package logbuf

import (
	"bytes"
	"sync"
)

const DefaultMax = 256 << 10 // 256 KiB

// Buffer is a concurrency-safe, size-capped bytes.Buffer replacement for engine stderr.
type Buffer struct {
	mu  sync.Mutex
	buf bytes.Buffer
	max int
}

func New(max int) *Buffer {
	if max <= 0 {
		max = DefaultMax
	}
	return &Buffer{max: max}
}

func (b *Buffer) Write(p []byte) (int, error) {
	if b == nil {
		return len(p), nil
	}
	b.mu.Lock()
	defer b.mu.Unlock()
	if b.buf.Len()+len(p) > b.max {
		// Drop oldest half to keep recent diagnostics.
		keep := b.buf.Bytes()
		if len(keep) > b.max/2 {
			keep = keep[len(keep)-b.max/2:]
		}
		b.buf.Reset()
		_, _ = b.buf.Write(keep)
		_, _ = b.buf.WriteString("\n… log truncated …\n")
	}
	return b.buf.Write(p)
}

func (b *Buffer) String() string {
	if b == nil {
		return ""
	}
	b.mu.Lock()
	defer b.mu.Unlock()
	return b.buf.String()
}

func (b *Buffer) Len() int {
	if b == nil {
		return 0
	}
	b.mu.Lock()
	defer b.mu.Unlock()
	return b.buf.Len()
}