rusqlite/src/functions.rs

//! Create or redefine SQL functions.
//!
//! # Example
//!
//! Adding a `regexp` function to a connection in which compiled regular expressions
//! are cached in a `HashMap`. For an alternative implementation that uses SQLite's
//! [Function Auxilliary Data](https://www.sqlite.org/c3ref/get_auxdata.html) interface
//! to avoid recompiling regular expressions, see the unit tests for this module.
//!
//! ```rust
//! extern crate libsqlite3_sys;
//! extern crate rusqlite;
//! extern crate regex;
//!
//! use rusqlite::{Connection, Error, Result};
//! use std::collections::HashMap;
//! use regex::Regex;
//!
//! fn add_regexp_function(db: &Connection) -> Result<()> {
//!     let mut cached_regexes = HashMap::new();
//!     db.create_scalar_function("regexp", 2, true, move |ctx| {
//!         let regex_s = try!(ctx.get::<String>(0));
//!         let entry = cached_regexes.entry(regex_s.clone());
//!         let regex = {
//!             use std::collections::hash_map::Entry::{Occupied, Vacant};
//!             match entry {
//!                 Occupied(occ) => occ.into_mut(),
//!                 Vacant(vac) => {
//!                     match Regex::new(&regex_s) {
//!                         Ok(r) => vac.insert(r),
//!                         Err(err) => return Err(Error::UserFunctionError(Box::new(err))),
//!                     }
//!                 }
//!             }
//!         };
//!
//!         let text = try!(ctx.get::<String>(1));
//!         Ok(regex.is_match(&text))
//!     })
//! }
//!
//! fn main() {
//!     let db = Connection::open_in_memory().unwrap();
//!     add_regexp_function(&db).unwrap();
//!
//!     let is_match: bool = db.query_row("SELECT regexp('[aeiou]*', 'aaaaeeeiii')", &[],
//!                                 |row| row.get(0)).unwrap();
//!
//!     assert!(is_match);
//! }
//! ```
use std::error::Error as StdError;
use std::ffi::CStr;
use std::mem;
use std::ptr;
use std::slice;
use libc::{c_int, c_double, c_char, c_void};

use ffi;
pub use ffi::sqlite3_context;
pub use ffi::sqlite3_value;
pub use ffi::sqlite3_value_type;
pub use ffi::sqlite3_value_numeric_type;

use types::{Null, FromSql, ValueRef};

use {Result, Error, Connection, str_to_cstring, InnerConnection};

/// A trait for types that can be converted into the result of an SQL function.
pub trait ToResult {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context);
}

macro_rules! raw_to_impl(
    ($t:ty, $f:ident) => (
        impl ToResult for $t {
            unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
                ffi::$f(ctx, *self)
            }
        }
    )
);

raw_to_impl!(c_int, sqlite3_result_int);
raw_to_impl!(i64, sqlite3_result_int64);
raw_to_impl!(c_double, sqlite3_result_double);

impl<'a> ToResult for bool {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        if *self {
            ffi::sqlite3_result_int(ctx, 1)
        } else {
            ffi::sqlite3_result_int(ctx, 0)
        }
    }
}


impl<'a> ToResult for &'a str {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        let length = self.len();
        if length > ::std::i32::MAX as usize {
            ffi::sqlite3_result_error_toobig(ctx);
            return;
        }
        match str_to_cstring(self) {
            Ok(c_str) => {
                ffi::sqlite3_result_text(ctx,
                                         c_str.as_ptr(),
                                         length as c_int,
                                         ffi::SQLITE_TRANSIENT())
            }
            // TODO sqlite3_result_error
            Err(_) => ffi::sqlite3_result_error_code(ctx, ffi::SQLITE_MISUSE),
        }
    }
}

impl ToResult for String {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        (&self[..]).set_result(ctx)
    }
}

impl<'a> ToResult for &'a [u8] {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        if self.len() > ::std::i32::MAX as usize {
            ffi::sqlite3_result_error_toobig(ctx);
            return;
        }
        ffi::sqlite3_result_blob(ctx,
                                 mem::transmute(self.as_ptr()),
                                 self.len() as c_int,
                                 ffi::SQLITE_TRANSIENT())
    }
}

impl ToResult for Vec<u8> {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        (&self[..]).set_result(ctx)
    }
}

impl<T: ToResult> ToResult for Option<T> {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        match *self {
            None => ffi::sqlite3_result_null(ctx),
            Some(ref t) => t.set_result(ctx),
        }
    }
}

impl ToResult for Null {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        ffi::sqlite3_result_null(ctx)
    }
}

/// Error indicating that a memory allocation failed.
#[derive(Copy,Clone)]
pub struct NoMem;

impl ToResult for NoMem {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        ffi::sqlite3_result_error_nomem(ctx)
    }
}

/// Error indicating that a string or BLOB is too long to represent.
#[derive(Copy,Clone)]
pub struct TooBig;

impl ToResult for TooBig {
    unsafe fn set_result(&self, ctx: *mut sqlite3_context) {
        ffi::sqlite3_result_error_toobig(ctx)
    }
}

impl<'a> ValueRef<'a> {
    pub unsafe fn from_value(value: *mut sqlite3_value) -> ValueRef<'a> {
        use std::slice::from_raw_parts;

        match ffi::sqlite3_value_type(value) {
            ffi::SQLITE_NULL => ValueRef::Null,
            ffi::SQLITE_INTEGER => ValueRef::Integer(ffi::sqlite3_value_int64(value)),
            ffi::SQLITE_FLOAT => ValueRef::Real(ffi::sqlite3_value_double(value)),
            ffi::SQLITE_TEXT => {
                let text = ffi::sqlite3_value_text(value);
                assert!(!text.is_null(), "unexpected SQLITE_TEXT value type with NULL data");
                let s = CStr::from_ptr(text as *const c_char);

                // sqlite3_value_text returns UTF8 data, so our unwrap here should be fine.
                let s = s.to_str().expect("sqlite3_value_text returned invalid UTF-8");
                ValueRef::Text(s)
            }
            ffi::SQLITE_BLOB => {
                let blob = ffi::sqlite3_value_blob(value);
                assert!(!blob.is_null(), "unexpected SQLITE_BLOB value type with NULL data");

                let len = ffi::sqlite3_value_bytes(value);
                assert!(len >= 0, "unexpected negative return from sqlite3_value_bytes");

                ValueRef::Blob(from_raw_parts(blob as *const u8, len as usize))
            }
            _ => unreachable!("sqlite3_value_type returned invalid value"),
        }
    }
}

unsafe extern "C" fn free_boxed_value<T>(p: *mut c_void) {
    let _: Box<T> = Box::from_raw(mem::transmute(p));
}

/// Context is a wrapper for the SQLite function evaluation context.
pub struct Context<'a> {
    ctx: *mut sqlite3_context,
    args: &'a [*mut sqlite3_value],
}

impl<'a> Context<'a> {
    /// Returns the number of arguments to the function.
    pub fn len(&self) -> usize {
        self.args.len()
    }
    /// Returns `true` when there is no argument.
    pub fn is_empty(&self) -> bool {
        self.args.is_empty()
    }

    /// Returns the `idx`th argument as a `T`.
    ///
    /// # Failure
    ///
    /// Will panic if `idx` is greater than or equal to `self.len()`.
    ///
    /// Will return Err if the underlying SQLite type cannot be converted to a `T`.
    pub fn get<T: FromSql>(&self, idx: usize) -> Result<T> {
        let arg = self.args[idx];
        let value = unsafe { ValueRef::from_value(arg) };
        FromSql::column_result(value).map_err(|err| match err {
            Error::InvalidColumnType => Error::InvalidFunctionParameterType,
            _ => err,
        })
    }

    /// Sets the auxilliary data associated with a particular parameter. See
    /// https://www.sqlite.org/c3ref/get_auxdata.html for a discussion of
    /// this feature, or the unit tests of this module for an example.
    pub fn set_aux<T>(&self, arg: c_int, value: T) {
        let boxed = Box::into_raw(Box::new(value));
        unsafe {
            ffi::sqlite3_set_auxdata(self.ctx,
                                     arg,
                                     mem::transmute(boxed),
                                     Some(free_boxed_value::<T>))
        };
    }

    /// Gets the auxilliary data that was associated with a given parameter
    /// via `set_aux`. Returns `None` if no data has been associated.
    ///
    /// # Unsafety
    ///
    /// This function is unsafe as there is no guarantee that the type `T`
    /// requested matches the type `T` that was provided to `set_aux`. The
    /// types must be identical.
    pub unsafe fn get_aux<T>(&self, arg: c_int) -> Option<&T> {
        let p = ffi::sqlite3_get_auxdata(self.ctx, arg) as *mut T;
        if p.is_null() { None } else { Some(&*p) }
    }
}

/// Aggregate is the callback interface for user-defined aggregate function.
///
/// `A` is the type of the aggregation context and `T` is the type of the final result.
/// Implementations should be stateless.
pub trait Aggregate<A, T>
    where T: ToResult
{
    /// Initializes the aggregation context. Will be called prior to the first call
    /// to `step()` to set up the context for an invocation of the function. (Note:
    /// `init()` will not be called if the there are no rows.)
    fn init(&self) -> A;

    /// "step" function called once for each row in an aggregate group. May be called
    /// 0 times if there are no rows.
    fn step(&self, &mut Context, &mut A) -> Result<()>;

    /// Computes and returns the final result. Will be called exactly once for each
    /// invocation of the function. If `step()` was called at least once, will be given
    /// `Some(A)` (the same `A` as was created by `init` and given to `step`); if `step()`
    /// was not called (because the function is running against 0 rows), will be given
    /// `None`.
    fn finalize(&self, Option<A>) -> Result<T>;
}

impl Connection {
    /// Attach a user-defined scalar function to this database connection.
    ///
    /// `fn_name` is the name the function will be accessible from SQL.
    /// `n_arg` is the number of arguments to the function. Use `-1` for a variable
    /// number. If the function always returns the same value given the same
    /// input, `deterministic` should be `true`.
    ///
    /// The function will remain available until the connection is closed or
    /// until it is explicitly removed via `remove_function`.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use rusqlite::{Connection, Result};
    /// # type c_double = f64;
    /// fn scalar_function_example(db: Connection) -> Result<()> {
    ///     try!(db.create_scalar_function("halve", 1, true, |ctx| {
    ///         let value = try!(ctx.get::<c_double>(0));
    ///         Ok(value / 2f64)
    ///     }));
    ///
    ///     let six_halved: f64 = try!(db.query_row("SELECT halve(6)", &[], |r| r.get(0)));
    ///     assert_eq!(six_halved, 3f64);
    ///     Ok(())
    /// }
    /// ```
    ///
    /// # Failure
    ///
    /// Will return Err if the function could not be attached to the connection.
    pub fn create_scalar_function<F, T>(&self,
                                        fn_name: &str,
                                        n_arg: c_int,
                                        deterministic: bool,
                                        x_func: F)
                                        -> Result<()>
        where F: FnMut(&Context) -> Result<T>,
              T: ToResult
    {
        self.db.borrow_mut().create_scalar_function(fn_name, n_arg, deterministic, x_func)
    }

    /// Attach a user-defined aggregate function to this database connection.
    ///
    /// # Failure
    ///
    /// Will return Err if the function could not be attached to the connection.
    pub fn create_aggregate_function<A, D, T>(&self,
                                              fn_name: &str,
                                              n_arg: c_int,
                                              deterministic: bool,
                                              aggr: D)
                                              -> Result<()>
        where D: Aggregate<A, T>,
              T: ToResult
    {
        self.db
            .borrow_mut()
            .create_aggregate_function(fn_name, n_arg, deterministic, aggr)
    }

    /// Removes a user-defined function from this database connection.
    ///
    /// `fn_name` and `n_arg` should match the name and number of arguments
    /// given to `create_scalar_function` or `create_aggregate_function`.
    ///
    /// # Failure
    ///
    /// Will return Err if the function could not be removed.
    pub fn remove_function(&self, fn_name: &str, n_arg: c_int) -> Result<()> {
        self.db.borrow_mut().remove_function(fn_name, n_arg)
    }
}

impl InnerConnection {
    fn create_scalar_function<F, T>(&mut self,
                                    fn_name: &str,
                                    n_arg: c_int,
                                    deterministic: bool,
                                    x_func: F)
                                    -> Result<()>
        where F: FnMut(&Context) -> Result<T>,
              T: ToResult
    {
        unsafe extern "C" fn call_boxed_closure<F, T>(ctx: *mut sqlite3_context,
                                                      argc: c_int,
                                                      argv: *mut *mut sqlite3_value)
            where F: FnMut(&Context) -> Result<T>,
                  T: ToResult
        {
            let ctx = Context {
                ctx: ctx,
                args: slice::from_raw_parts(argv, argc as usize),
            };
            let boxed_f: *mut F = mem::transmute(ffi::sqlite3_user_data(ctx.ctx));
            assert!(!boxed_f.is_null(), "Internal error - null function pointer");
            match (*boxed_f)(&ctx) {
                Ok(r) => r.set_result(ctx.ctx),
                Err(Error::SqliteFailure(err, s)) => {
                    ffi::sqlite3_result_error_code(ctx.ctx, err.extended_code);
                    if let Some(Ok(cstr)) = s.map(|s| str_to_cstring(&s)) {
                        ffi::sqlite3_result_error(ctx.ctx, cstr.as_ptr(), -1);
                    }
                }
                Err(err) => {
                    ffi::sqlite3_result_error_code(ctx.ctx, ffi::SQLITE_CONSTRAINT_FUNCTION);
                    if let Ok(cstr) = str_to_cstring(err.description()) {
                        ffi::sqlite3_result_error(ctx.ctx, cstr.as_ptr(), -1);
                    }
                }
            }
        }

        let boxed_f: *mut F = Box::into_raw(Box::new(x_func));
        let c_name = try!(str_to_cstring(fn_name));
        let mut flags = ffi::SQLITE_UTF8;
        if deterministic {
            flags |= ffi::SQLITE_DETERMINISTIC;
        }
        let r = unsafe {
            ffi::sqlite3_create_function_v2(self.db(),
                                            c_name.as_ptr(),
                                            n_arg,
                                            flags,
                                            mem::transmute(boxed_f),
                                            Some(call_boxed_closure::<F, T>),
                                            None,
                                            None,
                                            Some(free_boxed_value::<F>))
        };
        self.decode_result(r)
    }

    fn create_aggregate_function<A, D, T>(&mut self,
                                          fn_name: &str,
                                          n_arg: c_int,
                                          deterministic: bool,
                                          aggr: D)
                                          -> Result<()>
        where D: Aggregate<A, T>,
              T: ToResult
    {
        unsafe fn aggregate_context<A>(ctx: *mut sqlite3_context,
                                       bytes: usize)
                                       -> Option<*mut *mut A> {
            let pac = ffi::sqlite3_aggregate_context(ctx, bytes as c_int) as *mut *mut A;
            if pac.is_null() {
                return None;
            }
            Some(pac)
        }

        unsafe fn report_aggregate_error(ctx: *mut sqlite3_context, err: Error) {
            match err {
                Error::SqliteFailure(err, s) => {
                    ffi::sqlite3_result_error_code(ctx, err.extended_code);
                    if let Some(Ok(cstr)) = s.map(|s| str_to_cstring(&s)) {
                        ffi::sqlite3_result_error(ctx, cstr.as_ptr(), -1);
                    }
                }
                _ => {
                    ffi::sqlite3_result_error_code(ctx, ffi::SQLITE_CONSTRAINT_FUNCTION);
                    if let Ok(cstr) = str_to_cstring(err.description()) {
                        ffi::sqlite3_result_error(ctx, cstr.as_ptr(), -1);
                    }
                }
            }
        }

        unsafe extern "C" fn call_boxed_step<A, D, T>(ctx: *mut sqlite3_context,
                                                      argc: c_int,
                                                      argv: *mut *mut sqlite3_value)
            where D: Aggregate<A, T>,
                  T: ToResult
        {
            let boxed_aggr: *mut D = mem::transmute(ffi::sqlite3_user_data(ctx));
            assert!(!boxed_aggr.is_null(),
                    "Internal error - null aggregate pointer");

            let pac = match aggregate_context(ctx, ::std::mem::size_of::<*mut A>()) {
                Some(pac) => pac,
                None => {
                    ffi::sqlite3_result_error_nomem(ctx);
                    return;
                }
            };

            if (*pac).is_null() {
                *pac = Box::into_raw(Box::new((*boxed_aggr).init()));
            }

            let mut ctx = Context {
                ctx: ctx,
                args: slice::from_raw_parts(argv, argc as usize),
            };

            match (*boxed_aggr).step(&mut ctx, &mut **pac) {
                Ok(_) => {}
                Err(err) => report_aggregate_error(ctx.ctx, err),
            };
        }

        unsafe extern "C" fn call_boxed_final<A, D, T>(ctx: *mut sqlite3_context)
            where D: Aggregate<A, T>,
                  T: ToResult
        {
            let boxed_aggr: *mut D = mem::transmute(ffi::sqlite3_user_data(ctx));
            assert!(!boxed_aggr.is_null(),
                    "Internal error - null aggregate pointer");

            // Within the xFinal callback, it is customary to set N=0 in calls to
            // sqlite3_aggregate_context(C,N) so that no pointless memory allocations occur.
            let a: Option<A> = match aggregate_context(ctx, 0) {
                Some(pac) => {
                    if (*pac).is_null() {
                        None
                    } else {
                        let a = Box::from_raw(*pac);
                        Some(*a)
                    }
                }
                None => None,
            };

            match (*boxed_aggr).finalize(a) {
                Ok(r) => r.set_result(ctx),
                Err(err) => report_aggregate_error(ctx, err),
            };
        }

        let boxed_aggr: *mut D = Box::into_raw(Box::new(aggr));
        let c_name = try!(str_to_cstring(fn_name));
        let mut flags = ffi::SQLITE_UTF8;
        if deterministic {
            flags |= ffi::SQLITE_DETERMINISTIC;
        }
        let r = unsafe {
            ffi::sqlite3_create_function_v2(self.db(),
                                            c_name.as_ptr(),
                                            n_arg,
                                            flags,
                                            mem::transmute(boxed_aggr),
                                            None,
                                            Some(call_boxed_step::<A, D, T>),
                                            Some(call_boxed_final::<A, D, T>),
                                            Some(free_boxed_value::<D>))
        };
        self.decode_result(r)
    }

    fn remove_function(&mut self, fn_name: &str, n_arg: c_int) -> Result<()> {
        let c_name = try!(str_to_cstring(fn_name));
        let r = unsafe {
            ffi::sqlite3_create_function_v2(self.db(),
                                            c_name.as_ptr(),
                                            n_arg,
                                            ffi::SQLITE_UTF8,
                                            ptr::null_mut(),
                                            None,
                                            None,
                                            None,
                                            None)
        };
        self.decode_result(r)
    }
}

#[cfg(test)]
mod test {
    extern crate regex;

    use std::collections::HashMap;
    use libc::c_double;
    use self::regex::Regex;
    use std::f64::EPSILON;

    use {Connection, Error, Result};
    use functions::{Aggregate, Context};

    fn half(ctx: &Context) -> Result<c_double> {
        assert!(ctx.len() == 1, "called with unexpected number of arguments");
        let value = try!(ctx.get::<c_double>(0));
        Ok(value / 2f64)
    }

    #[test]
    fn test_function_half() {
        let db = Connection::open_in_memory().unwrap();
        db.create_scalar_function("half", 1, true, half).unwrap();
        let result: Result<f64> = db.query_row("SELECT half(6)", &[], |r| r.get(0));

        assert!((3f64 - result.unwrap()).abs() < EPSILON);
    }

    #[test]
    fn test_remove_function() {
        let db = Connection::open_in_memory().unwrap();
        db.create_scalar_function("half", 1, true, half).unwrap();
        let result: Result<f64> = db.query_row("SELECT half(6)", &[], |r| r.get(0));
        assert!((3f64 - result.unwrap()).abs() < EPSILON);

        db.remove_function("half", 1).unwrap();
        let result: Result<f64> = db.query_row("SELECT half(6)", &[], |r| r.get(0));
        assert!(result.is_err());
    }

    // This implementation of a regexp scalar function uses SQLite's auxilliary data
    // (https://www.sqlite.org/c3ref/get_auxdata.html) to avoid recompiling the regular
    // expression multiple times within one query.
    fn regexp_with_auxilliary(ctx: &Context) -> Result<bool> {
        assert!(ctx.len() == 2, "called with unexpected number of arguments");

        let saved_re: Option<&Regex> = unsafe { ctx.get_aux(0) };
        let new_re = match saved_re {
            None => {
                let s = try!(ctx.get::<String>(0));
                match Regex::new(&s) {
                    Ok(r) => Some(r),
                    Err(err) => return Err(Error::UserFunctionError(Box::new(err))),
                }
            }
            Some(_) => None,
        };

        let is_match = {
            let re = saved_re.unwrap_or_else(|| new_re.as_ref().unwrap());

            let text = try!(ctx.get::<String>(1));
            re.is_match(&text)
        };

        if let Some(re) = new_re {
            ctx.set_aux(0, re);
        }

        Ok(is_match)
    }

    #[test]
    #[cfg_attr(rustfmt, rustfmt_skip)]
    fn test_function_regexp_with_auxilliary() {
        let db = Connection::open_in_memory().unwrap();
        db.execute_batch("BEGIN;
                         CREATE TABLE foo (x string);
                         INSERT INTO foo VALUES ('lisa');
                         INSERT INTO foo VALUES ('lXsi');
                         INSERT INTO foo VALUES ('lisX');
                         END;").unwrap();
        db.create_scalar_function("regexp", 2, true, regexp_with_auxilliary).unwrap();

        let result: Result<bool> = db.query_row("SELECT regexp('l.s[aeiouy]', 'lisa')",
                                  &[],
                                  |r| r.get(0));

        assert_eq!(true, result.unwrap());

        let result: Result<i64> =
            db.query_row("SELECT COUNT(*) FROM foo WHERE regexp('l.s[aeiouy]', x) == 1",
                         &[],
                         |r| r.get(0));

        assert_eq!(2, result.unwrap());
    }

    #[test]
    #[cfg_attr(rustfmt, rustfmt_skip)]
    fn test_function_regexp_with_hashmap_cache() {
        let db = Connection::open_in_memory().unwrap();
        db.execute_batch("BEGIN;
                         CREATE TABLE foo (x string);
                         INSERT INTO foo VALUES ('lisa');
                         INSERT INTO foo VALUES ('lXsi');
                         INSERT INTO foo VALUES ('lisX');
                         END;").unwrap();

        // This implementation of a regexp scalar function uses a captured HashMap
        // to keep cached regular expressions around (even across multiple queries)
        // until the function is removed.
        let mut cached_regexes = HashMap::new();
        db.create_scalar_function("regexp", 2, true, move |ctx| {
            assert!(ctx.len() == 2, "called with unexpected number of arguments");

            let regex_s = try!(ctx.get::<String>(0));
            let entry = cached_regexes.entry(regex_s.clone());
            let regex = {
                use std::collections::hash_map::Entry::{Occupied, Vacant};
                match entry {
                    Occupied(occ) => occ.into_mut(),
                    Vacant(vac) => {
                        match Regex::new(&regex_s) {
                            Ok(r) => vac.insert(r),
                            Err(err) => return Err(Error::UserFunctionError(Box::new(err))),
                        }
                    }
                }
            };

            let text = try!(ctx.get::<String>(1));
            Ok(regex.is_match(&text))
        }).unwrap();

        let result: Result<bool> = db.query_row("SELECT regexp('l.s[aeiouy]', 'lisa')",
                                  &[],
                                  |r| r.get(0));

        assert_eq!(true, result.unwrap());

        let result: Result<i64> =
            db.query_row("SELECT COUNT(*) FROM foo WHERE regexp('l.s[aeiouy]', x) == 1",
                         &[],
                         |r| r.get(0));

        assert_eq!(2, result.unwrap());
    }

    #[test]
    fn test_varargs_function() {
        let db = Connection::open_in_memory().unwrap();
        db.create_scalar_function("my_concat", -1, true, |ctx| {
                let mut ret = String::new();

                for idx in 0..ctx.len() {
                    let s = try!(ctx.get::<String>(idx));
                    ret.push_str(&s);
                }

                Ok(ret)
            })
            .unwrap();

        for &(expected, query) in &[("", "SELECT my_concat()"),
                                    ("onetwo", "SELECT my_concat('one', 'two')"),
                                    ("abc", "SELECT my_concat('a', 'b', 'c')")] {
            let result: String = db.query_row(query, &[], |r| r.get(0)).unwrap();
            assert_eq!(expected, result);
        }
    }

    struct Sum;
    struct Count;

    impl Aggregate<i64, Option<i64>> for Sum {
        fn init(&self) -> i64 {
            0
        }

        fn step(&self, ctx: &mut Context, sum: &mut i64) -> Result<()> {
            *sum += try!(ctx.get::<i64>(0));
            Ok(())
        }

        fn finalize(&self, sum: Option<i64>) -> Result<Option<i64>> {
            Ok(sum)
        }
    }

    impl Aggregate<i64, i64> for Count {
        fn init(&self) -> i64 {
            0
        }

        fn step(&self, _ctx: &mut Context, sum: &mut i64) -> Result<()> {
            *sum += 1;
            Ok(())
        }

        fn finalize(&self, sum: Option<i64>) -> Result<i64> {
            Ok(sum.unwrap_or(0))
        }
    }

    #[test]
    fn test_sum() {
        let db = Connection::open_in_memory().unwrap();
        db.create_aggregate_function("my_sum", 1, true, Sum).unwrap();

        // sum should return NULL when given no columns (contrast with count below)
        let no_result = "SELECT my_sum(i) FROM (SELECT 2 AS i WHERE 1 <> 1)";
        let result: Option<i64> = db.query_row(no_result, &[], |r| r.get(0))
            .unwrap();
        assert!(result.is_none());

        let single_sum = "SELECT my_sum(i) FROM (SELECT 2 AS i UNION ALL SELECT 2)";
        let result: i64 = db.query_row(single_sum, &[], |r| r.get(0))
            .unwrap();
        assert_eq!(4, result);

        let dual_sum = "SELECT my_sum(i), my_sum(j) FROM (SELECT 2 AS i, 1 AS j UNION ALL SELECT \
                        2, 1)";
        let result: (i64, i64) = db.query_row(dual_sum, &[], |r| (r.get(0), r.get(1)))
            .unwrap();
        assert_eq!((4, 2), result);
    }

    #[test]
    fn test_count() {
        let db = Connection::open_in_memory().unwrap();
        db.create_aggregate_function("my_count", -1, true, Count).unwrap();

        // count should return 0 when given no columns (contrast with sum above)
        let no_result = "SELECT my_count(i) FROM (SELECT 2 AS i WHERE 1 <> 1)";
        let result: i64 = db.query_row(no_result, &[], |r| r.get(0)).unwrap();
        assert_eq!(result, 0);

        let single_sum = "SELECT my_count(i) FROM (SELECT 2 AS i UNION ALL SELECT 2)";
        let result: i64 = db.query_row(single_sum, &[], |r| r.get(0))
            .unwrap();
        assert_eq!(2, result);
    }
}