add example code using RandomAccessReader. closes #30

examples/unzip.js

@@ -2,14 +2,56 @@
 var yauzl = require("../");
 var path = require("path");
 var fs = require("fs");
+var util = require("util");
 var Transform = require("stream").Transform;
 
-var zipFilePath = process.argv[2];
-if (zipFilePath == null || /^-/.test(zipFilePath)) {
+var zipFilePath;
+var offsetArg;
+var lenArg;
+var endArg;
+var args = process.argv.slice(2);
+for (var i = 0; i < args.length; i++) {
+  var arg = args[i];
+  if (arg === "--offset") {
+    i += 1;
+    offsetArg = parseInt(args[i]);
+    if (isNaN(offsetArg)) throw new Error("--offset argument not parsable as an int");
+  } else if (arg === "--len") {
+    i += 1;
+    lenArg = parseInt(args[i]);
+    if (isNaN(lenArg)) throw new Error("--len argument not parsable as an int");
+  } else if (arg === "--end") {
+    i += 1;
+    endArg = parseInt(args[i]);
+    if (isNaN(endArg)) throw new Error("--end argument not parsable as an int");
+  } else if (["-h", "--help"].indexOf(arg) !== -1) {
+    // print help
+    zipFilePath = null;
+    break;
+  } else if (/^--/.test(arg)) {
+    throw new Error("unrecognized option: " + arg);
+  } else {
+    if (zipFilePath != null) throw new Error("too many arguments");
+    zipFilePath = arg;
+  }
+}
+if (zipFilePath == null || /^-/.test(zipFilePath) || (lenArg != null && endArg != null)) {
   console.log(
-    "usage: node unzip.js path/to/file.zip\n" +
+    "usage: node unzip.js [options] path/to/file.zip\n" +
+    "\n" +
+    "unzips the specified zip file into the current directory\n" +
     "\n" +
-    "unzips the specified zip file into the current directory");
+    "options:\n" +
+    "  --offset START\n" +
+    "  --len LEN\n" +
+    "  --end END\n" +
+    "    interprets the middle of the specified file as a zipfile.\n" +
+    "    starting START number of bytes in from the beginning (default 0).\n" +
+    "    end with length of LEN (default is all the way to the end of the file).\n" +
+    "    or end at byte offset END (exclusive) (default is the end of the file).\n" +
+    "    end can be negative to count backwards from the end of the file\n" +
+    "    (example, `--end -1` excludes the last byte of the file).\n" +
+    "");
   process.exit(1);
 }
 
@@ -26,7 +68,60 @@ function mkdirp(dir, cb) {
   });
 }
 
-yauzl.open(zipFilePath, {lazyEntries: true}, function(err, zipfile) {
+if (offsetArg != null || lenArg != null || endArg != null) {
+  openMiddleOfFile(zipFilePath, {lazyEntries: true}, offsetArg, lenArg, endArg, handleZipFile);
+} else {
+  yauzl.open(zipFilePath, {lazyEntries: true}, handleZipFile);
+}
+
+function openMiddleOfFile(zipFilePath, options, offsetArg, lenArg, endArg, handleZipFile) {
+  fs.open(zipFilePath, "r", function(err, fd) {
+    if (err != null) throw err;
+    fs.fstat(fd, function(err, stats) {
+      // resolve optional parameters
+      if (offsetArg == null) offsetArg = 0;
+      if (lenArg == null && endArg == null) endArg = stats.size;
+      if (endArg == null) endArg = lenArg + offsetArg;
+      else if (endArg < 0) endArg = stats.size + endArg;
+      // validate parameters
+      if (offsetArg < 0) throw new Error("--offset < 0");
+      if (lenArg < 0) throw new Error("--len < 0");
+      if (offsetArg > endArg) throw new Error("--offset > --end");
+      if (endArg > stats.size) throw new Error("--end/--len goes past EOF");
+
+      function adjustOffset(n) {
+        return n + offsetArg;
+      }
+
+      // extend RandomAccessReader
+      function MiddleOfFileReader() {
+        yauzl.RandomAccessReader.call(this);
+      }
+      util.inherits(MiddleOfFileReader, yauzl.RandomAccessReader);
+      // implement required and option methods
+      MiddleOfFileReader.prototype._readStreamForRange = function(start, end) {
+        return fs.createReadStream(null, {
+          fd: fd,
+          // shift the start and end offsets
+          start: start + offsetArg,
+          end: end + offsetArg - 1, // the -1 is because fs.createReadStream()'s end option is inclusive
+          autoClose: false,
+        });
+      };
+      MiddleOfFileReader.prototype.read = function(buffer, offset, length, position, callback) {
+        // shift the position
+        fs.read(fd, buffer, offset, length, position + offsetArg, callback);
+      };
+      MiddleOfFileReader.prototype.close = function(callback) {
+        fs.close(fd, callback);
+      };
+
+      yauzl.fromRandomAccessReader(new MiddleOfFileReader(), endArg - offsetArg, options, handleZipFile);
+    });
+  });
+}
+
+function handleZipFile(err, zipfile) {
   if (err) throw err;
 
   zipfile.readEntry();
@@ -85,5 +180,5 @@ yauzl.open(zipFilePath, {lazyEntries: true}, function(err, zipfile) {
       });
     }
   });
-});
+}