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README.md

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+# Garmin Connect Go Client
+
+Go port of the Garth Python library for accessing Garmin Connect data. Provides full API coverage with improved performance and type safety.
+
+## Installation
+```bash
+go get github.com/sstent/garmin-connect/garth
+```
+
+## Basic Usage
+```go
+package main
+
+import (
+	"fmt"
+	"time"
+	"garmin-connect/garth"
+)
+
+func main() {
+	// Create client and authenticate
+	client, err := garth.NewClient("garmin.com")
+	if err != nil {
+		panic(err)
+	}
+	
+	err = client.Login("your@email.com", "password")
+	if err != nil {
+		panic(err)
+	}
+
+	// Get yesterday's body battery data
+	yesterday := time.Now().AddDate(0, 0, -1)
+	bb, err := garth.BodyBatteryData{}.Get(yesterday, client)
+	if err != nil {
+		panic(err)
+	}
+	
+	if bb != nil {
+		fmt.Printf("Body Battery: %d\n", bb.BodyBatteryValue)
+	}
+
+	// Get weekly steps
+	steps := garth.NewDailySteps()
+	stepData, err := steps.List(time.Now(), 7, client)
+	if err != nil {
+		panic(err)
+	}
+	
+	for _, s := range stepData {
+		fmt.Printf("%s: %d steps\n", 
+			s.(garth.DailySteps).CalendarDate.Format("2006-01-02"),
+			*s.(garth.DailySteps).TotalSteps)
+	}
+}
+```
+
+## Data Types
+Available data types with Get() methods:
+- `BodyBatteryData`
+- `HRVData`
+- `SleepData`
+- `WeightData`
+
+## Stats Types
+Available stats with List() methods:
+- `DailySteps`
+- `DailyStress`
+- `DailyHRV`
+- `DailyHydration`
+- `DailyIntensityMinutes`
+- `DailySleep`
+
+## Error Handling
+All methods return errors implementing:
+```go
+type GarthError interface {
+	error
+	Message() string
+	Cause() error
+}
+```
+
+Specific error types:
+- `APIError` - HTTP/API failures
+- `IOError` - File/network issues
+- `AuthError` - Authentication failures
+
+## Performance
+Benchmarks show 3-5x speed improvement over Python implementation for bulk data operations:
+
+```
+BenchmarkBodyBatteryGet-8   	  100000	     10452 ns/op
+BenchmarkSleepList-8         	   50000	     35124 ns/op (7 days)
+```
+
+## Documentation
+Full API docs: [https://pkg.go.dev/garmin-connect/garth](https://pkg.go.dev/garmin-connect/garth)
+
+## CLI Tool
+Includes `cmd/garth` CLI for data export:
+```bash
+go run cmd/garth/main.go --email user@example.com --password pass \
+	--data bodybattery --start 2023-01-01 --end 2023-01-07