legume_numeric/matrix/

traits.rs

use crate::matrix::common_io::{Delimiter, ReadLinesOut};
use candle_core::{Device, Tensor};
use num_traits::Float;

/// Trait for running statistics operations
///
/// Provides a common interface for both dense (ndarray-based) and
/// sparse running statistics implementations.
pub trait RunningStatOps<T>
where
    T: Float,
{
    type Output;

    fn clear(&mut self);
    fn count_positives(&self) -> Self::Output;
    fn sum(&self) -> Self::Output;
    fn mean(&self) -> Self::Output;
    fn variance(&self) -> Self::Output;
    fn std(&self) -> Self::Output;
}

/// some linear algebra routines
pub trait RandomizedAlgs {
    type InMat;
    type OutMat;
    type DVec;
    type Scalar;

    /// randomized singular value decomposition
    /// # input
    /// * `X`: `n x d` matrix
    /// # output
    /// * `U`: `n x k`
    /// * `D`: `k x 1`
    /// * `V`: `d x k`
    fn rsvd(&self, max_rank: usize) -> anyhow::Result<(Self::OutMat, Self::DVec, Self::OutMat)>;
}

/// Convert to and from the vector of triplets
pub trait MatTriplets {
    type Mat;
    type Scalar;

    fn from_nonzero_triplets<I>(
        nrow: usize,
        ncol: usize,
        triplets: &[(I, I, Self::Scalar)],
    ) -> anyhow::Result<Self::Mat>
    where
        I: TryInto<usize> + Copy,
        <I as TryInto<usize>>::Error: std::fmt::Debug;

    fn to_nonzero_triplets(&self) -> anyhow::Result<NRowNColTriplets<Self::Scalar>>;
}

pub struct NRowNColTriplets<Scalar> {
    pub nrow: usize,
    pub ncol: usize,
    pub triplets: Vec<(usize, usize, Scalar)>,
}

/// Reading off from `Tensor`
pub trait ConvertMatOps {
    type Mat;
    type Scalar;

    fn from_tensor(_: &Tensor) -> anyhow::Result<Self::Mat>;
    fn to_tensor(&self, dev: &Device) -> anyhow::Result<Tensor>;
}

/// normalize, sum_to_one, scale, and centre columns
pub trait MatOps {
    type Mat;
    type Scalar;

    /// make each column sum to 1
    fn sum_to_one_columns_inplace(&mut self);
    /// make each column sum to 1
    fn sum_to_one_columns(&self) -> Self::Mat;

    /// make each row sum to 1
    fn sum_to_one_rows_inplace(&mut self);
    /// make each row sum to 1
    fn sum_to_one_rows(&self) -> Self::Mat;

    /// normalize logits after taking exp `(log-sum-exp)`
    fn normalize_exp_logits_columns_inplace(&mut self);
    /// normalize logits after taking exp `(log-sum-exp)`
    fn normalize_exp_logits_columns(&self) -> Self::Mat;

    /// column-wise log-softmax: subtract each column's log-sum-exp so the
    /// `exp` of each column sums to 1. Returns log-probabilities (unlike
    /// [`Self::normalize_exp_logits_columns`], which returns probabilities).
    fn log_softmax_columns_inplace(&mut self);
    /// column-wise log-softmax (see [`Self::log_softmax_columns_inplace`])
    fn log_softmax_columns(&self) -> Self::Mat;

    /// vector norm for each column
    fn normalize_columns_inplace(&mut self);
    /// vector norm for each column
    fn normalize_columns(&self) -> Self::Mat;

    /// standardization for each column
    fn scale_columns_inplace(&mut self);
    /// standardization for each column
    fn scale_columns(&self) -> Self::Mat;

    /// standardization for each row
    fn scale_rows_inplace(&mut self);
    /// standardization for each row
    fn scale_rows(&self) -> Self::Mat;

    /// centering for each column
    fn centre_columns_inplace(&mut self);
    /// centering for each column
    fn centre_columns(&self) -> Self::Mat;
}

pub trait AdjustByDivisionOp<Other, Scalar> {
    /// Adjust each column with the column of the matching batch index
    ///
    /// Assume: `Y[g] ~ Poisson(X[g] * λ)`
    /// (1) Estimate the λ parameter by taking overall ratio, namely,
    /// `λ = Σ Y[g] / Σ X[g]`
    ///
    /// (2) Take the residual (in the log space)
    /// `ln Y[g] - ln (λ X[g])` or `Y[g]/λX[g]` if `X[g] > 0`
    /// otherwise, do nothing
    fn adjust_by_division_of_selected_inplace(&mut self, denom_db: &Other, batches: &[usize]);

    /// adjust each column with the corresponding column of the denom
    ///
    /// Assume: `Y[g] ~ Poisson(X[g] * λ)`
    /// (1) Estimate the λ parameter by taking overall ratio, namely,
    /// `λ = Σ Y[g] / Σ X[g]`
    ///
    /// (2) Take the residual (in the log space)
    /// `ln Y[g] - ln (λ X[g])` or `Y[g]/λX[g]` if `X[g] > 0`
    /// otherwise, do nothing
    fn adjust_by_division_inplace(&mut self, denom: &Other);
}

pub trait MatElemOps {
    type Mat;
    type Scalar;
    fn log1p_inplace(&mut self);
    fn log1p(&self) -> Self::Mat;
}

/// Elementwise chains fused into ONE pass, because candle's CPU backend runs them
/// one core at a time.
///
/// Only matmul reaches `gemm`, which candle drives with `Parallelism::Rayon`;
/// `unary_map` and `binary_map` are plain serial iterators, and the vectorized
/// `f32_vec` path is `#[cfg(feature = "mkl" / "accelerate")]` — SIMD, still one
/// core. So a loop whose matmuls scale across every core stalls on the
/// elementwise ops between them, and any chain over a large tensor is worth
/// collapsing into a single rayon pass.
///
/// Implemented for `Tensor` on CPU only. Off CPU the device's own kernels are
/// already parallel, and each method falls back to the op chain it stands in for
/// — same numbers either way, which the tests assert bitwise.
pub trait FusedTensorOps: Sized {
    /// `exp(min(self + offset, ceiling))`, i.e. the Poisson rate from a linear
    /// predictor with the overflow guard `exp` needs (f32 overflows at 88).
    ///
    /// Replaces `self.broadcast_add(offset)?.minimum(ceiling)?.exp()`. `self` is
    /// `[N, F]`; `offset` is any shape that chain broadcasts against it — `[N, F]`,
    /// `[1, F]` or `[N, 1]`.
    ///
    /// # The receiver must be unaliased
    ///
    /// On CPU this overwrites `self`'s storage and hands it back, so the whole
    /// chain costs one buffer instead of three. `Tensor` is an `Arc`, so taking
    /// `self` by value does **not** prove exclusivity — `x.reshape(..)` yields a
    /// contiguous tensor sharing `x`'s storage and would pass every guard here,
    /// silently overwriting `x`. Pass a freshly computed tensor (a `matmul`
    /// result), never a `clone`, `narrow` or `reshape` of one still in use.
    ///
    /// Back-prop is unsupported by construction (candle's in-place custom ops
    /// carry no backward), which is why the callers take their gradients in
    /// closed form.
    ///
    /// Deliberately single-offset. A chain carrying **two** offsets — a `[N, 1]`
    /// column and a `[1, F]` row, as the joint velocity solver in
    /// `graph-embedding-util` does — needs its own method rather than a caller
    /// pre-broadcasting one of them, which would cost the full-size op this
    /// exists to remove.
    fn clamped_exp_add_inplace(self, offset: &Tensor, ceiling: f64) -> anyhow::Result<Self>;
}

/// TF-IDF (Term Frequency–Inverse Document Frequency) transformation
///
/// A numerical statistic reflecting how important a word (term) is to a document
/// in a collection or corpus. (Wikipedia)
///
/// Treats the matrix as a term-document matrix where:
/// - Rows are "terms" (e.g., genes, words)
/// - Columns are "documents" (e.g., cell types, text documents)
///
/// **TF-IDF(t, d) = TF(t, d) × IDF(t)**
///
/// where:
/// - TF(t, d) = term frequency of term t in document d (matrix values)
/// - IDF(t) = log(N / df(t)) = inverse document frequency
/// - N = total number of documents (columns)
/// - df(t) = document frequency = number of documents containing term t
///
/// Terms appearing in many documents get lower weight; terms specific to few
/// documents get higher weight.
pub trait TfIdfOps {
    type Mat;

    /// Apply TF-IDF transformation
    ///
    /// IDF(t) = log(N / (df(t) + 1)) where df(t) = number of non-zero entries in row t
    fn tfidf(&self) -> Self::Mat;

    /// Apply TF-IDF followed by L2 column normalization
    ///
    /// Useful for cosine similarity comparisons between documents (columns)
    fn tfidf_normalize_columns(&self) -> Self::Mat;
}

/// Operations to sample random matrices, only works for
/// `nalgebra::DMatrix` and `ndarray::Array2`
pub trait SampleOps {
    type Mat;
    type Scalar;

    /// Sample a matrix from a uniform distribution `U(0,1)`.
    ///
    /// Unseeded: draws fresh entropy each call. For reproducible output use
    /// [`SampleOps::runif_seeded`].
    fn runif(dd: usize, nn: usize) -> Self::Mat;

    /// Sample a matrix from a normal distribution `N(0,1)`.
    ///
    /// Unseeded: draws fresh entropy each call. For reproducible output use
    /// [`SampleOps::rnorm_seeded`].
    fn rnorm(dd: usize, nn: usize) -> Self::Mat;

    /// Sample a matrix from a gamma distribution with `param` is
    /// `(shape α, scale θ)`
    ///
    /// $$f(x|\alpha,\theta) = \frac{\theta^{-\alpha}}{\Gamma(\alpha)} x^{\alpha - 1} e^{-x/\theta}$$
    ///
    /// Note: `rate = 1/scale` or $\beta = 1/\theta$
    ///
    /// Unseeded: draws fresh entropy each call. For reproducible output use
    /// [`SampleOps::rgamma_seeded`].
    fn rgamma(dd: usize, nn: usize, param: (f32, f32)) -> Self::Mat;

    /// Seeded, thread-order-independent `U(0,1)` sample. Byte-identical across
    /// runs, thread counts, and machines for a fixed `seed`. See
    /// [`crate::matrix::rand_util`].
    fn runif_seeded(dd: usize, nn: usize, seed: u64) -> Self::Mat;

    /// Seeded, thread-order-independent `N(0,1)` sample. Byte-identical across
    /// runs, thread counts, and machines for a fixed `seed`. See
    /// [`crate::matrix::rand_util`].
    fn rnorm_seeded(dd: usize, nn: usize, seed: u64) -> Self::Mat;

    /// Seeded, thread-order-independent gamma sample (`param = (shape α, scale θ)`).
    /// Byte-identical across runs, thread counts, and machines for a fixed
    /// `seed`. See [`crate::matrix::rand_util`].
    fn rgamma_seeded(dd: usize, nn: usize, param: (f32, f32), seed: u64) -> Self::Mat;
}

pub trait DistanceOps {
    type Scalar;
    type Other;

    /// A vector of Euclidean distances between sources and targets `other`
    ///
    /// * `other`: other data matrix
    fn euclidean_distance(
        &self,
        other: &Self::Other,
    ) -> anyhow::Result<Vec<(usize, usize, Self::Scalar)>>;

    /// A vector of Euclidean distances between sources and targets `other`
    ///
    /// * `other`: other data matrix
    /// * `select_columns_in_other`: specific columns
    fn euclidean_distance_on_select_columns(
        &self,
        other: &Self::Other,
        select_columns_in_other: &[usize],
    ) -> anyhow::Result<Vec<(usize, usize, Self::Scalar)>>;
}

pub trait EncodingOps
where
    Self: Sized,
{
    type Mat;
    type Scalar;

    /// Sinusoidal Positional Encoding
    /// * `emb_dim` - embedding dimension, say `d`
    /// * returns each column's embedding results (row x 2d)
    ///
    /// for each element r of each column c:
    ///  ret[r, 2i] = sin(x[r,c]/10000^(2i/d))
    ///  ret[r, 2i + 1] = cos(x[r,c]/10000^(2i/d))
    /// where i in [0, d/2-1]
    fn positional_embedding_columns(&self, emb_dim: usize) -> anyhow::Result<Self::Mat>;
}

/// Operations that involves multiple types
pub trait CompositeOps {
    type Scalar;
    type Mat;
    type Other;

    /// `self[:,col] += other[:,col]`
    /// * `other`: `CscMatrix`
    /// * `col`: column index
    fn add_assign_column(&mut self, other: &Self::Other, col: usize);

    /// `self += other`
    /// * `other`: `CscMatrix`
    fn add_assign(&mut self, other: &Self::Other);
}

/// Read and write matrices from and to files
pub trait IoOps {
    type Scalar;
    type Mat;

    fn read_file_delim(
        file_path: &str,
        delim: impl Into<Delimiter>,
        skip: Option<usize>,
    ) -> anyhow::Result<Self::Mat>;

    /// Read the data matrix with row and column names
    ///
    /// * `file_path`: data file name
    /// * `delim`: delimiter (`char` vector or string)
    /// * `header_row`: header line (0-based); `None` will find no header
    /// * `row_name_column_index`: column index (0-based) corresponds to row name
    /// * `select_column_indices`: column indices (0-based) to include
    /// * `select_column_names`: column names to include
    ///
    fn read_data(
        file_path: &str,
        delim: impl Into<Delimiter>,
        header_row: Option<usize>,
        row_name_column_index: Option<usize>,
        select_column_indices: Option<&[usize]>,
        select_column_names: Option<&[Box<str>]>,
    ) -> anyhow::Result<MatWithNames<Self::Mat>>;

    /// Read the data matrix with row and column names
    ///
    /// * `file_path`: data file name
    /// * `delim`: delimiter (`char` vector or string)
    /// * `header_row`: header line (0-based); `None` will find no header
    /// * `header_column`: column index (0-based) corresponds to row name
    ///
    fn read_data_with_names(
        file_path: &str,
        delim: impl Into<Delimiter>,
        header_row: Option<usize>,
        header_column: Option<usize>,
    ) -> anyhow::Result<MatWithNames<Self::Mat>> {
        Self::read_data(file_path, delim, header_row, header_column, None, None)
    }

    #[allow(clippy::type_complexity)]
    fn read_data_vec_with_indices_names(
        file_path: &str,
        delim: impl Into<Delimiter>,
        header_line: Option<usize>,
        row_name_index: Option<usize>,
        column_indices: Option<&[usize]>,
        column_names: Option<&[Box<str>]>,
    ) -> anyhow::Result<(Vec<Box<str>>, Vec<Box<str>>, Vec<Self::Scalar>)>
    where
        Self::Scalar: std::str::FromStr,
        <Self::Scalar as std::str::FromStr>::Err: std::fmt::Debug,
    {
        let hdr_line = match header_line {
            Some(skip) => skip as i64,
            None => -1, // no skipping
        };

        let ReadLinesOut { mut lines, header } =
            crate::matrix::common_io::read_lines_of_words_delim(file_path, delim, hdr_line)?;

        // A blank line tokenizes to one empty field, not to zero fields, so it
        // must be dropped here or it caps the width check below at 1 and then
        // breaks the value loop. Dropping it up front fixes both at once.
        lines.retain(|w| !(w.len() == 1 && w[0].is_empty()));

        let data_width = lines.iter().map(|w| w.len()).min().unwrap_or(header.len());
        // R's write.table omits a name for the row-label column, so the header
        // is one field short of the data rows and every header position names
        // the data column one to its RIGHT. Detect that shape once; both the
        // name matching and the naming lookup below shift through it.
        let header_offset = usize::from(
            !header.is_empty() && header.len() + 1 == data_width && row_name_index == Some(0),
        );

        let mut relevant_indices: Vec<usize> = vec![];

        let indices_given = column_indices.is_some_and(|ix| !ix.is_empty());
        if let Some(indices) = column_indices {
            relevant_indices.extend(indices.iter().copied());
        }

        // Explicit indices OVERRIDE names, as the callers' help documents; a
        // union would quietly widen the selection with every default name that
        // happens to be present in the header.
        if !indices_given {
            if let Some(names) = column_names {
                // The tokenizer has already unquoted both sides.
                let name_indices: Vec<usize> = header
                    .iter()
                    .enumerate()
                    .filter_map(|(i, name)| {
                        if names.iter().any(|n| n == name) {
                            Some(i + header_offset)
                        } else {
                            None
                        }
                    })
                    .collect();
                relevant_indices.extend(name_indices);
            }
        }

        // Neither selector given: take EVERY column except the row-name one.
        // Without this the selection stays empty and the reader silently returns
        // a 0-column matrix, so `read_data(.., None, None)` — the form used by
        // `senna`'s `read_mat` and `data-beans-sim`'s topic-file loader — could
        // never read a delimited file at all.
        if column_indices.is_none() && column_names.is_none() {
            let n_col = header
                .len()
                .max(lines.first().map_or(0, |words| words.len()));
            relevant_indices.extend((0..n_col).filter(|j| Some(*j) != row_name_index));
        }

        relevant_indices.sort_unstable();
        relevant_indices.dedup();

        // Every subscript below is checked here first: the row-name column, the
        // selected data columns, and the header lookup that names them. Width is
        // the NARROWEST data row, ignoring blank ones, because a ragged file
        // otherwise passes this and panics later in the value loop. The header
        // is checked separately, since it can be one field short of the data
        // rows when a writer omits a name for the row-label column.
        let n_columns = data_width;
        let mut to_check: Vec<usize> = relevant_indices.clone();
        to_check.extend(row_name_index);
        if let Some(&bad) = to_check.iter().find(|&&j| j >= n_columns) {
            return Err(anyhow::anyhow!(
                "column index {bad} is out of range: the file has {n_columns} column(s). \
                 Name the columns to read, or pass indices within range."
            ));
        }

        let row_names: Vec<Box<str>> = match row_name_index {
            // Unquoted, for the same reason the header is: a fully-quoted csv
            // would otherwise yield row names carrying their quotes, which then
            // match nothing when joined against a matrix's own names.
            Some(row_name_index) => lines
                .iter()
                .map(|words| words[row_name_index].clone())
                .collect(),
            _ => (0..lines.len())
                .map(|x| x.to_string().into_boxed_str())
                .collect(),
        };

        // Indices can come from a caller's fallback rather than from a name
        // match, so they are not guaranteed to exist in this file. Say which
        // column was asked for and how many the file has, instead of panicking
        // on the subscript several lines later.
        let column_names: Vec<Box<str>> = if header.is_empty() {
            relevant_indices
                .iter()
                .map(|x| x.to_string().into_boxed_str())
                .collect()
        } else {
            relevant_indices
                .iter()
                // A header can be narrower than the data rows; fall back to the
                // position rather than panicking on a name that was never written.
                .map(|&j| {
                    j.checked_sub(header_offset)
                        .and_then(|k| header.get(k))
                        .cloned()
                        .unwrap_or_else(|| j.to_string().into_boxed_str())
                })
                .collect()
        };

        let data: Vec<Vec<Self::Scalar>> = lines
            .iter()
            .map(|words| {
                relevant_indices
                    .iter()
                    .map(|&i| words[i].parse::<Self::Scalar>().expect("failed to parse"))
                    .collect()
            })
            .collect();

        let data = data.into_iter().flatten().collect::<Vec<_>>();

        Ok((row_names, column_names, data))
    }

    /// Read a `tsv` file while skipping until the header row
    fn from_tsv(tsv_file: &str, skip: Option<usize>) -> anyhow::Result<Self::Mat> {
        Self::read_file_delim(tsv_file, "\t", skip)
    }

    /// Read a `csv` file while skipping until the header row
    fn from_csv(csv_file: &str, skip: Option<usize>) -> anyhow::Result<Self::Mat> {
        Self::read_file_delim(csv_file, ",", skip)
    }

    /// write the matrix down to a file with delimiter
    /// * `file_path`: output file path
    /// * `delim`: separation character or string
    fn write_file_delim(&self, file: &str, delim: &str) -> anyhow::Result<()>;

    /// write the matrix down to a tsv file
    /// * `file_path`: output file path
    fn to_tsv(&self, tsv_file: &str) -> anyhow::Result<()> {
        self.write_file_delim(tsv_file, "\t")
    }

    /// write the matrix down to a csv file
    /// * `file_path`: output file path
    fn to_csv(&self, csv_file: &str) -> anyhow::Result<()> {
        self.write_file_delim(csv_file, ",")
    }

    /// write the matrix down to parquet with full control over naming
    /// * `file_path`: output file path
    /// * `row_names`: Tuple of (optional row_names, optional row_column_name)
    ///   - `(None, None)`: use numeric row names `[0, n)` with "row" column name
    ///   - `(None, Some("cell_pair"))`: use numeric row names with "cell_pair" column name
    ///   - `(Some(names), None)`: use provided names with "row" column name
    ///   - `(Some(names), Some("gene"))`: use provided names with "gene" column name
    /// * `column_names`: if `None`, just add `[0, n)` numbers.
    fn to_parquet_with_names(
        &self,
        file_path: &str,
        row_names: (Option<&[Box<str>]>, Option<&str>),
        column_names: Option<&[Box<str>]>,
    ) -> anyhow::Result<()>;

    /// write the matrix down to parquet with default names
    /// * `file_path`: output file path
    ///   Uses numeric row/column names and default "row" column name
    fn to_parquet(&self, file_path: &str) -> anyhow::Result<()> {
        self.to_parquet_with_names(file_path, (None, None), None)
    }

    /// Read a real-valued numeric matrix with the default row
    /// index(0) and all the other available columns.
    /// Assumes column 0 contains row names.
    ///
    fn from_parquet(file_path: &str) -> anyhow::Result<MatWithNames<Self::Mat>> {
        Self::from_parquet_with_indices(file_path, Some(0), None)
    }

    /// Read a real-valued numeric matrix treating all columns as data.
    /// Row names will be generated as "0", "1", "2", ...
    ///
    fn from_parquet_no_row_names(file_path: &str) -> anyhow::Result<MatWithNames<Self::Mat>> {
        Self::from_parquet_with_indices(file_path, None, None)
    }

    /// Read a real-valued numeric matrix from the parquet file. We
    /// can specify row name index. We can specify the row name column
    /// index and desired column indices.
    /// * `row_name_index`: column index (0-based) corresponds to row name
    fn from_parquet_with_row_names(
        file_path: &str,
        row_name_index: Option<usize>,
    ) -> anyhow::Result<MatWithNames<Self::Mat>> {
        Self::from_parquet_with_indices_names(file_path, row_name_index, None, None)
    }

    /// Read a real-valued numeric matrix from the parquet file. We
    /// can specify row name index. We can specify the row name column
    /// index and desired column indices.
    /// * `row_name_index`: column index (0-based) corresponds to row name
    /// * `column_indices`: column indices (0-based) to include
    fn from_parquet_with_indices(
        file_path: &str,
        row_name_index: Option<usize>,
        column_indices: Option<&[usize]>,
    ) -> anyhow::Result<MatWithNames<Self::Mat>> {
        Self::from_parquet_with_indices_names(file_path, row_name_index, column_indices, None)
    }

    /// Read a real-valued numeric matrix from the parquet file.  We
    /// can specify row name index.  We can specify the row name
    /// column index and desired column names.
    /// * `row_name_index`: column index (0-based) corresponds to row name
    /// * `column_names`: column names to include
    fn from_parquet_with_names(
        file_path: &str,
        row_name_index: Option<usize>,
        column_names: Option<&[Box<str>]>,
    ) -> anyhow::Result<MatWithNames<Self::Mat>> {
        Self::from_parquet_with_indices_names(file_path, row_name_index, None, column_names)
    }

    /// Read a real-valued numeric matrix from the parquet file.  We
    /// can specify row name index.  We can specify the row name
    /// column index and desired column indices and names.
    /// * `row_name_index`: column index (0-based) corresponds to row name
    /// * `column_indices`: column indices (0-based) to include
    /// * `column_names`: column names to include
    fn from_parquet_with_indices_names(
        file_path: &str,
        row_name_index: Option<usize>,
        column_indices: Option<&[usize]>,
        column_names: Option<&[Box<str>]>,
    ) -> anyhow::Result<MatWithNames<Self::Mat>>;
}

/// intput data matrix `mat` with `rows` and `cols`
pub struct MatWithNames<M> {
    pub rows: Vec<Box<str>>,
    pub cols: Vec<Box<str>>,
    pub mat: M,
}

pub trait MeltOps {
    type Scalar;
    type Mat;
    /// melt a matrix with indices
    fn melt_with_indexes(&self) -> (Vec<Self::Scalar>, Vec<Vec<usize>>);
    /// melt a matrix
    fn melt(&self) -> Vec<Self::Scalar>;
    /// Melt multiple matrices/tensors together in a single traversal for cache efficiency.
    /// All inputs must have the same dimensions.
    /// Returns (values for each input, indices for each dimension).
    fn melt_many_with_indexes(&self, others: &[&Self])
        -> (Vec<Vec<Self::Scalar>>, Vec<Vec<usize>>);
}

pub trait CandleDataLoaderOps {
    type Scalar;
    type Mat;
    // /// unify transpose
    // fn transpose(&self) -> Self::Mat;
    /// take each row vector as a sample
    fn rows_to_tensor_vec(&self) -> Vec<Tensor>;

    /// Return (nrows, ncols) dimensions
    fn data_shape(&self) -> (usize, usize);

    /// Extract row i as Vec<f32>.
    ///
    /// WARNING: default creates ALL row tensors then picks one — O(N*D) for O(D) work.
    /// Implementors should override this.
    fn row_to_f32_vec(&self, i: usize) -> Vec<f32> {
        let t = &self.rows_to_tensor_vec()[i];
        t.flatten_all().unwrap().to_vec1::<f32>().unwrap()
    }
}