const std = @import("std");
const util = @import("util");

const postgres = @import("./engines/postgres.zig");
//const postgres = @import("./engines/null.zig");
const sqlite = @import("./engines/sqlite.zig");
//const sqlite = @import("./engines/null.zig");
const common = @import("./engines/common.zig");
const Allocator = std.mem.Allocator;

const errors = @import("./errors.zig").library_errors;

pub const AcquireError = OpenError || error{NoConnectionsLeft};
pub const OpenError = errors.OpenError;
pub const QueryError = errors.QueryError;
pub const RowError = errors.RowError;
pub const QueryRowError = errors.QueryRowError;
pub const BeginError = errors.BeginError;
pub const CommitError = errors.CommitError;

pub const QueryOptions = common.QueryOptions;

pub const Engine = enum {
    postgres,
    sqlite,
};

// TODO: make this suck less
pub const Config = union(Engine) {
    postgres: struct {
        pg_conn_str: [:0]const u8,
    },
    sqlite: struct {
        sqlite_file_path: [:0]const u8,
        sqlite_is_uri: bool = false,
    },
};

const RawResults = union(Engine) {
    postgres: postgres.Results,
    sqlite: sqlite.Results,

    fn finish(self: RawResults) void {
        switch (self) {
            .postgres => |pg| pg.finish(),
            .sqlite => |lite| lite.finish(),
        }
    }

    fn columnCount(self: RawResults) !u15 {
        return try switch (self) {
            .postgres => |pg| pg.columnCount(),
            .sqlite => |lite| lite.columnCount(),
        };
    }

    fn columnIndex(self: RawResults, name: []const u8) error{ NotFound, Unexpected }!u15 {
        return switch (self) {
            .postgres => |pg| pg.columnIndex(name),
            .sqlite => |lite| lite.columnIndex(name),
        } catch |err| switch (err) {
            error.OutOfRange => error.Unexpected,
            error.NotFound => error.NotFound,
        };
    }

    fn row(self: *RawResults) RowError!?Row {
        return switch (self.*) {
            .postgres => |*pg| if (try pg.row()) |r| Row{ .postgres = r } else null,
            .sqlite => |*lite| if (try lite.row()) |r| Row{ .sqlite = r } else null,
        };
    }
};

// Represents a set of results.
// row() must be called until it returns null, or the query may not complete
// Must be deallocated by a call to finish()
pub fn Results(comptime T: type) type {
    // would normally make this a declaration of the struct, but it causes the compiler to crash
    const fields = if (T == void) .{} else std.meta.fields(T);
    return struct {
        const Self = @This();

        underlying: RawResults,
        column_indices: [fields.len]u15,

        fn from(underlying: RawResults) QueryError!Self {
            if (std.debug.runtime_safety and fields.len != underlying.columnCount() catch unreachable) {
                std.log.err("Expected {} columns in result, got {}", .{ fields.len, underlying.columnCount() catch unreachable });
                return error.ColumnMismatch;
            }

            return Self{ .underlying = underlying, .column_indices = blk: {
                var indices: [fields.len]u15 = undefined;
                inline for (fields) |f, i| {
                    indices[i] = if (!std.meta.trait.isTuple(T))
                        underlying.columnIndex(f.name) catch {
                            std.log.err("Could not find column index for field {s}", .{f.name});
                            return error.ColumnMismatch;
                        }
                    else
                        i;
                }
                break :blk indices;
            } };
        }

        pub fn finish(self: Self) void {
            self.underlying.finish();
        }

        // Returns the next row of results, or null if there are no more rows.
        // Caller owns all memory allocated. The entire object can be deallocated with a
        // call to util.deepFree
        pub fn row(self: *Self, alloc: ?Allocator) RowError!?T {
            if (try self.underlying.row()) |row_val| {
                var result: T = undefined;
                var fields_allocated: usize = 0;
                errdefer inline for (fields) |f, i| {
                    // Iteration bounds must be defined at comptime (inline for) but the number of fields we could
                    // successfully allocate is defined at runtime. So we iterate over the entire field array and
                    // conditionally deallocate fields in the loop.
                    if (i < fields_allocated) util.deepFree(alloc, @field(result, f.name));
                };

                inline for (fields) |f, i| {
                    // TODO: Causes compiler segfault. why?
                    //const F = f.field_type;
                    const F = @TypeOf(@field(result, f.name));
                    @field(result, f.name) = row_val.get(F, self.column_indices[i], alloc) catch |err| {
                        std.log.err("SQL: Error getting column {s} of type {}", .{ f.name, F });
                        return err;
                    };
                    fields_allocated += 1;
                }

                return result;
            } else return null;
        }
    };
}

// Row is invalidated by the next call to result.row()
const Row = union(Engine) {
    postgres: postgres.Row,
    sqlite: sqlite.Row,

    // Returns a value of type T from the zero-indexed column given by idx.
    // Not all types require an allocator to be present. If an allocator is needed but
    // not required, it will return error.AllocatorRequired.
    // The caller is responsible for deallocating T, if relevant.
    fn get(self: Row, comptime T: type, idx: u15, alloc: ?Allocator) common.GetError!T {
        if (T == void) return;
        return switch (self) {
            .postgres => |pg| pg.get(T, idx, alloc),
            .sqlite => |lite| lite.get(T, idx, alloc),
        };
    }
};

pub const ConstraintMode = enum {
    deferred,
    immediate,
};

pub const ConnPool = struct {
    const max_conns = 4;
    const Conn = struct {
        engine: union(Engine) {
            postgres: postgres.Db,
            sqlite: sqlite.Db,
        },
        in_use: std.atomic.Atomic(bool) = std.atomic.Atomic(bool).init(false),
        current_tx_level: u8 = 0,
    };

    config: Config,
    connections: [max_conns]Conn,

    pub fn init(cfg: Config) OpenError!ConnPool {
        var self = ConnPool{
            .config = cfg,
            .connections = undefined,
        };
        var count: usize = 0;
        errdefer for (self.connections[0..count]) |*c| closeConn(c);
        for (self.connections) |*c| {
            c.* = try self.createConn();
            count += 1;
        }

        return self;
    }

    pub fn deinit(self: *ConnPool) void {
        for (self.connections) |*c| closeConn(c);
    }

    pub fn acquire(self: *ConnPool) AcquireError!Db {
        for (self.connections) |*c| {
            if (tryAcquire(c)) return Db{ .conn = c };
        }
        return error.NoConnectionsLeft;
    }

    fn tryAcquire(conn: *Conn) bool {
        const acquired = !conn.in_use.swap(true, .AcqRel);
        if (acquired) {
            if (conn.current_tx_level != 0) @panic("Transaction still open on unused db connection");
            return true;
        }

        return false;
    }

    fn createConn(self: *ConnPool) OpenError!Conn {
        return switch (self.config) {
            .postgres => |postgres_cfg| Conn{
                .engine = .{
                    .postgres = try postgres.Db.open(postgres_cfg.pg_conn_str),
                },
            },
            .sqlite => |lite_cfg| Conn{
                .engine = .{
                    .sqlite = if (lite_cfg.sqlite_is_uri)
                        try sqlite.Db.openUri(lite_cfg.sqlite_file_path)
                    else
                        try sqlite.Db.open(lite_cfg.sqlite_file_path),
                },
            },
        };
    }

    fn closeConn(conn: *Conn) void {
        if (conn.in_use.loadUnchecked()) @panic("DB Conn still open");
        switch (conn.engine) {
            .postgres => |pg| pg.close(),
            .sqlite => |lite| lite.close(),
        }
    }
};

pub const Db = Tx(0);

/// When tx_level == 0, the DB is operating in "implied transaction" mode where
/// every command is its own transaction
/// When tx_level >= 1, the DB has an explicit transaction open
/// When tx_level >= 2, the DB has (tx_level - 1) levels of transaction savepoints open
/// (like nested transactions)
fn Tx(comptime tx_level: u8) type {
    return struct {
        const Self = @This();
        const savepoint_name = if (tx_level == 0)
            @compileError("Transaction not started")
        else
            std.fmt.comptimePrint("save_{}", .{tx_level});
        const next_savepoint_name = Tx(tx_level + 1).savepoint_name;

        conn: *ConnPool.Conn,

        /// The type of SQL engine being used. Use of this function should be discouraged
        pub fn sqlEngine(self: Self) Engine {
            return self.conn.engine;
        }

        /// Return the connection to the pool
        pub fn releaseConnection(self: Self) void {
            if (tx_level != 0) @compileError("close must be called on root db");
            if (self.conn.current_tx_level != 0) {
                std.log.warn("Database released while transaction in progress!", .{});
                self.rollbackUnchecked() catch {};
            }

            if (!self.conn.in_use.swap(false, .AcqRel)) @panic("Double close on db conection");
        }

        // ********* Transaction management functions **********

        /// Start an explicit transaction
        pub fn begin(self: Self) !Tx(1) {
            if (tx_level != 0) @compileError("Transaction already started");
            if (self.conn.current_tx_level != 0) return error.BadTransactionState;

            try self.exec("BEGIN", {}, null);

            self.conn.current_tx_level = 1;

            return Tx(1){ .conn = self.conn };
        }

        /// Create a savepoint (nested transaction)
        pub fn savepoint(self: Self) !Tx(tx_level + 1) {
            if (tx_level == 0) @compileError("Cannot place a savepoint on an implicit transaction");
            if (self.conn.current_tx_level != tx_level) return error.BadTransactionState;

            try self.exec("SAVEPOINT " ++ next_savepoint_name, {}, null);

            self.conn.current_tx_level = tx_level + 1;

            return Tx(tx_level + 1){ .conn = self.conn };
        }

        /// Commit the entire transaction
        pub fn commit(self: Self) !void {
            if (tx_level == 0) @compileError("Transaction not started");
            if (tx_level >= 2) @compileError("Cannot commit a savepoint");
            if (self.conn.current_tx_level == 0) return error.BadTransactionState;

            try self.exec("COMMIT", {}, null);

            self.conn.current_tx_level = 0;
        }

        /// Release the current savepoint and all savepoints created from it.
        pub fn release(self: Self) !void {
            if (tx_level == 0) @compileError("Transaction not started");
            if (tx_level == 1) @compileError("Cannot release a transaction");
            if (self.conn.current_tx_level < tx_level) return error.BadTransactionState;

            try self.exec("RELEASE SAVEPOINT " ++ savepoint_name, {}, null);

            self.conn.current_tx_level = tx_level - 1;
        }

        /// Rolls back the entire transaction
        pub fn rollbackTx(self: Self) !void {
            if (tx_level == 0) @compileError("Transaction not started");
            if (tx_level >= 2) @compileError("Cannot rollback a transaction using a savepoint");
            if (self.conn.current_tx_level == 0) return error.BadTransactionState;

            try self.rollbackUnchecked();

            self.conn.current_tx_level = 0;
        }

        /// Attempts to roll back to a savepoint
        pub fn rollbackSavepoint(self: Self) !void {
            if (tx_level == 0) @compileError("Transaction not started");
            if (tx_level == 1) @compileError("Cannot rollback a savepoint on the entire transaction");
            if (self.conn.current_tx_level < tx_level) return error.BadTransactionState;

            try self.exec("ROLLBACK TO " ++ savepoint_name, {}, null);

            self.conn.current_tx_level = tx_level - 1;
        }

        /// Perform whichever rollback is appropriate for the situation
        pub fn rollback(self: Self) void {
            (if (tx_level < 2) self.rollbackTx() else self.rollbackSavepoint()) catch |err| {
                std.log.err("Failed to rollback transaction: {}", .{err});
                @panic("TODO: more gracefully handle rollback failures");
            };
        }

        pub const beginOrSavepoint = if (tx_level == 0) begin else savepoint;
        pub const commitOrRelease = if (tx_level < 2) commit else release;

        // Allows relaxing *some* constraints for the lifetime of the transaction.
        // You should generally not do this, but it's useful when bootstrapping
        // the initial admin community and cluster operator user.
        pub fn setConstraintMode(self: Self, mode: ConstraintMode) !void {
            if (tx_level == 0) @compileError("Transaction not started");
            if (tx_level >= 2) @compileError("Cannot set constraint mode on a savepoint");
            switch (self.sqlEngine()) {
                .sqlite => try self.exec(
                    switch (mode) {
                        .immediate => "PRAGMA defer_foreign_keys = FALSE",
                        .deferred => "PRAGMA defer_foreign_keys = TRUE",
                    },
                    {},
                    null,
                ),
                .postgres => try self.exec(
                    switch (mode) {
                        .immediate => "SET CONSTRAINTS ALL IMMEDIATE",
                        .deferred => "SET CONSTRAINTS ALL DEFERRED",
                    },
                    {},
                    null,
                ),
            }
        }

        // ********** Query Helpers ************

        /// Runs a command without returning results
        pub fn exec(
            self: Self,
            sql: [:0]const u8,
            args: anytype,
            alloc: ?std.mem.Allocator,
        ) !void {
            try self.execInternal(sql, args, alloc, true);
        }

        pub fn queryWithOptions(
            self: Self,
            comptime RowType: type,
            sql: [:0]const u8,
            args: anytype,
            options: QueryOptions,
        ) QueryError!Results(RowType) {
            return Results(RowType).from(try self.runSql(sql, args, options, true));
        }

        pub fn query(
            self: Self,
            comptime RowType: type,
            sql: [:0]const u8,
            args: anytype,
            alloc: ?Allocator,
        ) QueryError!Results(RowType) {
            return self.queryWithOptions(RowType, sql, args, .{ .prep_allocator = alloc });
        }

        /// Runs a query to completion and returns a row of results, unless the query
        /// returned a different number of rows.
        pub fn queryRow(
            self: Self,
            comptime RowType: type,
            q: [:0]const u8,
            args: anytype,
            alloc: ?Allocator,
        ) QueryRowError!RowType {
            var results = try self.query(RowType, q, args, alloc);
            defer results.finish();

            const row = (try results.row(alloc)) orelse return error.NoRows;
            errdefer util.deepFree(alloc, row);

            // execute query to completion
            var more_rows = false;
            while (try results.row(alloc)) |r| {
                util.deepFree(alloc, r);
                more_rows = true;
            }
            if (more_rows) return error.TooManyRows;

            return row;
        }

        // Inserts a single value into a table
        pub fn insert(
            self: Self,
            comptime table: []const u8,
            value: anytype,
            alloc: ?std.mem.Allocator,
        ) !void {
            const ValueType = comptime @TypeOf(value);

            const fields = std.meta.fields(ValueType);
            comptime var types: [fields.len]type = undefined;
            comptime var table_spec: []const u8 = table ++ "(";
            comptime var value_spec: []const u8 = "(";
            inline for (fields) |field, i| {
                // This causes a compile error. Why?
                //const F = field.field_type;
                const F = @TypeOf(@field(std.mem.zeroes(ValueType), field.name));
                // causes issues if F is @TypeOf(null), use dummy type
                types[i] = if (F == @TypeOf(null)) ?i64 else F;
                table_spec = comptime (table_spec ++ field.name ++ ",");
                value_spec = comptime value_spec ++ std.fmt.comptimePrint("${},", .{i + 1});
            }
            table_spec = comptime table_spec[0 .. table_spec.len - 1] ++ ")";
            value_spec = comptime value_spec[0 .. value_spec.len - 1] ++ ")";
            const q = comptime std.fmt.comptimePrint(
                "INSERT INTO {s} VALUES {s}",
                .{ table_spec, value_spec },
            );

            var args_tuple: std.meta.Tuple(&types) = undefined;
            inline for (fields) |field, i| {
                args_tuple[i] = @field(value, field.name);
            }
            try self.exec(q, args_tuple, alloc);
        }

        // internal helpers

        fn runSql(
            self: Self,
            sql: [:0]const u8,
            args: anytype,
            opt: QueryOptions,
            comptime check_tx: bool,
        ) !RawResults {
            if (check_tx and self.conn.current_tx_level != tx_level) return error.BadTransactionState;

            return switch (self.conn.engine) {
                .postgres => |pg| RawResults{ .postgres = try pg.exec(sql, args, opt) },
                .sqlite => |lite| RawResults{ .sqlite = try lite.exec(sql, args, opt) },
            };
        }

        fn execInternal(
            self: Self,
            sql: [:0]const u8,
            args: anytype,
            alloc: ?std.mem.Allocator,
            comptime check_tx: bool,
        ) !void {
            var results = try self.runSql(sql, args, .{ .prep_allocator = alloc }, check_tx);
            defer results.finish();

            while (try results.row()) |_| {}
        }

        fn rollbackUnchecked(self: Self) !void {
            try self.exec("ROLLBACK", {}, null);
        }
    };
}