velesdb-core 5.1.0

//! Half-precision floating point support for memory-efficient vector storage.
//!
//! This module provides f16 (IEEE 754 half-precision) and bf16 (bfloat16) support,
//! reducing memory usage by 50% compared to f32 with minimal precision loss.
//!
//! # Memory Savings
//!
//! | Dimension | f32 Size | f16 Size | Savings |
//! |-----------|----------|----------|---------|
//! | 768 (BERT)| 3.0 KB   | 1.5 KB   | 50%     |
//! | 1536 (GPT)| 6.0 KB   | 3.0 KB   | 50%     |
//! | 4096      | 16.0 KB  | 8.0 KB   | 50%     |
//!
//! # Format Comparison
//!
//! - **f16**: IEEE 754 half-precision, best general compatibility
//! - **bf16**: Brain float16, same exponent range as f32, better for ML
//!
//! # Usage
//!
//! ```rust
//! use velesdb_core::half_precision::{VectorData, VectorPrecision};
//!
//! // Create from f32
//! let v = VectorData::from_f32_slice(&[0.1, 0.2, 0.3], VectorPrecision::F16);
//!
//! // Convert back to f32 for calculations
//! let f32_vec = v.to_f32_vec();
//! ```

use half::{bf16, f16};
use serde::{Deserialize, Serialize};

/// Vector precision format.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
#[non_exhaustive]
pub enum VectorPrecision {
    /// 32-bit floating point (4 bytes per dimension)
    #[default]
    F32,
    /// 16-bit floating point IEEE 754 (2 bytes per dimension)
    F16,
    /// Brain float 16-bit (2 bytes per dimension, same exponent as f32)
    BF16,
}

impl VectorPrecision {
    /// Returns the size in bytes per dimension.
    #[must_use]
    pub const fn bytes_per_element(&self) -> usize {
        match self {
            Self::F32 => 4,
            Self::F16 | Self::BF16 => 2,
        }
    }

    /// Calculates total memory for a vector of given dimension.
    #[must_use]
    pub const fn memory_size(&self, dimension: usize) -> usize {
        self.bytes_per_element() * dimension
    }
}

/// Vector data supporting multiple precision formats.
///
/// Stores vectors in their native precision format to minimize memory usage.
/// Provides conversion methods for distance calculations.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[non_exhaustive]
pub enum VectorData {
    /// Full precision f32 vector
    F32(Vec<f32>),
    /// Half precision f16 vector (50% memory reduction)
    F16(Vec<f16>),
    /// Brain float bf16 vector (50% memory reduction, ML-optimized)
    BF16(Vec<bf16>),
}

impl VectorData {
    /// Creates a new `VectorData` from an f32 slice with the specified precision.
    ///
    /// # Arguments
    ///
    /// * `data` - Source f32 data
    /// * `precision` - Target precision format
    ///
    /// # Example
    ///
    /// ```
    /// use velesdb_core::half_precision::{VectorData, VectorPrecision};
    ///
    /// let v = VectorData::from_f32_slice(&[0.1, 0.2, 0.3], VectorPrecision::F16);
    /// assert_eq!(v.len(), 3);
    /// ```
    #[must_use]
    pub fn from_f32_slice(data: &[f32], precision: VectorPrecision) -> Self {
        match precision {
            VectorPrecision::F32 => Self::F32(data.to_vec()),
            VectorPrecision::F16 => Self::F16(data.iter().map(|&x| f16::from_f32(x)).collect()),
            VectorPrecision::BF16 => Self::BF16(data.iter().map(|&x| bf16::from_f32(x)).collect()),
        }
    }

    /// Creates a new `VectorData` from an f32 vec, taking ownership.
    ///
    /// For F32 precision, takes ownership with zero conversion overhead.
    /// For F16/BF16, delegates to [`from_f32_slice`](Self::from_f32_slice).
    #[must_use]
    pub fn from_f32_vec(data: Vec<f32>, precision: VectorPrecision) -> Self {
        if precision == VectorPrecision::F32 {
            Self::F32(data)
        } else {
            // RF-DEDUP: reuse from_f32_slice for the conversion path
            Self::from_f32_slice(&data, precision)
        }
    }

    /// Returns the precision of this vector.
    #[must_use]
    pub const fn precision(&self) -> VectorPrecision {
        match self {
            Self::F32(_) => VectorPrecision::F32,
            Self::F16(_) => VectorPrecision::F16,
            Self::BF16(_) => VectorPrecision::BF16,
        }
    }

    /// Returns the dimension (length) of the vector.
    #[must_use]
    pub fn len(&self) -> usize {
        match self {
            Self::F32(v) => v.len(),
            Self::F16(v) => v.len(),
            Self::BF16(v) => v.len(),
        }
    }

    /// Returns true if the vector is empty.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns the memory size in bytes.
    #[must_use]
    pub fn memory_size(&self) -> usize {
        self.precision().memory_size(self.len())
    }

    /// Converts the vector to f32 for calculations.
    ///
    /// For F32 vectors, this clones the data.
    /// For F16/BF16 vectors, this converts each element.
    #[must_use]
    pub fn to_f32_vec(&self) -> Vec<f32> {
        match self {
            Self::F32(v) => v.clone(),
            Self::F16(v) => v.iter().map(|x| x.to_f32()).collect(),
            Self::BF16(v) => v.iter().map(|x| x.to_f32()).collect(),
        }
    }

    /// Returns a reference to the underlying f32 data if precision is F32.
    ///
    /// Returns `None` for F16/BF16 vectors.
    #[must_use]
    pub fn as_f32_slice(&self) -> Option<&[f32]> {
        match self {
            Self::F32(v) => Some(v.as_slice()),
            Self::F16(_) | Self::BF16(_) => None,
        }
    }

    /// Converts to another precision format.
    #[must_use]
    pub fn convert(&self, target: VectorPrecision) -> Self {
        if self.precision() == target {
            return self.clone();
        }
        Self::from_f32_slice(&self.to_f32_vec(), target)
    }
}

impl From<Vec<f32>> for VectorData {
    fn from(data: Vec<f32>) -> Self {
        Self::F32(data)
    }
}

impl From<&[f32]> for VectorData {
    fn from(data: &[f32]) -> Self {
        Self::F32(data.to_vec())
    }
}

// =============================================================================
// Distance calculations for half-precision vectors
// =============================================================================

/// Applies a SIMD distance function over two `VectorData`.
///
/// RF-DEDUP: Eliminates 8+ per-precision-combination match arms. The F32*F32
/// case uses SIMD directly (zero-copy); all other combinations convert to f32
/// vecs first, then delegate to the same SIMD path.
///
/// Mixed-precision paths (F16, BF16) are not hot — the allocation cost of
/// `to_f32_vec()` is negligible compared to the element conversion overhead.
fn with_f32_simd(a: &VectorData, b: &VectorData, simd_fn: fn(&[f32], &[f32]) -> f32) -> f32 {
    match (a, b) {
        (VectorData::F32(va), VectorData::F32(vb)) => simd_fn(va, vb),
        _ => simd_fn(&a.to_f32_vec(), &b.to_f32_vec()),
    }
}

/// Computes dot product between two `VectorData` with optimal precision handling.
///
/// For F32 vectors, uses SIMD-optimized f32 path.
/// For F16/BF16 vectors, converts to f32 then delegates to SIMD.
#[must_use]
pub fn dot_product(a: &VectorData, b: &VectorData) -> f32 {
    with_f32_simd(a, b, crate::simd_native::dot_product_native)
}

/// Computes cosine similarity between two `VectorData`.
#[must_use]
pub fn cosine_similarity(a: &VectorData, b: &VectorData) -> f32 {
    if let (VectorData::F32(va), VectorData::F32(vb)) = (a, b) {
        crate::simd_native::cosine_similarity_native(va, vb)
    } else {
        let dot = dot_product(a, b);
        let norm_a = norm_squared(a).sqrt();
        let norm_b = norm_squared(b).sqrt();

        if norm_a < f32::EPSILON || norm_b < f32::EPSILON {
            0.0
        } else {
            (dot / (norm_a * norm_b)).clamp(-1.0, 1.0)
        }
    }
}

/// Computes Euclidean distance between two `VectorData`.
#[must_use]
pub fn euclidean_distance(a: &VectorData, b: &VectorData) -> f32 {
    with_f32_simd(a, b, crate::simd_native::euclidean_native)
}

/// Computes squared L2 norm without allocation for F32, with conversion for half-precision.
/// RF-DEDUP: F16 and BF16 share the same `to_f32_vec` -> SIMD norm path.
fn norm_squared(v: &VectorData) -> f32 {
    if let VectorData::F32(data) = v {
        let n = crate::simd_native::norm_native(data);
        n * n
    } else {
        let f32_vec = v.to_f32_vec();
        let n = crate::simd_native::norm_native(&f32_vec);
        n * n
    }
}

#[cfg(test)]
#[path = "half_precision_unit_tests.rs"]
mod tests;