package counter

import (
	"sync"
	"testing"
)

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

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

	c.value += delta
	return c.value
}

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

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

	return c.value
}

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

	c.value = 0
}

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

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

	if got := c.Inc(); got != 1 {
		t.Fatalf("Inc() = %d, want 1", got)
	}

	if got := c.Add(4); got != 5 {
		t.Fatalf("Add(4) = %d, want 5", got)
	}

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

func TestCounterConcurrentIncrements(t *testing.T) {
	var c Counter
	const goroutines = 20
	const increments = 1000

	var wg sync.WaitGroup
	wg.Add(goroutines)

	for range goroutines {
		go func() {
			defer wg.Done()
			for range increments {
				c.Inc()
			}
		}()
	}

	wg.Wait()

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