Add an optional offset to File.readBytes().

doc/site/modules/io/file.markdown

@@ -12,7 +12,7 @@ a file descriptor.
 
 ### File.**open**(path, fn)
 
-Opens the file at [path] and passes it to [fn]. After the function returns, the
+Opens the file at `path` and passes it to `fn`. After the function returns, the
 file is automatically closed.
 
     :::wren
@@ -22,7 +22,7 @@ file is automatically closed.
 
 ### File.**read**(path)
 
-Reads the entire contents of the file at [path] and returns it as a string.
+Reads the entire contents of the file at `path` and returns it as a string.
 
     :::wren
     File.read("words.txt")
@@ -33,13 +33,13 @@ whatever encoding the file uses.
 
 ### File.**size**(path)
 
-Returns the size in bytes of the contents of the file at [path].
+Returns the size in bytes of the contents of the file at `path`.
 
 ## Constructors
 
 ### File.**open**(path)
 
-Opens the file at [path] for reading.
+Opens the file at `path` for reading.
 
 ## Methods
 
@@ -61,6 +61,21 @@ Closes the file. After calling this, you can read or write from it.
 
 ### **readBytes**(count)
 
-Reads up to [count] bytes starting from the beginning of the file.
+Reads up to `count` bytes starting from the beginning of the file.
 
-(Allowing an offset to read elsewhere from the file isn't implemented yet.)
+    :::wren
+    // Assume this file contains "I am a file!".
+    File.open("example.txt") {|file|
+      System.print(file.readBytes(6)) //> I am a
+    }
+
+### **readBytes**(count, offset)
+
+Reads up to `count` bytes starting at `offset` bytes from the beginning of
+the file.
+
+    :::wren
+    // Assume this file contains "I am a file!".
+    File.open("example.txt") {|file|
+      System.print(file.readBytes(6, 2)) //> am a f
+    }

src/cli/modules.c

@@ -99,7 +99,7 @@ static ModuleRegistry modules[] =
       STATIC_METHOD("sizePath_(_,_)", fileSizePath)
       METHOD("close_(_)", fileClose)
       METHOD("descriptor", fileDescriptor)
-      METHOD("readBytes_(_,_)", fileReadBytes)
+      METHOD("readBytes_(_,_,_)", fileReadBytes)
       METHOD("size_(_)", fileSize)
     END_CLASS
     CLASS(Stdin)

src/module/io.c

@@ -228,12 +228,13 @@ static void fileReadBytesCallback(uv_fs_t* request)
 
   FileRequestData* data = (FileRequestData*)request->data;
   uv_buf_t buffer = data->buffer;
+  size_t count = request->result;
 
   // TODO: Having to copy the bytes here is a drag. It would be good if Wren's
   // embedding API supported a way to *give* it bytes that were previously
   // allocated using Wren's own allocator.
   schedulerResume(freeRequest(request), true);
-  wrenSetSlotBytes(getVM(), 2, buffer.base, buffer.len);
+  wrenSetSlotBytes(getVM(), 2, buffer.base, count);
   schedulerFinishResume();
 
   // TODO: Likewise, freeing this after we resume is lame.
@@ -242,19 +243,20 @@ static void fileReadBytesCallback(uv_fs_t* request)
 
 void fileReadBytes(WrenVM* vm)
 {
-  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 2));
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 3));
 
   int fd = *(int*)wrenGetSlotForeign(vm, 0);
   // TODO: Assert fd != -1.
 
   FileRequestData* data = (FileRequestData*)request->data;
   size_t length = (size_t)wrenGetSlotDouble(vm, 1);
+  size_t offset = (size_t)wrenGetSlotDouble(vm, 2);
 
   data->buffer.len = length;
   data->buffer.base = (char*)malloc(length);
 
-  // TODO: Allow passing in offset.
-  uv_fs_read(getLoop(), request, fd, &data->buffer, 1, 0, fileReadBytesCallback);
+  uv_fs_read(getLoop(), request, fd, &data->buffer, 1, offset,
+             fileReadBytesCallback);
 }
 
 void fileSize(WrenVM* vm)

src/module/io.wren

@@ -62,13 +62,19 @@ foreign class File {
     return Scheduler.runNextScheduled_()
   }
 
-  readBytes(count) {
+  readBytes(count) { readBytes(count, 0) }
+
+  readBytes(count, offset) {
     if (!isOpen) Fiber.abort("File is not open.")
     if (!(count is Num)) Fiber.abort("Count must be an integer.")
     if (!count.isInteger) Fiber.abort("Count must be an integer.")
     if (count < 0) Fiber.abort("Count cannot be negative.")
 
-    readBytes_(count, Fiber.current)
+    if (!(offset is Num)) Fiber.abort("Offset must be an integer.")
+    if (!offset.isInteger) Fiber.abort("Offset must be an integer.")
+    if (offset < 0) Fiber.abort("Offset cannot be negative.")
+
+    readBytes_(count, offset, Fiber.current)
     return Scheduler.runNextScheduled_()
   }
 
@@ -76,7 +82,7 @@ foreign class File {
   foreign static sizePath_(path, fiber)
 
   foreign close_(fiber)
-  foreign readBytes_(count, fiber)
+  foreign readBytes_(count, start, fiber)
   foreign size_(fiber)
 }
 

src/module/io.wren.inc

@@ -53,6 +53,8 @@ static const char* ioModuleSource =
 "    Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
+"  foreign descriptor\n"
+"\n"
 "  isOpen { descriptor != -1 }\n"
 "\n"
 "  size {\n"
@@ -62,13 +64,19 @@ static const char* ioModuleSource =
 "    return Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
-"  readBytes(count) {\n"
+"  readBytes(count) { readBytes(count, 0) }\n"
+"\n"
+"  readBytes(count, offset) {\n"
 "    if (!isOpen) Fiber.abort(\"File is not open.\")\n"
 "    if (!(count is Num)) Fiber.abort(\"Count must be an integer.\")\n"
 "    if (!count.isInteger) Fiber.abort(\"Count must be an integer.\")\n"
 "    if (count < 0) Fiber.abort(\"Count cannot be negative.\")\n"
 "\n"
-"    readBytes_(count, Fiber.current)\n"
+"    if (!(offset is Num)) Fiber.abort(\"Offset must be an integer.\")\n"
+"    if (!offset.isInteger) Fiber.abort(\"Offset must be an integer.\")\n"
+"    if (offset < 0) Fiber.abort(\"Offset cannot be negative.\")\n"
+"\n"
+"    readBytes_(count, offset, Fiber.current)\n"
 "    return Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
@@ -76,8 +84,7 @@ static const char* ioModuleSource =
 "  foreign static sizePath_(path, fiber)\n"
 "\n"
 "  foreign close_(fiber)\n"
-"  foreign descriptor\n"
-"  foreign readBytes_(count, fiber)\n"
+"  foreign readBytes_(count, start, fiber)\n"
 "  foreign size_(fiber)\n"
 "}\n"
 "\n"

test/io/file/read_bytes.wren

@@ -10,4 +10,10 @@ System.print(file.readBytes(7)) // expect: this is
 // Allows zero.
 System.print(file.readBytes(0).bytes.count) // expect: 0
 
+// A longer number reads the whole file.
+System.print(file.readBytes(100)) // expect: this is a text file
+
+// Reading past the end truncates the buffer.
+System.print(file.readBytes(100).bytes.count) // expect: 19
+
 file.close()

test/io/file/read_bytes_from.wren

@@ -0,0 +1,20 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+
+// Zero starts at the beginning.
+System.print(file.readBytes(3, 0)) // expect: thi
+
+// Starts at the offset.
+System.print(file.readBytes(8, 3)) // expect: s is a t
+
+// Allows zero.
+System.print(file.readBytes(0, 4).bytes.count) // expect: 0
+
+// A longer number length reads until the end.
+System.print(file.readBytes(100, 2)) // expect: is is a text file
+
+// An offset past the end returns an empty string.
+System.print(file.readBytes(100, 30).bytes.count) // expect: 0
+
+file.close()

test/io/file/read_bytes_from_after_close.wren

@@ -0,0 +1,6 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.close()
+
+file.readBytes(3, 0) // expect runtime error: File is not open.

test/io/file/read_bytes_from_count_negative.wren

@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(-1, 0) // expect runtime error: Count cannot be negative.

test/io/file/read_bytes_from_count_not_integer.wren

@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1.2, 0) // expect runtime error: Count must be an integer.

test/io/file/read_bytes_from_count_not_num.wren

@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes("not num", 0) // expect runtime error: Count must be an integer.

test/io/file/read_bytes_from_offset_negative.wren

@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1, -1) // expect runtime error: Offset cannot be negative.

test/io/file/read_bytes_from_offset_not_integer.wren

@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1, 1.2) // expect runtime error: Offset must be an integer.

test/io/file/read_bytes_from_offset_not_num.wren

@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1, "not num") // expect runtime error: Offset must be an integer.