Introduce a way to store the HNSW on multiple LMDB entries

milli/src/readable_slices.rs

@@ -0,0 +1,84 @@
+use std::io::{self, Read};
+use std::iter::FromIterator;
+
+pub struct ReadableSlices<A> {
+    inner: Vec<A>,
+    pos: u64,
+}
+
+impl<A> FromIterator<A> for ReadableSlices<A> {
+    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
+        ReadableSlices { inner: iter.into_iter().collect(), pos: 0 }
+    }
+}
+
+impl<A: AsRef<[u8]>> Read for ReadableSlices<A> {
+    fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
+        let original_buf_len = buf.len();
+
+        // We explore the list of slices to find the one where we must start reading.
+        let mut pos = self.pos;
+        let index = match self
+            .inner
+            .iter()
+            .map(|s| s.as_ref().len() as u64)
+            .position(|size| pos.checked_sub(size).map(|p| pos = p).is_none())
+        {
+            Some(index) => index,
+            None => return Ok(0),
+        };
+
+        let mut inner_pos = pos as usize;
+        for slice in &self.inner[index..] {
+            let slice = &slice.as_ref()[inner_pos..];
+
+            if buf.len() > slice.len() {
+                // We must exhaust the current slice and go to the next one there is not enough here.
+                buf[..slice.len()].copy_from_slice(slice);
+                buf = &mut buf[slice.len()..];
+                inner_pos = 0;
+            } else {
+                // There is enough in this slice to fill the remaining bytes of the buffer.
+                // Let's break just after filling it.
+                buf.copy_from_slice(&slice[..buf.len()]);
+                buf = &mut [];
+                break;
+            }
+        }
+
+        let written = original_buf_len - buf.len();
+        self.pos += written as u64;
+        Ok(written)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::ReadableSlices;
+    use std::io::Read;
+
+    #[test]
+    fn basic() {
+        let data: Vec<_> = (0..100).collect();
+        let splits: Vec<_> = data.chunks(3).collect();
+        let mut rdslices: ReadableSlices<_> = splits.into_iter().collect();
+
+        let mut output = Vec::new();
+        let length = rdslices.read_to_end(&mut output).unwrap();
+        assert_eq!(length, data.len());
+        assert_eq!(output, data);
+    }
+
+    #[test]
+    fn small_reads() {
+        let data: Vec<_> = (0..u8::MAX).collect();
+        let splits: Vec<_> = data.chunks(27).collect();
+        let mut rdslices: ReadableSlices<_> = splits.into_iter().collect();
+
+        let buffer = &mut [0; 45];
+        let length = rdslices.read(buffer).unwrap();
+        let expected: Vec<_> = (0..buffer.len() as u8).collect();
+        assert_eq!(length, buffer.len());
+        assert_eq!(buffer, &expected[..]);
+    }
+}