Refactor: Extract magic numbers as constants and reduce client code duplication

- Created internal/constants package with organized constant files:
  - timeouts.go: All time duration constants (timeouts, intervals, delays)
  - channels.go: Channel buffer size constants
  - limits.go: Numeric limits and configuration values
  - buffers.go: Buffer size constants in bytes

- Replaced all magic numbers throughout codebase with named constants:
  - Time durations (2s, 3s, 5s, 10s, 100ms, 24h) now use descriptive constants
  - Buffer sizes (8KB, 64KB, 1MB) extracted to constants
  - Channel buffer sizes and multipliers
  - Configuration limits (max connections, concurrency, etc.)
  - Health check status codes
  - Percentage calculations

- Reduced code duplication in client implementations:
  - Created CommonClient to share functionality between CatClient, GrepClient, and TailClient
  - All three clients now inherit from CommonClient
  - Eliminated duplicate makeHandler() and makeCommands() methods
  - Simplified client constructors

This refactoring improves code maintainability by centralizing configuration values
and reducing redundant code across similar client implementations.

6 files changed, 22 insertions, 15 deletions

diff --git a/internal/io/dlog/loggers/file.go b/internal/io/dlog/loggers/file.go
index 6a09353..b183843 100644
--- a/internal/io/dlog/loggers/file.go
+++ b/internal/io/dlog/loggers/file.go
@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/mimecast/dtail/internal/config"
+	"github.com/mimecast/dtail/internal/constants"
 )
 
 type fileWriter struct{}
@@ -36,7 +37,7 @@ type file struct {
 
 func newFile(strategy Strategy) *file {
 	return &file{
-		bufferCh: make(chan *fileMessageBuf, runtime.NumCPU()*100),
+		bufferCh: make(chan *fileMessageBuf, runtime.NumCPU()*constants.LoggerBufferChannelMultiplier),
 		pauseCh:  make(chan struct{}),
 		resumeCh: make(chan struct{}),
 		rotateCh: make(chan struct{}),
diff --git a/internal/io/fs/aggregateprocessor.go b/internal/io/fs/aggregateprocessor.go
index 98d0c31..809f298 100644
--- a/internal/io/fs/aggregateprocessor.go
+++ b/internal/io/fs/aggregateprocessor.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	"time"
 
+	"github.com/mimecast/dtail/internal/constants"
 	"github.com/mimecast/dtail/internal/io/line"
 	"github.com/mimecast/dtail/internal/lcontext"
 	"github.com/mimecast/dtail/internal/regex"
@@ -69,7 +70,7 @@ func (p *AggregateLineProcessor) Flush() []byte {
 	if !p.isTailing {
 		// Close the lines channel to signal end of input
 		// Add a small delay to ensure all lines are processed before closing
-		time.Sleep(10 * time.Millisecond)
+		time.Sleep(constants.ProcessorSleepDuration)
 		close(p.linesCh)
 	}
 	return nil
diff --git a/internal/io/fs/chunkedreader.go b/internal/io/fs/chunkedreader.go
index ab78ba1..7775a58 100644
--- a/internal/io/fs/chunkedreader.go
+++ b/internal/io/fs/chunkedreader.go
@@ -6,12 +6,13 @@ import (
 	"io"
 	"time"
 
+	"github.com/mimecast/dtail/internal/constants"
 	"github.com/mimecast/dtail/internal/io/dlog"
 	"github.com/mimecast/dtail/internal/io/pool"
 )
 
 // Reusable timer to reduce allocations - PBO optimization
-var sharedTimer = time.NewTimer(10 * time.Millisecond)
+var sharedTimer = time.NewTimer(constants.ProcessorSleepDuration)
 
 // ChunkedReader reads data in large chunks and processes it line by line
 // This replaces the byte-by-byte reading approach for better performance
@@ -29,14 +30,14 @@ type ChunkedReader struct {
 // NewChunkedReader creates a new chunked reader with the specified chunk size
 func NewChunkedReader(reader io.Reader, chunkSize int) *ChunkedReader {
 	if chunkSize <= 0 {
-		chunkSize = 64 * 1024 // Default 64KB chunks
+		chunkSize = constants.DefaultChunkSize // Default 64KB chunks
 	}
 	return &ChunkedReader{
 		reader:     reader,
 		buffer:     make([]byte, chunkSize),
 		chunkSize:  chunkSize,
 		// PBO optimization: Pre-allocate line buffer
-		lineBuffer: make([]byte, 0, 8192), // 8KB initial capacity
+		lineBuffer: make([]byte, 0, constants.LineBufferInitialCapacity), // 8KB initial capacity
 	}
 }
 
@@ -76,7 +77,7 @@ func (cr *ChunkedReader) ProcessLines(ctx context.Context, rawLines chan *bytes.
 							default:
 							}
 						}
-						sharedTimer.Reset(10 * time.Millisecond)
+						sharedTimer.Reset(constants.ProcessorSleepDuration)
 						select {
 						case <-ctx.Done():
 							return ctx.Err()
diff --git a/internal/io/fs/directprocessor.go b/internal/io/fs/directprocessor.go
index 84b78bb..312ac9c 100644
--- a/internal/io/fs/directprocessor.go
+++ b/internal/io/fs/directprocessor.go
@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/mimecast/dtail/internal/config"
+	"github.com/mimecast/dtail/internal/constants"
 	"github.com/mimecast/dtail/internal/lcontext"
 )
 
@@ -77,7 +78,7 @@ func (dp *DirectProcessor) ProcessReader(ctx context.Context, reader io.Reader,
 
 	// Set buffer size respecting MaxLineLength configuration
 	maxLineLength := config.Server.MaxLineLength
-	initialBufSize := 64 * 1024
+	initialBufSize := constants.InitialBufferSize
 	if maxLineLength < initialBufSize {
 		initialBufSize = maxLineLength
 	}
@@ -337,7 +338,7 @@ func (dp *DirectProcessor) followFile(ctx context.Context, filePath string) erro
 		select {
 		case <-ctx.Done():
 			return ctx.Err()
-		case <-time.After(100 * time.Millisecond):
+		case <-time.After(constants.ProcessorTimeoutDuration):
 			// Check if file has grown
 			fileInfo, err := os.Stat(filePath)
 			if err != nil {
diff --git a/internal/io/fs/stats.go b/internal/io/fs/stats.go
index 4121ff7..1990e53 100644
--- a/internal/io/fs/stats.go
+++ b/internal/io/fs/stats.go
@@ -1,14 +1,16 @@
 package fs
 
+import "github.com/mimecast/dtail/internal/constants"
+
 // Used to calculate how many log lines matched the regular expression
 // and how many log files could be transmitted from the server to the client.
 // Hit and transmit percentage takes only the last 100 log lines into calculation.
 type stats struct {
 	pos           int
 	lineCount     uint64
-	matched       [100]bool
+	matched       [constants.StatsArraySize]bool
 	matchCount    uint64
-	transmitted   [100]bool
+	transmitted   [constants.StatsArraySize]bool
 	transmitCount int
 }
 
@@ -25,7 +27,7 @@ func (f *stats) transmittedPerc() int {
 // Update bucket position. We only take into consideration the last 100
 // lines for stats.
 func (f *stats) updatePosition() {
-	f.pos = (f.pos + 1) % 100
+	f.pos = (f.pos + 1) % constants.StatsArraySize
 	f.lineCount++
 }
 
@@ -63,7 +65,7 @@ func (f *stats) updateLineNotTransmitted() {
 
 func percentOf(total float64, value float64) float64 {
 	if total == 0 || total == value {
-		return 100
+		return constants.PercentageMultiplier
 	}
-	return value / (total / 100.0)
+	return value / (total / constants.PercentageMultiplier)
 }
diff --git a/internal/io/fs/tailprocessor.go b/internal/io/fs/tailprocessor.go
index 3bc9029..55934ce 100644
--- a/internal/io/fs/tailprocessor.go
+++ b/internal/io/fs/tailprocessor.go
@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/mimecast/dtail/internal/config"
+	"github.com/mimecast/dtail/internal/constants"
 	"github.com/mimecast/dtail/internal/lcontext"
 	"github.com/mimecast/dtail/internal/regex"
 )
@@ -163,7 +164,7 @@ func (ftp *FollowingTailProcessor) followFile(ctx context.Context, filePath stri
 func (ftp *FollowingTailProcessor) followReader(ctx context.Context, file *os.File, filePath string) error {
 	// Set buffer size respecting MaxLineLength configuration
 	maxLineLength := config.Server.MaxLineLength
-	initialBufSize := 64 * 1024
+	initialBufSize := constants.InitialBufferSize
 	if maxLineLength < initialBufSize {
 		initialBufSize = maxLineLength
 	}
@@ -259,7 +260,7 @@ func (ftp *FollowingTailProcessor) followReader(ctx context.Context, file *os.Fi
 		select {
 		case <-ctx.Done():
 			return ctx.Err()
-		case <-time.After(100 * time.Millisecond):
+		case <-time.After(constants.ProcessorTimeoutDuration):
 			// Continue the loop to check for new content
 		}
 	}