Function build_quantize_lut

csrc/cpu_ops.cpp:501–520

static constexpr int kLUTCacheSlots = 4;

static void build_quantize_lut(const float* codebook, unsigned char* lut) {
    // codebook has 256 sorted entries in [-1, 1].
    // We discretize the [-1, 1] range into 65536 bins and find the nearest codebook entry for each.
    // Precompute midpoints between consecutive codebook entries for nearest-neighbor lookup.
    float midpoints[kCodebookSize - 1];
    for (int i = 0; i < kCodebookSize - 1; ++i) {
        midpoints[i] = 0.5f * (codebook[i] + codebook[i + 1]);
    }

    int code_idx = 0;
    for (int i = 0; i < kLUTSize; ++i) {
        // Map LUT index to normalized value in [-1, 1]
        float val = -1.0f + (2.0f * i) / (kLUTSize - 1);
        // Advance code_idx while the next midpoint is still below val
        while (code_idx < kCodebookSize - 1 && midpoints[code_idx] < val) {
            ++code_idx;
        }
        lut[i] = static_cast<unsigned char>(code_idx);
    }
}

// Per-thread LUT cache with multiple slots to avoid rebuilding when alternating codebooks
struct LUTCache {