Function encode

nodedb-codec/src/alp.rs:90–151

Encode a slice of f64 values using ALP compression. Finds the optimal decimal exponent, converts to integers, stores exceptions for values that don't round-trip, and bit-packs the integers via FastLanes.

(values: &[f64])

/// exceptions for values that don't round-trip, and bit-packs the integers
/// via FastLanes.
pub fn encode(values: &[f64]) -> Vec<u8> {
    let count = values.len() as u32;

    if values.is_empty() {
        let mut out = Vec::with_capacity(11);
        out.extend_from_slice(&0u32.to_le_bytes()); // count
        out.push(0); // encode exponent
        out.push(0); // decode exponent
        out.push(0); // mode
        out.extend_from_slice(&0u32.to_le_bytes()); // exception count
        return out;
    }

    // Step 1: Find optimal parameters by sampling.
    let params = find_best_params(values);
    let factor = POW10[params.encode_exp as usize];

    // Step 2: Encode values to integers, collect exceptions.
    let mut encoded_ints = Vec::with_capacity(values.len());
    let mut exceptions: Vec<(u32, u64)> = Vec::new();

    for (i, &val) in values.iter().enumerate() {
        let encoded = try_alp_encode(val, factor, params.decode_exp, params.mode);
        match encoded {
            Some(int_val) => {
                encoded_ints.push(int_val);
            }
            None => {
                // Exception: store original bits, use 0 as placeholder in int array.
                exceptions.push((i as u32, val.to_bits()));
                encoded_ints.push(0);
            }
        }
    }

    // Step 3: Build output.
    let exception_count = exceptions.len() as u32;
    let packed_ints = fastlanes::encode(&encoded_ints);

    let mut out = Vec::with_capacity(10 + exceptions.len() * 12 + packed_ints.len());

    // Header.
    out.extend_from_slice(&count.to_le_bytes());
    out.push(params.encode_exp);
    out.push(params.decode_exp);
    out.push(match params.mode {
        DecodeMode::MultiplyInverse => 0,
        DecodeMode::DivideByFactor => 1,
    });
    out.extend_from_slice(&exception_count.to_le_bytes());

    // Exceptions.
    for &(idx, bits) in &exceptions {
        out.extend_from_slice(&idx.to_le_bytes());
        out.extend_from_slice(&bits.to_le_bytes());
    }

    // Packed integers.