// Copyright (C) 2024 The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import {Filter} from './filters';
import {ColumnOrderClause, SqlColumn, SqlExpression} from './sql_column';

// The goal of this module is to generate a query statement from the list of columns, filters and order by clauses.
// The main challenge is that the column definitions are independent, and the columns themselves can reference the same join multiple times:
//
// For example, in the following query `parent_slice_ts` and `parent_slice_dur` are both referencing the same join, but we want to include only one join in the final query.

// SELECT
//    parent.ts AS parent_slice_ts,
//    parent.dur AS parent_slice_dur
// FROM slice
// LEFT JOIN slice AS parent ON slice.parent_id = parent.id

// Normalised sql column, where the source table is resolved to a unique index.
type NormalisedSqlColumn =
  | {
      kind: 'table_column';
      column: string;
      // If |sourceTableId| is undefined, then the columnName comes from the primary table.
      sourceTableId?: number;
    }
  | {
      kind: 'expression';
      index: number;
      op: (cols: string[]) => string;
      columns: NormalisedSqlColumn[];
    };

// Normalised source table, where the join constraints are resolved to a normalised columns.
type NormalisedSourceTable = {
  table: string;
  joinOn: {[key: string]: NormalisedSqlColumn};
  innerJoin: boolean;
};

// Checks whether two normalised columns are equivalent.
function normalisedSqlColumnsEqual(
  a: NormalisedSqlColumn | undefined,
  b: NormalisedSqlColumn | undefined,
): boolean {
  if (a === undefined) return false;
  if (b === undefined) return false;
  if (a.kind === 'table_column') {
    if (b.kind !== 'table_column') return false;
    return a.column === b.column && a.sourceTableId === b.sourceTableId;
  } else {
    if (b.kind !== 'expression') return false;
    // For expressions, first check that the underlying columns are equal.
    if (a.columns.length !== b.columns.length) return false;
    for (let i = 0; i < a.columns.length; ++i) {
      if (!normalisedSqlColumnsEqual(a.columns[i], b.columns[i])) return false;
    }
    // Subsitute the columns with dummy values to check if the expressions are equal.
    const cols = Array.from({length: a.columns.length}, (_, i) => `__$${i}`);
    return a.op(cols) === b.op(cols);
  }
}

// Checks whether two join constraints are equal to allow deduplication of joins.
function areJoinConstraintsEqual(
  a: {[key: string]: NormalisedSqlColumn},
  b: {[key: string]: NormalisedSqlColumn},
): boolean {
  if (Object.keys(a).length !== Object.keys(b).length) {
    return false;
  }

  for (const key of Object.keys(a)) {
    const aValue = a[key];
    const bValue = b[key];
    if (!normalisedSqlColumnsEqual(aValue, bValue)) return false;
  }
  return true;
}

// Class responsible for building a query and maintaing a list of normalised join tables.
class QueryBuilder {
  tables: NormalisedSourceTable[] = [];
  tableAlias: string;
  expressionIndex: number = 0;

  constructor(tableName: string) {
    this.tableAlias = `${tableName}_0`;
  }

  // Normalises a column, including adding if necessary the joins to the list of tables.
  normalise(column: SqlColumn): NormalisedSqlColumn {
    // Simple columns do not require any normalisation.
    if (typeof column === 'string') {
      return {
        kind: 'table_column',
        column: column,
      };
    }
    // Expressions require normalisation of the underlying columns.
    if (column instanceof SqlExpression) {
      return {
        kind: 'expression',
        index: this.expressionIndex++,
        op: column.op,
        columns: column.columns.map((column) => this.normalise(column)),
      };
    }
    // Otherwise, normalise join constraints.
    const normalisedJoinOn: {[key: string]: NormalisedSqlColumn} =
      Object.fromEntries(
        Object.entries(column.source.joinOn).map(([key, value]) => [
          key,
          this.normalise(value),
        ]),
      );

    // Check if this join is already present.
    for (let i = 0; i < this.tables.length; ++i) {
      const table = this.tables[i];
      if (
        table.table === column.source.table &&
        table.innerJoin === (column.source.innerJoin ?? false) &&
        areJoinConstraintsEqual(table.joinOn, normalisedJoinOn)
      ) {
        return {
          kind: 'table_column',
          column: column.column,
          sourceTableId: i,
        };
      }
    }

    // Otherwise, add a new join.
    this.tables.push({
      table: column.source.table,
      joinOn: normalisedJoinOn,
      innerJoin: column.source.innerJoin ?? false,
    });
    return {
      kind: 'table_column',
      column: column.column,
      sourceTableId: this.tables.length - 1,
    };
  }

  // Prints a reference to a column, including properly disambiguated table alias.
  printReference(column: NormalisedSqlColumn): string {
    if (column.kind === 'expression') {
      return column.op(
        column.columns.map((column) => this.printReference(column)),
      );
    }
    if (column.sourceTableId === undefined) {
      if (!/^[A-Za-z0-9_]*$/.test(column.column)) {
        // If this is an expression, don't prefix it with the table name.
        return column.column;
      }
      return `${this.tableAlias}.${column.column}`;
    }
    const table = this.tables[column.sourceTableId];
    // Dependent tables are 0-indexed, but we want to display them as 1-indexed to reserve 0 for the primary table.
    return `${table.table}_${column.sourceTableId + 1}.${column.column}`;
  }

  printJoin(joinIndex: number): string {
    const join = this.tables[joinIndex];
    const alias = `${join.table}_${joinIndex + 1}`;
    const clauses = Object.entries(join.joinOn).map(
      ([key, value]) => `${alias}.${key} = ${this.printReference(value)}`,
    );
    // Join IDs are 0-indexed, but we want to display them as 1-indexed to reserve 0 for the primary table.
    return `${join.innerJoin ? '' : 'LEFT '}JOIN ${join.table} AS ${alias} ON ${clauses.join(' AND ')}`;
  }
}

// Returns a query fetching the columns from the table, with the specified filters and order by clauses.
// keys of the `columns` object are the names of the columns in the result set.
export function buildSqlQuery(args: {
  table: string;
  columns: {[key: string]: SqlColumn};
  prefix?: string;
  filters?: Filter[];
  // List of columns to group by. Should be a subset of the keys of the `columns` object.
  groupBy?: SqlColumn[];
  orderBy?: ColumnOrderClause[];
}): string {
  const builder = new QueryBuilder(args.table);

  const normalisedColumns = Object.fromEntries(
    Object.entries(args.columns).map(([key, value]) => [
      key,
      builder.normalise(value),
    ]),
  );
  const normalisedFilters = (args.filters || []).map((filter) => ({
    op: filter.op,
    columns: filter.columns.map((column) => builder.normalise(column)),
  }));
  const normalisedOrderBy = (args.orderBy || []).map((orderBy) => ({
    order: orderBy.direction,
    column: builder.normalise(orderBy.column),
  }));
  const normalisedGroupBy = (args.groupBy || []).map((column) =>
    builder.normalise(column),
  );

  const formatFilter = (filter: {
    op: (cols: string[]) => string;
    columns: NormalisedSqlColumn[];
  }) => {
    return filter.op(
      filter.columns.map((column) => builder.printReference(column)),
    );
  };

  const filterClause =
    normalisedFilters.length === 0
      ? ''
      : `WHERE\n ${normalisedFilters.map(formatFilter).join('\n  AND ')}`;
  const joinClause = builder.tables
    .map((_, index) => builder.printJoin(index))
    .join('\n');
  const groupBys = normalisedGroupBy.map((column) =>
    builder.printReference(column),
  );
  const groupByClause =
    args.groupBy === undefined ? '' : `GROUP BY\n  ${groupBys.join(', ')}`;
  const orderBys = normalisedOrderBy.map(
    (orderBy) => `${builder.printReference(orderBy.column)} ${orderBy.order}`,
  );
  const orderByClause =
    normalisedOrderBy.length === 0 ? '' : `ORDER BY\n  ${orderBys.join(',  ')}`;

  return `
    ${args.prefix === undefined ? '' : args.prefix}
    SELECT
      ${Object.entries(normalisedColumns)
        .map(([key, value]) => `${builder.printReference(value)} AS ${key}`)
        .join(',\n  ')}
    FROM ${args.table} AS ${builder.tableAlias}
    ${joinClause}
    ${filterClause}
    ${groupByClause}
    ${orderByClause}
  `;
}