package counter

import (
	"sync"
	"testing"
)

// Counter is a thread-safe integer counter.
type Counter struct {
	mu    sync.RWMutex
	value int64
}

// Add changes the counter by delta and returns its new value.
func (c *Counter) Add(delta int64) int64 {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.value += delta
	return c.value
}

// Increment increases the counter by one and returns its new value.
func (c *Counter) Increment() int64 {
	return c.Add(1)
}

// Value returns the current counter value.
func (c *Counter) Value() int64 {
	c.mu.RLock()
	defer c.mu.RUnlock()

	return c.value
}

// Reset sets the counter to zero.
func (c *Counter) Reset() {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.value = 0
}

func TestCounter(t *testing.T) {
	var c Counter

	c.Increment()
	c.Add(4)

	if got, want := c.Value(), int64(5); got != want {
		t.Fatalf("Value() = %d, want %d", got, want)
	}

	c.Reset()
	if got := c.Value(); got != 0 {
		t.Fatalf("Value() after Reset() = %d, want 0", got)
	}
}

func TestCounterConcurrent(t *testing.T) {
	var c Counter
	var wg sync.WaitGroup

	const goroutines = 20
	const increments = 1_000

	wg.Add(goroutines)
	for range goroutines {
		go func() {
			defer wg.Done()
			for range increments {
				c.Increment()
			}
		}()
	}
	wg.Wait()

	if got, want := c.Value(), int64(goroutines*increments); got != want {
		t.Fatalf("Value() = %d, want %d", got, want)
	}
}