package counter

import (
	"sync"
	"testing"
)

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

// New returns a counter initialized to value.
func New(value int64) *Counter {
	return &Counter{value: value}
}

// Add increments 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 increments 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 value and returns the previous value.
func (c *Counter) Reset(value int64) int64 {
	c.mu.Lock()
	defer c.mu.Unlock()

	old := c.value
	c.value = value
	return old
}

func TestCounterBasic(t *testing.T) {
	c := New(2)

	if got := c.Inc(); got != 3 {
		t.Fatalf("Inc() = %d, want 3", got)
	}
	if got := c.Add(4); got != 7 {
		t.Fatalf("Add(4) = %d, want 7", got)
	}
	if old := c.Reset(10); old != 7 {
		t.Fatalf("Reset(10) old value = %d, want 7", old)
	}
	if got := c.Value(); got != 10 {
		t.Fatalf("Value() = %d, want 10", got)
	}
}

func TestCounterConcurrent(t *testing.T) {
	const goroutines = 32
	const increments = 1000

	c := New(0)
	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)
	}
}