moonfire-nvr/server/db/coding.rs

// This file is part of Moonfire NVR, a security camera network video recorder.
// Copyright (C) 2016 The Moonfire NVR Authors; see AUTHORS and LICENSE.txt.
// SPDX-License-Identifier: GPL-v3.0-or-later WITH GPL-3.0-linking-exception.

//! Binary encoding/decoding.

/// Zigzag-encodes a signed integer, as in [protocol buffer
/// encoding](https://developers.google.com/protocol-buffers/docs/encoding#types). Uses the low bit
/// to indicate signedness (1 = negative, 0 = non-negative).
#[inline(always)]
pub fn zigzag32(i: i32) -> u32 {
    ((i << 1) as u32) ^ ((i >> 31) as u32)
}

/// Zigzag-decodes to a signed integer.
/// See `zigzag`.
#[inline(always)]
pub fn unzigzag32(i: u32) -> i32 {
    ((i >> 1) as i32) ^ -((i & 1) as i32)
}

#[inline(always)]
pub fn decode_varint32(data: &[u8], i: usize) -> Result<(u32, usize), ()> {
    // Unroll a few likely possibilities before going into the robust out-of-line loop.
    // This aids branch prediction.
    if data.len() > i && (data[i] & 0x80) == 0 {
        return Ok((data[i] as u32, i + 1));
    } else if data.len() > i + 1 && (data[i + 1] & 0x80) == 0 {
        return Ok((
            ((data[i] & 0x7f) as u32) | ((data[i + 1] as u32) << 7),
            i + 2,
        ));
    } else if data.len() > i + 2 && (data[i + 2] & 0x80) == 0 {
        return Ok((
            ((data[i] & 0x7f) as u32)
                | (((data[i + 1] & 0x7f) as u32) << 7)
                | ((data[i + 2] as u32) << 14),
            i + 3,
        ));
    }
    decode_varint32_slow(data, i)
}

#[cold]
fn decode_varint32_slow(data: &[u8], mut i: usize) -> Result<(u32, usize), ()> {
    let l = data.len();
    let mut out = 0;
    let mut shift = 0;
    loop {
        if i == l {
            return Err(());
        }
        let b = data[i];
        if shift == 28 && (b & 0xf0) != 0 {
            return Err(());
        }
        out |= ((b & 0x7f) as u32) << shift;
        shift += 7;
        i += 1;
        if (b & 0x80) == 0 {
            break;
        }
    }
    Ok((out, i))
}

pub fn append_varint32(i: u32, data: &mut Vec<u8>) {
    if i < 1u32 << 7 {
        data.push(i as u8);
    } else if i < 1u32 << 14 {
        data.extend_from_slice(&[((i & 0x7F) | 0x80) as u8, (i >> 7) as u8]);
    } else if i < 1u32 << 21 {
        data.extend_from_slice(&[
            ((i & 0x7F) | 0x80) as u8,
            (((i >> 7) & 0x7F) | 0x80) as u8,
            (i >> 14) as u8,
        ]);
    } else if i < 1u32 << 28 {
        data.extend_from_slice(&[
            ((i & 0x7F) | 0x80) as u8,
            (((i >> 7) & 0x7F) | 0x80) as u8,
            (((i >> 14) & 0x7F) | 0x80) as u8,
            (i >> 21) as u8,
        ]);
    } else {
        data.extend_from_slice(&[
            ((i & 0x7F) | 0x80) as u8,
            (((i >> 7) & 0x7F) | 0x80) as u8,
            (((i >> 14) & 0x7F) | 0x80) as u8,
            (((i >> 21) & 0x7F) | 0x80) as u8,
            (i >> 28) as u8,
        ]);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_zigzag() {
        struct Test {
            decoded: i32,
            encoded: u32,
        }
        let tests = [
            Test {
                decoded: 0,
                encoded: 0,
            },
            Test {
                decoded: -1,
                encoded: 1,
            },
            Test {
                decoded: 1,
                encoded: 2,
            },
            Test {
                decoded: -2,
                encoded: 3,
            },
            Test {
                decoded: 2147483647,
                encoded: 4294967294,
            },
            Test {
                decoded: -2147483648,
                encoded: 4294967295,
            },
        ];
        for test in &tests {
            assert_eq!(test.encoded, zigzag32(test.decoded));
            assert_eq!(test.decoded, unzigzag32(test.encoded));
        }
    }

    #[test]
    fn test_correct_varints() {
        struct Test {
            decoded: u32,
            encoded: &'static [u8],
        }
        let tests = [
            Test {
                decoded: 1,
                encoded: b"\x01",
            },
            Test {
                decoded: 257,
                encoded: b"\x81\x02",
            },
            Test {
                decoded: 49409,
                encoded: b"\x81\x82\x03",
            },
            Test {
                decoded: 8438017,
                encoded: b"\x81\x82\x83\x04",
            },
            Test {
                decoded: 1350615297,
                encoded: b"\x81\x82\x83\x84\x05",
            },
        ];
        for test in &tests {
            // Test encoding to an empty buffer.
            let mut out = Vec::new();
            append_varint32(test.decoded, &mut out);
            assert_eq!(&out[..], test.encoded);

            // ...and to a non-empty buffer.
            let mut buf = Vec::new();
            out.clear();
            out.push(b'x');
            buf.push(b'x');
            buf.extend_from_slice(test.encoded);
            append_varint32(test.decoded, &mut out);
            assert_eq!(out, buf);

            // Test decoding from the beginning of the string.
            assert_eq!(
                (test.decoded, test.encoded.len()),
                decode_varint32(test.encoded, 0).unwrap()
            );

            // ...and from the middle of a buffer.
            buf.push(b'x');
            assert_eq!(
                (test.decoded, test.encoded.len() + 1),
                decode_varint32(&buf, 1).unwrap()
            );
        }
    }

    #[test]
    fn test_bad_varints() {
        let tests: &[&[u8]] = &[
            // buffer underruns
            b"",
            b"\x80",
            b"\x80\x80",
            b"\x80\x80\x80",
            b"\x80\x80\x80\x80",
            // int32 overflows
            b"\x80\x80\x80\x80\x80",
            b"\x80\x80\x80\x80\x80\x00",
        ];
        for (i, encoded) in tests.iter().enumerate() {
            assert!(decode_varint32(encoded, 0).is_err(), "while on test {i}");
        }
    }
}