 100644
--- a/kittens/pager/main.go
+++ b/kittens/pager/main.go
@@ -25,8 +25,10 @@ var debugprintln = tty.DebugPrintln
 var _ = debugprintln
 
 type input_line_struct struct {
-	line string
-	err  error
+	line                 string
+	num_carriage_returns int
+	is_a_complete_line   bool
+	err                  error
 }
 
 type global_state_struct struct {
@@ -59,7 +61,11 @@ func main(_ *cli.Command, opts_ *Options, args []string) (rc int, err error) {
 		}
 		global_state.input_file_name = args[0]
 	}
-	go read_input(input_file, global_state.input_file_name, input_channel)
+	follow := global_state.opts.Follow
+	if follow && global_state.input_file_name == "/dev/stdin" {
+		follow = false
+	}
+	go read_input(input_file, global_state.input_file_name, input_channel, follow, global_state.opts.Role == "scrollback")
 	return
 }
 
diff --git a/kittens/pager/main.py b/kittens/pager/main.py
index 5541fdaae..55d783e18 100644
--- a/kittens/pager/main.py
+++ b/kittens/pager/main.py
@@ -12,6 +12,11 @@
 default=pager
 choices=pager,scrollback
 The role the pager is used for. The default is a standard less like pager.
+
+
+--follow
+type=bool-set
+Follow changes in the specified file, automatically scrolling if currently on the last line.
 '''.format
 
 help_text = '''\