From c42be33ba94c47a7959bd1bb2ac5f5ed3b604836 Mon Sep 17 00:00:00 2001
From: "Sascha L. Teichmann" <sascha.teichmann@intevation.de>
Date: Tue, 16 Sep 2014 18:52:50 +0200
Subject: [PATCH] Generate ETags and encode PNG directly to response stream.

---
 cmd/mtwebmapper/TODO    |  1 -
 cmd/mtwebmapper/main.go | 90 +++++++++++++++++++++++++++--------------
 2 files changed, 59 insertions(+), 32 deletions(-)
 delete mode 100644 cmd/mtwebmapper/TODO

diff --git a/cmd/mtwebmapper/TODO b/cmd/mtwebmapper/TODO
deleted file mode 100644
index da890db..0000000
--- a/cmd/mtwebmapper/TODO
+++ /dev/null
@@ -1 +0,0 @@
-* Generate and use ETags to improve caching behavior.
diff --git a/cmd/mtwebmapper/main.go b/cmd/mtwebmapper/main.go
index 5b3b9b3..c1178d8 100644
--- a/cmd/mtwebmapper/main.go
+++ b/cmd/mtwebmapper/main.go
@@ -12,11 +12,10 @@ import (
 	"image/png"
 	"log"
 	"net/http"
+	"os"
 	"path/filepath"
 	"strconv"
 
-	"bytes"
-
 	"bitbucket.org/s_l_teichmann/mtredisalize/common"
 
 	"github.com/gorilla/mux"
@@ -64,6 +63,15 @@ func toUint(s string) uint {
 	return uint(x)
 }
 
+func createETag(path string) (etag string, err error) {
+	var fi os.FileInfo
+	if fi, err = os.Stat(path); err != nil {
+		return
+	}
+	etag = fmt.Sprintf("%x-%x", fi.ModTime().Unix(), fi.Size())
+	return
+}
+
 func main() {
 	var (
 		port   int
@@ -84,41 +92,38 @@ func main() {
 
 	r := mux.NewRouter()
 	r.HandleFunc("/map/{z:[0-9]+}/{x:[0-9]+}/{y:[0-9]+}.png", func(rw http.ResponseWriter, r *http.Request) {
+
 		vars := mux.Vars(r)
 		xs := vars["x"]
 		ys := vars["y"]
 		zs := vars["z"]
+
 		x, y, z := toUint(xs), toUint(ys), toUint(zs)
 		if z < 9 {
 			filename := fmt.Sprintf("%d/%d/%d.png", z, x, y)
 			http.ServeFile(rw, r, filepath.Join(mapDir, filename))
 			return
 		}
+
 		if z > 16 {
 			z = 16
 		}
-		//fmt.Printf("x %d\n", x)
-		//fmt.Printf("y %d\n", y)
-		//fmt.Printf("z %d\n", z)
 		tx := x >> (z - 8)
 		ty := y >> (z - 8)
-		rx := x & ^(^uint(0) << (z - 8))
-		ry := y & ^(^uint(0) << (z - 8))
-
-		parts := uint(1) << (z - 8)
-		//fmt.Printf("z %d\n", z)
-		//fmt.Printf("parts %d\n", parts)
-
-		w := uint(256) / parts
-		xo := w * rx
-		yo := w * (parts - 1 - ry)
-		//fmt.Printf("w %d\n", w)
-		//fmt.Printf("xo %d\n", xo)
-		//fmt.Printf("yo %d\n", yo)
-		//fmt.Printf("rx %d\n", rx)
-		//fmt.Printf("ry %d\n", ry)
 
 		baseTile := filepath.Join(mapDir, fmt.Sprintf("8/%d/%d.png", tx, ty))
+
+		var etag string
+		var err error
+
+		if ifNoneMatch := r.Header.Get("If-None-Match"); ifNoneMatch != "" {
+			if etag, err = createETag(baseTile); err == nil {
+				if ifNoneMatch == etag {
+					http.Error(rw, http.StatusText(http.StatusNotModified), http.StatusNotModified)
+					return
+				}
+			}
+		}
 		img := common.LoadPNG(baseTile)
 
 		type subImage interface {
@@ -129,24 +134,47 @@ func main() {
 		var ok bool
 		if si, ok = img.(subImage); !ok {
 			// Should not happen.
-			http.ServeFile(rw, r, filepath.Join(mapDir, baseTile))
+			http.Error(rw,
+				http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 			return
 		}
 
+		rx := x & ^(^uint(0) << (z - 8))
+		ry := y & ^(^uint(0) << (z - 8))
+
+		parts := uint(1) << (z - 8)
+
+		w := uint(256) / parts
+		xo := w * rx
+		yo := w * (parts - 1 - ry)
+		//fmt.Printf("x %d\n", x)
+		//fmt.Printf("y %d\n", y)
+		//fmt.Printf("z %d\n", z)
+		//fmt.Printf("z %d\n", z)
+		//fmt.Printf("parts %d\n", parts)
+		//fmt.Printf("w %d\n", w)
+		//fmt.Printf("xo %d\n", xo)
+		//fmt.Printf("yo %d\n", yo)
+		//fmt.Printf("rx %d\n", rx)
+		//fmt.Printf("ry %d\n", ry)
+
 		img = si.SubImage(image.Rect(int(xo), int(yo), int(xo+w), int(yo+w)))
 		img = blowUp(img)
 
-		var buf bytes.Buffer
-		if err := png.Encode(&buf, img); err != nil {
-			http.ServeFile(rw, r, filepath.Join(mapDir, baseTile))
-			return
-		}
-		b := buf.Bytes()
 		rw.Header().Set("Content-Type", "image/png")
-		// Don't set the content length to use chunked mode.
-		// rw.Header().Set("Content-Length", strconv.Itoa(len(b)))
-		if _, err := rw.Write(b); err != nil {
-			log.Printf("WARM: sending image failed: %s", err)
+		if etag == "" {
+			if etag, err = createETag(baseTile); err != nil {
+				log.Printf("Cannot create ETag: %s", baseTile)
+			} else {
+				rw.Header().Set("ETag", etag)
+			}
+		} else {
+			rw.Header().Set("ETag", etag)
+		}
+
+		if err = png.Encode(rw, img); err != nil {
+			log.Printf("WARN: encoding image failed: %s", err)
+			return
 		}
 	})