move: string projects

x-tba/project-png-parser/png-parser-project/main.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"runtime"
+)
+
+func main() {
+	args := os.Args[1:]
+	if len(args) == 0 {
+		fmt.Println("run with a PNG file")
+		return
+	}
+
+	// this is not the best way
+	// it's better only to read the first 24 bytes
+	// this reads the whole file into memory
+	img, err := ioutil.ReadFile(args[0])
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	report()
+	img = append([]byte(nil), img[:24]...)
+	// img = img[:24:24] // unnecessary
+	report()
+
+	// s.PrintBacking = true
+	// s.MaxPerLine = 8
+	// s.MaxElements = 24
+	// s.PrintBytesHex = true
+	// s.Show("first 24 bytes", img)
+
+	var (
+		pngHeader     = [...]byte{0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a}
+		ihdrChunkType = [...]byte{0x49, 0x48, 0x44, 0x52}
+	)
+
+	// ------------------------------------------
+	// Read the PNG header
+	// https://www.w3.org/TR/2003/REC-PNG-20031110/#5PNG-file-signature
+	// ------------------------------------------
+	lpng := len(pngHeader)
+	if !bytes.Equal(img[:lpng], pngHeader[:]) {
+		fmt.Println("missing PNG header")
+		return
+	}
+
+	// skip the png header
+	img = img[lpng:]
+
+	// ------------------------------------------
+	// Read the IHDR chunk header
+	// The IHDR chunk shall be the first chunk in the PNG datastream.
+	// https://www.w3.org/TR/2003/REC-PNG-20031110/#11IHDR
+	// ------------------------------------------
+	header := img[:8] // get the length and chunk type
+
+	// ensure that the chunk type is IHDR
+	if !bytes.Equal(header[4:8], ihdrChunkType[:]) {
+		fmt.Println("missing IHDR chunk")
+		return
+	}
+
+	// ------------------------------------------
+	// Read the IHDR Chunk Data
+	// ------------------------------------------
+	img = img[len(header):] // skip the IHDR chunk header data
+	ihdr := img[:8]         // read the width&height from the ihdr chunk
+
+	// All integers that require more than one byte shall be in:
+	// network byte order = Big Endian
+	// https://www.w3.org/TR/2003/REC-PNG-20031110/#7Integers-and-byte-order
+	fmt.Printf("dimensions: %dx%d\n",
+		// read the first 4 bytes (width)
+		binary.BigEndian.Uint32(ihdr[:4]),
+		// read the next 4 bytes (height)
+		binary.BigEndian.Uint32(ihdr[4:8]))
+}
+
+func report() {
+	var m runtime.MemStats
+	runtime.GC()
+	runtime.ReadMemStats(&m)
+	fmt.Printf(" > Memory Usage: %v KB\n", m.Alloc/1024)
+}