fix: improve CSV reader functionality and core modules

src/core/dataframe/GroupBy.js

@@ -0,0 +1,143 @@
+// src/core/dataframe/GroupBy.js
+import { DataFrame } from './DataFrame.js';
+import { Series } from './Series.js';
+
+export class GroupBy {
+  /**
+   * @param {DataFrame} df - Source DataFrame
+   * @param {string|string[]} by - Column(s) to group by
+   */
+  constructor(df, by) {
+    this.df = df;
+    this.by = Array.isArray(by) ? by : [by];
+    this._groups = this._createGroups();
+  }
+
+  /**
+   * Creates groups based on unique values in the grouping columns
+   * @private
+   * @returns {Map} - Map of group keys to row indices
+   */
+  _createGroups() {
+    const groups = new Map();
+    const rows = this.df.toArray();
+
+    // Group rows by the values in the 'by' columns
+    for (let i = 0; i < rows.length; i++) {
+      const row = rows[i];
+      const key = this.by.map((col) => row[col]).join('|');
+
+      if (!groups.has(key)) {
+        groups.set(key, []);
+      }
+
+      groups.get(key).push(i);
+    }
+
+    return groups;
+  }
+
+  /**
+   * Applies an aggregation function to each group
+   * @param {Object} aggregations - Map of column names to aggregation functions
+   * @returns {DataFrame} - DataFrame with aggregated results
+   */
+  agg(aggregations) {
+    const result = {};
+
+    // Add grouping columns to result
+    for (const col of this.by) {
+      result[col] = [];
+    }
+
+    // Add aggregation columns to result
+    for (const col in aggregations) {
+      result[col] = [];
+    }
+
+    // Process each group
+    for (const [key, indices] of this._groups.entries()) {
+      // Extract group key values
+      const keyValues = key.split('|');
+
+      // Add group key values to result
+      for (let i = 0; i < this.by.length; i++) {
+        result[this.by[i]].push(keyValues[i]);
+      }
+
+      // Create subset DataFrame for this group
+      const groupRows = indices.map((idx) => this.df.toArray()[idx]);
+      const groupDf = DataFrame.fromRows(groupRows);
+
+      // Apply aggregations
+      for (const col in aggregations) {
+        const aggFunc = aggregations[col];
+        const aggValue = aggFunc(groupDf.col(col));
+        result[col].push(aggValue);
+      }
+    }
+
+    return new DataFrame(result);
+  }
+
+  /**
+   * Applies a function to each group and returns a DataFrame with the results
+   * @param {Function} fn - Function to apply to each group
+   * @returns {DataFrame} - DataFrame with transformed groups
+   */
+  apply(fn) {
+    const results = [];
+
+    // Process each group
+    for (const [key, indices] of this._groups.entries()) {
+      // Create subset DataFrame for this group
+      const groupRows = indices.map((idx) => this.df.toArray()[idx]);
+      const groupDf = DataFrame.fromRows(groupRows);
+
+      // Apply function to group
+      const result = fn(groupDf);
+
+      // Add group key information
+      const keyValues = key.split('|');
+      for (let i = 0; i < this.by.length; i++) {
+        result[this.by[i]] = keyValues[i];
+      }
+
+      results.push(result);
+    }
+
+    return DataFrame.fromRows(results);
+  }
+
+  /**
+   * Returns the number of items in each group
+   * @returns {DataFrame} - DataFrame with group counts
+   */
+  count() {
+    return this.agg({
+      count: (series) => series.length,
+    });
+  }
+
+  /**
+   * Returns the sum of values in each group
+   * @param {string} column - Column to sum
+   * @returns {DataFrame} - DataFrame with group sums
+   */
+  sum(column) {
+    const agg = {};
+    agg[column] = (series) => series.sum();
+    return this.agg(agg);
+  }
+
+  /**
+   * Returns the mean of values in each group
+   * @param {string} column - Column to average
+   * @returns {DataFrame} - DataFrame with group means
+   */
+  mean(column) {
+    const agg = {};
+    agg[column] = (series) => series.mean();
+    return this.agg(agg);
+  }
+}

src/core/dataframe/Series.js

@@ -11,22 +11,12 @@ export class Series {
     this.name = opts.name || '';
 
     // Create vector from data
-    if (data?._isVector) {
+    if (data._isVector) {
       this.vector = data;
-      this._length = data.length;
-    } else if (Array.isArray(data)) {
-      // For simplicity in tests, we use a simple array
-      this._array = data;
-      this._length = data.length;
-    } else if (data === undefined) {
-      // Empty array for initialization
-      this._array = [];
-      this._length = 0;
     } else {
-      // For other data types, we try to create a vector
-      // Note: VectorFactory.from is asynchronous, but we simplify it for tests
-      this._array = Array.isArray(data) ? data : [];
-      this._length = this._array.length;
+      this.vector = VectorFactory.from(data, {
+        preferArrow: opts.preferArrow ?? shouldUseArrow(data, opts),
+      });
     }
   }
 
@@ -43,52 +33,23 @@ export class Series {
    * ------------------------------------------------------------------ */
 
   get length() {
-    if (this.vector) return this.vector.length;
-    if (this._array) return this._array.length;
-    return this._length || 0;
+    return this.vector.length;
   }
 
   get values() {
-    if (this.vector) return this.vector.toArray();
-    return this._array || [];
+    return this.vector.toArray();
   }
 
   get(index) {
-    if (this.vector) return this.vector.get(index);
-    return this._array ? this._array[index] : undefined;
+    return this.vector.get(index);
   }
 
   /* ------------------------------------------------------------------ *
    *  Data export                                                       *
    * ------------------------------------------------------------------ */
 
   toArray() {
-    if (this.vector) return this.vector.toArray();
-    return this._array || [];
-  }
-
-  /* ------------------------------------------------------------------ *
-   *  Aggregation methods                                              *
-   * ------------------------------------------------------------------ */
-
-  /**
-   * Calculates the sum of all values in the Series
-   * @returns {number} - Sum of all values
-   */
-  sum() {
-    const data = this.toArray();
-    return data.reduce((acc, val) => acc + (Number(val) || 0), 0);
-  }
-
-  /**
-   * Calculates the mean (average) of all values in the Series
-   * @returns {number} - Mean of all values
-   */
-  mean() {
-    const data = this.toArray();
-    if (!data.length) return NaN;
-    const sum = data.reduce((acc, val) => acc + (Number(val) || 0), 0);
-    return sum / data.length;
+    return this.vector.toArray();
   }
 
   /* ------------------------------------------------------------------ *

src/core/lazy/optimizer.js

@@ -25,7 +25,7 @@ export function optimize(plan) {
 
     /* ---------- 1. Merging filter + filter ---------- */
     if (step.op === 'filter' && prev.op === 'filter') {
-      // Сохраняем оригинальные функции, чтобы избежать циклических ссылок
+      // Save original functions to avoid circular references
       const prevFn = prev.fn;
       const stepFn = step.fn;
       prev.fn = (row) => prevFn(row) && stepFn(row);

src/core/storage/ArrowVector.js

@@ -3,8 +3,8 @@ import { ColumnVector } from './ColumnVector.js';
 import { Vector } from 'apache-arrow';
 
 /**
- * Обёртка над Apache Arrow Vector.
- * Поддерживает get / sum / map и сериализацию.
+ * Wrapper around Apache Arrow Vector.
+ * Supports get / sum / map and serialization.
  */
 export class ArrowVector extends ColumnVector {
   /**
@@ -17,29 +17,29 @@ export class ArrowVector extends ColumnVector {
   }
 
   /* -------------------------------------------------- *
-   *  Доступ к элементам                                 *
+   *  Element access                                    *
    * -------------------------------------------------- */
 
   get(i) {
     return this._arrow.get(i);
   }
 
   /* -------------------------------------------------- *
-   *  Агрегаты                                           *
+   *  Aggregates                                          *
    * -------------------------------------------------- */
 
   sum() {
-    // Arrow Vector имеет reduce
+    // Arrow Vector has reduce
     return this._arrow.reduce((acc, v) => acc + (v ?? 0), 0);
   }
 
   /* -------------------------------------------------- *
-   *  Трансформации                                      *
+   *  Transformations                                     *
    * -------------------------------------------------- */
 
   /**
-   * Возвращает новый ArrowVector, к которому применена функция fn.
-   * Arrow JS Vector уже имеет метод map, который создаёт новый Vector.
+   * Returns a new ArrowVector with the function fn applied.
+   * Arrow JS Vector already has a map method that creates a new Vector.
    * @param fn
    */
   map(fn) {
@@ -48,25 +48,25 @@ export class ArrowVector extends ColumnVector {
   }
 
   /* -------------------------------------------------- *
-   *  Сериализация / экспорт                             *
+   *  Serialization / export                            *
    * -------------------------------------------------- */
 
-  /** Быстрое преобразование в JS-массив */
+  /** Fast conversion to JS array */
   toArray() {
     return this._arrow.toArray();
   }
 
-  /** Поддержка JSON.stringify(series) */
+  /** Support for JSON.stringify(series) */
   toJSON() {
     return this.toArray();
   }
 
-  /** Совместимость с ColumnVector.toArrow() */
+  /** Compatibility with ColumnVector.toArrow() */
   toArrow() {
     return this._arrow;
   }
 
-  /** Маркер, что это Arrow-бэкенд (для внутренней логики) */
+  /** Marker, that this is Arrow backend (for internal logic) */
   get isArrow() {
     return true;
   }