package gzip

import (
	"compress/gzip"
	"io"
	"net/http"
	"path/filepath"
	"strconv"
	"strings"
	"sync"

	"github.com/gin-gonic/gin"
)

const (
	headerAcceptEncoding  = "Accept-Encoding"
	headerContentEncoding = "Content-Encoding"
	headerVary            = "Vary"
)

type gzipHandler struct {
	*config
	gzPool sync.Pool
}

func isCompressionLevelValid(level int) bool {
	return level == gzip.DefaultCompression ||
		level == gzip.NoCompression ||
		(level >= gzip.BestSpeed && level <= gzip.BestCompression)
}

func newGzipHandler(level int, opts ...Option) *gzipHandler {
	cfg := &config{
		excludedExtensions: DefaultExcludedExtentions,
	}

	// Apply each option to the config
	for _, o := range opts {
		o.apply(cfg)
	}

	if !isCompressionLevelValid(level) {
		// For web content, level 4 seems to be a sweet spot.
		level = 4
	}

	handler := &gzipHandler{
		config: cfg,
		gzPool: sync.Pool{
			New: func() interface{} {
				gz, _ := gzip.NewWriterLevel(io.Discard, level)
				return gz
			},
		},
	}
	return handler
}

// Handle is a middleware function for handling gzip compression in HTTP requests and responses.
// It first checks if the request has a "Content-Encoding" header set to "gzip" and if a decompression
// function is provided, it will call the decompression function. If the handler is set to decompress only,
// or if the custom compression decision function indicates not to compress, it will return early.
// Otherwise, it retrieves a gzip.Writer from the pool, sets the necessary response headers for gzip encoding,
// and wraps the response writer with a gzipWriter. After the request is processed, it ensures the gzip.Writer
// is properly closed and the "Content-Length" header is set based on the response size.
func (g *gzipHandler) Handle(c *gin.Context) {
	if fn := g.decompressFn; fn != nil && strings.Contains(c.Request.Header.Get("Content-Encoding"), "gzip") {
		fn(c)
	}

	if g.decompressOnly ||
		(g.customShouldCompressFn != nil && !g.customShouldCompressFn(c)) ||
		(g.customShouldCompressFn == nil && !g.shouldCompress(c.Request)) {
		return
	}

	gz := g.gzPool.Get().(*gzip.Writer)
	gz.Reset(c.Writer)

	c.Header(headerContentEncoding, "gzip")
	c.Writer.Header().Add(headerVary, headerAcceptEncoding)
	// check ETag Header
	originalEtag := c.GetHeader("ETag")
	if originalEtag != "" && !strings.HasPrefix(originalEtag, "W/") {
		c.Header("ETag", "W/"+originalEtag)
	}
	c.Writer = &gzipWriter{c.Writer, gz}
	defer func() {
		if c.Writer.Size() < 0 {
			// do not write gzip footer when nothing is written to the response body
			gz.Reset(io.Discard)
		}
		_ = gz.Close()
		if c.Writer.Size() > -1 {
			c.Header("Content-Length", strconv.Itoa(c.Writer.Size()))
		}
		g.gzPool.Put(gz)
	}()
	c.Next()
}

func (g *gzipHandler) shouldCompress(req *http.Request) bool {
	if !strings.Contains(req.Header.Get(headerAcceptEncoding), "gzip") ||
		strings.Contains(req.Header.Get("Connection"), "Upgrade") {
		return false
	}

	// Check if the request path is excluded from compression
	extension := filepath.Ext(req.URL.Path)
	if g.excludedExtensions.Contains(extension) ||
		g.excludedPaths.Contains(req.URL.Path) ||
		g.excludedPathesRegexs.Contains(req.URL.Path) {
		return false
	}

	return true
}