From 384644f5ea98e1218595e4e9f15ee152536b18a2 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Thu, 10 Nov 2022 22:45:51 +0100
Subject: [PATCH] Package up Indexer

---
 implementations/rust/oo/src/packed/view.rs | 115 ++++++++++++---------
 1 file changed, 69 insertions(+), 46 deletions(-)

diff --git a/implementations/rust/oo/src/packed/view.rs b/implementations/rust/oo/src/packed/view.rs
index 35fe35b..73fafcd 100644
--- a/implementations/rust/oo/src/packed/view.rs
+++ b/implementations/rust/oo/src/packed/view.rs
@@ -40,16 +40,6 @@ fn tag_at(packed: &[u8], i: usize) -> io::Result<Tag> {
     Ok(Tag::try_from(at(packed, i)?)?)
 }
 
-fn skip_annotations(packed: &[u8], mut i: usize) -> io::Result<usize> {
-    loop {
-        if tag_at(packed, i)? == Tag::Annotation {
-            i = skip_value(packed, i + 1)?;
-        } else {
-            return Ok(i);
-        }
-    }
-}
-
 fn varint(packed: &[u8], mut i: usize) -> io::Result<(u64, usize)> {
     let mut shift = 0;
     let mut acc: u64 = 0;
@@ -64,51 +54,84 @@ fn varint(packed: &[u8], mut i: usize) -> io::Result<(u64, usize)> {
     }
 }
 
-fn skip_value(packed: &[u8], mut i: usize) -> io::Result<usize> {
-    loop {
-        let next_i = match tag_at(packed, i)? {
-            Tag::False => i + 1,
-            Tag::True => i + 1,
-            Tag::Float => i + 5,
-            Tag::Double => i + 9,
-            Tag::End => Err(io::Error::new(io::ErrorKind::InvalidData, "Unexpected end tag"))?,
-            Tag::Annotation => {
-                i = skip_value(packed, i + 1)?;
-                continue;
+struct Indexer<'de> {
+    packed: &'de [u8],
+    offset: usize,
+}
+
+impl<'de> Indexer<'de> {
+    fn skip_annotations(&mut self) -> io::Result<()> {
+        loop {
+            if tag_at(&self.packed, self.offset)? == Tag::Annotation {
+                self.offset += 1;
+                self.skip_value()?;
+            } else {
+                return Ok(());
             }
-            Tag::Embedded => {
-                i = i + 1;
-                continue;
-            }
-            Tag::SmallInteger(_) => i + 1,
-            Tag::MediumInteger(n) => i + 1 + (n as usize),
-            Tag::SignedInteger | Tag::ByteString => {
-                let (n, i) = varint(packed, i + 1)?;
-                i + (n as usize)
-            }
-            Tag::String | Tag::Symbol => {
-                let (n, i) = varint(packed, i + 1)?;
-                std::str::from_utf8(&packed[i .. i + (n as usize)]).map_err(
-                    |_| io::Error::new(io::ErrorKind::InvalidData, "Invalid UTF-8"))?;
-                i + (n as usize)
-            }
-            Tag::Record | Tag::Sequence | Tag::Set | Tag::Dictionary => {
-                i = i + 1;
-                while tag_at(packed, i)? != Tag::End {
-                    i = skip_value(packed, i)?;
+        }
+    }
+
+    fn tag(&mut self) -> io::Result<Tag> {
+        let tag = tag_at(&self.packed, self.offset)?;
+        Ok(tag)
+    }
+
+    fn varint(&mut self) -> io::Result<u64> {
+        let (n, i) = varint(&self.packed, self.offset + 1)?;
+        self.offset = i;
+        Ok(n)
+    }
+
+    fn skip_value(&mut self) -> io::Result<()> {
+        loop {
+            match self.tag()? {
+                Tag::False => self.offset += 1,
+                Tag::True => self.offset += 1,
+                Tag::Float => self.offset += 5,
+                Tag::Double => self.offset += 9,
+                Tag::End => Err(io::Error::new(io::ErrorKind::InvalidData, "Unexpected end tag"))?,
+                Tag::Annotation => {
+                    self.offset += 1;
+                    self.skip_value()?;
+                    continue;
+                }
+                Tag::Embedded => {
+                    self.offset += 1;
+                    continue;
+                }
+                Tag::SmallInteger(_) => self.offset += 1,
+                Tag::MediumInteger(n) => self.offset += 1 + n as usize,
+                Tag::SignedInteger | Tag::ByteString => {
+                    let n = self.varint()?;
+                    self.offset += n as usize;
+                }
+                Tag::String | Tag::Symbol => {
+                    let n = self.varint()?;
+                    std::str::from_utf8(&self.packed[self.offset .. self.offset + (n as usize)])
+                        .map_err(|_| io::Error::new(io::ErrorKind::InvalidData, "Invalid UTF-8"))?;
+                    self.offset += n as usize;
+                }
+                Tag::Record | Tag::Sequence | Tag::Set | Tag::Dictionary => {
+                    self.offset += 1;
+                    while tag_at(&self.packed, self.offset)? != Tag::End {
+                        self.skip_value()?;
+                    }
+                    self.offset += 1;
                 }
-                i + 1
             }
-        };
-        return Ok(next_i);
+            return Ok(());
+        }
     }
 }
 
 impl<'de, Packed: AsRef<[u8]> + 'de> View<'de, Packed> {
     pub fn new(packed: Packed) -> io::Result<Self> {
         // println!("packed {:?}", &packed.as_ref());
-        let value_offset = skip_annotations(packed.as_ref(), 0)?;
-        let value_end = skip_value(packed.as_ref(), value_offset)?;
+        let mut indexer = Indexer { packed: packed.as_ref(), offset: 0 };
+        indexer.skip_annotations()?;
+        let value_offset = indexer.offset;
+        indexer.skip_value()?;
+        let value_end = indexer.offset;
         if value_end > packed.as_ref().len() { Err(io_eof())? }
         Ok(View { packed, value_offset, value_end, phantom: PhantomData })
     }