hiqlite 0.13.2

Hiqlite - highly-available, embeddable, raft-based SQLite + cache
Documentation 
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use std::{borrow::Cow, collections::{hash_map::Entry, HashMap}};

use rusqlite::{types::Value, Transaction};

use crate::Error;

/// Data structure for supporting Param::StmtOutput
#[derive(Default)]
pub struct TransactionEnv {
    /// observable statements: statements with row output.
    /// It is keyed by the statement index.
    observable_stmts: Vec<(usize, ObservableStatement)>,

    /// A cache of column name->index per statement
    column_index_cache: HashMap<usize, HashMap<Cow<'static, str>, usize>>
}

impl TransactionEnv {
    pub fn push_observable_stmt(&mut self, stmt_index: usize, sql: Cow<'static, str>, first_row: Vec<Value>) {
        self.observable_stmts.push((stmt_index, ObservableStatement { sql, first_row }));
    }
}

/// A context for looking up Param variables
pub struct TransactionParamContext<'a, 't> {
    pub txn: &'a Transaction<'t>,
    pub env: &'a mut TransactionEnv,
}

impl TransactionParamContext<'_, '_> {
    pub fn lookup_statement_output_indexed(&mut self, statement_index: usize, column_index: usize) -> Result<Value, Cow<'static, str>> {
        let executed_stmt = ObservableStatement::by_index(&self.env.observable_stmts, statement_index)?;
        Ok(executed_stmt.get_first_row_value(column_index)?.clone())
    }

    pub fn lookup_statement_output_named(&mut self, statement_index: usize, column_name: Cow<'static, str>) -> Result<Value, Cow<'static, str>> {
        let executed_stmt = ObservableStatement::by_index(&self.env.observable_stmts, statement_index)?;

        let cache = self.env.column_index_cache.entry(statement_index).or_default();
        let column_index = match cache.entry(column_name) {
            Entry::Occupied(occpied) => *occpied.get(),
            Entry::Vacant(vacant) => {
                // Need to re-prepare the statement.
                // Hopefully the statement will be cached.
                // The statement has already been prepared (and error-checked) earlier, so should not fail.
                let stmt = self.txn.prepare_cached(&executed_stmt.sql)
                    .map_err(|_| "re-preparation")?;

                let column_index = stmt.column_index(vacant.key())
                    .map_err(|err| format!("{err:?}"))?;

                // Cache the column index for later, in case the same value
                // is used many times in the same transaction, which is not unlikely.
                // This should avoid some statement re-preparation in larger transactions.
                *vacant.insert(column_index)
            },
        };

        Ok(executed_stmt.get_first_row_value(column_index)?.clone())
    }
}

/// A previously executed statement that has output columns
struct ObservableStatement {
    /// The executed SQL
    pub sql: Cow<'static, str>,
    /// The first returned row of the statement.
    /// This will be empty if the statement returned no rows.
    pub first_row: Vec<Value>,
}

impl ObservableStatement {
    fn by_index(statements: &[(usize, ObservableStatement)], index: usize) -> Result<&ObservableStatement, Cow<'static, str>> {
        // Currently doing a linear search.
        // Could dynamically change to binary search if the number of entries is large enough, or use hashmap.
        statements
            .iter()
            .find(|(stmt_index, _)| *stmt_index == index)
            .map(|(_, statement)| statement)
            .ok_or_else(|| format!("StmtIndex({index}) does not have observable row output, or index out of bounds").into())
    }

    fn get_first_row_value(&self, column_index: usize) -> Result<&Value, Cow<'static, str>> {
        Ok(self.first_row.get(column_index).ok_or("column index out of bounds")?)
    }
}