Add a tool for writing descriptors with lengths not known at compile …

…time

This is meant to be a starting topint for rust-embedded-community#45

src/descriptor.rs

@@ -43,6 +43,104 @@ pub struct DescriptorWriter<'a> {
     write_iads: bool,
 }
 
+/// Write arbitrary sequences of bytes to descriptors with tools for delayed length writing
+/// If there is an error it is delayed until the end of writing when finish is called
+pub struct ByteWriter<'a, 'b> {
+    parent_writer: &'a mut DescriptorWriter<'b>,
+    result: Result<()>,
+}
+
+impl<'a, 'b> ByteWriter<'a, 'b> {
+    fn new(parent: &'a mut DescriptorWriter<'b>) -> Self {
+        ByteWriter {
+            parent_writer: parent,
+            result: Ok(()),
+        }
+    }
+
+    /// Write a single byte
+    pub fn byte(&mut self, b: u8) -> &mut Self {
+        if self.result.is_ok() {
+            let mut parent = &mut self.parent_writer;
+            if (parent.position + 1) >= parent.buf.len() {
+                self.result = Err(UsbError::BufferOverflow);
+                return self; // Delayed Error
+            }
+
+            parent.buf[parent.position] = b;
+            parent.position += 1;
+        }
+        self
+    }
+
+    /// Write a slice of bytes
+    pub fn arr(&mut self, bs: &[u8]) -> &mut Self {
+        if self.result.is_ok() {
+            let mut parent = &mut self.parent_writer;
+            let length = bs.len();
+            if (parent.position + length) >= parent.buf.len() {
+                self.result = Err(UsbError::BufferOverflow);
+                return self; // Delayed Error
+            }
+
+            let start = parent.position;
+            parent.buf[start..start + length].copy_from_slice(bs);
+            parent.position += length;
+        }
+        self
+    }
+
+    /// current position in the parent buffer
+    pub fn position(&self) -> usize {
+        self.parent_writer.position
+    }
+
+    /// Rserve space, do some writes, produce a value, then write something in that space using that value
+    pub fn delayed_write<W, V, D>(&mut self, size: usize, w: W, delayed: D) -> &mut Self
+    where
+        W: FnOnce(&mut ByteWriter<'_, '_>) -> V,
+        D: FnOnce(&mut [u8], V),
+    {
+        if self.result.is_ok() {
+            if (self.parent_writer.position + size) >= self.parent_writer.buf.len() {
+                self.result = Err(UsbError::BufferOverflow);
+                return self; // Delayed Error
+            }
+
+            let start = self.parent_writer.position;
+            self.parent_writer.position += size;
+
+            let v = w(self);
+            delayed(&mut self.parent_writer.buf[start..start + size], v);
+        }
+
+        self
+    }
+
+    /// Store the current position and advance by 1, write some bytes
+    /// then when leaving the functon write the length of those bytes, including the length byte
+    pub fn delayed_length<W>(&mut self, w: W) -> &mut Self
+    where
+        W: FnOnce(&mut ByteWriter<'_, '_>),
+    {
+        let start = self.position();
+        self.delayed_write(
+            1,
+            |bw| {
+                w(bw);
+                (bw.position() - start) as u8
+            },
+            |buf, length| buf[0] = length,
+        );
+        self
+    }
+
+    #[must_use]
+    fn finish(self) -> Result<()> {
+        self.result
+    }
+}
+
 impl DescriptorWriter<'_> {
     pub(crate) fn new(buf: &mut [u8]) -> DescriptorWriter<'_> {
         DescriptorWriter {
@@ -79,6 +177,20 @@ impl DescriptorWriter<'_> {
         Ok(())
     }
 
+    /// Write an arbiutrary sequence of bytes without needing to know the length at compile time
+    pub fn writer<W>(&mut self, w: W) -> Result<()>
+    where
+        W: FnOnce(&mut ByteWriter<'_, '_>),
+    {
+        if self.position > self.buf.len() {
+            return Err(UsbError::BufferOverflow);
+        }
+
+        let mut bb = ByteWriter::new(self);
+        w(&mut bb);
+        bb.finish()
+    }
+
     pub(crate) fn device(&mut self, config: &device::Config) -> Result<()> {
         self.write(
             descriptor_type::DEVICE,