From 9835e2ae736235810b4ea1c162ca5e65c547e770 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sat, 18 May 2024 04:49:50 +0200
Subject: Merging upstream version 1.71.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 vendor/varisat-internal-proof/src/vli_enc.rs | 136 +++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)
 create mode 100644 vendor/varisat-internal-proof/src/vli_enc.rs

(limited to 'vendor/varisat-internal-proof/src/vli_enc.rs')

diff --git a/vendor/varisat-internal-proof/src/vli_enc.rs b/vendor/varisat-internal-proof/src/vli_enc.rs
new file mode 100644
index 000000000..058058e6e
--- /dev/null
+++ b/vendor/varisat-internal-proof/src/vli_enc.rs
@@ -0,0 +1,136 @@
+//! Variable length integer encoding.
+//!
+//! This uses a different encoding from the leb128 encoding used for binary DRAT and CLRAT. It uses
+//! the same amount of bytes for each input, but is faster to encode and decode.
+//!
+//! Numbers are encoded like this:
+//! ```text
+//! 1xxxxxxx for up to 7 bits
+//! 01xxxxxx xxxxxxxx for up to 14 bits
+//! 001xxxxx xxxxxxxx xxxxxxxx for up to 21 bits
+//! ...
+//! ```
+//!
+//! The x-bits store the number from LSB to MSB. This scheme allows faster encoding and decoding, as
+//! the bits are kept consecutive and the length can be determined from the first or first two
+//! bytes.
+//!
+//! The current implementations are not optimal but fast enough to not be the bottleneck when
+//! checking proofs.
+use std::{
+    convert::{TryFrom, TryInto},
+    io::{self, BufRead, Write},
+};
+
+/// Write an encoded 64 bit number.
+pub fn write_u64(target: &mut impl Write, mut value: u64) -> Result<(), io::Error> {
+    let bits = (64 - value.leading_zeros()) as u32;
+    let blocks = (bits * (64 / 7)) / 64;
+    if value < (1 << (8 * 7)) {
+        value = ((value << 1) | 1) << blocks;
+        let bytes = unsafe { std::mem::transmute::<u64, [u8; 8]>(value.to_le()) };
+        target.write_all(&bytes[..(blocks + 1) as usize])
+    } else {
+        let lo_data = ((value << 1) | 1) << blocks;
+        let lo_bytes = unsafe { std::mem::transmute::<u64, [u8; 8]>(lo_data.to_le()) };
+        let hi_data = value >> (64 - (blocks + 1));
+        let hi_bytes = unsafe { std::mem::transmute::<u64, [u8; 8]>(hi_data.to_le()) };
+
+        target.write_all(&lo_bytes)?;
+        target.write_all(&hi_bytes[..(blocks as usize) + 1 - 8])
+    }
+}
+
+/// Read an encoded 64 bit number, if at least 16 bytes lookahead are available.
+fn read_u64_fast(bytes: &[u8; 16]) -> (u64, usize) {
+    let lo_data = u64::from_le(unsafe {
+        std::mem::transmute::<[u8; 8], u64>(*<&[u8; 8]>::try_from(&bytes[..8]).unwrap())
+    });
+
+    let len = (lo_data | (1 << 9)).trailing_zeros() + 1;
+
+    if len <= 8 {
+        let result = lo_data & (!0u64 >> (64 - 8 * len));
+
+        let result = result >> len;
+
+        (result, len as usize)
+    } else {
+        let hi_data = u64::from_le(unsafe {
+            std::mem::transmute::<[u8; 8], u64>(*<&[u8; 8]>::try_from(&bytes[8..]).unwrap())
+        });
+
+        let hi_data = hi_data & (!0u64 >> (64 - 8 * (len - 8)));
+
+        let result = (lo_data >> len) | (hi_data << (64 - len));
+
+        (result as u64, len as usize)
+    }
+}
+
+/// Read an encoded 64 bit number from a buffered reader.
+///
+/// This uses [`read_u64_fast`] if the remaining buffer is larger than 16 bytes and falls back to a
+/// slower implementation otherwise.
+pub fn read_u64(source: &mut impl BufRead) -> Result<u64, io::Error> {
+    let buf = source.fill_buf()?;
+    if buf.len() >= 16 {
+        let next_bytes: &[u8; 16] = (&buf[..16]).try_into().unwrap();
+        let (value, advance) = read_u64_fast(next_bytes);
+        source.consume(advance);
+        Ok(value)
+    } else {
+        let mut scan = 1 << 9;
+        let mut byte: [u8; 1] = [0];
+        let mut begin = 1;
+        source.read_exact(&mut byte[..])?;
+        let mut result = byte[0] as u64;
+        scan |= byte[0] as u64;
+        if byte[0] == 0 {
+            begin += 1;
+            source.read_exact(&mut byte[..])?;
+            result |= (byte[0] as u64) << 8;
+            scan |= (byte[0] as u64) << 8;
+        }
+        let len = scan.trailing_zeros() + 1;
+
+        result >>= len;
+
+        for i in begin..len {
+            source.read_exact(&mut byte[..])?;
+            result |= (byte[0] as u64) << (8 * i - len);
+        }
+
+        Ok(result as u64)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use proptest::prelude::*;
+
+    proptest! {
+        #[test]
+        fn roundtrip (
+            numbers in prop::collection::vec(prop::num::u64::ANY, 0..10_000)
+        ) {
+            let mut buf = vec![];
+
+            for &num in numbers.iter() {
+                write_u64(&mut buf, num)?;
+            }
+
+            let mut read = std::io::BufReader::with_capacity(128, &buf[..]);
+
+            let mut out = vec![];
+
+            while let Ok(num) = read_u64(&mut read) {
+                out.push(num)
+            }
+
+            prop_assert_eq!(numbers, out);
+        }
+    }
+}
-- 
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